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QUEENSLAND UNIVERSITY OF TECHNOLOGY
SCHOOL OF PHYSICAL AND
CHEMICAL SCIENCES
Radiological Aspects of Petroleum Exploration
and Production in the Sultanate of Oman
Afkar Nadhim Al-Farsi
BSc (Hons), PGDip, MSc
A thesis submitted in partial fulfilment of the requirements of the degree of
Doctor of Philosophy
2008
i
Dedicated to my mother Sheikha Issa, and
To my late grant parents Jokha Ali and Issa Said
ii
Key words
NORM, radiological, petroleum, mining, dating, sludge farming, separation tanks, radium, thorium, radon, lead-210, actinium, potassium, gamma spectroscopy, gamma dose rate, 222Rn exhalation, sludge, oil scales, gas scales, evaporation pond sediment, ambient soil, Oman
iii
Abstract
This thesis is a study of naturally occurring radioactive materials (NORM)
activity concentration, gamma dose rate and radon (222Rn) exhalation from the
waste streams of large-scale onshore petroleum operations. Types of activities
covered included; sludge recovery from separation tanks, sludge farming,
NORM storage, scaling in oil tubulars, scaling in gas production and
sedimentation in produced water evaporation ponds. Field work was conducted
in the arid desert terrain of an operational oil exploration and production region
in the Sultanate of Oman.
The main radionuclides found were 226Ra and 210Pb (238U - series), 228Ra and
228Th (232Th - series), and 227Ac (235U - series), along with 40K. All activity
concentrations were higher than the ambient soil level and varied over several
orders of magnitude. The range of gamma dose rates at a 1 m height above
ground for the farm treated sludge had a range of 0.06-0.43 µSv h-1, and an
average close to the ambient soil mean of 0.086 ± 0.014 µSv h-1, whereas the
untreated sludge gamma dose rates had a range of 0.07-1.78 µSv h-1, and a mean
of 0.456 ± 0.303 µSv h-1. The geometric mean of ambient soil 222Rn exhalation
rate for area surrounding the sludge was 7.90.11.3 mBq m-2 s-1. Radon exhalation
rates reported in oil waste products were all higher than the ambient soil value
and varied over three orders of magnitude.
iv
This study resulted in some unique findings including: (i) detection of
radiotoxic 227Ac in the oil scales and sludge, (ii) need of a new empirical
relation between petroleum sludge activity concentrations and gamma dose
rates, and (iii) assessment of exhalation of 222Rn from oil sludge. Additionally
the study investigated a method to determine oil scale and sludge age by the use
of inherent behaviour of radionuclides as 228Ra:226Ra and 228Th:228Ra activity
ratios.
v
Contents Key words .................................................................................................................................... ii
Abstract ....................................................................................................................................... iii
Contents ....................................................................................................................................... v
List of Figures ........................................................................................................................... viii
List of Tables ............................................................................................................................... x
Statement of original authorship ............................................................................................. xii
Acknowledgements ................................................................................................................... xiii
CHAPTER 1 INTRODUCTION ............................................................ 1
1.1 Origin of petroleum ...................................................................................................... 1
1.2 History of NORM in the petroleum industry ............................................................. 2
1.3 Distribution of radioactivity in the petroleum exploration and production processes ........................................................................................................................ 3
1.4 Onshore operations ..................................................................................................... 11
1.5 Gaps in knowledge ...................................................................................................... 15
CHAPTER 2 LOCALITY AND OIL MINING ...................................... 17
2.1 The Sultanate of Oman ............................................................................................... 17
2.2 Mining sites .................................................................................................................. 18
2.3 The surrounding area ................................................................................................. 20
2.4 The oil mining process ................................................................................................ 21
2.5 Implications of Oman’s aging reservoirs .................................................................. 22
2.6 The future of oil exploration in Oman ...................................................................... 24
CHAPTER 3 SAMPLING AND MEASUREMENT TECHNIQUES .... 27
3.1 Introduction ................................................................................................................. 27
3.2 Dating of petroleum scale and sludge ........................................................................ 28
3.3 In-situ gamma spectroscopy ....................................................................................... 30
3.4 Laboratory gamma spectroscopy .............................................................................. 32 3.4.1 Sample collection and preparation ........................................................................... 32 3.4.2 Gamma spectroscopy measurement procedure ........................................................ 34
vi
3.5 Comparison between in-situ and laboratory gamma spectroscopy measurements .. ...................................................................................................................................... 37
3.6 In-situ gamma dose-rate measurements ................................................................... 40
3.7 Radon activity flux measurements using charcoal cups .......................................... 41
3.8 Radon exhalation rate measurements using the emanometer ................................. 43
CHAPTER 4 RADIOACTIVITY CONCENTRATION OF SCALE, SLUDGE AND SOIL SEDIMENT, FROM THE OIL FIELDS OF THE SOUTHERN OMAN DIRECTORATE .................................................... 47
4.1 Introduction ................................................................................................................. 47
4.2 Radioactivity in sludge ................................................................................................ 48 4.2.1 Sludge farming ......................................................................................................... 49 4.2.2 Radioactivity in ambient soil .................................................................................... 53 4.2.3 Radioactivity in the sludge recovered from a separator tank ................................... 55 4.2.4 Radioactivity in untreated piles at sludge farms ....................................................... 58 4.2.5 Radioactivity in treated sludge strips ....................................................................... 68
4.3 Bahja NORM store yard ............................................................................................ 76 4.3.1 Oil industry scales .................................................................................................... 78
4.3.1.1 Oil scale formation and removal ..................................................................... 78 4.3.1.2 Radioactivity in oil scales ............................................................................... 81
4.3.2 Gas industry scales ................................................................................................... 87 4.3.2.1 Gas scale formation and removal .................................................................... 87 4.3.2.2 Radioactivity in gas scales .............................................................................. 88
4.3.3 Comparison between oil and gas scales ................................................................... 91 4.3.4 Radioactivity in the sludge stored in barrels ............................................................ 92
4.4 Radioactivity in the sediments of Al-Noor evaporation ponds ................................ 96
4.5 Discussion and conclusions ....................................................................................... 100
CHAPTER 5 GAMMA DOSE RATES AT SLUDGE FARMS IN OILFIELDS OF THE SOUTHERN OMAN DIRECTORATE ................ 105
5.1 Introduction ............................................................................................................... 105
5.2 Terrestrial and cosmic gamma dose rates ............................................................... 105 5.2.1 Gamma dose rates in the petroleum industry ......................................................... 109
5.3 Materials and Methods ............................................................................................. 111
5.4 Results and discussion .............................................................................................. 112 5.4.1 Correlation between measured and predicted gamma dose rate ............................. 112 5.4.2 Development of a new gamma dose rate empirical model ..................................... 113 5.4.3 Gamma dose rate measurements ............................................................................ 117 5.4.4 Combining synthesised and measured gamma dose rates ...................................... 127
5.5 Conclusions ................................................................................................................ 130
vii
CHAPTER 6 RADON-222 EXHALATION FROM PETROLEUM INDUSTRY SCALE, SLUDGE AND SEDIMENT ................................ 133
6.1 Introduction ............................................................................................................... 133
6.2 Materials and methods ............................................................................................. 137
6.3 Results and discussion .............................................................................................. 138 6.3.1 Rn-222 exhalation rates .......................................................................................... 138
6.4 Conclusions ................................................................................................................ 153
CHAPTER 7 SUMMARY AND CONCLUSIONS ............................. 155
7.1 Summary .................................................................................................................... 155
7.2 Future directions ....................................................................................................... 167
References ................................................................................................................................ 170
viii
List of Figures Figure 1.1 Schematic on the precipitation of scales and sludge in production plant
and equipment where T: tubular, V: valves, W: wellheads, P: pumps, S: separation tank, H: water treatment vessel, G: gas treatment, O: oil storage tank ........................................................................................................................ 4
Figure 1.2 Primordial radioactive decay series (a) 238U, (b) 232Th and (c) 235U ........ 7 Figure 1.3 Produced water disposal in shallow, deep and producing reservoir wells
.............................................................................................................................. 10 Figure 1.4 Map of the Sultanate of Oman with Petroleum Development Oman’s
concession land ................................................................................................... 12 Figure 2.1 The five major oilfields studied during this research, all located within
PDO’s Southern Oman Directorate .................................................................. 19 Figure 2.2 Typical bedew rooming on their camels (picture courtesy of Trek Earth
http://www.trekearth.com/gallery/Middle_East/Oman/page19.htm) ............ 20 Figure 2.3 Daily oil, condensate and gas production in Oman over the last 11
years. .................................................................................................................... 23 Figure 3.1 The relative activity ratio of 228Th/228Ra and the relative decay of 228Ra
.............................................................................................................................. 29 Figure 3.2 (a) In situ gamma spectroscopy, and (b) A lead shield (designed and
poured at QUT) for shielding the NaI(Tl) probe of the portable gamma spectroscopy system. ........................................................................................... 31
Figure 3.3 Correlation between field portable NaI(Tl) and laboratory HPGe activity concentration readings for: (a) 226Ra Field vs 226Ra Lab, (b) 228Th Field vs 228Th Lab, (c) 40K Field vs 40K Lab and (d) 228Th Field vs 228Ra Lab............................ 39
Figure 3.4 Charcoal cups planted on a sludge pile ................................................... 42 Figure 3.5 (a) Schematic diagram of the emanometer, and (b) The emanometer in
its wooden box housing ...................................................................................... 44 Figure 3.6 (a) Emanometer’s airtight PVC sample chambers with FESTO valves
and Perspex cover, and (b) a sludge sample wrapped in perforated textile material, labelled and ready for 222Rn counting .............................................. 45
Figure 4.1 The sludge farming process: (a) sludge removed from a separation tank, (b) untreated sludge piles, (c) sludge piles after transport to the farming area, (d) a typical sludge strip, (e) watering the sludge strips, and (f) tilling the sludge strips. ................................................................................................. 52
Figure 4.2: Relation between 228Ra:226Ra and 228Th:228Ra activity ratios ............... 58 Figure 4.3: Relation between 226Ra and 228Ra for Marmul, Bahja and Nimr
untreated sludge piles. ........................................................................................ 65 Figure 4.4: 228Ra:226Ra and 228Th:228Ra mean activity ratios for Bahja, Nimr and
Marmul sludge farms. ........................................................................................ 66 Figure 4.5: Sludge pile activity concentration versus age (a) 226Ra, and (b) 228Ra. 67 Figure 4.6: NORM store yards: ................................................................................. 77 Figure 4.7: Average activity concentrations for radionuclides found in oil and gas
scales. ................................................................................................................... 92
ix
Figure 4.8: Section 2 of Al-Noor evaporation pond [picture courtesy of Mohammad Al-Masri] ....................................................................................... 97
Figure 5.1 Relation between measured and predicted gamma dose rates using UNSCEAR (2000) dose conversion factors. ................................................... 113
Figure 5.2 Relation between measured and empirically determined gamma dose rates. ................................................................................................................... 116
Figure 5.3 3D graph of gamma dose rate relation with both 226Ra and 228Ra activity concentrations. .................................................................................... 126
Figure 5.4 Measured and predicted gamma dose rate profiles at a 1 m height for untreated sludge piles and treated strips at Bahja, Nimr and Marmul sludge farms (where n denotes the total number of samples at each location). ...... 128
Figure 6.1 Emanometer to charcoal cup readings correlation .............................. 139 Figure 6.2 222Rn exhalation rate versus 226Ra activity concentration ................... 144 Figure 6.3 222Rn exhalation rate range and averages for the various sample types
............................................................................................................................ 150 Figure 6.4 Ratio of 222Rn exhalation rate to 226Ra activity concentration range and
averages for the various sample types ............................................................ 151
x
List of Tables Table 1.1 Typical 226Ra and 228Ra activity concentrations for various primary
production and power generation industries according to APPEA activity concentrations (kBq kg-1) .................................................................................. 14
Table 4.1: Bahja, Nimr and Marmul sludge farm locations, estimated volume of untreated sludge in piles and number of treated sludge Strips at the time of this study (Jan 2006 – June 2007) ..................................................................... 50
Table 4.2: Activity concentration (Bq kg-1) for ambient soils of Bahja, Al-Noor, Nimr and Marmul and the world average (UNSCEAR, 2000) (uncertainties represent counting error). .................................................................................. 54
Table 4.3: Activity concentration (Bq kg-1) and individual reading error of freshly removed sludge from a Nimr station separator tank ...................................... 56
Table 4.4: Sludge activity concentrations (Bq kg-1) and radioisotope ratios from Bahja, Nimr and Marmul untreated sludge piles ............................................ 60
Table 4.5: Activity concentrations (Bq kg-1) of untreated Bahja sludge piles ........ 61 Table 4.6 Activity concentrations (Bq kg-1) of untreated Nimr sludge piles
(analysed using the HPGe gamma spectroscopy system). ............................... 63 Table 4.7 Activity concentrations (Bq kg-1) of untreated Marmul sludge piles
(using the HPGe gamma spectroscopy system). .............................................. 64 Table 4.8 Activity concentration (Bq kg-1) of Bahja sludge strips. ......................... 71 Table 4.9 Activity concentrations (Bq kg-1) of Nimr sludge strips. ......................... 72 Table 4.10 Activity concentrations (Bq kg-1) of Marmul sludge strips. .................. 74 Table 4.11 Committed effective dose coefficients (µSv Bq-1) of selected
radionuclides likely to be present in petroleum scales (ICRP68) ................... 84 Table 4.12 Activity concentrations (Bq kg-1) of oil scale samples. .......................... 86 Table 4.13 Activity concentrations (Bq kg-1) of gas scale samples. ......................... 90 Table 4.14 Range of sludge 226Ra and 228Ra activity concentrations (kBq kg-1) for
oil exploration operations of several countries of the world ........................... 94 Table 4.15 Activity concentrations (Bq kg-1) for sludge stored in barrels. ............. 95 Table 4.16 Activity concentrations (Bq kg-1) of Al-Noor evaporation pond soil
sediments. ............................................................................................................ 99 Table 5.1 Air Kerma rate at 1 m height per disintegration rate (nGy h-1 per
Bq kg-1) of the parent nuclide per unit soil weight for natural sources uniformly distributed in the ground (adapted from Saito and Jacob). ....... 108
Table 5.2 Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles .......... 120
Table 5.3 Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips ............ 122
Table 5.4 Summary of field measured gamma dose rates (µSv h-1) at 1 m height for Bahja, Nimr and Marmul petroleum sludge treatment farms’ .............. 125
xi
Table 5.5 Number of samples, mean, median and range of the dose rates for untreated and treated sludge at Bahja, Nimr and Marmul obtained by both direct measurement and empirical relation. .................................................. 129
Table 6.1(a): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive waste. ............................... 140
Table 6.1(b): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive waste (emanometer measurements only). ......................................................................................... 142
Table 6.2: Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples........................................................................................... 148
Table 7.1 (a): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study ........................................................................................................... 159
Table 7.1 (b): Mean (± standard deviation), median and range of gamma dose rates in µSv h-1 for untreated and treated sludge in Bahja, Nimr and Marmul sludge farms, and ambient soil readings ........................................................ 162
Table 7.1 (c): Maximum, minimum and geometric mean of 222Rn exhalation rates in mBq m-2 s-1 and the geometric mean of radon exhalation to radium concentration ratio in mBq m-2 s-1/Bq kg-1 for the various sample types analysed in this study ...................................................................................... 164
xii
Statement of original authorship
The work contained in this thesis has not been previously submitted for a degree or a diploma at any other higher education institution. To the best of my knowledge and belief, the thesis contains no materials previously published or written by another person except where due reference is made. Signature: Date: 17 December 2008
xiii
Acknowledgements In the name of God, most Gracious, most Merciful. I initially would like to start by thanking God for the gift of life and for empowering his powerless creatures to realise (with hard work and dedication) their aspirations and dreams. Then I would like to have the honour of mentioning in my humble thesis the name of the greatest human being ever to walk on earth; “Mohammad” peace be upon him, and use one of his wise quotes: “He does not thank Allah, who does not thank people”. The success in carrying out various aspects of this work would not have been possible without the generous contributions in funding and volunteer support provided by many people. My heartfelt appreciation is given to all the persons involved. In the event of these acknowledgements failing to mention the names of any particular person(s) or organisation(s), I offer my sincerest apologies. I would like to thank Sultan Qaboos University for granting me the sabbatical to embark on the doctorate research degree, and also for allowing me use of Laboratory Instruments to analyse field collected samples of this research work. I would like to thank Queensland Univesity of Technology (QUT) for granting me the PhD Fee Waiver Scholarship, and for bearing the financial costs of the research’s instruments purchase and development along with international shipment between Brisbane (Australia) and Muscat (The Sultanate of Oman). I also would like to extend my thanks to Petroleum Development Oman (PDO) for the collaboration and logistic support during the research work, which included air transportation to and from the petroleum mining sites, land transportation within the mining sites, accommodation and meals. Not forgetting the special transportation arrangements made for the instruments by a cargo truck between Muscat and the desert mining locations. QUT academics, professionals and fellow students: Special thanks and gratitude are due to my principal supervisor: Dr. Riaz Akber for being my mentor, for his guidance, patience, expert feedback and enthusiastic help and support. Thanks are also due to my associate supervisor: Professor Lidia Morawska for being my mentor, for her guidance and encouragement. I am truly highly indebted to A/Professor Brian Thomas for his instrumental role in my enrolment at QUT, continuous encouragement, advice, help and support throughout my Master and Doctorate degrees. A lot of thanks are also due to my Master degree principle supervisor: Dr David Thiele for being a great mentor and bringing me up to the PhD degree doorstep. Thanks are also due to my Master degree associate supervisor: Dr Gregory Michael for his expert advice and support. Thanks and gratitude are also due to Ms. Rachael Robinson for the professional proof reading and Ms. Kerry Kruger for the final styling organisation of the thesis. Thanks are also due to the professional support of Mr. Jaya Dharmasiri, Mr. John Barrett, Ms. Elizabeth Stein, Ms. Magaret McBurney, Mr. Robert Organ and Mr. James Drysdale. I would also like to thank all my fellow students for their friendship, support and sharing educational experiences and especially: Hussein Kanaani, Yudi Wardoyo, Sade Fatokun, Diane Keogh, Mohammad Al-Roumy, Ezzat Abu Azza and Javaid Khan.
xiv
SQU academics and professionals: Many thanks are due to A/Professor Fadhil Mahdi Saleh, for his advice and recommendations on my career planning and development. Thanks are also due to Professor Lamk Al-Lamki, for his continuous support, help and encouragement. Many thanks to my colleagues; Dr Haddia Bererhi, Dr Nadir Atari, Mr Kirthi Jayasekara, Mr Hilal Al-Zheimi, Mr Mohammad Al-Subhi, Mrs Fatma Al-Maskery, Mrs Ibtisam Al-Maskery and Mrs Amaal Al-Rasby for their support and understanding with sharing the department’s facilities during this study’s sample analysis work. PDO professionals: Many thanks are due to Mr. Naaman Al-Naamany, Mr. Brett Young, Mr. Ahmed Al-Sabahi for their logistic support of the project and planning, providing transportation, accommodation and meals during the field work. The list of names for the people who offered volunteer help during the field work visits on the different locations is too long; to name a few: Mr. Said Saud Al-Maawaly, Mr. Salim Al-Rawahy, Mr. Nasser Abdullah Al-Kitany, Mr. Rashid Said Shinoun, Mr. Salim Al-Riyami, Mr. Rashid Al-Zakwani, Mr. Ahmed Al-Jabri and Mr. Seif Al-Habsy. To all the above and the others, I would like to say thank you for your help and support during the field visits. Other professionals: Many thanks are also due to Professor Mohammad Al Masri (Syrian Atomic Energy Agency) for advice and academic enrichment on the study during the field work, and collection of the sediment samples. Thanks are also due to Mr. Gert Jonkers (Shell Company) for providing us with bibliography on petroleum industry NORM. I would also like to thank Mr. Ibrahim Awad for his advice and help during sample collection. Family and friends: Thanks are also due to all my friends. Special thanks to all my family members for their support and encouragement. Finally, I would like to offer my sincere appreciation to my beloved wife: Ahlam Al-Adawi, for her relentless support, encouragement and sacrifice through this ostensibly jam-pact period of our lives.
1
Chapter 1 INTRODUCTION
1.1 Origin of petroleum
There are currently two plausible scientific theories that explain the process
of oil formation. The first is the biotic or biogenic theory, which states that oil
was formed hundreds of millions of years ago, following the extinction of
dinosaurs (i.e. terrestrial reptiles of the Mesozoic era) and algae that inhabited
the earth some 65-248 million years ago. The remaining organic matter was
then buried under many layers of sediment, and was exposed to high levels of
litho-spherical heat and pressure, which then transformed this preserved matter
into hydrocarbons (black gold or oil). According to geologists, this process is
thought to occur amid the earth’s solid rock layers, at temperatures ranging from
80-350 °C and pressures ranging from 0.8-2 kbar (Dyer and Graham, 2002,
Dutkiewicz et al., 2003). The oil then migrates and remains in porous stones
(such as limestone or sandstone, which have a porosity of about 20%) until it is
discovered.
The second theory is known as the abiotic or abiogenic theory, which states
that oil is not a fossil fuel, but that it was formed from inorganic materials deep
within the earth’s crust. According to this theory, hydrogen and carbon
molecules found in the earth’s mantle are subject to extremely high
temperatures and pressures, causing them to form hydrocarbon molecules,
which then migrate upward into oil reservoirs, through deep fracture networks
in the earth’s crust. Supporters of the abiogenic theory claim that the millions of
barrels of oil produced per day (1 barrel = 159 L) could not possibly be supplied
2
by the limited number of pre-historic animals (algae and dinosaurs) that existed.
For example, according to Morton (2004), the most productive oil well in Saudi
Arabia (Al-Ghawar) produces about 5x106 barrels of crude oil per day, with a
cumulative production of 5.5x1010 barrels of crude oil since 1951. Although
there is support for both theories, no one is mutually exclusive from the other,
and both may be equally as valid.
1.2 History of NORM in the petroleum industry
Kolb and Wojcik (1985) provide an interesting account of the discovery of
naturally occurring radioactive materials (NORM) in petroleum. The presence
of higher than background concentrations of radioactivity in crude petroleum
was reported for the first time more than a century ago, by Himstedt (1904) and
Burton (1904). In the 1920-1930’s, the presence of NORM was also reported in
numerous Russian and German research papers, however the first official
survey, from a radiation protection point of view, was not done until the early
1970’s.
Subsequent to the detection of NORM in a North Sea oil platform in 1981,
the presence of NORM in crude petroleum and petroleum industry waste has
been studied and reported by a number of authors worldwide, including Kolb
and Wajcik (1985), Smith (1987), Wilson and Scott (1992), Heaton and
Lambley (1995), Paschoa (1997), White and Rood (2001), Matta et al. (2002),
Godoy and Petinatti da Cruz (2003), Hamlat et al. (2003a), Hamlat et al.
(2003b), Smith et al. (2003), Al-Masri and Aba (2005) and Gazineu and Hazin
(2007).
3
In the petroleum industry, naturally occurring radionuclide concentrations
are often enhanced as a result of industrial operations. Whilst these materials are
formally referred to as technologically enhanced NORM (TENORM), the term
NORM is more widely used in both industry and the literature.
Examples of other industrial operations that have NORM present in their
primary material, products, by-products and waste include: uranium mining and
milling, metal mining and smelting, phosphate industries, coal mining and
power generation from coal, rare earth and titanium oxide industries, zirconium
and ceramics industries, building material disposal and the application of natural
radionuclides, such as radium and thorium. Whilst the activity levels of these
NORM are not always enhanced, simple chemical or physical changes can
sometimes take place, resulting in the radionuclides being more readily
available for transfer by various pathways (Heaton and Lambley, 1995,
UNSCEAR, 2001). For example, in uranium mining, simply bringing the
uranium to the surface can often leave radionuclide tailings, that may pose a
radiological threat to both the environment and the general public.
1.3 Distribution of radioactivity in the petroleum exploration and production processes
During the process of oil exploration and production (E&P), radioactive by-
products, such as scale and sludge, are formed and retained in the processing
equipment (Figure 1.1). Scales are formed in the electric submersible pumps,
down-hole tubular, upstream tubular and well heads (Testa et al., 1994, Al-
Masri and Aba, 2005, Othman et al., 2005), while its brittle nature can also
4
cause it to dislodge from the pipe walls and migrate to the oil-water separation
tanks.
Sludge, on the other hand, is found in the downstream tubular, water-oil
separation vessels, slops tanks of oil production facilities and storage tanks, and
as a result of the processes of tubular cleaning using ‘pig’ devices. It contains a
mixture of hydrocarbon, mud, natural radionuclides, sediments, bacterial
growth, corrosion particles and some scale debris (APPEA, 2002, Omar et al.,
2004). While the activity of radium in sludge is generally lower than that found
in scale (Vandenhove, 2002), it has still been found to be significantly above
background levels.
Figure 1.1: Schematic on the precipitation of scales and sludge in production plant and equipment where T: tubular, V: valves, W: wellheads, P: pumps, S: separation tank, H: water treatment vessel, G: gas treatment, O: oil storage tank.
S
H
O
G P
W
V
Scale
T
Formation water, oil and gas
DownstreamUpstream
Sludge
210Pb film
5
The estimated annual radium activity brought to the surface by global oil
exploration is in the order of 10 TBq (Lieser, 1995). This not only poses
significant health risks to the industry workers, but also to the community and
environment as a whole. In the United States, they are spending about
US$ 6 billion per year on the clean up and containment of such radioactive
waste (Harley, 2000).
The NORM found in scale and sludge are mainly from the 238U (T½: 4.5
billion years) and 232Th (T½: 14 billion years) natural radioactive decay series
(Figures 1.2 (a) and (b)). In contrast, Ac-227 (T½: 21.77 years) from the 235U
(T½: 0.7 billion years) natural radioactive decay series (Figure 1.2 (c)) was
detected for the first time during this research, and prior to this, it was only ever
mentioned in passing by Kolb and Wajcik (1985). The main isotopes found in
scale and sludge are those of 226Ra (T½: 1602 years; 238U primordial series), and
to a lesser degree 228Ra (T½: 5.75 years; 232Th primordial series). These two
radium isotopes are present as both sulphates and carbonates in the strontium,
barium and calcium mineral scales that develop in the tubular and other areas of
the extraction rigs (Wilson and Scott, 1992, Hamlat et al., 2001, Godoy and
Petinatti da Cruz, 2003, Al-Masri and Aba, 2005).
In the petroleum reservoirs, crude oil co-exists with underground water,
usually called ‘formation water’ or ‘formation brine’. While the initial oil
production process is usually dry (Smith, 1987), as the reservoir pressure falls
over time, water can also be co-produced with the crude oil, and this water is
given the name ‘produced water’. The amount of NORM formed in oil
6
producing fields and incorporated into the scale and sludge, is directly
proportional to the volume of produced water generated during the pumping of
the oil (Rood et al., 1998, Paranhos Gazineu et al., 2005).
238U
234Th
234Pa
234U
230Th
226Ra
222Rn
218Po
214Pb
210Po
210Bi
214Po
214Bi
206Pb 210Pb
Leach from reservoir rock into formation water
Transported with natural gas
α α
α
α
α
α α α
4.5 billion years
β β
ββ
β
β 24 days
1.2 minutes 240 years
77,000 years
1,602 years
3.8 days
3.1 minutes
27 minutes
20 minutes 160 micro-seconds 5 days
22.3 years
140 days
Precipitate on internal surfaces of petroleum equipment
(a)
7
Figure 1.2: Primordial radioactive decay series (a) 238U, (b) 232Th and (c) 235U.
(b)
310 nano- seconds
232Th 228Ac
228Ra
220Rn
224Ra
228Th
212Pb
216Po
208Tl
212Po
208Pb
212Bi
Leach from reservoir rock into formation water
Transported with natural gas
α 14 billion years
β 5.75 years
β6.1 hours
α
α
αα
α
α 1.9 years
3.7 days
56 seconds
0.15 seconds
β
β
β 11 hours
61 minutes (64%)
61 minutes (36%)
3.1 minutes
Precipitate on internal surfaces of petroleum equipment
235U
231Pa 231Th
α 700 million years
β 26 hours
1.8 milliseconds
219Rn
223Ra
227Th
211Pb
215Po
α
α
α
α 19 days
11 days
4 seconds
β36 minutes
207Tl
207Pb
211Bi
α
α
β
β22 years
(99%)
2.1 minutes
4.8 minutes
227Ac
223Fr
α
33,000 years
22 years (1%)
β22 minutes
Detected in oil and gas scales
(c)
8
The major radionuclides found in produced water include 226Ra, 228Ra, 224Ra
and 210Pb, in concentrations of up to a few hundred becquerels per litre (IAEA,
2003). This is because radium isotopes leach into the oil reservoir, as a result of
their high solubility in water, when compared to uranium and thorium. The high
temperature and pressure in the oil reservoirs also aid in the leaching of radium
from reservoir rock into the formation water. Because geological formations are
not always closed, this may allow 226Ra to migrate into the soil matrix and deposit
elsewhere outside the formation. If this takes place, the secular equilibrium of
226Ra with its parent will no longer exist, and the radium is then said to be
“unsupported”, meaning that its activity is not related to the activity of its series
predecessors (Paranhos Gazineu et al., 2005).
