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Microseismic monitoring of the hydraulic-fracture growth and geometry in the Upper Bahariya

member, Khalda concession, Western Desert, Egypt

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2014 J. Geophys. Eng. 11 045013

(http://iopscience.iop.org/1742-2140/11/4/045013)

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1 © 2014 Sinopec Geophysical Research Institute Printed in the UK

1. Introduction

The conjugate use of horizontal drilling and hydraulic frac-turing has dramatically increased production from shale gas reservoirs; however, the identification of fracture patterns remains the most influential factor on the success of such oper-ations. This is attributed to the fact that the efficient reservoir drainage is a function of fracture location, direction, and extent in three dimensions. Therefore, fracture maps are important

decisive data not only in selection of efficient well placement during early production phases but also in designing enhanced recovery programs (Phillips et al 1998). Hydraulic fracturing became a routine industrial practice to stimulate hydrocarbon production from tight reservoirs (King 2010). Besides gener-ating new flow paths, the pressured fluids of hydraulic frac-turing treatment are expected to displace hydrocarbons in small pores and cracks by imbibitions (capillary), leading to better mobility, and in consequence enhance production.

Journal of Geophysics and Engineering

Microseismic monitoring of the hydraulic-fracture growth and geometry in the Upper Bahariya member, Khalda concession, Western Desert, Egypt

Abdulaziz M Abdulaziz

Mining, Petroleum, and Metallurgical Engineering Department, Faculty of Engineering, Cairo University, Giza, Egypt

E-mail: amabdul@miners.utep.edu

Received 4 January 2014, revised 20 May 2014Accepted for publication 25 June 2014Published 31 July 2014

AbstractThree-component, downhole geophones are installed in production wells at the Khalda and Kahraman sites to monitor hydraulic fracturing treatment in a nearby well. Locatable microseismic events were distributed in space around the treatment well using the hypocenter-velocity inversion method. The E–W oriented microseismic pattern aligns with the proposed specifications of fracture model design at both the Khalda and Kahraman sites. Due to the small magnitude and long separation distance between the treatment well and the recording string, microseismicity was dominantly observed during the main fracturing operation at both the Khalda and Kahraman sites, with calculated magnitudes less than −0.3 Mw. In addition, the calculated confidence for locatable events was generally average or low. The seismic zone through the mainfrac treatment was estimated to spread over 58 m height and 320 m length at the Khalda site, while the zone of the Kahraman site was enclosed within 25 m height and 250 m length. The stimulated reservoir volume (SRV) at the Khalda site was estimated to be 433 200 m3, with asymmetrical wings around the stimulation well, but at the Kahraman site the stimulation imaging was marginally successful and the stimulated SRV was only 247 000 m3. In general, the two microseismic experiments at Khalda and Kahraman were relatively successful in locating the microseismic events and calculating the SRV within the producing horizon, elucidating the importance of the microseismic technique in monitoring reservoir stimulation.

Keywords: microseismic, hydraulic fracturing monitoring, Bahariya formation, Western Desert, Egypt

(Some figures may appear in colour only in the online journal)

A M Abdulaziz

Printed in the UK

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doi:10.1088/1742-2132/11/4/045013J. Geophys. Eng. 11 (2014) 045013 (10pp)

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Classically, fracture mechanics models provide the foundation for fracture treatments that determine fluid and proppant volumes, injection stages, and other treat-ment parameters (Urbancic et al 1999). Since propagation of seismic waves in crystalline hard rocks is more efficient compared to sedimentary rocks, microseismic records col-lected in geothermal reservoirs are usually qualitatively superior to those recorded in hydrocarbon reservoirs. However, selecting sensitive geophones to be installed in a close monitoring well(s) could significantly improve the seismic record quality of petroleum reservoirs (Warpinski et al 1996).