The produced water associated with the explored oil is usually saline,
containing potentially high levels of mineral salts, such as sodium chloride
(NaCl). They contain not only elements of low potential toxicity (Na, K, Ca, Ba,
Sr and Mg), but also more toxic elements, such as Pb, Zn, Cd and Hg. Other
minerals that may be present in the produced water include traces of oil, metals
and noxious gases. The reported salinities of produced water vary from 1-
400 g L-1 (USGS, 1997). For comparison, seawater has a salinity of around
35 g L-1 (USGS, 1997). The United States Environmental Protection Agency
(USEPA, 2005), Australia’s National Health and Medical Research Council
(NHMRC, 1996), and the World Heath Organisation (WHO, 2004) all
recommended that the level of salinity or total dissolved solids (TDS) in safe
drinking water should be less than 0.5 g L-1, though up to 1 g L-1 is palatable.
9
The salinity of the produced water may increase over the production lifetime
of an oil well, suggesting the co-production of brine. As such, the dissolution of
radium and other group II elements from the formation rock may be enhanced by
the higher salinity of the produced water, in a manner similar to that which occurs
as a result of injecting seawater to enhance recovery. This indicates that NORMs
may be absent at the start of production, but may appear in the later stages of the
well’s lifetime. The quantities of scale and sludge produced vary significantly
between reservoirs, individual wells and production conditions. Therefore, there
is no typical radionuclide concentration in NORMs from oil and gas production,
nor is there a typical quantity of scale and sludge produced annually or over the
lifetime of an oil well.
Petroleum companies have several management options with regard to the
produced water, after it has been separated from the crude oil. These include,
injection of the produced water into deep abandoned oil wells, pumping it into
evaporation ponds, or injecting it back into reservoirs to aid in the recovery of
more crude oil (Figure 1.3). The shallow disposal management option has been
consistently reduced in order to minimise produced water surface environmental
impact.
10
621 674 696 730 764 733
497550 547 575
645 732
385312 276 217
152 122
02 03 04 05 06 07
Shallow disposalDeep disposalProducing resevoir
Figure 1.3: Produced water disposal in shallow, deep and producing reservoir wells.
Another alternative was also found when a new technology called “Solar
Dew” was introduced by Petroleum Development Oman (PDO), designed to
purify the produced water. This system purifies water using a solar-driven, non-
porous membrane distillation process. Water purified using this process passes
World Health Organisation (WHO) and Oman standards for drinking water.
However, it does require the addition of a number of minerals before it is deemed
adequate for human consumption. A desert greening project has also been
successfully launched in Oman using solar dew purified water for irrigation
(PDO, 2001).
Year
Vol
ume
(mill
ion
of b
arre
ls)
11
1.4 Onshore operations
Oman discovered oil in 1962, and the first oil export consignment took place
in 1967. Oil is the country’s major source of income; it comprises 80% of the
export income, and 40% of the Gross Domestic Product (GDP). To date, the
largest oil producing company in Oman, Petroleum Development Oman (PDO),
has only been undertaking onshore oil exploration and production. It produces
90% of the country’s crude oil and almost all of the country’s natural gas.
PDO’s major shareholders are: the Oman Government (60%), Royal Dutch
Shell (34%), Total (4%) and Partex (2%).
The facilities operated by PDO cover approximately 114,000 km2 of
concession land (PDO, 2001), and the Oman government has granted PDO the
use of this land (Figure 1.4) for oil and gas exploration until 2044. Within this
area, PDO executes a range of activities, such as seismic surveys, drilling and
production. The area comprises 120 producing oil fields, 3,750 producing wells,
59 gathering and production stations and over 7,000 km of pipes and flow lines,
and these operations invariably create localised disturbances within the
environment.
12
Figure 1.4: Map of the Sultanate of Oman with Petroleum Development Oman’s
concession land.
13
An internal PDO inventory of oily waste, conducted in 2001, revealed that it
has in excess of 100,000 tonnes of oily sand/sludge, stored at 8 waste
management centres in the county’s interior regions, and a further 30,000 tonnes
was being treated at the time (PDO, 2001). These sludge volumes would have
substantially increased since then, due to the fact that an estimated 72,000
tonnes of oily sludge are produced annually (Al-Futaisi et al., 2007).
At PDO, one of the major issues they face is the handling of radioactive
sludge material and its eventual disposal. As mentioned earlier, the sludge
contains radioactivity, mainly due to radioisotopes of radium (226Ra and 228Ra).
As radium isotopes and their progeny are strong gamma emitters, the external
radiation dose in the vicinity of separation tanks increases as sludge builds up.
Moreover, frequent cleaning and replacing the lining of these tanks further
increases the external radiation dose. This work is generally carried out by
personnel accessing the interior of the tank, and because of the confined
environment within the tank, 222Rn and 220Rn tend to build up, leading to
significant air concentrations of hazardous radioactive material.
Petroleum industry sludge disposal methods are similar to those used to
dispose of tailings from the mining and milling of uranium ores. The average
ore grade at Roxby and ERA Ranger uranium mines in Australia are such that
the 226Ra activity concentrations in their tailing repositories are expected to be
6.6 and 31 kBq kg-1, respectively (Sonter et al., 2002). These values are
comparable to the PDO sludge activity concentration range of 0.15-1 kBq kg-1
(Salih et al., 2005). The Australian Petroleum Production and Exploration
14
Association (APPEA, 2002) also reported similar activity concentrations of
228Ra and 226Ra found in the NORMs of primary production and power
generation industries (Table 1.1).
Table 1.1: Typical 226Ra and 228Ra activity concentrations for various primary production and power generation industries according to APPEA activity concentrations (kBq kg-1)
Material/Grade 228Ra 226Ra
Uranium ore (1% U) - 120
Monazite 200 – 290 1.2 – 37
Xenotime 61 48
Fly ash 0.56 0.56
Phosphate rocks 4.8 0.12
To date in Oman, sludge with 226Ra activity concentrations > 1 kBq kg-1 is
contained in barrels, which are temporarily stored in confined concrete fenced
areas. The more oily sludge is stored in recently engineered hold up ponds
(100 x 10 x 1 m), lined with special geo-textile material. However, the issue of
long-term disposal of sludge remains and current methods need to be thoroughly
evaluated in order to ascertain their merits and demerits. On the other hand,
sludge with 226Ra activity concentration < 1 kBq kg-1 undergoes sludge farming
process.
The removal of scale from NORM contaminated tubular also generates
radioactive waste. This is commonly done by human operated machines that use
mechanical arms and pneumatic pressure to scrape and blow away the scale
15
from the inner surface of the pipes. However, significant amounts of dust are
also suspended during this process, resulting in the inhalation and ingestion of
radioactive particles by personnel. The total radiation dose can then be derived
from the internal dose exposure pathways and the external dose pathways.
PDO has thousands of NORM contaminated tubular and processing pieces of
equipment that require the scale removal process. This de-scaling project is still
under review by PDO’s Health, Safety and Environment (HSE) department.
1.5 Gaps in knowledge
Studies of NORM enhancement in the scale, sludge and produced water of
typical petroleum industries have been conducted in the past (Kolb and Wojcik,
1985, Heaton and Lambley, 1995, Paschoa, 1997, Spitz et al., 1997, Shawky et
al., 2001, White and Rood, 2001, Jerez Veguería et al., 2002, Paschoa and
Godoy, 2002, Al-Masri, 2006), however, most of these studies focus on the
offshore exploration and production operations conducted in the North Sea and
Brazil. According to an assessment reported by the European Commission
(Oman, 2008), the petroleum industry now contributes more radioactivity to the
North Sea than the nuclear power industry, which includes numerous power
stations and processing plants.
The main objective of this research project is to conduct a comprehensive
assessment and evaluation of the activity concentration and gamma dose rate of
large-scale onshore petroleum operations in ‘the Sultanate of Oman’. To the
best of our knowledge, no comprehensive and systematic studies on the
radiological impact of the industry have ever been carried out, particularly in the
16
Middle East and the Gulf region, which is the largest producer of oil worldwide.
This research also investigates radon gas exhalation from petroleum treated and
untreated sludge, oil and gas scales samples, and from evaporation pond
sediment soils. This assessment of radon gas exhalation in the petroleum
industry is the first of its kind; however it is somewhat limited in scope, due to
the fact that access permits were only granted to a limited number of sites in the
area.
17
Chapter 2 LOCALITY AND OIL MINING
2.1 The Sultanate of Oman
The Sultanate of Oman is one of six oil producing countries that make up the
Gulf Cooperation Council (GCC). These countries include the Sultanate of Oman,
the Kingdom of Saudi Arabia, the Kingdom of Bahrain, the United Arab
Emirates, Qatar and Kuwait. Oman is situated in the south eastern corner of the
Middle East (Arabian Peninsula), located between latitudes 16.40-26.20°N and
longitudes 51.50-59.40°E. It has an area of 309,500 km2 and a population of
2.6 million (Oman, 2008). It is bordered by the United Arab Emirates to the
northwest, the Kingdom of Saudi Arabia to the west and the Republic of Yemen
to the southwest. Oman also has a coast line of approximately 1700 km, adjoining
the Oman Gulf along its north eastern boarder and the Arabian Sea to the south
east. The capital, Muscat, is located on the country’s north eastern border, near
the Tropic of Cancer.
To date, the largest oil producing company in Oman, ‘Petroleum
Development Oman (PDO)’, produces 90% of the country’s crude oil and almost
all of the country’s natural gas. In 2006, a workforce of 5,000 people were
managing and handing the oil mining process at PDO (PDO, 2006).
18
2.2 Mining sites
PDO have divided their operations into two regions, otherwise known as the
“Northern Oman Directorate (NOD)” and the “Southern Oman Directorate
(SOD)”. The NOD accounts for half of PDO’s production and it contains four
clusters of oil fields, namely the Lekwair, Fahud, Yibal and Qarn Alam clusters.
The SOD accounts for the other half of production and it contains five clusters,
namely the Bahja, Marmul, Nimr, Harweel and Rahab Thuleilat Qaharir (RTQ)
clusters. The major oilfields studied during this research included Al Noor,
Bahja, Marmul, Nimr and Zuliya, which were all located within PDO’s SOD
(see Figure 2.1).
19
Figure 2.1: The five major oilfields studied during this research, all located within PDO’s Southern Oman Directorate.
20
2.3 The surrounding area
Inhabitants from the surrounding areas are predominantly bedews
(shepherds), who traditionally moved around with their herds of sheep and
camels to wherever water and food could be found (Figure 2.2). However, a
recent shift towards a more semi-urban lifestyle has seen many bedews
beginning to settle in permanent wooden or concrete houses. And in addition to
relying on their livestock for food, they are now beginning to rely on the
increasing number of grocery stores that have opened up in scattered locations
throughout the local area.
Figure 2.2: Typical bedew rooming on their camels (picture courtesy of Trek Earth http://www.trekearth.com/gallery/Middle_East/Oman/page19.htm).
These newly established residential areas are located in close proximity
(within 10km) to the sludge farms of Bahja, Nimr and Marmul, as well as the
21
Bahja NORM store yard. These facilities are surrounded by wire mesh boundary
fences, with guarded gates, in order to ensure that the local community, and
straying animals, are kept out of these contaminated areas.
2.4 The oil mining process
Oil mining can be divided into four major phases: exploration, drilling,
production, and rehabilitation and restoration. In the exploration phase, seismic
waves are used to detect the presence of underground hydrocarbons in the
surrounding area. Rock samples are also taken for laboratory analysis and
exploration wells are then drilled, in order to confirm the existence of oil
reservoirs. Once the presence of oil has been confirmed, the drilling of production
wells can then commence. This drilling process is a precise science, which can be
expensive and extremely hazardous, since many oil and gas reservoirs exist at
very high temperatures and pressures, ranging from 80-350 °C and 0.8-2 kbar,
respectively (Dyer and Graham, 2002, Dutkiewicz et al., 2003).
Once the wells have been drilled, they are then secured and capped, before
finally being connected to a collection of valves called the wellhead. These valves
channel the flow of crude oil (which coexists with both saline produced water and
natural gas) from the reservoir into distribution pipelines. After travelling along
these pipelines, which can range from a few kilometres to tens of kilometres long,
the crude oil eventually reaches the gathering station, where it enters large
separation tanks (1200 m3) that allow for the crude oil, natural gas and produced
water to separate. The dehydrated, degassed crude oil is then pumped to Mina
Al-Fahal sea port in Muscat (which is up to 800 km from some gathering stations)
22
for export on large oil tankers. In contrast, the natural gas component is delivered
to a number of Oman Government agencies for local use, as well as to the Oman
and Qalhat Liquefied Natural Gas (LNG) plants, near Sur sea port, for export on
LNG transporting tankers. The produced water is dealt with in a number of ways
that have already been discussed in detail in Chapter 1.
2.5 Implications of Oman’s aging reservoirs
When oil flows to the surface naturally, as a result of its own reservoir
pressure, it is by far the most convenient and cost effective means of oil
extraction. However, when an oil well begins to age and the reservoir pressure
falls, it becomes necessary to use artificial methods of extraction, such as
electrical submerged pumps or beam pumps (nodding donkeys). In general,
petroleum companies only recover 30-40% of the oil contained in a given
reservoir using this method (Chierici, 1992, Carrero et al., 2007).
While the oil production process is essentially anhydrous, with time, and as
the reservoir pressure falls, produced water is often co-produced along with the
crude oil. Hence, as a well ages, the ratio of produced water to crude oil increases,
sometimes reaching as high as 95% of the total production volume, while still
remaining economically viable (Oil & Gas_UK, 2008).
In 2002, PDO produced 3.774 million barrels (6 x 105 m3) of produced water
per day, while its daily crude oil production was 0.849 million barrels
(1.35 x 105 m3) – a total produced water volume of 82%. However, by the end of
2006, PDO’s produced water had increased to 88% of the total production volume
23
(PDO, 2006), indicating that PDO’s oil reservoirs may have already reached their
peak production capacities (see Figure 2.3).
0
300
600
900
1200
97 98 99 00 01 02 03 04 05 06 07
GasCondensate Black oil
Figure 2.3: Daily oil, condensate and gas production in Oman over the last 11 years.
This problem of aging oil wells is not restricted only to Oman. In 1993, the
United States total annual produced water volume was 25 billion barrels
(4 x 109 m3), whereas the crude oil total volume was only 2.5 billion barrels
(4 x 108 m3) – a total produced water volume of 91%. In response to this global
problem, PDO have started to investigate the use of enhanced oil recovery (EOR)
technologies, which work by flooding the reservoir with substances such as: (i)
reinjected produced water; (ii) a special water/polymer mix; (iii) gas (otherwise
known as ‘gas lifting’); or (iv) steam, in order to force the remaining, more
viscous oil into adjacent producing wells.
Year
Ave
rage
dai
ly p
rodu
ctio
n (th
ousa
nds o
f bar
rels
of o
il eq
uiva
lent
)
24
After flooding a well with reinjected produced water, the amount of
remaining oil may still be as high as 70% of the original oil volume, as the oil is
often too viscous and heavy to be moved by the water alone. However, the
addition of an alkaline surfactant polymer to the produced water, prior to
reinjecting it into the reservoir, increases the viscosity of the injected fluid and
increases the oil recovery factor, along with the final volume of oil produced
(Carrero et al., 2007).
Gas lifting, on the other hand, works by pumping natural gas into the
reservoir, which then mixes with the oil, making it less viscous and thus, more
mobile. Once the oil has been extracted, the gas is then recovered and reinjected
back to the reservoir to extract more oil. Similarly, injecting steam into the
reservoir also acts to decrease the viscosity of heavy oil, thus also increasing
yields.
Whilst the complete removal of oil from reservoirs is not possible with any of
the existing EOR technologies, together, these four methods have the potential to
enhance the average oil recovery factor to well over 50% (Doscher and Wise,
1976, Carrero et al., 2007).
2.6 The future of oil exploration in Oman
A report by the Oman Ministry of National Economics revealed that, despite
the Government’s efforts to expand the country’s range of income sources,
petroleum products remain the dominant income earner and driver of the Oman
25
economy. In 2006, oil and gas exports accounted for approximately 80% of all
export earning revenue and more than 20 international companies are now
exploring for oil and gas throughout the country. As an example, in early 2008,
a new offshore exploration contract was signed by the Oman Ministry of Oil
and Gas, which extended the Gulf of Oman offshore concession area by
23,850 km2, in addition to the 21,000km2 already allocated in Block 18 (Prabhu,
2007).
The current world oil price makes exploration of geologically challenging
reservoirs economical. The Oman Government’s pursuit of new offshore
explorations and embarking on new EOR technologies is a mark of commitment
to a sustainable oil supply (Al-Shaibany, 2007).
26
27
Chapter 3 SAMPLING AND MEASUREMENT TECHNIQUES
3.1 Introduction
Various measurements were performed on scales, sludge and soil sediment,
both in-situ and on physical samples collected from the study locations. The in-
situ analysis consisted of: (a) collecting gamma spectra measurements using a
portable gamma spectroscopy system consisting of a Pb shielded 2 ¼” NaI(Tl)
scintillation crystal connected to a multi-channel analyser (MCA, model 7000
Rainbow) with 1024 channels; (b) gamma dose rate measurements using an
energy compensated Geiger-Müller (GM) probe Mini Instrument Type 6-80;
and (c) charcoal cups analysis, for determining radon exhalation rates. On the
other hand, the physical samples underwent: (a) gamma spectroscopy analysis
using an Ortec EG&G high purity germanium detector (HPGe); and (b)
determination of radon exhalation, using a specially designed emanometer.
Gamma dose rate measurements were carried out along with gamma
spectroscopy measurements, on both treated and untreated sludge. Onsite total
gamma counts were also collected over a preset duration of 600 s and charcoal
cups were planted to determine 222Rn exhalation at the same locations. Physical
samples were also collected from piles and strips, which were then transported
to the Medical Physics Unit Laboratory in Muscat, where they underwent
gamma spectroscopy and 222Rn activity flux analysis.
28
3.2 Dating of petroleum scale and sludge
Radionuclide activity ratios can be used to date scale and sludge samples,
using two different methods. The first method makes use of the 228Ra:226Ra
activity ratio, where the former has a shorter half life than the latter. This
method is valid, provided that the radium isotopes incorporated into a radium
insoluble mineral approximate a chemically closed system (Zielinski et al.,
2001, Al-Masri and Aba, 2005). Observations made by Al-Masri (2006) and
Zielinski et al. (2001) have shown that this ratio is fairly constant for specific
formations and varies from 0.5-2 for nascent samples, which corresponds to
Th:U mass ratio of 1.5 to 6. Ahmad et al. (2003) and Al-Masri (2006) have
illustrated a potential use of this ratio, whereby the ratio which is obtained from
produced water samples can be used to determine if two oil wells are sharing the
same reservoir. Similarly, if the ratio for a certain reservoir changes over time,
this can indicate produced water breakthrough from a nearby water source. We
did not use 228Ra:226Ra activity ratio for the purpose of reservoir fingerprinting.
The second method for dating petroleum sludge and scale samples used the
228Th:228Ra ratio. In this method, a zero activity concentration of 228Th is
assumed at the start of sludge formation. This is because the produced water co-
extracted with crude oil is thorium free and an ingrowth of 228Th takes place
with the decay of 228Ra, whereby the parent-progeny transient equilibrium ratio
approaches 1.5 (Figure 3.1).
29
0.00
0.25
0.50
0.75
1.00
1.25
1.50
0 5 10 15 20 25
Th-228/Ra-228 activity ratio
Ra-228 decay
Th-228 accumulation
Figure 3.1: The relative activity ratio of 228Th/228Ra and the relative decay of 228Ra.
The first method can be used to find the age of scale and sludge samples up
to 40 years, provided the initial 228Ra/226Ra ratio is known. Beyond this, the
228Ra would have gone through more than seven half lives, and may no longer
be detectable enough to make accurate estimates of age. In contrast, the second
method is more suitable for estimating the age of relatively new scale or sludge,
typically less than 10 years. This is because as the change in relative activity
beyond 10 years approaches the 1.5 transient equilibrium ratio, it becomes more
difficult to resolve age with acceptable uncertainty.
Time (years)
Rel
ativ
e A
ctiv
ity
30
3.3 In-situ gamma spectroscopy
A portable gamma spectroscopy system was used to collect gamma spectra
measurements in the field (Figure 3.2 (a)). The spectrometer was energy
calibrated by Amersham Cs-137 and Co-60 standard reference sources at the
Queensland University of Technology (QUT) Radiological Laboratory,
Brisbane, Australia. Calibration by 232Th, 238U and 40K radionuclides was
performed at the CSIRO Core Library, Sydney, Australia. The calibration
facility comprised of five concrete circular slabs, namely background,
potassium, uranium, thorium and mixed, each being 0.8 m thick and 2.0 m
diameter.
In order to shield the detector from adjacent cosmic radiation and gamma
rays, and ensure consistent detection geometry over the measured area of
interest (being 1.5 m2 when the system was set at a height of 1 m), a Pb casket
was designed to surround the NaI(Tl) detector (Figure 3.2 (b)). At 3.5 cm thick
and 10.75 cm deep, the shield was able to attenuate approximately 82% of
2.615 MeV (208Tl ), 84% of 1.765 MeV (214Bi) and 85% of 1.461 MeV (40K) of
incident gamma radiation. Although the 18 kg shield was not able to eliminate
the effects of incident radiation completely, a larger shield would not have been
easily portable, so a compromise had to be made between practicality and
accuracy of the gamma spectroscopy measurements. A tripod was used to
mount the portable NaI(Tl) gamma spectroscopy system, including the Pb
shield, at a height of 1 m, and the spectra were recorded for a preset duration of
600 s.
31
Figure 3.2: (a) In situ gamma spectroscopy, and (b) A lead shield (designed and poured at QUT) for shielding the NaI(Tl) probe of the portable gamma spectroscopy system.
The spectra were then downloaded from the MCA to a laptop and an energy
calibration was applied. Carefully determined regions of interest (ROI) were
used to obtain total counts under the three major peaks: 1.461 MeV (40K),
1.765 MeV (214Bi) and 2.615 MeV (208Tl ). The peaks had a range of 1.43-1.49,
1.69-1.87 and 2.55-2.67 MeV, respectively. An algorithm was also developed to
find factors by a 3x3 inverse matrix method, that were applied on 40K, 214Bi and
208Tl peak counts in order to strip peak cross-talk and obtain activity
concentrations of these radionuclides in Bq kg-1.
In ambient soils, 238U and 232Th are in secular equilibrium with their
progeny, however because the 238U and 232Th series in the petroleum sludge
samples started from 226Ra and 228Ra, respectively, it was evident that the
equilibriums of the two series had been disturbed. The 238U progeny 214Po, 214Bi,
214Pb, 218Po and 222Rn were in secular equilibrium with unsupported 226Ra, while
the 232Th progeny 208Tl, 212Po, 212Bi, 212Pb, 216Po, 220Rn and 224Ra were in
(b) (a)
32
secular equilibrium with supported 228Th. Further, 228Th and 228Ac were also in
transient and secular equilibrium with 228Ra, respectively. In the above two
series, 222Rn and 220Rn radioactive gases were assumed to have been fully
retained in the surveyed samples. For ambient soil, because secular equilibrium
is assumed with 232Th, the branching ratio corrected 208Tl activity concentration
would represent all of its predecessor radionuclides. However, because the
series starts from 228Ra in sludge, transient equilibrium exists between 228Th and
228Ra, and thus the branching ratio corrected 208Tl activity concentration would
only represent predecessors up to 228Th. This makes determination of 228Ra by
the portable NaI(Tl) system for petroleum sludge complex because; (a) 228Th is
only present by ingrowth, and therefore a fresh sludge may have a significant
amount of 228Ra, but zero 208Tl due to zero 228Th, resulting in false activity
concentration for 228Ra, (b) Due to the ingrowth of 228Th into 228Ra, the ratio of
228Th:228Ra will vary according to sample age until 228Th reaches transient
equilibrium with 228Ra with an equilibrium factor of 1.5. This makes it
impossible to accurately determine the activity concentration of 228Ra unless the
ratio of 228Th:228Ra is known by other means.
3.4 Laboratory gamma spectroscopy
3.4.1 Sample collection and preparation
Sludge Samples: A number of 100-300 g surface sludge samples were
collected from freshly removed sludge, sludge piles, sludge strips, sludge
storage barrels, a sand waste and beads. Those samples that were exposed to
sunlight for an extended period of time were quite dry, while those that were
33
recently removed from separation tanks or stored in barrels, tended to be more
oily. Following collection, each sludge sample was placed in a standard 500 mL
sample bag. The bags were sealed, labelled with the date, time, sample type and
samplers name, and then transported to the laboratory for analysis. The oily
sludge samples were dried in the laboratory at 110 °C for 24 h. The dry samples
were then crushed, homogenised and passed through a 2 mm sieve. The samples
were then packed and pressed into petri dishes, which were sealed using
adhesive tape. Typical sample mass in the petri dish was about 0.1 kg. In some
cases, coarse non-crushable gravel with greater than 2 mm diameter was left
over. This material was monitored for radioactivity by a portable Mini
Instrument 900 series scintillation count rate meter and was found to emit
radioactivity at ambient levels, which may have resulted in an over estimation
of the radioactivity concentration of some sludge samples by up to 10%.
Scale Samples: Scales are generally chunky, hard, and in some cases brittle.
The scales collected in this study were from pipes previously used in the Fahud
oil fields of the Northern Oman Directorate. These pipes were used for pumping
produced water to injection pump stations, either for disposal or for re-injection
into reservoirs, as part of the EOR ‘water flooding’ process. They were
decommissioned because they became clogged with scales, and were
transported to Bahja NORM store yard in 1999. A spatula was used to collect
100-300 g scale samples from each pipe, penetrating the entire depth of the
scale deposited along the pipes internal wall. Following collection, each scale
sample was placed in a standard 500 mL sample bag, as per the methodology
outlined above. As the collected scale samples were already dry, solid and
34
brittle, they did not require drying. The samples were then crushed,
homogenised and packed in petri dishes, also according to the methodology
outlined above.
A further 12 samples of gas scale were also collected, however these
samples were only 50-100 g in weight, since many of the pipes they were
collected from were tightly capped, and thus we were unable to obtain samples
from many of the pipes. In addition, those pipes that were accessible only had
small amounts of gas scale that could be sampled. The gas scale samples
consisted of dry, flat flakes, about 1 mm thick and ranging from 0.01-20 cm2 in
area. These samples were then handled according to the same methodology used
for the original scale samples.
Sediment Samples: Six sediment samples, ranging from 100-300 g, were
collected from Al Noor evaporation pond, using a metal scooper. The samples
were placed in plastic bottles and transported to the laboratory in Muscat, where
they were dried at 110 °C for 24 h, before being crushed, homogenised and
passed through a 2 mm sieve. The homogenised samples were then packed and
handled according to the same methodology outlined above. In order to avoid
cross-contamination, sample preparation instruments were wipe-cleaned after
handling each individual sample.
3.4.2 Gamma spectroscopy measurement procedure
The high purity germanium (HPGe) detector system that was used to
measure the gamma spectra of the collected samples was housed in Laboratory
35
2043 at Sultan Qaboos University (SQU), Muscat. The system consisted of an
EG&G Ortec spectrometer, with a pop top semi-conductor detector (diameter:
59.0 mm, length: 74.2 mm), with 30% relative efficiency. It had a ‘full width at
half maximum’ (FWHM) of 0.807 keV for the 122 keV peak of 57Co, and
1.71 keV for the 1.33 MeV peak of 60Co. The detector was shielded against
background radiation by a 10 cm thick Pb castle.
The HPGe was energy calibrated daily, using a geometry reference source
standard in a 1000 mL marinelli beaker with known isotopes. This standard
complies with the requirements for traceability to the National Institute for
Standards and Technology (NIST), and was also used for efficiency calibration
of the system. However, because the samples used in this research were placed
in petri dishes for analysis, a separate geometry correction calibration was
carried out on the HPGe, using known volumes and activities of standard
radionuclides, in order to accommodate for the petri dish geometry. Background
spectra were also recorded and corrected during sample spectrum analysis.
The main radioisotopes identified by this study in petroleum NORM are
radium isotopes and their progeny, along with 40K and 227Ac. Because of the
226Ra strong overlap with the 235U 185.7 keV peak (actinium series - emission
intensity 57.5%), many gamma spectrometers do not rely on the main
186.1 keV peak of 226Ra (uranium series - emission intensity 3.5%) to determine
its activity concentration. Therefore, gamma emitting 226Ra progeny (214Pb,
T½: 26.8 minutes and 214Bi, T½: 19.9 minutes) are used to obtain 226Ra activity
concentration. However, due to the presence of the gaseous intermediate 222Rn
36
(refer to 238U decay series in Chapter 1, Figure 1.2 (a)) the samples had to be
sealed for 21 days, in order to allow the 222Rn and its progeny to reach
equilibrium with their parent radionuclide. The 226Ra activity could then be
obtained by calculating the error weighted average of gamma emitting 214Pb and
214Bi. Assuming similar activity concentrations of 238U and 232Th, and a 1:20
ratio between 235U and 238U, the contribution of 235U in the 226Ra peak was
found to be 2.7%. Due to the HPGe’s 30% relative efficiency, along with its
energy efficiency peak at 160 keV, the system was found to have a low
efficiency for radionuclides with low gamma energy, such as 210Pb (46.5 keV).
However, samples with a large activity concentration of 210Pb were an
exception, as was the case with the oil and gas scale samples collected during
this research. Activity concentrations of 210Pb, 228Th and 40K were also
determined directly from their respective gamma lines. However, activity
concentrations of radioisotopes with a weak or no gamma signal (for example;
227Ac and 228Ra) were determined by their progeny (227Th and 228Ac) gamma
lines, respectively.
The gamma spectrum for each sample was obtained over a 17 hour
timeframe. SQU’s HPGe system utilises Gamma Vision 5.1 software for
spectrum analysis. This software adjusts for the effects of interfering peaks,
should the presence of a second radionuclide be confirmed. As such, the
Gamma Vision software was able to adjust for the contribution of 235U to the
observed 186 keV peak, thus allowing for the 226Ra activity concentration to be
determined immediately. However, because SQU’s HPGe system was also used
for the University’s own work, the analysis of field samples was sometimes
37
delayed for up to three months. Even though 228Th has a relatively short half life
of 1.9 years, it is in transient equilibrium with 228Ra (T½: 5.75 years); and since
radioactivity is transported by produced water and 228Th is not water soluble, an
initial activity concentration of zero was assumed at the time of sludge/scale
formation. However, all reported gamma spectroscopy data were still needed to
be corrected for 228Ra and 228Th decay. To correct for 228Ra decay it is a straight
forward exponential relationship, however for the 228Th decay correction, the
age of the sample had to be calculated based on an analysis of the 228Ra/228Th
activity ratio at the time of collection. Thus, using the 228Ra activity
concentration at the collection date, 228Th activity concentration was calculated
for each sample (see Section 3.2).
3.5 Comparison between in-situ and laboratory gamma
spectroscopy measurements
Overall, 24 sampling locations were analysed for gamma spectra, using both
in-situ and laboratory gamma spectroscopy. The corresponding results for each
location were then compared and checked for correlation. The main
radionuclides that were compared are illustrated in Figures 3.3 (a-d). From these
figures it can be seen that the field and laboratory 226Ra, 228Th and 40K activity
concentrations compared well, giving linear fit and correlation coefficients as
follows:
ARa-226 Field = (1.05 ± 0.05) ARa-226 Lab (R2 = 0.96), (3.1)
ATh-228 Field = (1.09 ± 0.05) ATh-228 Lab (R2 = 0.96), and (3.2)
AK-40 Field = (1.10 ± 0.12) AK-40 Lab (R2 = 0.78) (3.3)
38
However, the linear fit of the field 228Th to laboratory 228Ra activity
concentration comparison was significantly further away from 1:
ATh-228 Field = (1.57 ± 0.11) ARa-228 Lab (R2 = 0.91) (3.4)
This difference is because, due to the absence of the primordial series parent
232Th in the oil industry NORM, 228Th reaches transient (and not secular)
equilibrium with 228Ra – hence the ratio approaches 1.5, instead of 1. This fact
was considered while applying in-situ gamma spectroscopy techniques for
NORM measurements.
39
0
1
2
3
4
5
6
0 1 2 3 4 5 0
300
600
900
1200
1500
1800
0 300 600 900 1200 1500
0
175
350
525
700
875
1,050
0 150 300 450 600 750 0
100
200
300
400
500
600
0 70 140 210 280 350 Figure 3.3: Correlation between field portable NaI(Tl) and laboratory HPGe activity concentration readings for: (a) 226Ra Field vs 226Ra Lab, (b) 228Th Field vs 228Th Lab, (c) 40K Field vs 40K Lab and (d) 228Th Field vs 228Ra Lab .
(a) AField = (1.05 ± 0.05) ALab R2 = 0.96, n = 24
(b)
(c) AField = (1.10 ± 0.12) ALab R2 = 0.78, n = 24
Lab. HPGe 40K (Bq kg-1) Lab. HPGe 228Ra (Bq kg-1)
Lab. HPGe - 226Ra (kBq kg-1)
Fiel
d N
aI -
226 R
a (k
Bq
kg-1
)
Fiel
d N
aI -
228 Th
(Bq
kg-1
)
Fiel
d N
aI -
40K
(Bq
kg-1
)
(d)
AField = (1.09 ± 0.05) ALab R2 = 0.96, n = 23
Fiel
d N
aI -
228 Th
(Bq
kg-1
) Lab. HPGe 228Th (Bq kg-1)
AField = (1.57 ± 0.11) ALab R2 = 0.91, n = 20
40
3.6 In-situ gamma dose-rate measurements
The Mini-Instrument 6-80 dose rate meter was used to assess gamma
radiation dose rate above piles and strips, located at the Bahja, Nimr and
Marmul sludge farms. The meter consisted of an electrometer, connected to an
energy compensated Geiger-Müller (GM) tube, which was mounted on a tripod
at a height of 1 m above ground. The electrometer provided two separate
readings, an instantaneous absorbed dose rate in air (in µGy h-1) on an analogue
scale, and pre-set time total counts at 600 s, on a digital display. The count rates
(s-1) were divided by a calibration factor of 17.1 to obtain the effective dose rate
in µSv h-1. This factor was obtained through calibrating the system against an
Amersham certified standard (Cs-137 – source number 3702GF) and the
measured dose rates were then verified, by calculating the external exposure
rates using (UNSCEAR, 2000) dose coefficients.
The 226Ra, 228Ra and 40K activity concentrations (in Bq kg-1) obtained from
both in-situ and laboratory gamma spectroscopy measurements were also used
to calculate the dose rate in air D (in µGy h-1) at a 1 m height from the ground,
according to the following equation:
D = (0.462 A[238U] + 0.604 A[232Th] + 0.0417 A[40K])/1000 (3.5)
where A[238U], A[232Th] and A[40K] are the activity concentrations (in Bq kg-1)
for 226Ra, 228Ra and 40K, respectively.
41
3.7 Radon activity flux measurements using charcoal
cups
Activated charcoal is known to adsorb radon gas onto its surface. Brass
charcoal canisters, or cups, containing 25 g of fine activated charcoal (secured
by a wire spring over metal mesh) were used to collect passive field readings of
222Rn exhalation from the sludge piles. These cylindrical cups were open at one
end, with a height and base area of 0.080 m and 0.0029 m2, respectively. Prior
to use, the charcoal cups are annealed for 8-10 h, at a temperature of 110 ºC.
The cups were then allowed to cool for 20 min, before being covered by a
polyethylene lid and sealed with adhesive tape.
Before field measurements were conducted, a ‘standard’ cup was prepared
using a 25 g sludge sample, with 226Ra and 228Ra activity concentrations of 4030
± 21 and 343 ± 7 Bq kg-1, respectively. An epoxy resin was then poured over the
seal, in order to ensure that the 222Rn would not escape. When the 222Rn reached
secular equilibrium with its parent 226Ra three weeks later, the standard was
ready for use.
In the field, the polyethylene lids were removed from the annealed cups and
they were planted upside down on the sludge piles, with the rim of the cup
buried approximately 1 cm into the ground (Figure 3.4). Two cups were planted
for each location, where the mean value was then used. The cups were left in the
field for four days, before being removed and immediately sealed. They were
then allowed to sit for a minimum of four hours, so that the 222Rn could reach
equilibrium with its short lived gamma emitting progeny 214Pb and 214Bi.
42
Figure 3.4: Charcoal cups planted on a sludge pile.
A 600 s gamma spectrum was then collected from the cups using the
NaI(Tl) detector. Planting, removal and counting dates and times were noted, in
order to calculate exposure (te), delay (td) and counting (tc) time intervals. The
net count rate was obtained from the spectra region of interest (ROI) covering
the gamma peak energies of 222Rn progeny (Pb-214 peaks at 242, 295 and
352 keV and Bi-214 peak at 609 keV). The set range was from 223-725 keV,
corresponding to channels 60-195 on the MCA. Rn-222 exhalation rate in the
charcoal cup was then interpolated from its gamma emitting progeny 214Pb (T½:
26.8 min) and 214Bi (T½: 19.9 min) using the following equation (Spehr and
Johnston, 1983):
)1()1(
2
ec
d
tt
tc
eeaetR
J λλ
λ
ελ
−− −⋅−⋅⋅⋅⋅⋅
= (3.6)
where te, td and tc are the exposure time in the field, the delay time between
retrieval from the field and counting in the laboratory, and the counting time (s);
J is the average 222Rn exhalation rate (Bq m-2 s-1) over the exposure time te; R is
the net count rate (s-1) post background subtraction obtained during tc; λ is the
43
decay constant (s-1) for 222Rn; a is the surface area covered by the charcoal
canister (m2); and ε is the counting efficiency of the detector system (s-1 Bq-1).
3.8 Radon exhalation rate measurements using the
emanometer
The emanometer was calibrated using certified Pylon radon gas (Model RN-
1025). Figure 3.5 (a) shows a schematic diagram of the emanometer, which was
housed in a secure wooden box with all of its components secured in
predetermined slots, as shown in Figure 3.5 (b). Due to time constraints during
the field visits, only a limited number of charcoal cup applications could be
performed. Therefore, laboratory assessment of 222Rn exhalation rates was also
undertaken for twenty corresponding field samples. These measurements were
then cross-referenced with the charcoal cup measurements, in order to verify the
accuracy of the emanometer. All of the reported radon exhalation rates (apart
from two out of the four Oman ambient soil samples) in Chapter 6 are from
values obtained using the emanometer.
44
Flow rate meter
ZnS(Ag) scintillation chamber
PM-tube
Emanometer assembly schematic
8 mm tube
Electronics
Pump
Absolute filter(Cotton wool)
Signal and power lines
Valves
inflow
outflo
w
Sample
Sample chamber
Table
Figure 3.5: (a) Schematic diagram of the emanometer, and (b) The emanometer in its wooden box housing.
This technique for the determination of 222Rn exhalation rate made use of
ZnS(Ag) scintillation chambers, coupled with photo-multiplier tubes. The main
components of the emanometer were: 2 x ZnS(Ag) scintillation chambers,
(a)
(b)
45
2 x photo-multiplier tubes coupled to the scintillation chambers, 2 x DayBreak
digital power supplies and counters, 2 x pumps with flow-rate meters, 2 x cotton
wool absolute filters and a suite of sample chambers. Each sample chamber was
a 3.5 L PVC cylinder with a clear Perspex lid (Figure 3.6 (a)). The cover was
fitted with inlet and outlet valves, and an O-ring, along with six screws and
butterfly nuts were fastened to the cover, in order to obtain an air tight seal
between the chamber and the lid. An 8 mm capillary tube connected the sample
chamber to the rest of the system.
Figure 3.6: (a) Emanometer’s airtight PVC sample chambers with FESTO valves and Perspex cover, and (b) a sludge sample wrapped in perforated textile material, labelled and ready for 222Rn counting.
Sample collection has been described in Section 3.4.1. The samples are kept
as intact as practically possible in order to obtain the same 222Rn exhalation rate
found in the field. The samples analysed by the emanometer included petroleum
scales, treated and untreated sludge from sludge farms, sludge stored in barrels,
sediment soil from a produced water evaporation pond and ambient soil. Each
0.25 kg sample was wrapped in 225 cm2 of perforated textile, tied with a wire
ribbon, labelled and placed on a plastic table inside the sample chamber
(Figure 3.6 (b)). The chamber was then sealed for 24 h to allow for 222Rn gas
(a) (b)
46
accumulation, before being connected to the system. Air was then pumped
through a closed loop, at a rate of 6 L min-1, moving from the sample chamber,
through a wool cotton absolute filter, into the ZnS(Ag) scintillation chamber,
through the pump and the flow rate meter, then back to the sample chamber.
47
Chapter 4 RADIOACTIVITY CONCENTRATION OF SCALE, SLUDGE AND SOIL SEDIMENT, FROM THE OIL FIELDS OF THE SOUTHERN OMAN DIRECTORATE
4.1 Introduction
The presence of technologically enhanced NORMs in petroleum industry
scales and sludge has been reported by many of the world’s oil producing
countries. In this work, we have attempted to characterise and quantify this
radiation, based on samples taken from the oil fields of the Southern Oman
Directorate (SOD). This chapter will report the radioactivity concentrations of
sludge, oil and gas scales, and evaporation pond soil sediment, as well as the use
of radioisotopes to date scales and sludge.
Gamma spectroscopy measurements were performed on various solid waste
samples, including: (1) nascent sludge and sludge in Bahja, Nimr and Marmul
sludge farms; (2) oil and gas scales, and sludge stored in barrels in Bahja
NORM store yard; and (3) sediment in Al-Noor evaporation pond. Sludge
samples were also tracked from its initial point of accumulation at the separator
tanks, to its final destination at the sludge farm or in barrels at the NORM store
yard.
48
4.2 Radioactivity in sludge
Oman generates approximately 7.2 x 104 tonnes of sludge per annum (Al-
Futaisi et al., 2007). As mentioned in Chapter 1, Section 1.3, the word ‘sludge’
refers to a mixture of hydrocarbon, mud, natural radionuclides, sediments,
bacterial growth, corrosion particles and some scale debris. In Oman, the
presence of petroleum industry NORMs was first discovered in a sludge sample
removed by a pig device from the Hasirah of Zauliyah line, in the Bahja cluster
in 1997. Today, PDO conduct an analysis of all sludge removed from its storage
and separator tanks, using a Mini-Instrument 900 series count rate meter, with a
gamma scintillation probe (Model 44A). The nominal background surface count
rate for these meters is 3 CPS and sludge samples with count rates 5 CPS higher
than the nominal background (i.e. 8 CPS) are considered to be NORM
contaminated. According to an unpublished paper, presented at the ‘PDO
Workshop on NORM’ in Muscat (2005), PDO’s total accumulated sludge was
more than 3.3 x 105 tonnes and out of this mass, almost 2.0 x 104 tonnes was
NORM contaminated.
In a national ambient soil radioactivity survey carried out by Goddard
(2002), the average activity concentrations of 226Ra and 232Th in the upper 1 cm
of exposed surface soil or rock were reported as 29.7 ± 8.9 and
15.9 ± 7.6 Bq kg-1, respectively. Assuming equilibrium between 232Th and
228Ra, the latter reported average could be considered the national average for
228Ra activity concentration. A report on 226Ra and 224Ra activity concentrations
in 50 oily sludge samples collected from the northern Oman oilfields was also
published by Salih et al. (2005). The activity concentrations obtained using
49
laboratory HPGe gamma spectroscopy ranged from 0.15-1 kBq kg-1 for 226Ra,
and 0.1-0.6 kBq kg-1 for 224Ra.
4.2.1 Sludge farming
The presence of long lived radioisotopes (e.g. 226Ra, T½: 1602 years; 210Pb,
T½: 22.26 years and 228Ra, T½: 5.75 years) in sludge, at levels higher than those
found in ambient soil, poses a significant radiological hazard to the
environment. Shailubhai (1986) discussed various options for the disposal of
oily sludge, including ocean disposal, incineration and land farming. However,
the presence of toxic chemicals in the oily sludge means that ocean disposal is
not a desirable option, since aquatic organisms may be poisoned by these toxins.
Since global warming has become an issue, incineration is no longer desirable
either, because it is not only energy intensive, but it also contributes
significantly to air pollution. Land farming, on the other hand, is highly cost
effective, provided that the sludge does not contaminate clean soil and seep into
underground water supplies. Many other studies have also investigated options
for sludge farming; however none of them have addressed the issue of
radioactivity (Arora et al., 1982, Couillard et al., 1991, Prado-Jatar et al., 1993,
Brown et al., 1998, Vasudevan and Rajaram, 2001, Mater et al., 2006).
In Oman, oily sludge is currently being disposed of at sludge farms, which
utilise micro-organisms to biodegrade the complex hydrocarbons found in oil,
into naturally occurring by-products, such as carbon dioxide and water. This
process requires minimal machinery and labour inputs, and since it is able to
50
occur above ground, in areas exposed to high levels of solar radiation, it is the
most cost effective method of sludge disposal, particularly in the remote desert
oil fields of Oman.
At the time of the field studies conducted during this research, the total land
area of Bahja, Nimr and Marmul sludge farms was about 30 hectares. These
farms usually consisted of two areas – one where the excavated sludge was
heaped, and the other, a flat plot of machine compacted land dedicated for
spreading the sludge. The sludge was spread in rows, otherwise known as
‘strips’, measuring approximately 6 m in width, 75 m in length and 0.4 m in
height, and were separated by 4-8 m of open space, to allow passage for the
water tanker and other service vehicles. Table 4.1 shows the GPS locations,
estimated volume of material in untreated sludge piles and number of treated
sludge strips at Bahja, Nimr and Marmul sludge farms, at the time of our final
visit to Oman (21 April - 8 May 2007). The total volume of sludge found in the
three farms was estimated to be around 34,000 m3.
Table 4.1: Bahja, Nimr and Marmul sludge farm locations, estimated volume of untreated sludge in piles and number of treated sludge Strips at the time of this study (Jan 2006 – June 2007) Sludge farm Geographical location Sludge pile
volume (m3) Number of Strips Latitude Longitude
Bahja N 19° 52.8’ E 56° 02.0’ 10,000 42
Nimr N 18° 33.3’ E 55° 51.9’ 17,000 147
Marmul N 18° 12.6’ E 55° 17.5’ 7,000 64
Total 34,000 253
51
After the sludge is delivered to the sludge farms, the heaps are transported to
the strip site, where the sludge is spread, before being mixed with clean soil by
an earth moving machine (skid loader). Alternatively, sludge can be mixed with
clean sand before it is transported to the strip site. The strips are then tilled and
watered on daily basis, in order to maintain optimal aeration and moisture
conditions for the biodegradation and evaporation of volatile compounds
(Figure 4.1 (a-f)). Typically, the soil to sludge ratio ranges from 1-5:1,
depending on the 226Ra activity concentration of the sludge. Along with daily
tilling, this not only serves to assist bioremediation, but it also helps to dilute the
radioactivity present in the sludge.
A total of 55 untreated sludge samples were analysed from the three sludge
farms - 25 from Bahja, 14 from Nimr and 16 from Marmul. A further 57 treated
sludge samples were also analysed - 12 from Bahja, 16 from Nimr and 29 from
Marmul (see Chapter 3, Sections 3.2.1 and 3.2.2 for details on sampling and
measurement procedures).
52
Figure 4.1: The sludge farming process: (a) sludge removed from a separation tank, (b) untreated sludge piles, (c) sludge piles after transport to the farming area, (d) a typical sludge strip, (e) watering the sludge strips, and (f) tilling the sludge strips.
b
e
d c
a
f
53
4.2.2 Radioactivity in ambient soil
The radioactivity of ambient soil samples from Bahja, Al-Noor, Nimr and
Marmul are shown in Table 4.2. The mean activity concentrations of 226Ra,
228Ra, 228Th and 40K were 34.2 ± 3.8, 8.1 ± 1.3, 7.1 ± 0.8 and 151 ± 55 Bq kg-1,
respectively. Bahja 40K activity concentration (293 ± 8 Bq kg-1) was three times
higher than the mean activity concentration for the other three sites
(104 ± 6 Bq kg-1). As expected, the mean 228Th:228Ra activity ratio for the four
sites was close to one (0.89 ± 0.09), since 228Th and 228Ra were likely to be in
secular equilibrium with 232Th.
A detailed study of Oman’s ambient terrestrial radioactivity concentrations
was performed by Goddard (2002). The study reported mean activity
concentrations for 15 samples from the Wusta region (the same region where
this research was conducted), being 36.2 ± 8.2, 16.4 ± 5.5 and 166 ± 27 Bq kg-1
for 226Ra, 232Th and 40K, respectively. Taking into account the uncertainties of
these types of measurements, the ambient soil concentrations found in this
research are similar to those found by Goddard in 2002. However, a study
conducted by the United Nations (UNSCEAR, 2000) reported population
weighted natural radionuclide activity concentrations in soils to be 35, 30 and
400 Bq kg-1 for 238U, 232Th and 40K, respectively. In comparison to these
findings, the desert environment of Oman seems to have lower 232Th and 40K
concentrations than those found elsewhere in the world.
54
Table 4.2: Activity concentration (Bq kg-1) for ambient soils of Bahja, Al-Noor, Nimr and Marmul and the world average (UNSCEAR, 2000) (uncertainties represent counting error) Sample ID Geographical location 226Ra 228Ra 228Th 40K 137Cs 228Ra:226Ra 228Th:228Ra
Latitude Longitude
Bahja N 19° 53.047' E 56° 01.753' 38.1 ± 5.4 11.3 ± 1.4 9.1 ± 1.4 293 ± 8 1.02 ± 0.23 0.30 ± 0.08 0.81 ± 0.22
Al Noor N 18° 41.391' E 55° 30.376' 30.4 ± 5.2 7.7 ± 0.8 7.1 ± 0.4 108 ± 5 0.58 ± 0.18 0.25 ± 0.07 0.92 ± 0.15
Nimr N 18° 33.257' E 55° 51.864' 27.2 ± 1.3 5.7 ± 0.4 5.7 ± 0.4 111 ± 2 < 0.12 0.21 ± 0.03 1.00 ± 0.14
Marmul N 18° 12.562' E 55° 17.337' 41.2 ± 4.9 7.8 ± 1.1 6.5 ± 1.5 93 ± 5 < 0.44 0.19 ± 0.05 0.83 ± 0.31
Mean ± SE 34.2 ± 3.8 8.1 ± 1.3 7.1 ± 0.8 151 ± 55 - 0.24 (0.03) 0.89 (0.05)
Standard
Deviation
6.5 2.3 1.5 95
-
0.05 0.09
World Median
35 30# 30# 400
# Assuming secular equilibrium of 228Ra and 228Th with 232Th
55
4.2.3 Radioactivity in the sludge recovered from a separator
tank
PDO cleans its separator tanks approximately every 5 years, depending on
the sludge accumulation rate at the bottom of the tank. After cleaning, NORM
contaminated sludge samples are sent to Sultan Qaboos University (SQU) for
gamma spectroscopy analysis, in order to determine 226Ra activity
concentration, and thus, the fate of the sludge. NORM contaminated sludge,
with a 226Ra activity concentration equal to or higher than 1 kBq kg-1, is
transported to the Bahja NORM store yard for storage. NORM contaminated
sludge with a 226Ra activity concentration less than 1 kBq kg-1 is sent to the
Bahja, Nimr and Marmul sludge farms.
Six sludge samples were collected from a newly cleaned separator tank at
Nimr sludge farm. They were assessed for 226Ra, 228Ra, 228Th and 40K
radioactivity and 228Ra:226Ra and 228Th:228Ra ratios (refer to Chapter 3, Section
3.2.1 and 3.2.2 for details on sampling and measurement procedures), in order
to determine the initial radioactivity and the age of the sludge sediment. From
Table 4.3 it can be seen that the mean 226Ra, 228Ra, 228Th and 40K activity
concentrations were 588 ± 106, 264 ± 53, 296 ± 52 and 109 ± 20 Bq kg-1,
respectively. While the mean 40K activity concentration was similar to the
ambient soil value for Nimr sludge farm, the 226Ra, 228Ra and 228Th activity
concentrations were significantly higher than the Nimr ambient soil
concentrations.
56
Table 4.3: Activity concentration (Bq kg-1) and individual reading error of freshly removed sludge from a Nimr station separator tank Sample ID 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra 228Ra:226Ra
ratio at deposition Age of sludge
(years)
NFS 1 446 ± 17 206 ± 4 221 ± 5 149 ± 8 0.46 ± 0.03 1.08 ± 0.04 0.86 0.96 0.77 5.2 5.7
4.8
NFS 2 391 ± 15 163 ± 3 186 ± 4 89 ± 7 0.42 ± 0.02 1.15 ± 0.05 0.85 0.97 0.76 6.0 6.6
5.5
NFS 3 985 ± 24 446 ± 5 496 ± 7 119 ± 12 0.45 ± 0.02 1.13 ± 0.03 0.89 0.96 0.83 5.8 6.1
5.5
NFS 4 697 ± 24 352 ± 5 346 ± 6 117 ± 13 0.51 ± 0.02 0.99 ± 0.03 0.86 0.93 0.79 4.5 4.8
4.2
NFS 5 363 ± 12 139 ± 3 207 ± 5 32 ± 6 0.38 ± 0.02 1.49 ± 0.07 7.0a 7.4 6.6
NFS 6 648 ± 19 280 ± 5 321 ± 6 151 ± 10 0.43 ± 0.02 1.16 ± 0.04 0.89 1.00 0.80 6.2 6.7
5.7
Maximum 985 446 496 151 0.51 1.49
Minimum 363 139 186 32 0.38 0.99
Median 547 243 271 118 0.44 1.14
Standard Deviation 238 118 117 44 0.04 0.17
Mean ± SE 588 ± 106 264 ± 53 296 ± 52 109 ± 20 0.44 ± 0.02 1.16 ± 0.08 0.87 0.97 0.79 5.8 6.3
5.4
Nimr Ambient Soilb 27.2 ± 1.3 5.7 ± 0.4 5.7 ± 0.4 111 ± 2 0.21 ± 0.03 1.00 ± 0.14
a Based on 228Ra:226Ra activity concentration ratio. Not included in the average b From Table 4.2
57
From Table 4.3 it can also be seen that the average 228Th:228Ra activity ratio
was 1.16 ± 0.17, indicating that the sludge was approximately 5.8 3.64.5 years old,
which is consistent with the company’s separator tank cleaning interval of
5 years. The age of individual samples was then used to determine the
228Ra:226Ra ratio at the time of deposition. Since 226Ra (T½: 1602 years) decay
would be insignificant in 5.8 years, no decay correction was necessary when
calculating the mean 228Ra:226Ra activity ratio at the time of deposition, which
was found to be 0.87 97.079.0 . In addition, the deposition activity ratio corresponded
to a 232Th:238U mass ratio of 2.6. These values were similar to those found by
Al-Masri and Aba (2005), who reported a mean 228Ra:226Ra activity ratio and
232Th:238U mass ratio of 0.76 and 2.3, respectively.
As outlined in Chapter 3, Section 3.2, the initial formation of sludge is
usually free of 228Th. Build up, and ingrowth of 228Th (T½: 1.9 years) takes place
as 228Ra (T½: 5.75 years) decays, until a transient equilibrium is reached. In
contrast, considering the difference in the half lives, 228Ra activity reduces
rapidly relative to 226Ra, which explains the inverse trend that was demonstrated
by the 228Ra:226Ra and 228Th:228Ra activity ratios (Figure 4.2).
58
0.3
0.4
0.5
0.6
0.8 1.0 1.2 1.4 1.6
Figure 4.2: Relation between 228Ra:226Ra and 228Th:228Ra activity ratios.
4.2.4 Radioactivity in untreated piles at sludge farms
A summary of the activity concentrations found in untreated sludge from the
Bahja, Nimr and Marmul sludge farms, is presented in Table 4.4, while the
individual values are displayed in Tables 4.5, 4.6 and 4.7.
Bahja Sludge Farm: A preliminary survey of the untreated sludge piles,
using a portable gamma count rate meter, revealed that about 75% of the total
piles had gamma counts close to ambient soil, and therefore no samples were
collected from those piles. The remaining 25% had enhanced activity
concentrations equal to or greater than the set limit of 1 kBq kg-1 (Petroleum
Development Oman, 2005), which meant that they were not suitable to be used
in the sludge farming process . These high activity sludge piles were segregated
228Th:228Ra activity ratio
228 R
a:22
6 Ra
activ
ity ra
tio
59
from the rest of the piles, in an area measuring approximately 600 m2 and they
will remain there until an alternative disposal or treatment method is approved.
The mean (± standard error) 226Ra, 228Ra, 228Th and 40K activity concentrations
for these high activity piles were found to be 3289 ± 264, 261 ± 19, 338 ± 26
and 427 ± 50 Bq kg-1, respectively. The mean 228Th:228Ra and 228Ra:226Ra
activity ratios were then used to estimate the age of the sludge, which was found
to be 9.0 ± 0.4 and 15 years (assuming the initial 228Ra:226Ra ratio is the same as
Nimr nascent sludge), respectively. As expected, these age estimates for Bahja
farmed sludge were 3-9 years more than the age of sludge freshly removed from
the tank, as determined in Section 4.2.3.