Hydraulic fracture monitoring using downhole micro-seismic events was first applied in geothermal reservoirs to help understand the developed fracture system (Baria and Green 1986, Batchelor, Baria and Hearn 1983, Fehler and Phillips 1991, House 1987), fault zone analysis (Cornet and Scotti 1993, Fehler, House and Kaieda et al 1987), pore pressure, and reservoir control (Niitsumi et al 1987, Pearson 1981). Recently, moderate to large-magnitude seismicity has been historically reported as a few events associating hydrocarbon production operations in spe-cific fields, but the relatively common events fall within microseismicity (Suckale 2010). The recurrence of this microseismicity follows earthquake frequency–magnitude statistics, as it increases 10- to 100-fold as the magnitude decreases by one unit (Fehler and Phillips 1991). Over the past decade microseismic approaches have been applied, especially in North America, to map the hydraulic frac-ture geometry and evaluate the efficiency of the hydraulic fracturing treatment (Abdulaziz 2013, Mohammad and Miskimins 2012). In addition, information on fracture slip dimensions, event magnitude/energy, and failure charac-teristics can be delineated through recorded wave form analysis (Urbancic and Trifu 1996). Furthermore, real time microseismic monitoring allows effective hydraulic

fracturing stimulation and minimizes the risk of reservoir deterioration though updated images of the fracture growth (Abdulaziz 2013).

In 2008, two microseismic experiments were conducted in the Khalda concession in the Western Desert, representing the first microseismic experiment in Egypt, to test the capacity of microseismic imaging in mapping fracture growth and geom-etry. Such information is significant to improve hydraulic fracturing designs to optimize production well distribution and improve the production methods. In this research these experiments are presented in details to show integration of spatial and temporal records of events in mapping fracture growth in three dimensions. These results are important in evaluation of the SRV and the efficiency of the treatment process.

2. Geological setting

The Khalda concession is located in the northern part of the Western Desert (figure 1) and occupies a structural closure that engages three stratigraphic levels of hydrocarbon poten-tial in the Bahariya, Kharita, and Alam El-Bueib formations (figure 2). Bahariya Formation is shallow marine sandstones with intercalations of shale that can be distinguished as Upper and Lower Bahariya. Upper Bahariya comprises tid-ally dominated shale intercalations with slight sand deposits that tend to be a low to moderate energy inner shelf envi-ronment. It is slightly dipping, ~4°, in the north direction, but the dipping angle may dramatically increase close to fault zones. The average porosity of Upper Bahariya falls between 18% and 21% while the permeability is notably tight (4–5 mD). Lower Bahariya is an inter-tidal to sub-tidal sandstone–shale intercalation with average porosity 25% and net sand 40–55% in the upper 30 m of the section in the study area. Kharita Formation consists of consolidated sandstone

Figure 1. Location map of Khalda concession.

N

25O 00' 00'' E 30O 00' 00''E 35O 00' 00''E

Mediterranean Sea

Sinai

Red Sea

Cairo

Alexandria

30O

00' 0

0'' N

25O 0

0' 0

0'' N

Western Desert Eastern Desert

0 150 30075km

EGYPT

Kahraman site

Khalda site

Khalda Concession

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of fluvio-deltaic facies that provides a good quality reser-voir of 18–25% porosity and hundreds of millidarcys per-meability. Alam El-Bueib Formation (Φ ≈ 8–17%) belongs to the Lower Cretaceous transgressive cycle that started at the Neocomian with fluvio-continental series and finished

with transitional near-shore to deltaic deposits at the Lower Aptian–Albian.

At Bahariya and Kharita levels the structure closure is a simple three way dip with multiple prospects falling within a series of NW–SE aligned highs with local small scale syn-thetic faults. The charge of hydrocarbon is believed to be from the mature shale of the Khatatba and Alam El-Bueib formations deposited in the Shushan Basin. The study area was covered with a good quality 3D seismic pattern acquired in 2001 with only basic processing applied (figure 3). At the top of Bahariya Formation a structure closure of 235 ft was recorded with approximate fill to spill areal closure of 1214 acres (figure  4). Oil production in Kahraman Field is predominantly from Bahariya formation (mostly the Upper Bahariya unit), but occasionally it has contributions from Kharita formation and Alam El-Bueib formation (AEB 1, AEB3A, and 3D units).