Nimr Sludge Farm: All of the untreated sludge samples had 226Ra activity
concentrations less than 1 kBq kg-1 and the distribution was also less dispersed
between piles. Averages for 226Ra, 228Ra, 228Th and 40K activity concentrations
were 343 ± 35, 129 ± 13, 123 ± 14 and 433 ± 27 Bq kg-1, respectively.
Marmul Sludge Farm: On the basis of a gamma dose rate survey of the
Marmul untreated sludge piles, about 10% were expected to have activity
concentrations exceeding 1 kBq kg-1, and sample collection was biased towards
the higher activity piles. The maximum activity concentrations detected in
Marmul sludge were 3690 ± 60, 6036 ± 20, 5164 ± 20 and 720 ± 13 Bq kg-1 for
226Ra, 228Ra, 228Th and 40K, respectively. When weighted according to the area
of higher and lower concentration piles, the mean (± standard error) activity
concentrations for 226Ra, 228Ra, 228Th and 40K were 326 ± 153, 394 ± 205, 342
± 179 and 360 ± 79 Bq kg-1, respectively.
60
Table 4.4: Sludge activity concentrations (Bq kg-1) and radioisotope ratios from Bahja, Nimr and Marmul untreated sludge piles Location 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra Bahja Median 3164 268 344 272 0.083 1.29
Mean 3289 261 338 427 0.082 1.29 Standard Error 264 19 26 50 0.004 0.01 Standard Deviation 1295 93 125 243 0.015 0.05 Number of Samples 25
Nimr Median 323 123 113 448 0.37 0.95 Mean 343 129 123 433 0.40 0.95 Standard Error 35 13 14 27 0.03 0.02 Standard Deviation 128 46 51 99 0.11 0.07 Number of Samples 14
Marmul * Mean 356 394 342 360 0.60 0.85 * Standard Error 153 205 179 79 0.12 0.04 * Standard Deviation 342 421 428 247 0.29 0.10 Number of Samples 16
* Area weighted values due to distinct low and high radioactivity sections
61
Table 4.5: Activity concentrations (Bq kg-1) of untreated Bahja sludge piles Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra
Easting Northing
BHJP 1 * 398306 2198873 2210 ± 40 191 ± 7 240 ± 10 185 ± 21 0.086 ± 0.004 1.26 ± 0.10 BHJP 2 * 398318 2198877 1310 ± 20 103 ± 5 132 ± 7 252 ± 15 0.079 ± 0.005 1.28 ± 0.13 BHJP 3 * 398316 2198885 2980 ± 30 203 ± 6 255 ± 9 229 ± 18 0.068 ± 0.003 1.25 ± 0.09 BHJP 4 * 398318 2198888 2150 ± 70 143 ± 11 180 ± 19 264 ± 27 0.066 ± 0.007 1.26 ± 0.23 BHJP 5 * 398318 2198898 2240 ± 40 150 ± 5 190 ± 11 254 ± 21 0.067 ± 0.003 1.27 ± 0.11 BHJP 6 * 398328 2198909 4520 ± 50 200 ± 10 376 ± 15 209 ± 23 0.066 ± 0.003 1.25 ± 0.09 BHJP 7 * 398363 2198960 4000 ± 40 253 ± 9 330 ± 16 217 ± 23 0.063 ± 0.003 1.32 ± 0.11 BHJP 8 * 5670 ± 50 373 ± 9 501 ± 13 182 ± 22 0.066 ± 0.002 1.34 ± 0.06 BHJP 9 * 5180 ± 40 345 ± 7 454 ± 3 187 ± 12 0.067 ± 0.002 1.32 ± 0.04 BHJP 10 * 5300 ± 40 345 ± 7 506 ± 6 200 ± 19 0.065 ± 0.002 1.47 ± 0.05 BHJP 11 * 1090 ± 20 92 ± 4 112 ± 5 232 ± 12 0.088 ± 0.005 1.22 ± 0.10 BHJP 12 * 4580 ± 50 309 ± 8 392 ± 12 192 ± 22 0.065 ± 0.002 1.27 ± 0.07 BHJP 13 * 3164 ± 56 253 ± 11 306 ± 20 272 ± 31 0.084 ± 0.005 1.22 ± 0.14 BHJP 14 # 398367 2198958 1955 ± 56 172 ± 14 221 ± 18 524 ± 122 0.088 ± 0.010 -
62
Table 4.5 (Continued): Activity concentrations (Bq kg-1) of untreated Bahja sludge piles Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra
Easting Northing
BHJP 15 # 398360 2198953 2981 ± 69 289 ± 18 372 ± 24 597 ± 150 0.097 ± 0.008 - BHJP 16 # 398355 2198950 3221 ± 71 271 ± 18 349 ± 23 566 ± 153 0.084 ± 0.007 - BHJP 17 # 398351 2198944 3799 ± 77 315 ± 19 405 ± 25 591 ± 165 0.083 ± 0.007 - BHJP 18 # 398342 2198933 3875 ± 79 375 ± 21 484 ± 27 779 ± 171 0.097 ± 0.007 - BHJP 19 # 398340 2198929 4336 ± 82 366 ± 21 470 ± 26 767 ± 178 0.084 ± 0.006 - BHJP 20 # 398333 2198920 4934 ± 89 472 ± 23 607 ± 30 758 ± 190 0.096 ± 0.006 - BHJP 21 # 398330 2198915 2908 ± 68 258 ± 17 332 ± 22 638 ± 147 0.089 ± 0.008 - BHJP 22 # 398320 2198906 3445 ± 73 287 ± 18 369 ± 23 954 ± 162 0.083 ± 0.007 - BHJP 23 # 398310 2198896 2444 ± 63 268 ± 18 344 ± 23 742 ± 140 0.109 ± 0.010 - BHJP 24 # 398309 2198890 2274 ± 60 200 ± 15 257 ± 20 409 ± 129 0.088 ± 0.009 - BHJP 25 # 398300 2198878 1651 ± 53 198 ± 15 255 ± 19 469 ± 115 0.120 ± 0.013 - Maximum 5670 470 607 954 0.120 1.47 Minimum 1090 92 112 182 0.063 1.22 Number of Samples 25
* Analysis by HPGe gamma spectroscopy system
# Analysis by NaI portable gamma spectroscopy system – 228Ra value were determined by using 228Th:228Ra ratio of 1.29
63
Table 4.6: Activity concentrations (Bq kg-1) of untreated Nimr sludge piles (analysed using the HPGe gamma spectroscopy system) Sample ID Location 40QUTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra
Easting Northing NMRP 1 383329 2051550 531 ± 13 139 ± 3 151 ± 4 446 ± 10 0.26 ± 0.01 1.08 ± 0.06 NMRP 2 383337 2051562 403 ± 17 138 ± 5 130 ± 5 487 ± 14 0.34 ± 0.03 0.94 ± 0.07 NMRP 3 383356 2051569 291 ± 14 118 ± 3 106 ± 4 405 ± 10 0.41 ± 0.03 0.90 ± 0.06 NMRP 4 383353 2051598 314 ± 10 114 ± 3 109 ± 3 457 ± 9 0.36 ± 0.02 0.96 ± 0.05 NMRP 5 383309 2051584 320 ± 11 123 ± 2 115 ± 3 457 ± 9 0.38 ± 0.02 0.94 ± 0.04 NMRP 6 383296 2051560 285 ± 10 95 ± 3 94 ± 3 439 ± 9 0.33 ± 0.02 0.99 ± 0.07 NMRP 7 383347 2051609 639 ± 17 270 ± 4 281 ± 5 134 ± 10 0.42 ± 0.02 1.04 ± 0.04 NMRP 8 73 ± 7 55 ± 2 48 ± 2 595 ± 10 0.75 ± 0.10 0.87 ± 0.08 NMRP 9 309 ± 19 130 ± 4 119 ± 6 392 ± 14 0.42 ± 0.04 0.91 ± 0.07 NMRP 10 284 ± 14 141 ± 4 109 ± 5 451 ± 14 0.50 ± 0.04 0.77 ± 0.06 NMRP 11 361 ± 17 136 ± 5 125 ± 6 469 ± 16 0.38 ± 0.03 0.91 ± 0.07 NMRP 12 327 ± 18 111 ± 4 109 ± 6 442 ± 14 0.34 ± 0.03 0.98 ± 0.09 NMRP 13 339 ± 14 112 ± 3 110 ± 4 480 ± 11 0.33 ± 0.02 0.98 ± 0.06 NMRP 14 334 ± 17 123 ± 4 117 ± 5 406 ± 14 0.37 ± 0.03 0.96 ± 0.08 Maximum 639 270 281 595 0.75 1.08 Minimum 73 55 48 134 0.26 0.77 Number of Samples 14
64
Table 4.7: Activity concentrations (Bq kg-1) of untreated Marmul sludge piles (using the HPGe gamma spectroscopy system) Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra
Easting Northing
MRLP 1 # 319390 2014275 195 ± 20 83 ± 12 67 ± 10 47 ± 43 0.35 ± 0.09 0.81 ± 0.24 MRLP 2 319393 2014288 36 ± 6 9 ± 1 9 ± 1 119 ± 6 0.26 ± 0.09 0.99 ± 0.30 MRLP 3 319385 2014287 42 ± 4 10 ± 1 9 ± 1 135 ± 5 0.24 ± 0.05 0.91 ± 0.20 MRLP 4 319378 2014292 36 ± 4 7 ± 1 8 ± 1 118 ± 5 0.20 ± 0.05 1.07 ± 0.33 MRLP 5 319370 2014292 38 ± 4 9 ± 1 7 ± 2 103 ± 5 0.24 ± 0.06 0.76 ± 0.27 MRLP 6 319365 2014314 125 ± 9 92 ± 3 84 ± 3 639 ± 11 0.73 ± 0.08 0.90 ± 0.06 MRLP 7 319350 2014320 46 ± 6 23 ± 2 17 ± 2 683 ± 11 0.49 ± 0.10 0.76 ± 0.13 MRLP 8 319385 2014271 918 ± 21 845 ± 7 773 ± 7 548 ± 15 0.92 ± 0.03 0.91 ± 0.02 MRLP 9 319392 2014271 1022 ± 24 1557 ± 7 1380 ± 8 359 ± 16 1.52 ± 0.04 0.89 ± 0.01 MRLP 10 27 ± 3 7 ± 1 5 ± 1 84 ± 5 0.26 ± 0.07 0.77 ± 0.18 MRLP 11 146 ± 9 123 ± 3 93 ± 3 653 ± 11 0.84 ± 0.07 0.76 ± 0.04 MRLP 12 196 ± 13 162 ± 3 120 ± 3 720 ± 13 0.83 ± 0.07 0.74 ± 0.04 MRLP 13 3690 ± 60 6036 ± 20 5164 ± 20 519 ± 20 1.64 ± 0.03 0.86 ± 0.01 MRLP 14 2290 ± 40 2211 ± 10 2023 ± 13 477 ± 19 0.97 ± 0.02 0.91 ± 0.01 MRLP 15 1310 ± 30 1444 ± 10 1236 ± 11 443 ± 17 1.10 ± 0.03 0.86 ± 0.01 MRLP 16 166 ± 9 139 ± 3 103 ± 3 601 ± 11 0.84 ± 0.06 0.74 ± 0.04 Maximum 3690 6036 5164 720 1.64 1.07 Minimum 27 7 5 47 0.20 0.74 Number of Samples 16 # Analysis by NaI portable gamma spectroscopy system
65
Comparison of Activity Concentrations between Sludge Farms: The data
presented in this section shows that the 226Ra and 228Ra activity concentrations
for the Marmul untreated sludge piles varied significantly (by two and three
orders of magnitude, respectively), compared to the activity concentrations for
the Bahja and Nimr untreated sludge piles, which were within one order of
magnitude (Figure 4.3). Figure 4.3 also illustrates that the two radium isotopes
had a direct proportionality, implying that the encountered increase in gamma
count rate was a result of a simultaneous activity concentration increase for both
226Ra and 228Ra.
1
10
100
1000
10000
1 10 100 1000 10000
MarmulBahjaNimr
Figure 4.3: Relation between 226Ra and 228Ra for Marmul, Bahja and Nimr untreated sludge piles.
228 R
a (B
q kg
-1)
226Ra (Bq kg-1)
66
The mean ages of Nimr and Marmul untreated sludge piles, as estimated by
the 228Th:228Ra activity ratio, were 4.2 ± 0.3 and 3.6 ± 0.4 years, respectively,
indicating that Nimr and Marmul untreated sludge piles were relatively new and
similar in age. The Bahja untreated sludge piles, on the other hand, were
estimated to be as old as 15 years, which is not unexpected, since the bulk of
high activity untreated sludge piles have been at the farm for quite some time,
and will remain there until an alternative disposal or treatment method is
approved. As a result of these age differences, an inverse relationship was
observed between 228Ra:226Ra and 228Th:228Ra mean activity ratios for the three
sludge farms (Figure 4.4). On the other hand, as shown in Figures 4.5 (a) and
(b), no correlation was observed between sludge age and the 226Ra and 228Ra
activity concentrations for each individual farm.
0.00
0.25
0.50
0.75
1.00
0.7 0.9 1.1 1.3 1.5
Figure 4.4: 228Ra:226Ra and 228Th:228Ra mean activity ratios for Bahja, Nimr and Marmul sludge farms.
228 R
a:22
6 Ra
activ
ity ra
tio
228Th:228Ra activity ratio
Bahja
Nimr
Marmul
67
10
100
1000
10000
0 2 4 6 8 10
BahjaNimr
Marmul
1
10
100
1000
10000
0 2 4 6 8 10
Bahja
Nimr
Marmul
Figure 4.5: Sludge pile activity concentration versus age (a) 226Ra, and (b) 228Ra.
Sludge pile age (years)
226 R
a ac
tivity
con
cent
ratio
n (B
q kg
-1)
228 R
a ac
tivity
con
cent
ratio
n (B
q kg
-1)
Sludge pile age (years)
a
b
68
4.2.5 Radioactivity in treated sludge strips
Gamma spectroscopy results for Bahja, Nimr and Marmul treated sludge
strips are presented in Tables 4.8, 4.9 and 4.10, respectively. The bulk of the
data was collected in the field by the portable NaI gamma spectroscopy system,
while the rest was obtained from analysing samples in the laboratory, using an
HPGe gamma spectroscopy system. As outlined in Chapter 3, Section 3.2, 228Ra
activity concentration was not directly measured in the field, but was calculated
from the 228Th:228Ra activity ratio obtained from laboratory measurements for
samples collected from same locations.
Bahja Sludge Farm: Overall, Bahja’s sludge strips had the lowest mean
(± standard error) activity concentration for 226Ra, 228Ra and 228Th, being 55 ± 3,
14 ± 1 and 11 ± 1 Bq kg-1, respectively. However, these activity concentrations
were still higher than the corresponding ambient soil activity concentrations of
38 ± 5, 11 ± 1 and 9 ± 1 Bq kg-1, respectively (Table 4.2). No localised spots of
higher activity (hotspots: often 1-2 m2 surface area and about 0.5 m deep) were
detected on the treated sludge strips. This was due to the fact that only the
sludge piles with activity concentrations similar to ambient soil radioactivity
were approved for the sludge farming process (refer to discussion in
Section 4.2.4.).
Nimr and Marmul Sludge Farms: Despite dilution by the farming process,
the mean and maximum 226Ra activity concentrations for Nimr and Marmul’s
treated sludge strips were about three and ten times higher than their respective
ambient soil activity concentrations.
69
Nimr and Marmul’s treated sludge strips were also found to contain
hotspots, even after being mixed with clean soil, which may have been the result
of a non-uniform activity distribution in the sludge. At Nimr sludge farm, a
hotspot was detected on strip 61, with 226Ra, 228Ra and 228Th activity
concentrations of 1340 ± 26, 736 ± 7 and 441 ± 8 Bq kg-1, respectively. At
Marmul sludge farm, two hotspots were detected. The first was on strip 44
(analysed by HPGe), with 226Ra, 228Ra and 228Th activity concentrations of
2080 ± 40, 184 ± 5 and 160 ± 6 Bq kg-1, respectively, and the second on strip 7
(analysed by portable NaI), with 226Ra and 228Th activity concentrations of
1920 ± 54 and 165 ± 16 Bq kg-1, respectively.
It is unlikely that these hotspots will disappear any time soon, since both
228Ra and 226Ra will require about 7 half lives to decay to ambient soil activity
concentrations. Taking into account the 232Th half life, supported 228Ra found in
ambient soil will remain virtually constant while the unsupported sludge 228Ra
will decay to 0.78 % of its original activity over approximately four decades.
Ra-226, however, has a longer half life, and will require at least 10,000 years to
decay to 0.78 % of its original activity.
In addition, Nimr and Marmul’s ambient soil 228Th:228Ra activity ratios were
1.00 ± 0.14 and 0.83 ± 0.31, respectively, indicating that 228Th and 228Ra were in
transient equilibrium with 232Th. However, at the previously identified hotspots
for both farms, 228Th was in disequilibrium with 228Ra. In Nimr, strip 61’s
228Th:228Ra activity ratio of 0.6 was characteristic of new sludge activity ratios,
where 228Th is in an ingrowth phase with 228Ra. A similar argument could be
70
made for strips 7 and 44 in Marmul, indicating that these high activity
concentrations were due to the sludge and not the ambient soil.
When calculating the mean and median activity concentrations for the two
farms, these hotspots were excluded from the analysis, due to their potential to
bias the results. As such, the overall results suggest that, on average, Nimr and
Marmul sludge 226Ra activity concentration had been reduced by factors of 4.6
± 1.6 and 3.1 ± 1.9, respectively, as a result of the mixing and tilling process.
Finally, it should be mentioned that 40K activity concentration did not follow
a specific trend for samples collected from the treated sludge strips. The activity
concentrations of 40K for Bahja, Nimr and Marmul were very similar, with mean
(± standard error) values of 175 ± 10, 146 ± 19 and 167 ± 21 Bq kg-1,
respectively.
71
Table 4.8: Activity concentration (Bq kg-1) of Bahja sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K
Easting Northing BHJS 005 * 48 ± 6 20 ± 2 15 ± 2 161 ± 6 BHJS 008 * 52 ± 6 18 ± 2 15 ± 2 258 ± 8 BHJS 018 398766 2198993 51 ± 10 13 ± 10 10 ± 4 159 ± 30 BHJS 030 398675 2198899 55 ± 10 13 ± 10 10 ± 4 175 ± 32 BHJS 039 * 47 ± 6 15 ± 2 12 ± 2 164 ± 7 BHJS 043 398775 2198963 68 ± 10 8 ± 7 6 ± 3 175 ± 32 BHJS 065 398736 2198925 58 ± 10 11 ± 9 9 ± 4 163 ± 31 BHJS 070 398698 2198876 71 ± 11 13 ± 10 10 ± 4 107 ± 29 BHJS 073 398682 2198837 52 ± 10 17 ± 12 13 ± 4 191 ± 33 BHJS 076 398659 2198802 37 ± 9 15 ± 11 12 ± 4 178 ± 30 BHJS 078 398637 2198780 62 ± 10 11 ± 9 9 ± 4 172 ± 32 BHJS 080 398617 2198763 63 ± 10 11 ± 9 9 ± 4 198 ± 33 Maximum 71 20 15 258 Minimum 37 8 6 107 Median 54 13 10 174 Mean ± SE 55 ± 3 14 ± 1 11 ± 1 175 ± 10 Standard Deviation 10 3 3 35 Number of Samples 12
* Analysis by HPGe gamma spectroscopy system
72
Table 4.9: Activity concentrations (Bq kg-1) of Nimr sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K
Easting Northing
NMRS 001 * 133 ± 8 54 ± 2 59 ± 3 200 ± 7 NMRS 024 * 108 ± 7 47 ± 2 41 ± 3 128 ± 7 NMRS 026 * 197 ± 10 103 ± 3 96 ± 3 369 ± 10 NMRS 029 383548 2051927 18 ± 9 22 ± 8 21 ± 6 82 ± 24 NMRS 039 383612 2051829 56 ± 11 19 ± 7 18 ± 5 75 ± 27 NMRS 044a 383648 2051790 22 ± 10 26 ± 9 25 ± 6 102 ± 27 NMRS 044b * 260 ± 10 130 ± 3 136 ± 3 150 ± 7 NMRS 045 * 52 ± 5 17 ± 2 14 ± 2 221 ± 7 NMRS 052 383579 2051742 34 ± 10 20 ± 8 19 ± 5 109 ± 27 NMRS 085 383370 2052107 49 ± 10 12 ± 6 12 ± 4 84 ± 26 NMRS 100 383387 2051795 36 ± 8 8 ± 4 7 ± 3 184 ± 30 NMRS 111 383498 2051667 85 ± 13 29 ± 10 28 ± 6 126 ± 34 NMRS 116 383401 2051590 46 ± 9 12 ± 6 12 ± 4 137 ± 29 NMRS 119 0383375 2051622 37 ± 5 10 ± 1 10 ± 2 118 ± 6 NMRS 126 383332 2051726 17 ± 9 23 ± 8 22 ± 6 132 ± 27 NMRS 143 383291 2051664 32 ± 8 12 ± 6 12 ± 4 113 ± 26
73
Table 4.9 (Continued): Activity concentrations (Bq kg-1) of Nimr sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K
Easting Northing Maximum 260 130 136 369 Minimum 17 8 7 75 Median 47 21 20 127 Mean ± SE 74 ± 18 34 ± 9 33 ± 9 146 ± 19 Standard Deviation 69 35 36 73 Number of Samples 16
* Analysis by HPGe gamma spectroscopy system
74
Table 4.10: Activity concentrations (Bq kg-1) of Marmul sludge strips Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K
Easting Northing
MRLS 005 319306 2014175 38 ± 10 16 ± 6 18 ± 5 119 ± 28 MRLS 011 319230 2014186 66 ± 12 17 ± 7 19 ± 5 54 ± 27 MRLS 013 319161 2014080 63 ± 11 13 ± 6 15 ± 5 114 ± 30 MRLS 015 * 9.5 ± 0.7 1.9 ± 0.2 2.3 ± 0.2 16 ± 1 MRLS 018 319120 2014219 62 ± 12 25 ± 8 28 ± 6 39 ± 27 MRLS 019 * 414 ± 13 117 ± 3 138 ± 4 188 ± 9 MRLS 020 * 295 ± 19 76 ± 3 96 ± 4 467 ± 13 MRLS 021 319058 2014135 56 ± 11 19 ± 7 21 ± 6 58 ± 27 MRLS 024 319079 2014233 52 ± 12 24 ± 8 27 ± 6 53 ± 27 MRLS 025a 319352 2014140 153 ± 17 28 ± 9 31 ± 7 141 ± 40 MRLS 025b 319352 2014140 181 ± 16 9 ± 5 10 ± 4 240 ± 44 MRLS 027 * 175 ± 9 60 ± 3 65 ± 3 455 ± 11 MRLS 031 * 79 ± 7 23 ± 2 26 ± 2 163 ± 6 MRLS 033 * 205 ± 11 116 ± 3 116 ± 4 313 ± 10 MRLS 046 * 455 ± 15 121 ± 4 109 ± 5 264 ± 9 MRLS 062 * 115 ± 8 61 ± 2 69 ± 2 166 ± 7 MRLS 069 319026 2014173 80 ± 12 15 ± 6 16 ± 5 284 ± 39 MRLS 102a 319037 2014148 67 ± 15 51 ± 14 56 ± 9 214 ± 40
75
Table 4.10 (Continued): Activity concentrations (Bq kg-1) of Marmul sludge strips
Sample ID Location 40Q UTM 226Ra 228Ra 228Th 40K
Easting Northing MRLS 102b 319042 2014179 92 ± 20 100 ± 22 111 ± 13 204 ± 47 MRLS 102c 319046 2014183 180 ± 24 130 ± 27 144 ± 15 137 ± 53 MRLS 112 319143 2014210 56 ± 11 21 ± 8 24 ± 6 35 ± 26 MRLS 115 319174 2014200 22 ± 10 23 ± 8 25 ± 6 166 ± 30 MRLS 118 319202 2014078 59 ± 11 15 ± 6 16 ± 5 110 ± 29 MRLS 148 319125 2014094 27 ± 11 31 ± 10 34 ± 7 103 ± 29 MRLS 183 319261 2014182 37 ± 12 31 ± 10 34 ± 7 54 ± 27 MRLS 186 319091 2014110 42 ± 9 12 ± 5 13 ± 4 79 ± 25 MRLS 189 319199 2014192 44 ± 10 15 ± 6 16 ± 5 93 ± 27 MRLS 192 319318 2014095 118 ± 13 5 ± 3 6 ± 3 205 ± 38 MRLS 193 319283 2014108 121 ± 14 11 ± 5 12 ± 4 197 ± 38 Maximum 455 130 144 467 Minimum 9.5 1.9 2.3 16 Median 67 23 26 141 Mean ± SE 116 ± 21 41± 7 45 ± 8 163 ± 22 Standard Deviation 110 39 42 114 Number of Samples 29
* Analysis by HPGe gamma spectroscopy system
76
4.3 Bahja NORM store yard
As mentioned in Section 4.2, any sludge or scale with a 226Ra activity
concentration higher than 1 kBq kg-1 is transported to the Bahja NORM storage
facility, in order to safely segregate the NORM contaminated scales and sludge
until a suitable disposal method is found. The yard is also used to store
decommissioned oil and gas distribution pipes before they are sent for cleaning,
along with a number of submersible electrical pumps, well heads and other
surface equipment.
The Bahja store yard consists of approximately 6 acres of flat compacted
land, surrounded by a boundary fence, with a locked gate. Figure 4.3 (a) shows
some of the sludge barrels, originally stored in Zuliya NORM store yard,
located some 70 km northwest of Bahja (refer to map in Chapter 2, Figure 2.1),
which were relocated to the Bahja NORM store yard in February 2007. At the
Bahja store yard, some of the oily sludge is also stored in one of two storage
pits, lined by a geo-textile material, designed to prevent seepage into the
surrounding earth (Figure 4.3 (b)). These pits are about 1 m deep, 10 m wide
and 100 m long. At the time of our last site visit (21 April – 8 May 2007), there
were approximately 5,000 decommissioned gas pipes (with a length and an
inner diameter of 8 m and 9 cm, respectively) and 20 decommissioned oil pipes
(of varying lengths and shapes, with an inner diameter of 35 cm) in the store
yard (Figure 4.3 (c)).
77
Figure 4.6: NORM store yards: (a) Sludge stored in 120 steel barrels with 226Ra activity content ≥ 1 kBq kg-1 (Zuliya store yard) (b) Oily sludge in one of the two geo-textile lined pits (Bahja store yard), and (c) Oil and gas pipes and other oil processing equipment contaminated with NORM at (Bahja store yard).
(a) (c) (b)
78
4.3.1 Oil industry scales
4.3.1.1 Oil scale formation and removal
The initial extraction of oil from the reservoir is usually water-free,
however, as the well ages and more oil is extracted from the formation rock, the
temperature and pressure in the reservoir decreases and the natural water present
in the reservoir will begin to be co-produced with the oil (Smith, 1987).
Subsequent to extraction, the dissolved radium present in the produced water is
co-precipitated with calcium and barium, in the form of carbonates and
sulphates, resulting in the formation of hard and highly insoluble scale deposits
on the interior walls of both the pipes, and other production equipment (Testa et
al., 1994, Hamlat et al., 2001, Godoy and Petinatti da Cruz, 2003, Paschoa,
2003, Hamlat et al., 2003b, Hamilton et al., 2004, Al-Masri and Aba, 2005,
Bader, 2006). The predominant scale constituents are those of calcium
carbonate (CaCO3) and calcium and barium sulphate (Ca/BaSO4), however,
barium and magnesium carbonate (Ba/MgCO3), along with strontium sulphate
(SrSO4) may also be found to a lesser degree (Hamilton et al., 2004) . Rn-222
(T½: 3.824 days, alpha energy 5.49 MeV) and 220Rn (T½: 55.6 seconds, alpha
energy 6.29 MeV), along with their progeny, may also be exhaled from scales
and/or build up in the crude oil and gas streams (Rood et al., 1998, Worden et
al., 2000). Like barium and strontium, radium is an alkaline earth metal that
belongs to the group II elements in the periodic chart, and as such, it also
displays similar chemical properties.
79
According to Wilson and Scott (1992), the scale formation process occurs in
three stages. Firstly, a scale molecule is formed, when there is a supersaturation
or other chemical disequilibrium in the environment. Secondly, these molecules
come together, forming microcrystalline nuclei that grow and coalesce to form
clusters. When the clusters reach a specific size, precipitation takes place.
Finally, the precipitate adheres to the inner walls of the production equipment,
including the well fluid handling system, down-hole pipes, well heads,
subsurface safety valves, manifolds, separators, oil coolers and produced water
distribution pipes. Although the sticking mechanism is not well understood,
initial surface nucleation seems to be an important factor, before the
precipitation can take place.
In addition to increasing production costs, as a result of the maintenance and
downtime associated with their removal, these scales also reduce efficiency by
clogging valves, restricting flow and damaging equipment (IAEA, 2003,
Hamilton et al., 2004). The removal of scale not only requires time and money,
but also significant expertise, especially since most scales are NORM
contaminated. As such, appropriate radiation protection measures should be
enforced according to the method adopted for scale removal. In Oman, PDO
have approved two main methods of scale removal. Initially, partially clogged
flow line pipes are physically cleared of solid scale deposits while still in
operation, using a process called ‘pigging’. This process utilises a rotating
plastic or rubber plug, termed a ‘pig’, which is launched upstream of the main
flow and then recovered (along with the removed scale) using a ‘pig trap’,
80
located further down-stream (IAEA, 2003). The removed scale/sludge is then
transported to the nearest sludge farm for storage and/or disposal.
When the flow lines become completely clogged and pigging is no longer an
option, the pipes are decommissioned and transported to Bahja NORM store
yard. It is then possible for scale build-up to be removed, through the
application of a mechanically controlled metal rod, used to break up the scale.
This would then be followed by the application of high pressure water jets
(1500-2500 bar), designed to clear the broken scale from the pipes (Al-Masri
and Aba, 2005). However, at the time of writing this thesis, PDO was still in the
process of finding an appropriate contractor to build a decontamination facility
for the removal of these scales, and until such time as this occurs, these pipes
and contaminated equipment are unable to be re-used or melted down and
recycled.
In addition to the number of other scale decontamination and disposal
options discussed by Rood et al. (1998), IAEA (2003) and Hamilton et al.
(2004), IAEA (2003) also discussed the possibility of preventing scale
precipitation, by introducing scale-inhibiting chemicals into the seawater
injection systems, which would act to prevent sulphate and carbonate scale
deposition. The prevention of scale build-up would mean that the radium
isotopes would actually move through production system, and would only be
found in the final produced water by-product. However, this theory was
challenged by several authors (Rajaretnam and Spitz, 2000, Shawky et al.,
81
2001), who argued that scale prevention or remediation processes are not only
difficult, but also very expensive to manage.
4.3.1.2 Radioactivity in oil scales
Seven oil scale samples were collected from decommissioned flow line
pipes (35 cm in diameter), originally used to pump produced water from the
separator tanks (see Chapter 3, Section 3.2.1 and 3.2.2 for details on sampling
and measurement procedures). The radioactivity of these samples was then
analysed using the HPGe system and the full range of gamma spectroscopy
results for the scale samples are presented in Table 4.12.
Radium 226 and 228: The main isotopes found during this analysis were
226Ra (T½: 1602 years; 238U primordial series), and to a lesser degree 228Ra
(T½: 5.75 years; 232Th primordial series), with the highest activity concentrations
for 226Ra and 228Ra being about 500 times higher than the mean ambient soil
activity concentrations found in the south of Oman (Table 4.2).
This finding can be explained by the fact that radium is somewhat soluble in
water, when placed under the high temperatures and pressures found in
petroleum reservoirs, 226Ra tends to leach into co-existing brines or formation
water (Rajaretnam and Spitz, 2000, Shawky et al., 2001). Upon 226Ra decay, the
product, 222Rn, migrates back into the organic liquid and natural gas phases
(Jerez Vegueria et al., 2002b). In contrast, uranium and thorium are part of the
rock matrix core, and since neither are highly soluble in water, only trace
82
amounts of 238U, 232Th, 228Th, 210Pb and 210Po are generally found in reservoir
fluids (White and Rood, 2001).
Overall, 226Ra, 228Ra and 228Th activity concentrations ranged between
3.4-17.3, 1.4-4.3 and 1.4-5.7 kBq kg-1, respectively, and Ra-228 activity
concentrations were less than 226Ra values, with a mean (± standard error) for
228Ra:226Ra activity ratio of 0.307 ± 0.026. A number of other studies also
reported radium activity in scales. For example, in Malaysia, Omar et al. (2004)
reported maximum activity concentrations of 434 and 479 kBq kg-1 for 226Ra
and 228Ra, respectively, while Godoy and Petinatti da Cruz (2003) reported
activity concentrations of 19.1-323 kBq kg-1 for 226Ra and 4.21-235 kBq kg-1 for
228Ra, in Brazil. Al-Masri and Aba (2005) also investigated activity
concentrations for oil scales in the Republic of Syria, and found maximum
(mean) radium isotope activity concentrations of 1520(174), 868(91) and
780(67) kBq kg-1 for 226Ra, 228Ra and 224Ra, respectively.
While the radium activity concentration values reported in this study were
within the typical range reported internationally of 0.1-15,000 kBq kg-1 (Jonkers
et al., 1997, IAEA, 2003), it was found that the activity concentrations observed
in Oman were somewhat lower than the values reported in the above mentioned
studies. This could either be attributed to the difference in Oman geological
formations to those found elsewhere in the world, or an indication that
radioactive scale formation is still in its early stages, since older reservoirs tend
to have higher produced water to crude oil volume ratio, and hence higher
quantity of NORM (Rood et al., 1998). Another possible reason for the
83
discrepancy is that the scale samples collected in this study were taken from the
available downstream pipes, used for handling produced water once it was
separated from the crude oil. This can be explained by other reported findings,
which suggest that radium isotopes activity concentrations are higher in up-
stream scales that form between the reservoir and the well head, compared to
down-stream scales that form in the distribution pipes (Hamilton et al., 2004,
Al-Masri and Aba, 2005).
Actinium 227: In addition to radium isotopes, unsupported 227Ac (T½:
21.77 years; 235U primordial series) was also detected in the oil scale samples.
Whilst no other activity concentrations for 227Ac in oil scales have been reported
to date, its association with naturally occurring brines in the environment has
been reported by a number of authors, such as Dickson (1991), Lieser et al.
(1999), and Martin and Akber (1999). For example, Martin and Akber (1999)
studied 227Ac behaviour in aqueous solutions containing seepage from the
tailings impoundments of a uranium mine, by looking at 227Ac:223Ra ratio, and
came to the conclusion that not all of the 223Ra in solution was supported by
227Ac, due to the absorption of the intermediate radionuclide 227Th
(T½: 18.7 days).
In contrast, results for the solid scales analysed in this research showed
223Ra (T½: 11.4 days) in equilibrium with 227Ac at the time of measurement, and
since our samples were collected from pipes which had been decommissioned
several years prior, any unsupported 223Ra would have been completely
decayed, indicating that the only possible source of the detected 223Ra was the
84
disintegration process of 227Ac. This newly quantified presence of 227Ac in oil
and gas scales is highly important, since it can not only pose a significant
radiological hazard, but it also has a higher committed effective dose coefficient
for inhalation and ingestion compared to the other radionuclides found in scales
and sludge (Table 4.11).
Table 4.11: Committed effective dose coefficients (µSv Bq-1) of selected radionuclides likely to be present in petroleum scales (ICRP68) Mode of Entry ↓
Radioisotope →
227Ac 228Ra 226Ra 210Pb + 210Po
Inhalation for 1 µm AMAD 540 2.6 3.2 1.5
Ingestion 1.1 0.67 0.28 0.92
As such, the presence of 227Ac in oil scales is thought to be a result of its
affinity to the brines found in oil reservoirs, which would then allow it
precipitate with the other minerals on the internal surface of the pipes. The
maximum 227Ac activity concentration, of 123 Bq kg-1, was more than 70 times
higher than the theoretically calculated ambient soil value of 1.7 Bq kg-1.
Lead 210: The oil scale samples analysed in this study were collected from
pipes decommissioned in 1999. Considering that these pipes had been in service
for 10 years prior to decommission, this means that the scales would have been
in the pipes for at least 18 years. As such, assuming zero 210Pb activity at the
time the scales were forming, the expected 210Pb:226Ra activity ratio would be
0.42. Moreover, dating the oil scales using the 228Th:228Ra activity ratio gave an
average age of 15 years, indicating that the expected 210Pb:226Ra activity ratio
was 0.36. However, both of these calculations were significantly lower than the
85
actual mean (± standard error) for 210Pb:226Ra activity ratio measured in this
study, being 0.64 ± 0.08. This discrepancy indicates that there was higher than
expected 210Pb in the oil scale samples collected in this study, possibly brought
to the surface from the reservoir rock by the produced water.
86
Table 4.12: Activity concentrations (Bq kg-1) of oil scale samples Sample ID 226Ra 210Pb 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra 210Pb:226Ra
FHDOS 1 4360 ± 50 3060 ± 610 1550 ± 10 2070 ± 380 40 ± 7 < 28 0.356 ± 0.006 1.34 ± 0.25 0.70 ± 0.15 FHDOS 2 13000 ± 100 4990 ± 1410 3230 ± 20 4800 ± 720 68 ± 14 < 54 0.248 ± 0.003 1.49 ± 0.23 0.38 ± 0.11 FUDOS 3 3380 ± 40 3310 ± 470 1360 ± 10 1930 ± 390 47 ± 7 < 27 0.402 ± 0.008 1.42 ± 0.30 0.98 ± 0.15 FHDOS 4 11800 ± 100 6450 ± 1490 3100 ± 20 4950 ± 740 67 ± 13 < 52 0.263 ± 0.004 1.60 ± 0.25 0.55 ± 0.13 FHDOS 5 6340 ± 60 4330 ± 870 2250 ± 10 2920 ± 420 34 ± 9 < 33 0.355 ± 0.005 1.30 ± 0.19 0.68 ± 0.14 FHDOS 6 17300 ± 100 7590 ± 1640 4310 ± 20 6810 ± 950 123 ± 17 < 65 0.249 ± 0.003 1.58 ± 0.23 0.44 ± 0.10 FHDOS 7 6380 ± 50 4720 ± 570 1740 ± 10 2610 ± 440 71 ± 9 < 32 0.273 ± 0.004 1.50 ± 0.26 0.74 ± 0.10 Maximum 17300 7590 4310 6810 123 0.402 1.60 0.98 Minimum 3380 3060 1360 1930 34 0.248 1.30 0.38 Median 6380 4720 2250 2920 67 0.273 1.49 0.68 Mean ± SE 8940 ± 2110 4920 ± 670 2510 ± 440 3730 ± 750 64 ± 12 0.307 ± 0.026 1.46 ± 0.05 0.64 ± 0.08 Standard Deviation 5160 1630 1080 1830 30 0.063 0.11 0.20 Number of Samples 7
87
4.3.2 Gas industry scales
4.3.2.1 Gas scale formation and removal
Radon is a noble gas, also known to be soluble in organic matter, and its
association with natural gas streams has been known for almost 100 years
(Satterly and McLennan, 1918). From a radiological hazard point of view, 220Rn
(Ra-228 progeny, T½: 55.6 seconds) does not pose as high a risk in oil industry,
as 222Rn (Ra-226 progeny, T½: 3.824 days), the reason being that the former
only has a few short-lived progeny, whereas the latter has two long-lived
progeny, 210Pb and 210Po, with half-lives of 22.3 years and 138.4 days,
respectively.
The precipitation of 210Pb results in a thin radioactive film forming on the
internal walls of gas pipes, pumps and vessels, with a reported specific activity
higher than 1 kBq g-1 (Hamlat et al., 2003a, Al-Masri and Shwiekani, 2008).
Although Pb-210 has a soft gamma of 46.5 keV, which cannot even be detected
by a conventional dose rate meter outside the pipes, it has an extremely high
toxicity value if inhaled or ingested via dust particles, during plant maintenance.
Both stable lead and 210Pb are known to be mobilised from the reservoir rock,
however the mechanism by which this occurs is not yet well understood
(Jonkers et al., 1997, IAEA, 2003). Condensates derived from natural gas have
also shown elevated 210Po relative to its grandparent 210Pb, suggesting a direct
emanation from the reservoir rock (IAEA, 2003).
88
In addition, because 222Rn has a boiling point between ethane and propane,
unsupported 222Rn is also known to migrate with the ethane and propane
fractions of the natural gas stream. As such, high levels of 222Rn daughters
(210Pb, 210Bi and 210Po) are often found in the internal walls of ethane and
propane processing pumps and vessels.
4.3.2.2 Radioactivity in gas scales
The radioactivity of gas scale samples was also analysed using the HPGe
system. Scale samples were collected from 12 pipes, according to the sampling
and measurement procedures outlined in Chapter 3, Section 3.2.1 and 3.2.2. The
main gamma emitting radionuclides identified during the analysis were 210Pb,
227Ac, 40K and 226Ra. The full range of gamma spectroscopy results are shown in
Table 4.13.
Radium isotopes and Lead 210: The 210Pb activity concentration of the
samples ranged from 0.959-66.4 kBq kg-1, with a mean value of
30.56 ± 7.83 kBq kg-1, however the mean 226Ra activity concentration of
75 ± 10 Bq kg-1 was too low to support such activity. This suggests that the high
210Pb radioactivity may be the result of 222Rn decay, which is known to migrate
with the gaseous organic phase. While the activity ratio of 226Ra:210Pb ranged
between 1.2 x 10-3 - 2.3 x 10-2, with a mean of (7.5 ± 2.5) x 10-3, Jonkers et al.
(1997) reported detection of 210Pb activity concentrations higher than 226Ra,
with a 226Ra:210Pb activity ratio of 0.1. As such, the ratios reported in this study
were approximately two orders of magnitude less than Jonkers et al. (1997)
ratio. Mean 228Ra activity concentration was four times lower than mean 226Ra
89
activity concentration. Gas scales mean activity concentrations for both 226Ra
and 228Ra were two times higher than in ambient soils.
Thorium 228 and Potassium 40: Th-228 activity concentrations were
below HPGe system’s MDA, and therefore the data presented in Table 4.16 for
228Th, are the error weighted averages derived from its progeny; 224Ra, 212Pb,
212Bi and 208Tl. This resulted in relatively higher uncertainties in the 228Th:228Ra
activity ratios in the gas scales compared to oil scales. The mean (± standard
error) 228Th:228Ra activity ratio of 1.47 ± 0.12 results in an average gas scale age
of approximately 16 years.
Actinium 227: As mentioned in Section 4.2.2, the mean 226Ra activity
concentration in ambient soils is 34 Bq kg-1. Assuming equilibrium and uranium
mass balance, this would indicate an average 227Ac activity concentration of
1.7 Bq kg-1 in ambient soils. Higher than ambient 227Ac activity concentrations
were detected in 10 out of the 12 gas scale samples, ranging from 4-181 Bq kg-1.
Hence the maximum activity concentration of 227Ac in the gas scales was more
than 100 times higher than the ambient soil value. To date, the only other
numerical value reported for 227Ac activity concentration in gas scale was
2.5 Bq g-1, published by Kolb and Wojcik (1985).
90
Table 4.13: Activity concentrations (Bq kg-1) of gas scale samples Sample ID 226Ra 210Pb 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra 226Ra:210Pb
ZULGS 1 114 ± 25 66405 ± 882 25 ± 2 39 ± 5 69 ± 4 73 ± 7 0.22 ± 0.06 1.58 ± 0.34 (1.7 ± 0.4)x10-3
ZULGS 2 125 ± 13 49013 ± 703 26 ± 2 41 ± 5 71 ± 4 89 ± 7 0.21 ± 0.04 1.58 ± 0.31 (2.6 ± 0.3)x10-3
ZULGS 3 33 ± 6 6241 ± 200 7.8 ± 1.1 7.4 ± 2.1 5 ± 1 347 ± 8 0.24 ± 0.08 0.95 ± 0.40 (5.3 ± 1.1)x10-3
ZULGS 4 51 ± 21 43377 ± 633 5.6 ± 1.0 7.6 ± 2.0 9 ± 4 26 ± 4 0.11 ± 0.07 1.37 ± 0.60 (1.2 ± 0.5)x10-3
ZULGS 5 53 ± 13 < 541 < 6.3 - < 6.9 76 ± 15 - - - ZULGS 6 83 ± 12 5643 ± 353 19 ± 2 41 ± 5 24 ± 3 157 ± 11 0.23 ± 0.06 2.20 ± 0.58 (1.5 ± 0.3)x10-2
ZULGS 7 84 ± 15 5991 ± 538 24 ± 4 42 ± 7 21 ± 4 212 ± 17 0.28 ± 0.10 1.76 ± 0.62 (1.4 ± 0.4)x10-2 ZULGS 8 82 ± 15 9110 ± 457 16 ± 3 13 ± 4 65 ± 4 236 ± 14 0.20 ± 0.07 0.80 ± 0.37 (9.0 ± 2.1)x10-3
ZULGS 9 < 32 54039 ± 710 6.1 ± 0.9 9.9 ± 2.2 < 7.6 30 ± 4 - 1.65 ± 0.59 - ZULGS 10 84 ± 13 52678 ± 658 24 ± 2 35 ± 5 181 ± 4 98 ± 7 0.28 ± 0.07 1.48 ± 0.32 (1.6 ± 0.3)x10-3 ZULGS 11 22 ± 4 959 ± 108 2.3 ± 0.6 2.9 ± 1.3 4 ± 1 61 ± 4 0.11 ± 0.05 1.26 ± 0.90 ( 2.3 ± 0.7)10-2
ZULGS 12 92 ± 24 42716 ± 502 32 ± 3 48 ± 5 11 ± 5 190 ± 10 0.35 ± 0.12 1.51 ± 0.30 (2.2 ± 0.6)x10-3
Maximum 125 66405 32 48 181 347 0.35 2.20 2.3 x 10-2
Minimum 22 959 2.3 2.9 4 26 0.11 0.80 1.2 x 10-3
Median 83 42716 19 35 22 94 0.22 1.51 3.9 x 10-3
Mean ± SE 75 ± 10 30561 ± 7830 17 ± 3 26 ± 6 46 ± 18 133 ± 29 0.22 ± 0.02 1.47 ± 0.12 (7.5 ± 2.5)x10-3
Standard Deviation
32 24759 10 18 55 97 0.07 0.38 7.4 x 10-3
Number of Samples
12
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4.3.3 Comparison between oil and gas scales
Figure 4.7 shows a comparison between average radionuclide activity
concentration for the oil and gas scales analysed in this study. Ac-227 activity
concentrations for both oil and gas scales were within the same order of
magnitude, whilst the 226Ra activity concentration for oil scales was two orders
of magnitude higher than for gas scales. In contrast, the 210Pb activity
concentration for gas scales was one order of magnitude higher than for oil
scales. The 226Ra:210Pb activity ratio of gas scales, being (7.5 ± 2.5) x 10-3,
supports the theory outlined earlier in this chapter, that excess 210Pb was
transported in the natural gas stream as a result of 222Rn decay, as well as being
directly mobilised from the reservoir rock. Similarly, the 210Pb:226Ra activity
ratio for oil scales, being 0.64 ± 0.08, indicates that the ratio is higher than
expected by 210Pb ingrowth from 226Ra decay, given the scale age, suggesting
direct mobilisation of 210Pb from the reservoir rock by the produced water.
92
0.01
0.1
1
10
100
Ac-227 Ra-226 Pb-210 Ra-228 Th-228 K-40
Gas scalesOil scales
Figure 4.7: Average activity concentrations for radionuclides found in oil and gas scales.
Although 228Ra and 228Th activity concentrations for oil scales were two
orders of magnitude higher than for gas scales, the 228Th:228Ra mean (± standard
error) activity ratios of 1.46 ± 0.05 and 1.47 ± 0.12, respectively, indicate a
similarity in their average age. While the mean (± standard error) for gas scales
40K was 133 ± 29 Bq kg-1, oil scale 40K activity concentrations were less than
the HPGe system’s MDA.
4.3.4 Radioactivity in the sludge stored in barrels
According to PDO’s Health, Safety and Environment Specifications (2005),
NORM contaminated sludge, with a 226Ra activity concentration equal to or
higher than 1 kBq kg-1, is separated from the rest of the sludge, packed in steel
Act
ivity
con
cent
ratio
n (B
q g-1
)
Radionuclide
93
barrels and transported to the Bahja NORM store yard as soon as possible. At
the time of our last visit to the site (21 April – 8 May 2007), there were more
than 200 barrels stored at the Bahja NORM store yard, and this number will
continue to increase, until a suitable disposal method is found for this
radioactive oily sludge.
Barrel surface dose rates of up to 40 µSv h-1 were encountered during the
field work, with six sludge samples collected from the barrels and taken back to
the laboratory for further analysis (see Chapter 3, Section 3.2.1 and 3.2.2 for
details on sampling and measurement procedures). Single samples were also
collected from separate barrels containing beads and sand. All samples were
analysed using the HPGe system and the results are shown in Table 4.14.
Radium and Thorium Isotopes: 226Ra radioactivity concentrations for the
barrel stored sludge varied over a wide range of at least two orders of
magnitude. The highest activity concentrations were 223 kBq kg-1 for 226Ra,
34 kBq kg-1 for 228Ra and 45 kBq kg-1 for 228Th, which were similar to values
measured in Brazil, reported by Matta et al. (2002), which were up to
331 kBq kg-1 for 226Ra, 245 kBq kg-1 for 228Ra and 272 kBq kg-1 for 228Th
(Table 4.15). Possible reasons for this difference have already been outlined in
Section 4.3.1.2.
Gazineu et al. (2005) also reviewed some of the existing international data
for 226Ra and 228Ra activity concentrations in sludge samples from various oil
94
exploration operators. Table 4.15 provides a summary of their findings,
alongside the findings from Matta et al. (2002) and the findings from this study.
Table 4.14: Range of sludge 226Ra and 228Ra activity concentrations (kBq kg-1) for oil exploration operations of several countries of the world
Country Material 226Ra 228Ra
Brazil * sludge 0.13 – 331 0.01 – 245
Brazil sludge 0.36 – 367 0.25 – 343
Norway sludge 0.1 – 4.7 0.1 – 4.6
Algeria sludge 0.069 – 0.393 -
Oman ** sludge (in store) 1.7 – 223 1.2 – 34.4
Oman ** sludge (separator tank) 0.36 – 0.99 0.14 – 0.45
Oman ** sludge (piles) 0.027 – 5.670 0.007 – 6.036
* Matta et al. (2002); ** This study; All other data are from Gazinue et al.
Actinium 227: Both the mean and maximum 227Ac activity concentrations,
of 188 Bq kg-1 and 614 Bq kg-1, were two orders of magnitude higher than our
theoretically calculated value of 1.7 Bq kg-1, for Oman’s ambient soils. In
addition the mean is an order of magnitude higher than both oil and gas scale
averages. However, the maximum activity concentration obtained was four
times less than the single value reported by Kolb and Wojcik (1985), for a
CaCO3 gas field scale in North Germany.
Further, the 228Th:228Ra activity ratio indicates that the sludge found in the
barrels was quite new, with a mean age of 4 years (except for sample 6, which
gave an average age of 8.3 years).
95
Table 4.15: Activity concentrations (Bq kg-1) for sludge stored in barrels Sample ID 226Ra 228Ra 228Th 227Ac 40K 228Ra:226Ra 228Th:228Ra
BHJB 1 14000 ± 100 10489 ± 32 8975 ± 37 141 ± 18 336 ± 32 0.74 ± 0.01 0.88 ± 0.01 BHJB 2 1700 ± 30 1212 ± 10 916 ± 9 286 ± 8 1480 ± 30 0.70 ± 0.02 0.80 ± 0.01 BHJB 3 5560 ± 69 3747 ± 18 3320 ± 20 40 ± 20 952 ± 33 0.66 ± 0.01 0.92 ± 0.01 BHJB 4 2000 ± 40 1478 ± 10 1218 ± 11 15 ± 7 953 ± 23 0.73 ± 0.02 0.86 ± 0.01 BHJB 5 6700 ± 70 4639 ± 20 4041 ± 21 31 ± 17 765 ± 25 0.68 ± 0.01 0.91 ± 0.01 BHJB 6 223000 ± 355 34413 ± 47 44639 ± 83 614 ± 66 < 170 0.15 ± 0.00 1.30 ± 0.00 Beads * 38012 ± 131 39491 ± 39 29109 ± 36 247 ± 24 < 80 1.04 ± 0.00 0.74 ± 0.00 Sand * 35386 ± 162 69406 ± 58 53653 ± 516 440 ± 49 324 ± 54 1.96 ± 0.01 0.77 ± 0.01 Maximum 223000 34413 44639 614 1480 0.74 1.30 Minimum 1700 1212 916 15 336 0.15 0.80 Median 6130 4193 3680 91 952 0.69 0.89 Mean ± standard error 42160 ± 39670 9330 ± 5696 10518 ± 7587 188 ± 104 897 ± 206 0.61 ± 0.10 0.94 ± 0.08 Standard deviation 88705 12738 16965 232 412 0.22 0.18 Sample count 8
* Not included in the sum calculations
96
4.4 Radioactivity in the sediments of Al-Noor
evaporation ponds
Like many other industrial operations, the petroleum industry makes use of
ponds, trenches or pits to store or evaporate liquid wastes generated during oil
production. Al-Noor evaporation pond (Figure 4.7) is used for the evaporation
of produced water generated by the dehydration of crude oil at nearby Al-Noor
station (refer to map in Chapter 2, Figure 2.1). Al-Noor evaporation pond is
secured by a boundary net with a lockable gate; it has an area of 14,400 m2, a
depth of about 1.5 m and is lined by high density polyethylene thermoplastic.
The pond is comprised of two sections (Section 1 and Section 2),
interconnected by four channels, which allow water to flow from Section 1 to
Section 2. Approximately 10 m3 of produced water is pumped into Section 1 on
a daily basis, and this waste water is contaminated by NORM, heavy metals,
volatile organic compounds, polycyclic aromatic hydrocarbons and other toxic
compounds. Evaporation concentrates the NORM activity content of the
produced water, which then crystallises and eventually leads to scale formation
on the ponds internal walls. Six samples were collected from the pond for
analysis, according to the sampling and measurement procedures described in
Chapter 3, Section 3.2.1 and 3.2.2. The radioactivity of these sediment samples
was analysed using the HPGe system.
97
Figure 4.8: Section 2 of Al-Noor evaporation pond [picture courtesy of Mohammad Al-Masri].
During this study, 227Ac was detected for the first time in oil scales and
sludge, however it was not detected in produced water evaporation pond
sediments, because it is most likely transported as a vector and deposited with
scale before reaching the ponds. The samples were found to contain both 238U
and 232Th progeny, with maximum 226Ra and 228Ra activity concentrations of
5.26 and 0.58 kBq kg-1, respectively (Table 4.16). This activity was localised at
the drainage point of the pond. The median and maximum 226Ra activity
concentrations were also more than 10 and 170 times higher than the Al-Noor
ambient soil radioactivity, respectively.
At the time of our last visit to the site (21 April – 8 May 2007) the pond had
also accumulated a 50 cm thick layer of sediment. At the time of deposition, this
98
sediment was found to have a mean 228Ra:226Ra activity ratio of 0.12. This
corresponded to a 232Th:238U mass ratio of 0.36, which is approximately half the
value obtained for nascent sludge at Nimr (0.73), which may serve to illustrate
the difference in geological formation fingerprints between the two regions.
99
Table 4.16: Activity concentrations (Bq kg-1) of Al-Noor evaporation pond soil sediments Sample ID 226Ra 228Ra 228Th 40K 228Ra:226Ra 228Th:228Ra
NOR 1 5260 ± 48 585 ± 9 188 ± 6 193 ± 20 0.11 0.32 NOR 2 354 ± 13 35 ± 2 17 ± 2 109 ± 8 0.10 0.48 NOR 3 119 ± 7 10 ± 1 5 ± 1 30 ± 5 0.08 0.46 NOR 4 743 ± 16 77 ± 3 31 ± 4 429 ± 13 0.10 0.40 NOR 5 379 ± 12 46 ± 2 8 ± 3 379 ± 9 0.12 0.17 NOR 6 107 ± 6 13 ± 2 5 ± 2 837 ± 12 0.12 0.40 Maximum 5260 583 205 837 0.12 0.48 Minimum 107 10 5 30 0.08 0.17 Median 367 41 13 286 0.11 0.40 Mean ± standard error 1160 ± 904 127 ± 100 46 ± 35 330 ± 131 0.11 ± 0.01 0.37 ± 0.05 Standard deviation 2022 225 79 292 0.01 0.11 Sample number 6
100
4.5 Discussion and conclusions
This chapter outlined the radioactivity concentrations found in oilfields of
the Southern Oman Directorate, where the radioactivity of untreated and treated
sludge, oil and gas scales, barrel stored sludge and evaporation pond sediment,
was assessed using both in-situ and laboratory gamma spectrometers. This is the
first comprehensive survey of radioactivity concentrations to be conducted for
the petroleum industry, in the Sultanate of Oman, and it is also the first ever
study to report the detection of 227Ac in oil sludge and in both oil and gas scales.
Ac-227 half life (21.8 years) is similar to that of 210Pb (22.3 years), but
because it is unsupported it would decay to ambient levels in seven to nine half
lives. On the other hand, the 226Ra supported 210Pb is a longer term radiological
hazard. Because once it reached secular equilibrium with its parent radionuclide
by ingrowth in about 100 years, it would decay at a 226Ra half life of 1602 years.