3. Methods

Microseismic imaging of Upper Bahariya was accomplished by a temporary string of triaxial geophones (ten in Kahraman and eleven in Khalda) deployed in a monitoring well to evaluate the progression of the fracture system developed by hydraulic fracturing at 60 m intervals within the Upper Bahariya member (figure 5). The triaxial geophones with a downhole amplification system were installed at a depth of 1895 m in a 22° deviated wellbore (Khalda-53) and 1928 m in the Kahraman well (figure 5). Perforation, 11.68 mm in diam-eter and with 60° phasing, of stimulation wells (five intervals of 8.5 m in Khalda and three intervals of 6.7 m in Kahraman) is expected to enable a complete zonal coverage and max-imum stimulation through sand intervals. The perforation shots usually release strong SH compared to SV signals (figure  6). Therefore, P-wave propagation in the horizontal plane is expected to be sufficiently large during hydraulic fracturing compared to the vertical component of the shear waves (Fehler and Pearson 1984). To define a velocity model for the two experiments, reliable geologic boundaries were delineated using a gamma-ray log of the monitoring and treatment wells. The geophone orientation in the toolstring was determined using the released seismic energy during per-foration shots in the treatment well (table 1).

Microseismic event location is a function of arrival time and hodogram orientation, a record of the orientation of P-wave particle motion that defines the ray-path orientation. Hodograms provide the mean to determine the event direc-tion relative to the monitoring well, while the recorded Δt arrival time (the lag time between P and S-wave arrivals ema-nating from a common event) with the velocity model con-strains the separation distance between the monitoring station and the fracture location. The hodogram is computed using eigenvector analysis (Flinn 1965) of the first cycle of the P wave and the hodogram azimuth is determined using the two horizontal components. The quality of azimuth measurements (Vidale 1986) is evaluated using hodogram consistency anal-ysis (figure 7). To pick the arrival time, horizontal component

Figure 2. Composite geologic column for the study area.

AppoloniaEo

cene

Khom

an

Maa

strich

tian

U p

p e

r

C r e

t a

c e o

u s

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nian-

Sant

onian

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man

ianA

b u

R

aw

a s h

Baha

riya

K h

a r i

t a

DahabAlamein

A l a

m

E l -

B u

e i b

Masajid

AB1

B2

CDEF

G

Upper

Lowe

r

1

2

3

4

56

A l

b i

a n

Aptia

nBa

rrem

ianHa

uter

ivian

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w e

r

C

r e t

a c e

o u

sTe

rtiar

y

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- - - - - - - - - - - - - - -

- - - - - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

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- - - - -- - - -- - - - -- - - -

- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -- - - - -

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Age Formation Lithology

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seismograms (figure 6) are manually separated into radial and tangential components based on the calculated azimuths. The SV component showed better response compared to the SH component and therefore it was preferred in calcula-tions. Accepting ±7° as a representative azimuth error of the hodogram ensures minimization of residual arrival time and/or mismatch between forward modeled and observed arrival times; the time pick error was within 1.5 and 2.7 ms for P and

SV waves respectively. After refinement of the initial velocity model (figure 8) and geophone orientation (table 1), event location was identified using hypocenter-velocity inversion as described by Rutledge et al (1998) for high quality recorded events. Since the axis of symmetry for anisotropy in the pre-sent study is vertical (layered geology model), the seismic velocity is independent of propagation azimuth. Therefore, it is acceptable to apply the isotropic velocity model for ray

Figure 3. Seismic section showing a prospect trap in the Kahraman field with allocation of formation tops; the location of the seismic line is presented in figure 4.

Figure 4. Depth structure contour map to the top of the Upper Bahariya member constructed from the seismic data. High level is equivalent to low depth.

Kahraman Field

0 1 2Km

28o 37' 27'' 28o 39' 52'' 28o 42' 17''

30o 5

2' 2

0''

High level Low level

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paths from a common source and similar incidence angle (Phillips et al 1998). During inversion the velocity, arrival time, and azimuth were fine tuned using the iterative least-square technique to fit the recorded arrival time and hodogram data. If a consistent location, within a predefined constraint, is obtained, the solution is accepted as the proper event location; otherwise, the documented fracture system is applied to con-strain the multiple solution into the most similar one. Event magnitude was estimated using the peak amplitudes of the tri-axial signals as described by Abdulaziz (2013). To facilitate obtaining a representative event magnitude data set, a higher magnitude threshold of less than −0.4 Mw has to be used.