This study successfully made use of a portable NaI(Tl) gamma spectroscopy
system for measuring 226Ra and 228Ra activity concentration, however the
system did have its advantages and disadvantages. One obvious advantage was
the instantaneous radioactivity information that could be obtained in the field,
without having to transport radioactive samples for hundreds of kilometres back
to the laboratory. Another advantage was the relatively short acquisition time
for readings, due to the high efficiency of the scintillating NaI(Tl) 2¼” crystal
(600 s) compared to the semi-conductor HPGe crystal (60,000 s). However,
including the Pb shield, the system weighed approximately 30 kg, which was a
101
significant disadvantage. In addition, the system had a lower energy resolution
compared to the HPGe detectors.
As a result of the findings presented in this work, it was found that whilst
sludge farming was useful in eliminating harmful hydrocarbons from the sludge,
it still comes up short in the reduction of radioactivity concentrations down to
ambient soil levels. For example, although dilution factors of up to 5 times were
used, in an effort to reduce the activity concentration of 226Ra, the treated strips
still showed activity concentrations up to ten times higher than the ambient
levels found at both Nimr and Marmul sludge farms. Despite the daily tilling of
the treated sludge, some hotspots > 1 kBq kg-1 also remained on the strips.
More specifically, this study found that sludge activity concentrations from
Bahja, Nimr and Marmul sludge farms ranged from 0.03-3.69, 0.01-6.04,
0.01-5.16 and 0.05-0.95 kBq kg-1 for 226Ra, 228Ra, 228Th and 40K, respectively.
In addition, 25% and 10% of the untreated sludge piles had 226Ra activity
concentration exceeding 1 kBq kg-1, at Bahja and Marmul farms, respectively.
Bahja sludge farm had the highest mean 226Ra activity concentration of
3289 Bq kg-1, while Nimr and Marmul sludge farms had similar 226Ra mean
activity concentrations of 343 and 356 Bq kg-1, respectively. Nimr and Marmul
mean activity concentrations were found to be lower than the mean activity
concentrations for Bahja sludge piles, by one order of magnitude. Similarly, the
Bahja sludge was found to be the oldest, with a mean age of 9.0 ± 0.4 years,
while the Nimr and Marmul sludge had similar ages of 4.2 ± 0.3 and
3.6 ± 0.4 years, respectively. The nascent Nimr sludge radioactivity results can
102
now be used as a baseline with which to compare future nascent oil sludge
originating from Nimr station. They can also be used as a reference point when
formulating trends in relation to the different types of new technologies that
may be used in the oil recovery process.
This study also found that the average radium isotope activity concentration
in Oman oil scales fell within the lower end of the range of activity
concentrations reported elsewhere (0.1-15,000 kBq kg-1). The range of 226Ra
and 228Ra were 3.38-17.30 and 1.36-4.31 kBq kg-1, respectively. The mean 226Ra
activity concentration of oil scales (8.940 kBq kg-1) was also found to be higher
than the mean activity concentration of the farm sludge found in Bahja, Nimr
and Marmul, which were 3.289, 0.343 and 0.356 kBq kg-1, respectively.
Both oil and gas scales contained detectable levels of 210Pb, however only
the gas scales were characterised by the presence of high activity concentrations
of 210Pb. In gas scales, the activity concentration of 210Pb ranged from
0.959-66.405 kBq kg-1, while the corresponding range for the oil scales was
3.06-7.59 kBq kg-1. Oil and gas scales, and barrel stored sludge also contained
227Ac, with maximum activity concentrations of 123 ± 17, 181 ± 4 and 641
± 66 Bq kg-1, respectively. As such, care should be taken when clearing these
scales from oil and gas pipes, particularly in terms of exposure pathways that
involve direct irradiation, inhalation and ingestion.
Many petroleum companies in Oman dispose of excess produced water in
evaporation in ponds, or by pumping it into producing reservoirs, and
103
abandoned deep and shallow wells. As outlined above, during this study, 227Ac
was detected for the first time in oil scales and sludge, however it was not
detected in produced water evaporation pond sediments, because it is most
likely transported as a vector and deposited with scale before reaching the
ponds. However, 238U and 232Th progeny along with 40K were detected in the
Al-Noor evaporation pond sediment with activity concentrations for 226Ra,
228Ra, 228Th and 40K ranging from 0.107-3.260, 0.010-0.583, 0.005-0.205 and
0.030-0.837 kBq kg-1, respectively. These activity concentrations are similar to
the activity concentrations of untreated sludge piles found in Bahja, Nimr and
Marmul. Hence, this work established that pond sediment is also contaminated
by enhanced NORM and caution should be exercised during its disposal.
In conclusion, this study provided the first ever information on the
radioactivity concentration of treated and untreated sludge, oil and gas scales,
barrel stored sludge and evaporation pond sediment for oilfields in the Southern
Oman Directorate. The information provided in this chapter may be used as
reference for NORM activity concentration assessments in the Oman oil
industry and it may also be used as a baseline for possible future assessments in
both the onshore and offshore oil rigs of the Northern Oman Directorate.
Finally, this pioneering study is also important since similar data is yet to be
published for neighbouring oil producing Gulf countries.
104
105
Chapter 5 GAMMA DOSE RATES AT SLUDGE FARMS IN OILFIELDS OF THE SOUTHERN OMAN DIRECTORATE
5.1 Introduction
This chapter presents findings in relation to gamma dose rates measured in
the air at and around sludge farms in the southern Oman Directorate. The
chapter begins with a brief outline of the terrestrial and cosmic components of
gamma dose in the air, followed by an overview of gamma dose rates in the
petroleum industry. It then goes on to present gamma dose rates for the sludge
farms, over both untreated sludge piles and treated sludge strips (refer to
Chapter 4, Section 4.2.1 for details of the sludge farming process), as well as the
gamma dose rate correlations with the main radionuclides present in the sludge,
namely 226Ra, 228Ra and 40K.
5.2 Terrestrial and cosmic gamma dose rates
Gamma dose rates in the air, from both cosmic and terrestrial sources, have
been the focus of a large number of international studies over the last four
decades, with many factors found to affect the measured rate. The terrestrial
component of the gamma dose rate is dependent on the depth and lateral
distribution profile of activity concentrations of the primordial series 232Th, 238U
and 235U, and the primordial radionuclide 40K, along with soil type, composition
and moisture content. Measurements performed on Anthropomorphic phantoms,
in order to determine dose to organ, and effective dose, have shown that there is
106
a strong dependence of measured dose on radiation energy and incident angle
(Saito and Jacob, 1995). The cosmic component of the gamma dose rate, on the
other hand, is dependent on the altitude at which the measurements are carried
out. The worldwide outdoor altitude adjusted value for the cosmic component of
gamma dose rate is 460 µSv y-1, reported by UNSCEAR (2000).
A survey on outdoor terrestrial gamma radiation in the Sultanate of Oman,
conducted by Goddard (2002), reported that most of Oman’s surface rock is
limestone, which is low in primordial uranium and thorium. Hence, the average
gamma dose rate, measured at the conventional height of 1 m above the ground,
was lower than the world average of 0.45 mSv y-1, with the mean population
weighted dose rate for Oman found to be 0.30 mSv y-1. Goddard also tested the
validity of measured ambient dose rates, using both the ICRU (1995) and Saito
and Jacob’s (1995) dose coefficient models, on activity concentration of soil or
rock collected from the in situ measurement locations, and developed the
following equation, which gave a strong linear correlation (R2) of 0.84:
Dm = 0.89 Dc - 0.17 (5.1)
where Dm and Dc are the measured and predicted dose rates.
In addition to Goddard (2002), many other authors have also modelled
gamma dose rates based on activity concentrations that arise from natural
sources in the ground (Lovborg et al., 1979, Kocher and Sjoreen, 1985,
Battaglia and Bramati, 1988, Deworm et al., 1988, Chen, 1991, Clark et al.,
1993, Saito and Jacob, 1995, UNSCEAR, 2000, Losana et al., 2001, Ajayi,
2002). Carter and Sonter (2003) reviewed much of this literature and tabulated
107
the conversion coefficients used in each study, to find dose rates from 238U,
232Th and 40K activity concentrations. The conversion coefficients differed for
the three radionuclides and varied by a factor of about two, ranging from
0.28-0.52 µSv h-1 per Bq g-1 for 238U, 0.295-0.73 for 232Th, and 0.029-0.05 for
40K.
Saito and Jacob (1995) also calculated Kerma coefficient factors at 1 m
height per disintegration for each individual radionuclide in the main primordial
series and 40K (see Table 5.1). These conversion factors allowed for the
calculation of air Kerma for individual radionuclides, which showed that 98% of
the 238U series air Kerma was due to only two radionuclides, being 214Pb and
214Bi. Further calculations also showed that 90% of the 232Th series total air
Kerma was due to 228Ac and 208Tl. This finding indicates that, while the series is
in equilibrium, 226Ra and 228Ra, along with their precursors, only make a small
contribution to the total measured gamma dose rate.
108
Table 5.1: Air Kerma rate at 1 m height per disintegration rate (nGy h-1 per Bq kg-1) of the parent nuclide per unit soil weight for natural sources uniformly distributed in the ground (adapted from Saito and Jacob)
238U series
Kerma rate per unit activity
(µGy h-1 per Bq g-1)
232Th series
Kerma rate per unit activity
(µGy h-1 per Bq g-1)
235U series
Kerma rate per unit activity
(µGy h-1 per Bq g-1)
40K Kerma rate per unit activity
(µGy h-1 per Bq g-1) 238U 4.33x10-5 232Th 4.78x10-5 235U 3.06x10-2 40K 4.17x10-2
234Th 9.47x10-4 228Ra 5.45x10-5 231Th 1.80x10-3 234Pam 4.30x10-3 228Ac 2.21x10-1 231Pa 6.89x10-3 234Pa 4.49x10-4 228Th 3.44x10-4 227Ac 3.54x10-5 234U 5.14x10-5 224Ra 2.14x10-3 227Th 2.10x10-2
230Th 6.90x10-5 220Rn 1.73x10-4 223Fr 1.15x10-4 226Ra 1.25x10-3 212Pb 2.77x10-2 223Ra 2.39x10-2 222Rn 8.78x10-5 212Bi 2.72x10-2 219Rn 1.25x10-2 214Pb 5.46x10-2 208Tl 3.26x10-1 215Po 5.11x10-5 214Bi 4.01x10-1 211Pb 1.70x10-2 210Tl 1.51x10-4 211Bi 1.08x10-2 210Pb 2.07x10-4 207Tl 5.67x10-4
Total 4.63x10-1 6.05x10-1 1.25x10-1 4.17x10-2
109
5.2.1 Gamma dose rates in the petroleum industry
In the petroleum industry, it is recommended that external gamma radiation
dose surveys on installations should be conducted both periodically and during
shutdowns. As discussed in Chapters 1 and 4, in addition to 40K and 227Ac (235U
series), it is mainly radium isotopes and 210Pb (one of 226Ra progeny – 238U
series) from the 238U and 232Th series, which are brought to the surface during
the oil extraction process. These radium isotopes and their progeny accumulate
on the internal walls of pipes and wellheads and the external radiation dose at
these sites is mainly from the gamma energies of 226Ra and its short-lived
daughters, primarily 214Pb and 214Bi. Higher energy photons of 208Tl (one of
228Ra progeny – 232Th series) can also be encountered when the scale has
accumulated over a period of several months. Pb-210, on the other hand, has a
soft gamma of 46 keV, which cannot be detected outside the pipes by a
conventional dose rate meter. However this isotope is toxic if inhaled or
ingested in the form of dust particles, during plant maintenance.
Ac-227 was also detected in sludge, oil and gas scales, and this also
contributes to the radio-toxicity of these materials. Ac-227 has nine radioactive
progeny (refer to Chapter 1, Figure 1.2 (c)), with 227Th, 223Ra and 219Rn being
the main gamma emitting radionuclides. Ac-227 and its progeny are responsible
for 69% of 235U series air Kerma (Table 5.1). As outlined in Chapter 4,
Section 4.3.1.2 and Table 4.11 adapted from (ICRP68), the committed effective
dose coefficient from 227Ac, through the inhalation pathway, is 169, 360 and
208 times higher than that of 226Ra, 210Pb and its progeny, and 228Ra,
110
respectively. Although the committed effective dose coefficient from 227Ac
through the ingestion pathway is in the same order of magnitude as those for
226Ra, 210Pb and progeny, and 228Ra, its annual ingestion radioactivity limits are
80 and 10 times lower than those of 238U and 226Ra, respectively (ICRP, 1991).
As outlined in recently published literature (Paranhos Gazineu et al., 2005,
Salih et al., 2005), the type of duties given to temporary contract workers in the
petroleum industry is one area that requires significant attention. These duties
tend to include equipment maintenance, clearing radioactive sludge from
separation tanks, scale scraping and other general cleaning, which often pose
considerable risks from a radiation protection point of view. Whilst dose rates
outside closed vessels and pipes are usually less than 7.5 µSv h-1, dose rates as
high as 80 μSv h-1 have been reported, which can be attributed to radon
daughter deposits on the surface of some equipment (Kolb and Wojcik, 1985).
As reported by Hamlat et al. (2001), during wellhead maintenance and the
clearing of sludge from separation tanks, the dose rates can be much higher. For
example, the annual effective dose rates due to gamma radiation were found to
be 0.48, 0.04 and 0.6 mSv y-1, for normal activities in the oil sector, 1 m away
from the pipes and separation tanks, and in storage tanks and wellheads,
respectively. However, the dose rates inside these structures will be higher
again, by a factor of at least 5, depending on the extent and configuration of the
contamination. The annual effective dose received by such workers can
therefore be greater than 3 mSv y-1 (Smith, 1987).
111
A comprehensive NORM survey of all PDO station equipment, manifolds,
wellheads and sludge farms is conducted by a scintillation count rate meter
(mini-900 meter coupled with an A44 scintillation probe) at least once every
four years (Petroleum Development Oman, 2005). During these surveys, a
NORM contaminated location is indicated by a reading of five CPS above the
background count rate of three CPS, and any facilities and equipment found to
be NORM contaminated are labelled and recorded, in order to alert PDO
personnel and contract workers to the risks (Petroleum Development Oman,
2005).
5.3 Materials and Methods
Gamma dose rates were measured directly from both untreated sludge piles
and treated sludge strips in Bahja, Nimr and Marmul sludge farms.
Measurements were carried out at a height of 1 m, using portable energy
compensated GM-tube, coupled with Mini-Instrument Type 6-80 (refer to
Chapter 3, Section 3.5 for meter calibration and measurement technique details).
The instrument was calibrated before and after the field measurements and the
readings were within a 10% error limit. Calibrations were conducted using a
certified Amersham gamma radiation source of 137Cs.
A total of 77 direct gamma dose rates were recorded, 34 of which were from
untreated sludge piles and 43 of which were from treated sludge strips. The
main radionuclides found in the sludge were 226Ra (T½: 1602 years), 228Ra (T½:
5.75 years) and their progeny, as well as 40K. Gamma dose rates were correlated
112
with 226Ra, 228Ra and 40K activity concentrations, which were obtained by both
portable NaI(Tl) gamma spectroscopy and laboratory HPGe systems (refer to
Chapter 3, Sections 3.3 and 3.4). In-situ gamma spectroscopy measurements
were performed at the same locations as the gamma dose rate measurements,
and additional gamma dose rates were also derived from activity concentrations
using an empirical relation (equation 5.2 below).
5.4 Results and discussion
5.4.1 Correlation between measured and predicted gamma dose rate
The cosmic ray component of 0.06 ± 0.01 µSv h-1 (equivalent to 530
± 90 µSv y-1) derived by this study was reasonably close to the worldwide
outdoor, altitude adjusted value of 460 µSv y-1 (UNSCEAR, 2000). A study
conducted by Bouville and Lowder (1988) on global cosmic radiation,
calculated the collective dose equivalent for the world’s population as a function
of altitude and geographic latitude, in various large cities around the world.
Pakistan was the closest country to Oman that was included in the above study,
and it had an annual dose equivalent of 430 µSv, which is also close to the
empirically derived value obtained in this study (530 ± 90 µSv y-1).
Measured and predicted gamma dose rates using conversion factors
published by UNSCEAR (2000) were also compared, as illustrated in
Figure 5.1. On average, the gamma dose rates were overestimated by 64% using
UNSCEAR (2000) formula, however, there was a strong linear correlation
coefficient (R2) of 0.95 between measured and predicted gamma dose rates.
113
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 5.1: Relation between measured and predicted gamma dose rates using UNSCEAR (2000) dose conversion factors.
5.4.2 Development of a new gamma dose rate empirical model
In order to find more accurate dose coefficients, a new correlation between
226Ra, 228Ra and 40K activity concentrations and gamma dose rate was defined
by multiple regressions using the SigmaPlot (Version 10.0) program. The
program utilises the Marquardt-Levenberg Algorithm to find the coefficients of
the independent variables that give the best fit between the equation and the
data. This resulted in the development of the following empirical relation, which
includes uncertainties and gave a correlation coefficient (R2) of 0.95. This
equation was used to determine the effective gamma dose rate for petroleum
treated and untreated sludge, as a result of 226Ra, 228Ra and 40K activity
concentrations:
Mea
sure
d ga
mm
a do
se ra
te (µ
Sv h
-1)
Predicted gamma dose rate (µSv h-1)
114
H• = (0.178 ± 0.007) CRa-226 + (0.230 ± 0.038) CRa-228 + (0.016 ± 0.046) CK-40 + HCosmic
(5.2)
where H• is the effective dose rate in µSv h-1, CRa-226, CRa-228 and CK-40 are the
activity concentrations of 226Ra, 228Ra and 40K in Bq g-1, respectively, and
HCosmic is the cosmic component of gamma dose rate, which was equal to 0.06
± 0.01 µSv h-1.
The corresponding UNSCEAR (2000) relationship used to generate
Figure 5.1 was:
H• = (0.462) CU-238 + (0.604) CTh-232 + (0.0417) CK-40 (5.3)
where H• is the effective dose rate in µSv h-1, and CU-238, CTh-232 and CK-40 are
the activity concentrations of 226Ra, 228Ra and 40K in Bq g-1 respectively.
The empirical relationship developed in this study generated approximately
30% variation between 226Ra and 228Ra dose conversion coefficients, which
compared well to the UNSCEAR (2000) 238U:232Th coefficient ratio, as well as
the total air Kerma ratio of 238U and 232Th presented in Table 5.1. However, the
calculated dose conversion coefficients were lower than the range found in
literature and 60% lower than dose coefficients reported by UNSCEAR (2000),
which may be due to one or a combination of the following reasons:
• The ambient soil radionuclides exist in unperturbed soil, whereas the
sludge piles are in a perturbed form, where the newly dumped, less
compacted sludge soil density may be lower than the ambient soil
density;
115
• The presence of heavy metal sediments and corrosive particles in the
petroleum sludge (APPEA, 2002, Omar et al., 2004) may lead to a
greater gamma attenuation coefficient relative to ambient soil;
• The ambient soil measurements were usually taken from flat extended
land, whereas untreated sludge piles are pyramidal, with a height of
1.75 ± 0.25 m and a base area of 4.0 ± 0.5 m2, and the measurements
were taken from the top of the piles. Similarly, the treated sludge strips
were laid over compacted land and had an average height of 0.4 ± 0.1 m.
The strips are 6 ± 2 m in width and 75 ± 10 m in length, separated by
4-8 m of open space, and measurements were usually taken near the
centre of the strips;
• Ambient soil radionuclides are usually found with the complete
primordial series existing in secular equilibrium (ignoring radon isotopes
exhalation) with the parent radionuclide, whereas the petroleum sludge
natural series starts from radium isotopes and not all radionuclides may
have reached equilibrium at the time of measurement; and
• There is a transient rather than secular equilibrium in the 232Th decay
chain between 228Ra and 228Th.
The empirical relation also resulted in a strong correlation between
measured and predicted gamma dose rates, as illustrated in Figure 5.2, with a
linear correlation coefficient (R2) of 0.96 and a correlation equation of:
H•
Measured = (1 ± 0.025) H•Predicted + (0 ± 0.009) (5.4)
where H•
Measured is the measured effective gamma dose rate, and H•Predicted is the
predicted or synthesised effective gamma dose rate.
116
0
0.2
0.4
0.6
0.8
1
1.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Figure 5.2: Relation between measured and empirically determined gamma dose rates.
Dose coefficients for the ambient soil measurements conducted during this
study were higher than those found for untreated and treated sludge
(equation 5.2 above). In fact, the calculated dose coefficients were close to the
coefficients reported by UNSCEAR (2000). Using multiple regression, the
below equation 5.5 was derived for the ambient soil samples taken from the
Southern Oman Directorate, which gave a correlation coefficient (R2) of 0.66.
However, the coefficient uncertainties were relatively high, which is likely a
result of the small number of available readings:
H• = (0.455) CRa-226 + (0.595) CRa-228 + (0.041) CK-40 + HCosmic (5.5)
where the symbols carry the same meaning as those in equation 5.2.
Predicted gamma dose rate (µSv h-1)
Mea
sure
d ga
mm
a do
se ra
te (µ
Sv h
-1)
117
Separating the untreated from the treated sludge farm data produced
somewhat different dose coefficients (see below), where the treated sludge dose
coefficients were about 20% higher than the untreated sludge values. However,
the values did overlap within the uncertainties:
H•
Untreated = (0.178 ± 0.011) CRa-226 + (0.232 ± 0.057) CRa-228 + (0.016 ± 0.076) CK-40 + HCosmic (5.6) H•
Treated = (0.217 ± 0.031) CRa-226 + (0.283 ± 0.048) CRa-228 + (0.020 ± 0.019) CK-40 + HCosmic (5.7)
This finding supports the five prior assumptions about untreated pile
geometry, soil constituent and density differences. Since the treated sludge
strips were a mixture of petroleum sludge and ambient soil, and were laid on flat
land, their derived dose coefficients were higher than untreated sludge values.
This is important when explaining the low dose coefficients that were obtained
for untreated sludge, and demonstrates that reported literature values for soil
radioactivity and gamma radiation dose do not strictly apply to petroleum
industry sludge piles.
5.4.3 Gamma dose rate measurements
During this field work, the highest measured gamma dose rate was recorded
at Bahja NORM store yard, from the sludge stored in barrels, where the
maximum barrel surface reading was 40 µSv h-1. These barrels contained
petroleum sludge with 226Ra activity concentrations as high as 223 kBq kg-1
(refer to Chapter 4, Section 4.3.4 for radioactivity in the sludge stored in
118
barrels). As expected, dose rates increased with radionuclide activity
concentrations of 226Ra and 228Ra and their progeny.
Goddard (2002) surveyed natural radioactivity in eight governorates of the
Sultanate of Oman, namely Muscat, Sharqiyah, Batinah, Dakhliyah, Wusta
(where this work was conducted), Dhahirah, Dhofar and Musandam. The mean
national average gamma dose rate from terrestrial gamma sources was
33.2 nGy h-1 and using a conversion factor between absorbed dose in air and
effective dose of 0.86 Sv Gy-1 (Clark et al., 1993), the national average effective
gamma dose rate becomes 28.6 nSv h-1. Specific to the Al-Wusta region, where
Al-Noor, Bahja, Nimr and Marmul were situated, the gamma dose rate due to
terrestrial sources was 33.6 ± 5.0 nGy h-1 (n = 72), within a range of
19.4-102.4 nGy h-1 (Goddard, 2002). Using the same conversion factor as
above, this translates to an effective gamma dose rate of 28.9 ± 4.3 nSv h-1. Our
measured Al-Noor, Bahja, Nimr and Marmul average ambient soil effective
gamma dose rate value, excluding the cosmic component of 60 ± 11 nSv h-1,
was 25.5 ± 13.8 nSv h-1, which is close to Goddard’s average value for the
region.
A total of 77 measured gamma dose rates from untreated sludge piles and
treated sludge strips are presented in Tables 5.2 and 5.3, respectively, along with
their sample identification, geographical location, gamma dose rate and the
corresponding 226Ra, 228Ra and 40K activity concentrations. A summary of the
gamma dose rates for both untreated and treated sludge at the three farms
(excluding hotspots in Marmul’s treated sludge strip numbers 7 and 44) is also
given in Table 5.4. The maximum gamma dose rates for the untreated sludge
119
piles at Bahja, Nimr and Marmul were 1.116, 0.238 and 0.613 µSv h-1,
respectively. The mean values for the three locations were 0.667 ± 0.209, 0.169
± 0.033 and 0.101 ± 0.207 µSv h-1, respectively. The values were all above the
natural background value of 0.086 ± 0.014 µSv h-1 and Bahja had the highest
gamma dose rates, due to the relatively higher 226Ra and 228Ra activity
concentrations, with ranges of 1-5 and 0.1-0.5 kBq kg-1, respectively.
120
Table 5.2: Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles Sample ID Location 40QUTM Gamma dose rate
(µSv h-1) 226Ra
(Bq kg-1) 228Ra
(Bq kg-1) 40K
(Bq kg-1) Easting Northing Bahja G01 0398306 2198873 0.496 ± 0.007 2210 ± 40 191 ± 7 185 ± 21 G02 0398318 2198877 0.464 ± 0.007 1310 ± 20 103 ± 5 252 ± 15 G03 0398316 2198885 0.591 ± 0.008 2980 ± 30 203 ± 6 229 ± 18 G04 0398318 2198888 0.530 ± 0.007 2150 ± 70 143 ± 11 264 ± 27 G05 0398318 2198898 0.596 ± 0.008 2240 ± 40 150 ± 5 254 ± 21 G06 0398328 2198909 1.116 ± 0.010 4520 ± 50 200 ± 10 209 ± 23 G07 0398363 2198960 0.550 ± 0.007 4000 ± 40 253 ± 9 217 ± 23 G14 0398367 2198958 0.440 ± 0.007 1955 ± 56 172 ± 14 524 ± 122 G15 0398360 2198953 0.637 ± 0.008 2981 ± 69 289 ± 18 597 ± 150 G16 0398355 2198950 0.686 ± 0.008 3221 ± 71 271 ± 18 566 ± 153 G17 0398351 2198944 0.740 ± 0.008 3799 ± 77 315 ± 19 591 ± 165 G18 0398342 2198933 0.851 ± 0.009 3875 ± 79 375 ± 21 779 ± 171 G19 0398340 2198929 0.949 ± 0.010 4336 ± 82 366 ± 21 767 ± 178 G20 0398333 2198920 1.098 ± 0.010 4934 ± 89 472 ± 23 758 ± 190 G21 0398330 2198915 0.693 ± 0.008 2908 ± 68 258 ± 17 638 ± 147 G22 0398320 2198906 0.744 ± 0.009 3445 ± 73 287 ± 18 954 ± 162 G23 0398310 2198896 0.591 ± 0.008 2444 ± 63 268 ± 18 742 ± 140 G24 0398309 2198890 0.529 ± 0.007 2274 ± 60 200 ± 15 409 ± 129 G25 0398300 2198878 0.378 ± 0.006 1651 ± 53 198 ± 15 469 ± 115
121
Table 5.2 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul untreated sludge piles Sample ID Location 40QUTM Gamma dose rate
(µSv h-1) 226Ra
(Bq kg-1)228Ra
(Bq kg-1)40K
(Bq kg-1) Easting Northing Nimr N01 0383329 2051550 0.156 ± 0.004 531 ± 13 139 ± 3 446 ± 10 N02 0383337 2051562 0.151 ± 0.004 403 ± 17 138 ± 5 487 ± 14 N03 0383356 2051569 0.159 ± 0.004 291 ± 14 118 ± 3 405 ± 10 N04 0383353 2051598 0.177 ± 0.004 314 ± 10 114 ± 3 457 ± 9 N05 0383309 2051584 0.166 ± 0.004 320 ± 11 123 ± 2 457 ± 9 N06 0383296 2051560 0.134 ± 0.004 285 ± 10 95 ± 3 439 ± 9 N07 0383347 2051609 0.238 ± 0.005 639 ± 17 270 ± 4 134 ± 10 Marmul M01 0319390 2014275 0.150 ± 0.004 195 ± 20 83 ± 12 47 ± 43 M02 0319393 2014288 0.076 ± 0.003 36 ± 6 9 ± 1 119 ± 6 M03 0319385 2014287 0.079 ± 0.003 42 ± 4 10 ± 1 135 ± 5 M04 0319378 2014292 0.076 ± 0.003 36 ± 4 7 ± 1 118 ± 5 M05 0319370 2014292 0.077 ± 0.003 38 ± 4 9 ± 1 103 ± 5 M06 0319365 2014314 0.123 ± 0.003 125 ± 9 92 ± 3 639 ± 11 M08 0319385 2014271 0.613 ± 0.008 918 ± 21 845 ± 7 548 ± 15 M09 0319392 2014271 0.448 ± 0.007 1022 ± 24 1557 ± 7 359 ± 16 Maximum 1.116 4934 1557 954 Minimum 0.076 36 7 47 Median 0.480 1803 195 443 Mean (standard deviation) 0.456 (0.303) 1836 (1557) 245 (281) 421 (234) Number 34
122
Table 5.3: Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate
(µSv h-1) 226Ra
(Bq kg-1)228Ra
(Bq kg-1)40K
(Bq kg-1) Easting Northing Bahja Strip # 018 0398766 2198993 0.081 ± 0.003 51 ± 10 13 ± 10 159 ± 30 Strip # 030 0398675 2198899 0.083 ± 0.003 55 ± 10 13 ± 10 175 ± 32 Strip # 043 0398775 2198963 0.085 ± 0.003 68 ± 10 8 ± 7 175 ± 32 Strip # 065 0398736 2198925 0.083 ± 0.003 58 ± 10 11 ± 9 163 ± 31 Strip # 070 0398698 2198876 0.084 ± 0.003 71 ± 11 13 ± 10 107 ± 29 Strip # 073 0398682 2198837 0.085 ± 0.003 52 ± 10 17 ± 12 191 ± 33 Strip # 076 0398659 2198802 0.076 ± 0.003 37 ± 9 15 ± 11 178 ± 30 Strip # 078 0398637 2198780 0.082 ± 0.003 62 ± 10 11 ± 9 172 ± 32 Strip # 080 0398617 2198763 0.086 ± 0.003 63 ± 10 11 ± 9 198 ± 33 Nimr Strip # 029 0383548 2051927 0.072 ± 0.003 18 ± 9 22 ± 8 82 ± 24 Strip # 039 0383612 2051829 0.074 ± 0.003 56 ± 11 19 ± 7 75 ± 27 Strip # 044 0383648 2051790 0.077 ± 0.003 22 ± 10 26 ± 9 102 ± 27 Strip # 052 0383579 2051742 0.080 ± 0.003 34 ± 10 20 ± 8 109 ± 27 Strip # 085 0383370 2052107 0.077 ± 0.003 49 ± 10 12 ± 6 84 ± 26 Strip # 100 0383387 2051795 0.078 ± 0.003 36 ± 8 8 ± 4 184 ± 30 Strip # 111 0383498 2051667 0.094 ± 0.003 85 ± 13 29 ± 10 126 ± 34 Strip # 116 0383401 2051590 0.083 ± 0.003 46 ± 9 12 ± 6 137 ± 29 Strip # 119 0383375 2051622 0.076 ± 0.003 37 ± 5 10 ± 1 118 ± 6 Strip # 126 0383332 2051726 0.075 ± 0.003 17 ± 9 23 ± 8 132 ± 27 Strip # 143 0383291 2051664 0.078 ± 0.003 32 ± 8 12 ± 6 113 ± 26
123
Table 5.3 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate
(µSv h-1) 226Ra
(Bq kg-1)228Ra
(Bq kg-1)40K
(Bq kg-1) Easting Northing Marmul Strip # 005 0319306 2014175 0.078 ± 0.003 38 ± 10 16 ± 6 119 ± 28 Strip # 007 h 0319248 2014072 0.327 ± 0.006 1920 ± 54 190 ± 30 300 ± 116 Strip # 011 0319230 2014186 0.076 ± 0.003 66 ± 12 17 ± 7 54 ± 27 Strip # 013 0319161 2014080 0.078 ± 0.003 63 ± 11 13 ± 6 114 ± 30 Strip # 018 0319120 2014219 0.086 ± 0.003 62 ± 12 25 ± 8 39 ± 27 Strip # 021 0319058 2014135 0.081 ± 0.003 56 ± 11 19 ± 7 58 ± 27 Strip # 024 0319079 2014233 0.081 ± 0.003 52 ± 12 24 ± 8 53 ± 27 Strip # 025a 0319352 2014140 0.101 ± 0.003 153 ± 17 28 ± 9 141 ± 40 Strip # 025b 0319352 2014140 0.108 ± 0.003 181 ± 16 9 ± 5 240 ± 44 Strip # 044 h - - 0.362 ± 0.006 2080 ± 40 184 ± 5 136 ± 15 Strip # 069 0319026 2014173 0.091 ± 0.003 80 ± 12 15 ± 6 284 ± 39 Strip # 102a 0319037 2014148 0.095 ± 0.003 67 ± 15 51 ± 14 214 ± 40 Strip # 102b 0319042 2014179 0.105 ± 0.003 92 ± 20 100 ± 22 204 ± 47 Strip # 102c 0319046 2014183 0.114 ± 0.003 180 ± 24 130 ± 27 137 ± 53 Strip # 112 0319143 2014210 0.077 ± 0.003 56 ± 11 21 ± 8 35 ± 26 Strip # 115 0319174 2014200 0.079 ± 0.003 22 ± 10 23 ± 8 166 ± 30 Strip # 118 0319202 2014078 0.084 ± 0.003 59 ± 11 15 ± 6 110 ± 29 Strip # 148 0319125 2014094 0.079 ± 0.003 27 ± 11 31 ± 10 103 ± 29 Strip # 183 0319261 2014182 0.081 ± 0.003 37 ± 12 31 ± 10 54 ± 27 Strip # 186 0319091 2014110 0.078 ± 0.003 42 ± 9 12 ± 5 79 ± 25 Strip # 189 0319199 2014192 0.079 ± 0.003 44 ± 10 15 ± 6 93 ± 27 Strip # 192 0319318 2014095 0.106 ± 0.003 118 ± 13 5 ± 3 205 ± 38 Strip # 193 0319283 2014108 0.099 ± 0.003 121 ± 14 11 ± 5 197 ± 38
124
Table 5.3 (Continued): Measured gamma dose rates and 226Ra, 228Ra and 40K activity concentrations for Bahja, Nimr and Marmul treated sludge strips Sample ID Location 40QUTM Gamma dose rate
(µSv h-1) 226Ra
(Bq kg-1) 228Ra
(Bq kg-1) 40K
(Bq kg-1) Easting Northing
Maximum 0.114 181 130 284 Minimum 0.072 17 5 35
Median 0.081 56 15 126
Mean (standard deviation) 0.085 (0.010) 63 (39) 22 (23) 134 (58)
Number 43 h: hotspots measured on treated sludge strips, not included in the final statistical calculations
125
Table 5.4: Summary of field measured gamma dose rates (µSv h-1) at 1 m height for Bahja, Nimr and Marmul petroleum sludge treatment farms’
Bahja Nimr Marmul
Untreated
piles
Treated
strips
Untreated
piles
Treated
strips
Untreated
piles
Treated
strips
Maximum 1.116 0.086 0.238 0.094 0.613 0.114
Minimum 0.378 0.076 0.134 0.072 0.076 0.076
Median 0.596 0.083 0.159 0.077 0.205 0.081
Mean 0.667 0.083 0.169 0.079 0.101 0.089
S.D. * 0.209 0.003 0.033 0.006 0.207 0.011
n # 19 9 7 11 8 19
* is the standard deviation # is the number of readings
The maximum recorded gamma dose rates for treated sludge strips at Bahja,
Nimr and Marmul (excluding Marmul’s hotspots on treated sludge strips 7 and
44) were 0.086, 0.094 and 0.114 µSv h-1, respectively, while the mean gamma
dose rates were 0.083 ± 0.003, 0.079 ± 0.006 and 0.089 ± 0.011 µSv h-1,
respectively. The average values for the treated sludge strips were within the
average range of measured ambient soil gamma dose rates, being 0.086
± 0.014 µSv h-1. The treated sludge hotspot readings for strips 7 and 44 at
Marmul were 0.327 ± 0.006 and 0.362 ± 0.006 µSv h-1, respectively. The direct
correlation between gamma dose rate and both 226Ra and 228Ra activity
concentrations are shown in Figure 5.3.
126
0.0
0.2
0.4
0.6
0.8
1.0
1.2
10002000
30004000
50000
100
200
300
400
500
Gam
ma
dose
rate
(µSv
h-1
)
226Ra (Bq kg-1)22
8 Ra
(Bq
kg-1 )
Bahja untreated pilesBahja treated stripsNimr untreated pilesNimr treated stripsMarmul untreated pilesMarmul treated strips
Figure 5.3: 3D graph of gamma dose rate relation with both 226Ra and 228Ra activity concentrations.
127
Although a strong correlation was observed between the measured gamma
dose rate and 226Ra and 228Ra, the correlation with 40K was only moderate. The
average measured gamma dose rate for untreated sludge in the Southern Oman
Directorate sludge farms was 0.456 ± 0.303 µSv h-1, while the average
measured gamma dose rate for treated sludge (excluding hotspots) was 0.085
± 0.010 µSv h-1. This difference in values was due to the sludge farming process
used at the farms, and that only sludge with activity concentrations < 1 kBq kg-1
was approved for the farming process. The average gamma dose rate for the
treated sludge was within the average range of measured ambient soil dose rate,
being 0.086 ± 0.014 µSv h-1.
5.4.4 Combining synthesised and measured gamma dose rates
Using the mathematical model developed in this study, it was possible to
derive gamma dose rates for locations which only had activity concentration
data recorded. This produced additional 36 data points for gamma dose rates in
the oilfields of the Southern Oman Directorate. A box plot of the entire gamma
dose rate data is shown in Figure 5.4, where the total number of samples for
each location is reported beneath each box. The box plot shows that Bahja,
Nimr and Marmul untreated sludge piles have different gamma dose rate
profiles, however the gamma dose rates of the three treated sludge farm strips
are not significantly different. The location, number of samples, mean, median
and dose rate ranges are also summarised in Table 5.5.
128
Location
Bahj
a pi
les
Bah
ja s
trips
Nim
r pile
s
Nim
r stri
ps
Mar
mul
pile
s
Mar
mul
stri
ps
Gam
ma
dose
rate
(µS
v h-
1 )
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Figure 5.4: Measured and predicted gamma dose rate profiles at a 1 m height for untreated sludge piles and treated strips at Bahja, Nimr and Marmul sludge farms (where n denotes the total number of samples at each location).
Above 90th percentile 90th percentile
Median
75th percentile
25th percentile 10th percentile Below 10th percentile
n = 12n = 23 n = 14 n = 17 n = 16 n = 31
Mean
129
Table 5.5: Number of samples, mean, median and range of the dose rates for untreated and treated sludge at Bahja, Nimr and Marmul obtained by both direct measurement and empirical relation
Location Number of
samples
Mean dose rate
(µSv h-1)
Median dose rate
(µSv h-1)
Dose rate range
(µSv h-1) Bahja piles 23 0.702 ± 0.250 0.637 0.281 - 1.116 Nimr piles 14 0.166 ± 0.026 0.166 0.126 – 0.238 Marmul piles 16 0.345 ± 0.454 0.146 0.072 – 1.781 Bahja strips 12 0.084 ± 0.004 0.084 0.076 – 0.091 Nimr strips 17 0.109 ± 0.084 0.080 0.072 – 0.426 Marmul strips 31 0.115 ± 0.068 0.091 0.063 – 0.362 Total 113
Since no other published data was found on gamma dose rates in petroleum
sludge treatment farms, we could not make any comparisons with our
measurements and our empirically derived equations for dose coefficients. The
only related scientific literature found was by Smith et al. (1998), who
developed a theoretical model to find absorbed dose rates over a period of time
(up to 1000 years), using RESRAD computer code version 5.782, in order to
assess different scenarios of sludge farm remediated land, including residential,
agricultural, industrial and recreational use. For a 226Ra activity concentration of
185 Bq kg-1 above background, the expected dose was 0.6 mSv y-1, while the
empirical relationship for rehabilitated land derived through the experimental
work of this study (equation 5.7) gives a dose of 0.4 mSv y-1 for the same
activity concentration of 226Ra.
130
5.5 Conclusions
This work derived a new empirical relation between petroleum sludge
activity concentration and gamma dose rates at a height of 1 m above ground.
The derived conversion coefficients were at the lower end of the range found in
literature, for normal soils, which may have been caused by one or a
combination of the following factors: (1) the presence of heavy metal sediments
and corrosive particles in the petroleum sludge, leading to a greater attenuation
coefficient; (2) the radionuclides may not have reached equilibrium at the time
of measurement; (3) the ambient soil measurements are usually conducted on
flat extended land, whereas untreated sludge piles are in small heaps; and (4) the
greater soil density of ambient soil compared to less compacted untreated and
treated sludge. This resulted in a new empirical relation being developed for
petroleum sludge, in order to determine effective gamma dose rate from
radionuclide activity concentrations.
A strong correlation was observed between the measured gamma dose rate
and 226Ra and 228Ra, however the correlation with 40K was only moderate. The
average measured gamma dose rate for untreated sludge in the Southern Oman
Directorate sludge farms was 0.456 ± 0.303 µSv h-1, while the average
measured gamma dose rate for treated sludge (excluding hotspots) was 0.085
± 0.010 µSv h-1. This difference in values was due to the sludge farming process
used at the farms, in that only sludge with activity concentrations < 1 kBq kg-1
was approved for the farming process. The average dose rate for the treated
sludge was within the average range of measured ambient soil gamma dose rate,
being 0.086 ± 0.014 µSv h-1.
131
The effective reduction of petroleum sludge gamma dose rates down to
ambient soil levels, using the sludge farming process, was clearly evidenced by
the results obtained in this study. However, hotspots were still present, which
still raise concern, considering that the desert terrain of mainly limestone
surface rock naturally contains only low concentrations of radionuclides. As
such, this study contributes important and significant knowledge on gamma
dose rates and the derivation of dose coefficients in the petroleum industry,
since no similar work has been reported to date.
132
133
Chapter 6 RADON-222 EXHALATION FROM PETROLEUM INDUSTRY SCALE, SLUDGE AND SEDIMENT
6.1 Introduction
Radon is a naturally occurring, highly mobile, chemically inert radioactive
gas. Its isotope 222Rn is part of the 238U series, produced by the radioactive
decay of 226Ra. Because radium is widely distributed in the earth’s crust, radon
is widely distributed too, with reports of radon detection in dwellings
throughout the world (UNSCEAR, 2001). Once formed by the radioactive decay
of 226Ra, it migrates freely as a gas or is dissolved in water, without being
trapped or removed by chemical reactions.
Whilst radon is inhaled on a regular basis by people all over the world, its
inert characteristics mean that it doesn’t interact with the respiratory system
lining, and a large proportion of the inhaled radon is usually exhaled. However,
the remaining inhaled portion, along with any ingested radon, does pose a
radiological risk from internal exposure of organs, because once inhaled or
ingested, radon and its short-lived progeny will deposit their entire alpha
particle energy in an immediate localised area. From the radiological hazard
point of view, 220Rn (224Ra progeny) does not pose as high a risk as that posed
by 222Rn, since 220Rn only has a few short-lived progeny, whereas 222Rn has two
long-lived progeny, 210Pb and 210Po, with half-lives of 22.3 years and
138.4 days, respectively (refer to Figure 1.2 in Chapter 1, Section 1.3 for 238U
and 232Th primordial radioactive decay series).
134
Radon has long been known as one of the causes of lung cancer and the
radiological dangers of 222Rn reside in its four short-lived radioisotope progeny,
which can attach to ambient aerosols and be inhaled into the bronchial system.
Two of the 222Rn progeny (218Po and 214Po) are alpha emitters with energies of
6.00 MeV and 7.69 MeV respectively. The other two (214Pb and 214Bi) are beta-
gamma emitters, which also contribute to the radiation dose (Wilson and Scott,
1993). On average, 222Rn is the highest single contributor to the ambient
radiation dose to the global human population, with inhalation exposure dose
from radon and its progeny estimated to be 1.2 mSv y-1 of the total of
2.4 mSv y-1 (UNSCEAR, 2000). The other contributors are external terrestrial
radiation, cosmic and cosmogenic, and ingestion exposure.
Due to its gaseous nature, radon can also be exhaled from the ground, as
well as from various building materials, and being chemically inert, it does not
react with other elements, as it finds its way to the atmosphere. The mechanism
of radon generation and transport, along with factors contributing to its
emanation from mineral grains, have been studied by many authors, e.g. Tanner
(1964, , 1980) Semkow (1990), Morawska and Phillips (1992), Greeman and
Rose (1996), Gomez Escobar et al. (1999), and Holdsworth and Akber (2004),
and generally involves processes such as emanation, diffusion, convection,
absorption (in the liquid phase) and adsorption (onto solid surfaces)
(UNSCEAR, 2000).
135
The rate at which radon exhales from a material depends (along with other
factors) on the radon emanation fraction, which is the fraction of radon atoms
formed in a solid that escape and are free to migrate. This fraction varies
depending on several factors, but is largely determined by the physical
properties of the material bearing the radium nuclide. These include (i) radium
distribution within the material, (ii) porosity, (iii) granule surface area to volume
ratio, and (iv) the effective radon diffusion coefficient in the material. These
factors often interact in complex and counter-intuitive ways, such that the
process of radon emanation (and its consequent exhalation) tend to follow a
log-normal distribution (Rood et al., 1998, White and Rood, 2001).
Radon exhalation into the atmosphere is a two stage process – the first stage
involves emanation from the material into the porous space of the ground (as
described above), while in the second stage, the radon diffuses, either by a
process of molecular diffusion or by advection and/or convection for up to
several metres in the interstitial space, before emerging into the atmosphere.
The emergence of radon from the interstitial space to the atmosphere is termed
‘exhalation’.
As outlined in Chapter 4, Section 4.3.1.1, as oil is extracted from the
formation rock and brought to the surface, its temperature and/or pressure
decrease, which allows the solutes contained within the produced water to
precipitate. Consequently, during the extraction process, radium is co-
precipitated with barium, strontium and calcium, in the form of sulphates and
carbonates (Kolb and Wojcik, 1985, Jerez Vegueria et al., 2002a, Hamilton et
136
al., 2004, Al-Masri and Aba, 2005). This precipitation results in the formation
of hard and highly insoluble scale deposits on the interior walls of the tubular
and other production equipment. The 222Rn and 220Rn along with their progeny,
which exhaled from the scales, may also be present or build up in the crude oil
and gas streams (Rood et al., 1998, Worden et al., 2000). In addition to scale
formation, once the crude oil is transported from oil wells to processing stations,
oily sludge can form in separation tanks, as a result of the oil, water and gas
separation process. The precipitated oily sludge also contains enhanced
concentrations of radium isotopes and as a result, the exhaled 222Rn and 220Rn,
along with their progeny, may also be present and build up in the separation
tanks. However the separation tanks are ventilated prior to sludge clearing, in
order to remove H2S gas, which suggests that any radon accumulated in the
separation tank would also escape. Radon is also known to be soluble in both
organic matter and water, and therefore, the separated natural gas and produced
water streams would continue transporting radon isotopes even after the oil has
been extracted. The 222Rn long-lived decay product (210Pb) also forms a thin
radioactive film on the internal walls of gas pipes and vessels, with a reported
specific activity greater than 1 kBq g-1 (Hamlat et al., 2003a).
The earliest account of radon gas found in the hydrocarbon industry noted
its presence in Canadian natural gas, as reported by McLennan in 1904 (Satterly
and McLennan, 1918). Many authors have since investigated and published
reports on the presence of 222Rn in the natural gas industry. Whilst such
investigations are still ongoing, with the most recent work by authors including
Al-Masri and Shwiekani (2008) and Hamlat et al. (2003a), who evaluated
137
activity flux and distribution of 222Rn and radiation exposure rates in the gas
industry, few have measured emanation factors of 222Rn from petroleum scales
e.g. White and Rood (2001) and Rood et al. (1998). In addition, this study is the
first ever to evaluate 222Rn exhalation rates from barrel stored oily sludge,
treated and untreated petroleum sludge and from sediments of a produced water
evaporation pond. This study also measured 222Rn exhalation rates from
petroleum scales and ambient soils, in order to make a preliminary assessment
of 222Rn exhalation from petroleum sludge samples, and then to compare
differences in both radon exhalation rates and radon exhalation to radium
concentration ratios for the different sample types.
6.2 Materials and methods
Overall, radon emanation rates were determined for 72 petroleum industry
samples and four ambient soil samples. A further six ambient soil locations
were assessed in Australia, for comparison. Field, as well as laboratory
measurements, were performed for six different sample types, including
ambient soils, petroleum scales, barrel stored oily sludge, treated sludge,
untreated sludge and evaporation pond sediments. The ambient soils were
collected from Al-Noor, Bahja, Nimr and Marmul (refer to map in Chapter 2,
Figure 2.1), while the petroleum scale and barrel stored oily sludge samples
were collected from Bahja NORM store yard. Treated and untreated sludge
samples were collected from Bahja, Nimr and Marmul sludge farms, and the
sediment samples were collected from the Al-Noor station produced water
evaporation pond. Grab sediment samples were collected using a metal scooper,
138
to a depth of about 5 cm, and placed in plastic bottles, along with a single bead
(up to 1 cm in diameter) and a single sand sample collected from barrels of an
unknown origin, that were stored in the Bahja NORM store yard. The reason for
collecting these two additional samples was because of their high gamma count
rate. The quantity of material collected for the entire set of samples varied from
100-300 g and all samples were then transported to the laboratory in Muscat for
radon emanation assessment and spectroscopy analysis. In order to preserve
their pore and grain structure, as well as the moisture and organic content, the
samples were not subjected to any vigorous preparation procedure before being
placed in the emanometer. A detailed outline of the materials and methods used
in this study can be found in Chapter 3, Section 3.6, which includes information
on the instruments used, as well as techniques applied for measuring 222Rn
exhalation rate.
6.3 Results and discussion
6.3.1 Rn-222 exhalation rates
Tables 6.1(a) and 6.1(b) present the 222Rn exhalation rate, 226Ra activity
concentration and 222Rn exhalation to 226Ra activity ratio for untreated sludge
piles, treated sludge strips, oil scales, sediments and barrel stored oily sludge.
Only emanometer 222Rn exhalation rates were used to determine the 222Rn to
226Ra ratio for samples with both charcoal cups and emanometer values.
Table 6.1(a) displays 222Rn exhalation rates determined by both the charcoal
cups and the emanometer, which were generally similar and within
measurement uncertainties. A good linear correlation (R2 = 0.92) was also found
139
between the emanometer and the charcoal cup readings (Figure 6.1), according
to the following correlation equation:
XEmanometer = (0.95 ± 0.05) XCharcoal Cup (6.1)
0
150
300
450
600
750
900
0 150 300 450 600 750 900
Figure 6.1: Emanometer to charcoal cup readings correlation.
Table 6.1(b) displays 222Rn exhalation rates determined by only the
emanometer. Table 6.1(b) also includes the results from the additional bead and
sand samples, however these values were not included in the averages.
Eman
omet
er 22
2 Rn
exha
latio
n ra
te
(mB
q m
-2 s-1
)
Charcoal cup 222Rn exhalation rate (mBq m-2 s-1)
140
Table 6.1(a): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive wastes
Sample ID
222Rn (mBq m-2 s-1)c
222Rn (mBq m-2 s-1)e
226Ra (Bq kg-1)
222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)
Bahja untreated sludge piles
G01 579 ± 99 598 ± 79 2210 ± 40 0.27 ± 0.04
G02 316 ± 55 218 ± 35 1310 ± 20 0.17 ± 0.03
G03 303 ± 53 360 ± 47 2980 ± 30 0.12 ± 0.02
G04 249 ± 44 300 ± 40 2150 ± 70 0.14 ± 0.02
G05 301 ± 53 189 ± 24 2240 ± 40 0.08 ± 0.01
G06 691 ± 117 579 ± 69 4520 ± 50 0.13 ± 0.02
G07 325 ± 57 447 ± 49 4000 ± 40 0.11 ± 0.01
Bahja NORM store yard pit sludge
Gp1 45 ± 12 - 1245 ± 36 0.04 ± 0.01
Gp2 33 ± 9 - 1385 ± 36 0.02 ± 0.01
Nimr untreated sludge piles
N01 7.7 ± 3.0 39 ± 9 531 ± 13 0.07 ± 0.02
N02 14 ± 4 37 ± 7 403 ± 17 0.09 ± 0.02
N03 8.6 ± 3.4 26 ± 5 291 ± 14 0.09 ± 0.02
N04 16 ± 5 23 ± 5 314 ± 10 0.07 ± 0.02
N05 6.3 ± 2.8 19 ± 4 320 ± 11 0.06 ± 0.02
N06 52 ± 11 23 ± 4 285 ± 10 0.08 ± 0.02
N07 2.8 ± 2.1 15 ± 3 639 ± 17 0.024 ± 0.005
N08 - 16 ± 4 73 ± 7 0.22 ± 0.08
N09 - 76 ± 21 309 ± 19 0.25 ± 0.08
141
Table 6.1(a) (Continued): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for various petroleum industry radioactive wastes
Sample ID
222Rn (mBq m-2
s-1)c
222Rn (mBq m-2
s-1)e
226Ra (Bq kg-1)
222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)
N10 - 37 ± 9 284 ± 14 0.13 ± 0.04
N11 - 42 ± 10 361 ± 17 0.12 ± 0.03
N12 - 13 ± 3 327 ± 18 0.04 ± 0.01
N13 - 31 ± 8 339 ± 14 0.09 ± 0.03
N14 - 18 ± 4 334 ± 17 0.05 ± 0.02
Marmul untreated sludge piles
M02 4.3 ± 2.4 18 ± 11 36 ± 6 0.49 ± 0.38
M03 - 12 ± 7 42 ± 4 0.29 ± 0.20
M04 4.3 ± 2.4 12 ± 6 36 ± 4 0.33 ± 0.21
M05 - 17 ± 10 38 ± 4 0.44 ± 0.31
M06 13 ± 4 54 ± 17 125 ± 9 0.43 ± 0.17
M07 5.5 ± 2.6 14 ± 7 46 ± 6 0.30 ± 0.19
M08 56 ± 12 63 ± 16 918 ± 21 0.07 ± 0.02
M09 10 ± 4 27 ± 11 1022 ± 24 0.03 ± 0.01
M10 - 13 ± 8 27 ± 3 0.48 ± 0.37
M11 - 66 ± 18 146 ± 9 0.45 ± 0.15
M12 - 38 ± 9 196 ± 13 0.19 ± 0.06
M13 - 82 ± 20 3690 ± 60 0.02 ± 0.01
M14 - 155 ± 30 2290 ± 40 0.07 ± 0.01 c Determined by charcoal cups e Determined by the emanometer
142
Table 6.1(b): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for petroleum industry’s various radioactive wastes (emanometer measurements only)
Sample ID
222Rn (mBq m-2 s-1)e
226Ra (Bq kg-1)
222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)
Bahja treated sludge strips
Strip # 005 23 ± 12 48 ± 6 0.48 ± 0.30
Strip # 008 10 ± 5 52 ± 6 0.19 ± 0.12
Strip # 039 20 ± 12 47 ± 6 0.42 ± 0.32
Nimr treated sludge strips
Strip # 001 37 ± 11 133 ± 8 0.28 ± 0.10
Strip # 024 10 ± 3 108 ± 7 0.09 ± 0.03
Strip # 026 14 ± 4 197 ± 10 0.07 ± 0.02
Strip # 044 35 ± 10 260 ± 10 0.14 ± 0.04
Strip # 045 19 ± 9 52 ± 5 0.36 ± 0.20
Strip # 061 22 ± 6 1340 ± 26 0.016 ± 0.004
Strip # 119 6 ± 3 37 ± 5 0.15 ± 0.09
Marmul treated sludge strips
Strip # 015 12 ± 2 10 ± 1 1.23 ± 0.32
Strip # 018 34 ± 6 414 ± 13 0.08 ± 0.02
Strip # 020 12 ± 3 295 ± 19 0.04 ± 0.01
Strip # 027 13 ± 5 175 ± 9 0.08 ± 0.03
Strip # 031 7 ± 3 79 ± 7 0.09 ± 0.04
Strip # 033 24 ± 5 205 ± 11 0.12 ± 0.03
Strip # 044 9 ± 3 2080 ± 40 0.005 ± 0.002
Strip # 046 11 ± 4 455 ± 15 0.02 ± 0.01
Strip # 062 16 ± 4 115 ± 8 0.14 ± 0.05
143
Table 6.1(b) (Continued): 222Rn exhalation rate, 226Ra activity concentration and 222Rn to 226Ra ratio for petroleum industry’s various radioactive wastes (emanometer measurements only)
Sample ID
222Rn (mBq m-2 s-1)e
226Ra (Bq kg-1)
222Rn:226Ra ratio (mBq m-2 s-1/Bq kg-1)
Scale samples
FHDOS 1 583 ± 66 4360 ± 50 0.13 ± 0.02
FHDOS 2 2760 ± 242 13000 ± 100 0.21 ± 0.02
FUDOS 3 274 ± 39 3380 ± 40 0.08 ± 0.01
FHDOS 4 1509 ± 156 11800 ± 100 0.13 ± 0.01
FHDOS 5 3209 ± 260 6340 ± 60 0.51 ± 0.05
FHDOS 6 5107 ± 415 17300 ± 100 0.30 ± 0.03
FHDOS 7 2779 ± 231 6380 ± 50 0.44 ± 0.04
Al-Noor evaporation pond sediments
NOR 1 102 ± 20 5260 ± 48 0.019 ± 0.004
NOR 2 108 ± 35 354 ± 13 0.31 ± 0.11
NOR 3 84 ± 35 119 ± 7 0.71 ± 0.34
NOR 4 120 ± 40 743 ± 16 0.16 ± 0.06
NOR 5 77 ± 31 379 ± 12 0.20 ± 0.09
NOR 6 31 ± 15 107 ± 6 0.29 ± 0.15
Barrel stored oily sludge, beads and sand
BHJB 1 152 ± 20 14000 ± 100 0.011 ± 0.001
BHJB 2 40 ± 7 1700 ± 30 0.023 ± 0.005
BHJB 3 79 ± 12 5560 ± 69 0.014 ± 0.002
BHJB 4 26 ± 5 2000 ± 40 0.013 ± 0.003
Beads 18 ± 3 38012 ± 131 0.0005 ± 0.0001
Sand 712 ± 87 35386 ± 162 0.020 ± 0.003 e Determined by the emanometer
144
A general upward trend was observed for 222Rn exhalation rate versus 226Ra
activity concentration (Figure 6.2). The overall slope and correlation coefficient
(R2) for 222Rn exhalation rate to 226Ra activity concentration were 0.095
± 0.014 mBq m-2 s-1/Bq kg-1 and 0.37, respectively. These results confirm a
general consensus between 222Rn exhalation and 226Ra activity concentration.
0.1
1.0
10.0
100.0
1,000.0
10,000.0
1 10 100 1,000 10,000 100,000
Bahja Strips
Bahja Piles
Nimr Strips
Nimr Piles
Marmul Strips
Marmul Piles
Al-Noor Sediments
Barrel Sludge
Scales
Figure 6.2: 222Rn exhalation rate versus 226Ra activity concentration.
Because radon emanation and its consequent exhalation is a random process,
depending upon multiple variables, it tends to follow a log-normal distribution
(refer to Section 6.1). Therefore, a summary of the exhalation data would be
better presented as a geometric mean rather than an arithmetic mean. However,
in this study; both arithmetic and geometric means have been presented.
Table 6.2 presents a summary of all of the measurements carried out in oilfields
of the Southern Oman Directorate. It includes 222Rn exhalation rate arithmetic
226Ra activity concentration (Bq kg-1)
222 R
n ex
hala
tion
rate
(mB
q m
-2 s-1
)
145
and geometric mean, along with the maximum and minimum measured values
for scales, evaporation pond sediments, barrel stored oily sludge, sludge piles,
sludge strips and ambient soil. It also contains the calculated arithmetic and
geometric mean 222Rn exhalation rate to 226Ra activity concentration ratio for
the samples.
Radon Exhalation Rate: Ambient soil radon exhalation rate measurements
were performed in Bahja, Al-Noor, Nimr and Marmul, and the arithmetic and
geometric mean values for the above locations were 3.7 ± 2.1 and
7.90.11.3 mBq m-2 s-1, respectively. For comparison purposes, six environmental
measurements of radon exhalation rates were also carried out in Jimboomba,
Queensland, Australia using activated charcoal cups. The calculated arithmetic
and geometric mean radon exhalation rates were 4.5 ± 0.7 and
9.51.33.4 mBq m-2 s-1, respectively. Given measurement uncertainties, the two
ambient soil 222Rn exhalation rates were similar.
The range, median and both arithmetic and geometric means of the 222Rn
exhalation rate for the six different sample types are illustrated in Figure 6.3.
Overall, the radon exhalation rates for the treated sludge strips were higher than
the rates for ambient soil, which can be attributed to the presence of 226Ra in the
sludge strips, introduced by mixing of sludge with clean soil. However, the
treated sludge strips exhalation rates ( 26915 mBq m-2 s-1) were lower than for the
untreated sludge piles ( 1541547 mBq m-2 s-1). The 222Rn exhalation rate for the
scales was three orders of magnitude higher than the ambient soil, and barrel
146
stored oily sludge had comparable radon exhalations to the Al-Noor water pond
sediments.
Rn-222 Exhalation Rate to 226Ra Activity Concentration Ratio:
Figure 6.4 presents the range, median and both arithmetic and geometric means
of the 222Rn exhalation rate to 226Ra activity concentration ratio for the
individual sample types. Because of the potential affects of the log-normal
distribution of data and the modest sample size on F-statistics: the Kruskal-
Wallis non-parametric test, followed by Mann-Whitney post-hoc analysis was
used to compare sample median scores across the groups. The analysis showed
that the difference between the averages for the 222Rn exhalation rate to 226Ra
activity concentration ratio for ambient soil, treated and untreated sludge, scales
and pond sediments was not statistically significant. The geometric mean 222Rn
exhalation rate to 226Ra activity concentration ratio for treated sludge strips and
untreated sludge piles were comparable ( 37.003.010.0 and
29.005.012.0 mBq m-2 s-1/Bq kg-1, respectively), however the Bahja barrel stored
oily sludge geometric mean ( 020.0011.0015.0 mBq m-2 s-1/Bq kg-1) was one order of
magnitude lower than the means for the rest of the samples and this difference
was found to be statistically significant (p < 0.05). As previously outlined in
Chapter 3, the collected sludge was either dry (if exposed to open sun for an
extended period of time) or oily (if recently removed from separation tanks or
stored in barrels). As such, the significantly lower 222Rn exhalation rate to 226Ra
activity concentration ratio for the Bahja barrel stored oily sludge might be a
result of its higher oily hydrocarbon content.
147
The radon exhalation to radium concentration ratios for both pond sediments
and scales ( 62.006.019.0 and 42.0
11.021.0 mBq m-2 s-1/Bq kg-1, respectively) were about
twice those obtained for ambient soil, treated sludge and untreated sludge. In
pond sediments, the 226Ra (originating from produced water) is thought to be
present as a surface coating on the sediment grains, resulting in a non-uniform
distribution, which may explain the greater fraction of 222Rn emanation per
226Ra activity concentration for the pond sediment samples. In contrast, a
uniform distribution of 226Ra is expected in the scale grains throughout the bulk
volume of the scale where it is contained within lattice of the barite or sulphite
(White and Rood, 2001), but because the scale grains are finer relative to sludge
grains, hence higher grain surface to volume ratio which may also explain the
greater fraction of 222Rn emanation per 226Ra activity concentration of the scale
samples.
148
Table 6.2: Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples
Number of
samples
222Rn 222Rn:226Ra ratio
Sample type arithmetic mean a geometric mean b maximum minimum arithmetic meana geometric meanb
(mBq m-2 s-1) (mBq m-2 s-1/Bq kg-1)
Scales 7 2317 ± 684 46355721628 5107 274 0.26 ± 0.07 42.0
11.021.0
Al-Noor pond sediment 6 87 ± 14 1324980 120 31 0.28 ± 0.10 62.0
06.019.0
Barrel stored sludge 4 74 ± 33 1292759 152 26 0.015 ± 0.003 020.0
011.0015.0
Bahja piles 9 309 ± 74 62075216 598 33 0.12 ± 0.03 21.0
05.010.0
Nimr piles 13 30 ±5 431627 76 13 0.10 ± 0.02 16.0
05.008.0
Marmul piles 14 44 ± 12 731331 155 12 0.28 ± 0.05 57.0
06.019.0
All pile samples 36 104 ± 27 1541547 598 12 0.17 ± 0.00 29.0
05.012.0
Bahja strips 3 18 ± 5 261117 23 10 0.36 ± 0.11 56.0
21.034.0
Nimr strips 9 20 ± 5 29817 37 6 0.16 ± 0.05 32.0
04.011.0
Marmul strips 7 15 ± 3 22914 34 7 0.20 ± 0.14 32.0
02.007.0
All strip samples 19 18 ± 2 26915 37 6 0.20 ± 0.02 37.0
03.010.0
149
Table 6.2 (Continued): Arithmetic mean, geometric mean maximum and minimum 222Rn exhalation rates, and arithmetic and geometric means of 222Rn:226Ra ratio for the various samples
Number of
samples
222Rn 222Rn:226Ra ratio
Sample type arithmetic mean a geometric mean b Maximum Minimum arithmetic meana geometric meanb
(mBq m-2 s-1) (mBq m-2 s-1/Bq kg-1)
Ambient soil 4 3.7 ± 2.1 7.90.11.3 8.2 0.7 0.13 ± 0.06 26.0
03.009.0
Ambient soilc 6 4.5 ± 0.7 9.51.33.4 6.3 2.9 - -
Beads 1 18 ± 3 d - - - 0.0005 ± 0.0001 d -
Sand 1 712 ± 87 d - - - 0.020 ± 0.003 d - a The errors reported are the arithmetic standard error b The upper and lower values geometric standard deviation from the geometric mean c Measurements carried out in Jimboomba, SE Queensland, Australia, included for comparison d Uncertainties represent counting error
150
Sample type
Am
bien
t soi
l
Trea
ted
slud
ge s
trip
Unt
reat
ed s
ludg
e pi
le
Bar
rel s
tore
d sl
udge
Pon
d se
dim
ent
Sca
le
222 R
n ex
hala
tion
rate
(mB
q m
-2 s
-1)
1
10
100
1000
10000
Figure 6.3: 222Rn exhalation rate range and averages for the various sample types.
95th percentile 90th percentile
Median
75th percentile
25th percentile 10th percentile
5th percentile
n = 19n = 4 n = 36 n = 6 n = 6 n = 7
Mean
Geomean
151
Sample type
Am
bien
t soi
l
Barre
l sto
red
slud
ge
Unt
reat
ed s
ludg
e pi
le
Trea
ted
slud
ge s
trip
Pond
sed
imen
t
Sca
le
222 R
n ex
hala
tion
rate
(mB
q m
2 s-1
) / 22
6 Ra
activ
ity c
once
ntra
tion
(Bq
kg-1
)
0.01
0.1
1
Figure 6.4: Ratio of 222Rn exhalation rate to 226Ra activity concentration range and averages for the various sample types.
90th percentile
5th percentile
75th percentile
95th percentile
10th percentile
Median
25th percentile
n = 6 n = 7n = 6 n = 36 n = 19n = 4
Mean
Geomean
152
Many authors have studied the effect of moisture content on radon
exhalation rate (Stranden et al., 1984, Hart and Levins, 1986, King and
Minissale, 1994, Shweikani et al., 1995, Sun and Furbish, 1995, Menetrez et al.,
1996, Nielson et al., 1996, Jha et al., 2000, Barillon et al., 2005, Faheem and
Matiullah, 2008), and they all agreed that as moisture content is increased,
radon exhalation rate increases to a maximum point, followed by a subsequent
decrease. This is because the presence of a small amount of water increases the
emanation rate by stopping the recoiled radon in the interstitial space, where
some of the additional stopped radon atoms would escape from water and
diffuse to the surface. However as the amount of water in the sample increases,
it leads to more radon getting trapped, and hence a decrease in the exhalation
rate.
It has been known since the early 1900s that radon is more soluble in
organic liquids than it is in water (Tanner, 1980), however there is no published
data available on the effect of hydrocarbon content on radon exhalation rate for
petroleum industry sludge. The results of this study suggest that samples with an
appreciable amount of oily hydrocarbon content tended to exhale less 222Rn,
which could be due to the fact that the higher the organic liquid content in the
sludge sample, the greater the proportion of recoiled radon that is absorbed and
retained.
153
6.4 Conclusions
Results of this study showed a direct relationship between 222Rn exhalation
and 226Ra activity concentration, along with a variation in 222Rn exhalation rates
up to three orders of magnitude for the various types of samples investigated.
The geometric mean of 222Rn exhalation rate for the surveyed samples, was
greatest for scales, followed by soil sediments, barrel stored oily sludge,
untreated sludge, treated sludge and ambient soil. The observed lower 222Rn
exhalation rates in treated sludge ( 26915 mBq m-2 s-1), when compared to
untreated sludge ( 1541547 mBq m-2 s-1), can be attributed to the lower 226Ra
activity concentration in the treated sludge, as a result of mixing the sludge with
clean soil at the sludge farms.
An investigation of 222Rn exhalation rate and 226Ra activity concentration
ratios showed that apart from barrel stored oily sludge, there was no statistically
significant difference between the ratios for ambient soil and the rest of sample
types. The significant difference observed for barrel stored oily sludge is most
likely due to the absorption of radon in the liquid hydrocarbon organic content
of the barrel stored oily sludge.
No published data is currently available on the effect of hydrocarbon content
on radon exhalation rate for petroleum industry sludge and when coupled with
the complexity of radon exhalation, which is a function of multiple parameters,
it is evident that further experimental work is required to verify the relation
154
between radon exhalation rate and liquid hydrocarbon organic content of the
samples.
155
Chapter 7 SUMMARY AND CONCLUSIONS
7.1 Summary
This study is the first comprehensive assessment and evaluation of the
activity concentration, gamma dose rate and radon exhalation of large-scale
onshore petroleum operations in the Sultanate of Oman. It was carried out in the
arid desert terrain of an operational oil exploration and production region of
Oman, namely the Southern Oman Directorate (SOD), with the main locations
visited during the study being Al-Noor, Bahja, Nimr and Marmul.
This study focused on the radioactive waste products that are generated
during oil exploration and production. The assessment covered:
(i) Concentration of naturally occurring radionuclides;
(ii) External radiation dose rates in areas where waste products are
accumulated and disposed; and
(iii) Radon (222Rn) exhalation rates.
The types of activities covered included:
• Sludge recovery from separation tanks
• Sludge farming
• NORM storage
• Scaling in oil tubulars
• Scaling in gas production
• Sedimentation in produced water evaporation ponds
156
Crude oil is usually co-produced with high salinity produced water, which
coexists with the crude oil in oil reservoirs. Because radium is soluble in water,
it is transported through oil production and processing installations as dissolved
radium in the produced water. It is then co-precipitated with calcium and
barium, in the form of carbonates and sulphates, as hard and highly insoluble
scale deposits on the interior walls of the pipes, and as sludge at the bottom of
separation tanks. In addition to increasing production costs, as a result of the
maintenance and downtime associated with scaled equipment replacement and
sludge removal, the scales also reduce efficiency by clogging valves, restricting
flow and damaging equipment.
After extraction, one of the disposal methods of the excess produced water
is pumping into evaporation ponds, and this waste water is contaminated by
NORM, heavy metals, volatile organic compounds, polycyclic aromatic
hydrocarbons and other toxic compounds. Evaporation concentrates the NORM
activity content of the produced water, which then crystallises and eventually
leads to scale formation on the ponds internal walls. Therefore, radium isotopes
were expected to be the major contributor to the activity in scale, sludge and
produced water evaporation pond sediments. Pb-210 was also expected to be
present in the older oil scales and sludge, as a result of ingrowth with the decay
of 226Ra.
For ease of interpretation, the results are provided in three separate tables.
Tables 7.1 (a-c) summarise findings for the identified radionuclide activity
concentrations (measured by both HPGe and portable NaI(Tl) gamma
157
spectrometers), gamma dose rates (measured by an energy compensated GM-
tube) and radon exhalation rates (measured by both charcoal cups and an
emanometer), respectively.
Table 7.1 (a) presents the range, median and mean radionuclide activity
concentrations for the various sample types. From the table it can be seen that
the gamma spectroscopy analysis of sludge, oil scale, gas scale and pond
sediment showed a large spectrum of radionuclides. These radionuclides are
progeny of the naturally occurring primordial series 238U, 235U and 232Th, along
with 40K. All activity concentrations were higher than the ambient soil level and
varied over several orders of magnitude.
The results also show excess activity of 210Pb in the oil scales, suggesting
that as well as being supported by 226Ra, mobilisation from oil reservoirs by the
produced water also took place. However the excess activity of 210Pb in the gas
scales is not supported by 226Ra, but is a result of 222Rn decay as it migrates in
the organic gaseous stream, and some authors have also suggested direct
mobilisation of 210Pb from the gas reservoir rock, in a process that is not yet
understood.
An interesting feature of our findings is the detection 227Ac, which was not
supported by 235U. During this study, Ac-227 was detected for the first time in
oil scales and sludge, however it was not detected in produced water
evaporation pond sediments, because it is most likely transported as a vector
and deposited with scale before reaching the ponds. Ac-227 half life
158
(21.8 years) is similar to that of 210Pb (22.3 years), but because it is unsupported
it would decay to ambient levels in seven to nine half lives. On the other hand,
the 226Ra supported 210Pb is a long term radiological hazard. Its activity
concentration will increase by ingrowth with 226Ra decay, reaching secular
equilibrium with 226Ra in about 100 years, and consequently decaying at 226Ra
half life of 1602 years.
The average oil scales 226Ra activity concentration (8.9 kBq kg-1) in Oman
fell within the lower end of the world wide range reported by other studies (0.1-
15,000 kBq kg-1). The stored sludge activity concentrations were also on par
with the world wide reported activities, and the 226Ra activity concentrations
were found to be similar to those reported for Australian uranium mining
activities.
The mean 228Ra:226Ra in sludge at time of deposition in a Nimr separation
tank was found to be 0.87 97.079.0 . The freshly removed sludge age calculated by
228Th:228Ra activity ratio was 5.8 years, which was consistent with the industry’s
separation tank clearance frequency of 5 years. No correlation was observed
between sludge age and the 226Ra and 228Ra activity concentrations for each
individual farm, indicating the radioactivity levels had been consistent over the
years. Dating the oil and gas scales using the 228Th:228Ra activity ratio gave an
average age of 15 and 16 years, respectively.
159
Table 7.1 (a): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study
Activity Concentrations Number
Sample type Median (Range) Mean of
226Ra 210Pb 228Ra 228Th 227Ac 40K samples
Sludge
Stored in barrels 6130(1700-223000)42160 - 4193(1212-34413)9330 3680(916-44639)10518 91(15-614)188 952(336-1480)897 6
Freshly removed from separation tank
547(363-985)588 - 243(139-446)264 271(186-496)296 - 118(32-151)109 6
Untreated piles
Bahja 3164(1090-5670)3289 - 286(92-470)261 344(112-607)338 - 272(182-954)427 25
Nimr 323(73-639)343 - 123(55-270)129 113(48-281)123 - 448(134-595)433 14
Marmul a (27-3690)356 - (7-6036)394 (5-5164)342 - (47-720)360 16
Treated strips
Bahja 54(37-71)55 - 13(8-20)14 10(6-15)11 - 174(107-258)175 12
Nimr b 47(16-260)74 - 21(8-130)34 20(7-136)33 - 127(75-369)146 16
Marmul strips b 67(9.5-455)116 - 23(1.9-130)41 26(2.3-144)45 - 141(16-467)163 29
160
Table 7.1 (a) (Continued): Range median and mean activity concentrations of 226Ra, 210Pb, 228Ra, 228Th, 227Ac and 40K in Bq kg-1, for the various sample types analysed in this study
Activity concentrations Number
Sample type Median (Range) Mean of
226Ra 210Pb 228Ra 228Th 227Ac 40K samples
Oil tubular scales
6380(3380-17300)8940 4720(3060-7590)4920 2250(1360-4310)2510 2920(1930-6810)3730 67(34-123)64 - 7
Gas tubular scales
83(22-125)75 42716(959-66405)30561 19(2.3-32)17 35(2.9-48)26 22(4-181)46 94(26-347)133 12
Produced water evaporation sediments
367(107-5260)1160 - 41(10-583)127 13(5-205)46 - 286(30-837)330 6
Ambient soil 34(27–41)34 - 8(6-11)8 7(6-9)7 - 110(93-293)151 4 a area weighted mean values are reported due to distinct low and high radioactivity sections b excluding hotspots
161
Field surveys of gamma radiation dose rates were carried out in open spaces
where ‘sludge farming’ occurred. Sludge farming is the name given to the
biodegradation process which utilises naturally occurring micro-organisms to
reduce the complex hydrocarbon components of sludge into carbon dioxide and
water. This process occurs after the sludge has been mixed with clean soil and
has undergone frequent tilling and watering. The difference in radioactivity
concentrations between untreated and treated sludge is evident from the values
presented in Table 7.1 (b). Despite obtaining a 226Ra mean activity
concentration and 222Rn exhalation rate at least two times and five times higher
than the ambient soils, respectively (Table 7.1 (a and c)), the treated sludge strip
gamma dose rate averages were close to the ambient soil levels, although some
‘spots’ were detected at Nimr and Marmul sludge farms, which gave higher
gamma dose rates. The reason for this discrepancy in activity concentration and
radon exhalation rate to gamma dose rate averages might be due to the finite
thickness (40 ± 10 cm) of the treated layer of sludge. Therefore, the gamma
dose rate readings cannot be used as a reliable indicator in the assessment of the
radiological contamination of treated sludge.
162
Table 7.1 (b): Mean (± standard deviation), median and range of gamma dose rates in µSv h-1 for untreated and treated sludge in Bahja, Nimr and Marmul sludge farms, and ambient soil readings
Sample type Dose rates (µSv h-1) Number
Mean Median Range of readings
Untreated sludge piles
Bahja 0.702 ± 0.250 0.637 0.281 - 1.116 23
Nimr 0.166 ± 0.026 0.166 0.126 – 0.238 14
Marmul 0.345 ± 0.454 0.146 0.072 – 1.781 16
Treated sludge strips
Bahja 0.084 ± 0.004 0.084 0.076 – 0.091 12
Nimr 0.109 ± 0.084 0.080 0.072 – 0.426 17
Marmul 0.115 ± 0.068 0.091 0.063 – 0.362 31
Ambient soil 0.086 ± 0.014 0.082 0.074 - 0.105 4
Table 7.1 (c) presents 222Rn exhalation rates and radon exhalation to radium
concentration ratios for the various samples analysed in this study. The
geometric mean of ambient soil exhalation rate for area surrounding the sludge
was 7.90.11.3 mBq m-2 s-1. Radon exhalation rates reported in oil waste products
were all higher than the ambient soil value and varied over three orders of
magnitude. Rn-222 exhalation to 226Ra concentration ratios for sludge farm
treated and untreated sludge were similar to the ambient soil value, whereas the
oil scale and pond sediment values were twice as high. The reason for this
difference might be that the scale grains are finer than the sludge grains,
resulting in a higher emanation fraction. In the pond sediment, the 226Ra
(originating from produced water) is thought to be present as a surface coating
on the sediment grains, resulting in a non-uniform distribution, which may
explain the greater fraction of 222Rn emanation per 226Ra activity concentration.
163
In contrast, the oily sludge had a low radon exhalation to radium concentration
ratio. Because radon is known to be soluble in water and organic liquids, the
results suggest a greater proportion of recoiled radon in the oily sludge samples
is absorbed and retained, hence resulting in the lower 222Rn:226Ra ratio.
164
Table 7.1 (c): Maximum, minimum and geometric mean of 222Rn exhalation rates in mBq m-2 s-1 and the geometric mean of radon exhalation to radium concentration ratio in mBq m-2 s-1/Bq kg-1 for the various sample types analysed in this study Sample type 222Rn exhalation rate(mBq m-2 s-1) 222Rn:226Ra ratio Number
Maximum Minimum Geomean Geomean of readings
Oil scales 5107 274 46355721628 42.0
11.021.0 7
Pond sediments 120 31 1324980 62.0
06.019.0 6
Untreated sludge piles 598 12 1541547 29.0
05.012.0 36
Treated sludge strips 37 6 26915 37.0
03.010.0 19
Barrel stored sludge 52 26 1292759 020.0
011.0015.0 6
Ambient soil 8.2 0.7 7.90.11.3 26.0
03.009.0 4
165
A total of five site visits, each ranging from 3-17 days, were made for in-situ
measurements and sample collection. Due to a number of difficulties, only a
limited amount of time could be spent in the field. For example, limited
transport was available for both the 260 kg of equipment and researchers to get
to the site, and finding accommodation in the remote mining camps was not
always easy. Also, in order to avoid transporting the radioactive materials back
to Australia, all of the collected samples had to be analysed in the Medical
Physics Laboratory of Sultan Qaboos University (SQU), Muscat, using both the
SQU facilities and the equipment transported from QUT.
Many factors also needed to be taken into consideration when designing the
measurement and analysis procedures used in the study. For example, where
portable gamma spectroscopy is used for detecting activity concentration of
228Ra through its progeny, it was important to understand that, in petroleum
waste products, its immediate daughter, 228Th, is found in transient rather than
secular equilibrium with 228Ra, while in ambient soils, the equilibrium is
secular.
Despite these difficulties, many of the features of this study are novel and
unique. Firstly, most of the previously published studies relate to offshore
operations. Therefore this study is unique in being the first large-scale onshore
study on petroleum exploration and production radiological assessment and in
being conducted in the Sultanate of Oman. Therefore, the information provided
in this thesis may be used as an important reference for NORM activity
concentration assessments in the oil industry. In addition, because this study
166
was conducted in the Southern Oman Directorate, the findings would also be a
good source of knowledge for possible future assessments in oil rig operations
of the Northern Oman Directorate as well.
Further, this was the first ever study to perform an assessment of radon
(222Rn) exhalation from oil sludge samples. It was also the first ever study to
quantify the presence of 227Ac in oil sludge and scales, and to date, it is only the
second study to ever quantify the presence of 227Ac in gas scales. Ac-227 has
high inhalation and ingestion dose coefficients compared to 226Ra, 228Ra and
210Pb, particularly since its committed effective dose coefficient for inhalation is
two orders of magnitude higher than 226Ra, 228Ra and 210Pb. Therefore, proper
radiological protection measures should be adhered to during the
decontamination of scaled pipes, as well as during the disposal of scale and
sludge.
An empirical relation was also derived between petroleum sludge activity
concentrations and gamma dose rates. The coefficients of this relationship
turned out to be different to those reported by various authors who investigated
such relationships of infinite thickness and infinite dimension slabs of soils. The
reasons for the lower conversion coefficients obtained in this study are thought
to be due to: (1) the presence of heavy metal sediments and corrosive particles
in the petroleum sludge, leading to a greater attenuation coefficient; (2) the
radionuclides may not have reached equilibrium at the time of measurement;
(3) the ambient soil measurements are usually conducted on flat extended land,
whereas untreated sludge piles are in small heaps; and (4) the greater soil
167
density of ambient soil compared to less compacted untreated and treated
sludge.
7.2 Future directions
As outlined above, this was the first ever study to perform an assessment of
radon (222Rn) exhalation from oil sludge samples, and the results of this study
suggest the samples with an appreciable amount of oily hydrocarbon content
tended to exhale less 222Rn. As such, further investigations are recommended to
establish this hypothesis. In addition, Ac-227 was detected for the first time in
oil scales and sludge, possibly as a result of the residues left behind from the
produced water. However, further investigations into the formation and ultimate
fate of Ac-227 are also required, in order to develop a better understanding of its
presence in the petroleum industry.
It is well known that the measurement capabilities of HPGe systems vary
significantly, and unfortunately, the SQU pop-up detector used in this study had
a poor sensitivity for the 210Pb gamma energy of 46.5 keV, resulting in high
uncertainties and/or no detection of 210Pb in a number of samples. As such, it is
recommended that future studies use a planner HPGe system instead, in order to
increase the detection capabilities for 210Pb.
Scaling sedimentation and deposition in various sections of the extraction,
transport and storage equipment is a major problem in the petroleum industry,
costing the oil production companies significant time and money, due to
168
maintenance and replacement costs, as well as losses in efficiency and the down
time associated with its removal. Thus, the ability to section the scale, and
measure the age of individual sections (using 228Ra:226Ra and/or 228Th:228Ra
activity ratios) would contribute greatly to existing knowledge on the scale
deposition process. However, field conditions during this study were such that
sectioning the depositions was not possible, leaving many opportunities for
further measurements and analysis in the future.
To summarise, the findings of this study highlight that considerably more
work is still required and that there are many gaps in knowledge yet to be
explored, including:
• Radiological health impact due to the petroleum mining activities
• Investigation of dust re-suspension pathway in the arid desert
environment
• Investigation of the social aspects of bedwen life style to model the
likely radiation dose to critical groups in the long term
• Modelling the likely radiation dose to be incurred by direct gamma,
inhalation and injection pathways during future oil and gas scale
removal activities
• Further investigation on 227Ac migration, deposition and fate in the oil
and gas systems
• Investigation of scale deposition rate by use of inherent radiological
behaviour of 228Ra:226Ra and/or 228Th:228Ra activity ratio(s)
169
• Assessment of 220Rn activity flux, since the presence of 228Ra may
make it an inhalation hazard, especially in confined areas such as
separation tanks
• Measurement of 220Rn and 222Rn in the air
• Use of a planner HPGe system for improved detection of 210Pb
• Detection of 210Po and its applications for health hazard and/or
estimating the age of 210Pb depositions
• Determination of petroleum samples moisture and hydrocarbon
content and its implications on radon exhalation
• Determination of sample mineralogy using X-ray fluorescence
spectrometer (XRF) and diffractometer (XRD)
170
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