4. Results

The seismic energy of perforation shots and sonic log proved helpful in constructing a representative velocity model of

the stimulated interval (figure 8). SV velocity was reported as 2.1  ±  0.05 km s−1 in Upper Bahariya, while it fluctuated between 1.94 and 2.2 km s−1 in Lower Abu Rawash and kept above 2.1 km s−1 in Lower Bahariya (figure 8). Fluctuation in the velocity record in Lower Abu Rawash formation was dis-tinct in P waves (2.8–3.9 km s−1) compared to the VS record, while both Upper and Lower Bahariya are well correlated in the two-velocity record (figure 8). During minifrac of the Upper Bahariya, seven microseisms were recorded at the Khalda site during pumping of the cross-linked gel through perforations, but in the Kahraman well no events were recorded probably due to severe depletion. During the mainfrac, more than 120 microseismic events in proximity to the Khalda site and 85 events at the Kahraman site showed relatively clear P, SV,

Figure 5. Data acquisition string from microseismic experiment at Khalda and Kahraman sites showing shot location and geophone array centered on the Upper Bahariya reservoir.

0200300400500 100

1740

1770

1800

1830

1860

1890

1920

1950

1980

2010

UpperBahariya

MonitoringWell

TreatmentWell

Khalda experimentGRlog

GRlog

Lateral:Depth ratio (1.86:1)

met

ermeter

TriaxialGeophones

Perf.Shots

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1980

2010

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TreatmentWell

MonitoringWell

UpperBahariya

Lateral:Depth ratio (1:1)

GRlog

TriaxialGeophones

PerforationShots

met

er

meter

Kahraman experiment

Figure 6. Seismogram records for a microseismic event at the Khalda site showing vertical, radial, and transverse components of ground motion. P, SV, and SH first arrival is identified on an arbitrary scale for arrival time and an identical amplitude scale for the three components.

P SV

P SV

SH

0 25 50 75 100 125Time (ms)

Vertical

Radial

Transverse

-Ve

+Ve

-Ve

+Ve

-Ve

+Ve

Table 1. Toolstring configuration for Khalda and Kahraman wells.

Tool Number

Khalda (KH-53) Kahraman (KHR C144)

TVD (m)

Orientation (˚)

TVD (m)

Orientation (˚)

1 1826 79 1890 662 1840 −52 1906 713 1854 47 1921 −3.24 1868 −20 1936 585 1882 57 1951 736 1896 −23 1967 887 1911 78 1982 418 1925 41 1997 509 1939 9 2012 −1110 1953 39 2028 −7311 1967 −45 — —

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and SH arrivals, representing respectively vertical, radial, and transverse components (figure 6). Due to the different sources, the wave forms of the perforation shots were distinctly dif-ferent from the microseismic events of hydraulic fracturing. However, all recorded data are rigorously analyzed to enhance

the signal-to-noise ratio. The recorded seismicity and average noise level, calculated using the root mean square (RMS) average amplitude method, showed good correlation with the treatment parameters, particularly well head pressure, over the entire data record (figure 9).

Figure 7. Plot of hodogram consistency analysis for all recording geophones at Khalda and Kahraman sites.

0 30-30 -10-20 10 20

Sen

sor

num

ber

1

2

3

4

5

6

7

8

9

10

11

Hodogram consistency (degrees)

Khalda-56

0 30-30 -10-20 10 20

Sen

sor

num

ber

1

2

3

4

5

6

7

8

9

10

Hodogram consistency (degrees)

KHR C144

Figure 8. Layered velocity model of P and S waves constructed for Khalda site.

25 50 75 100 125

1830

1890

1950

Gamma Ray Log Depth (m)

1786

1817

1835

1849

1875188218881899

1919

1932193819481958

1975

19891998

1500 3000 4500 6000Seismic Velocity (m/s)

1940

2167

1987

216220602220

2070

2120

21832150

2161

2040

2216

2352

3932

2745

3298

2847

327630153260

3147

3181

32033124

3208

3140

3178

VpVsToolstring

Depth(m)

Khalda-56Khalda-56

Upp

er B

ahar

iya

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Due to small event magnitudes and the long separation dis-tance between the treatment and observation wells, the locat-able microseismic events are dominantly developed during the main fracturing operation in both the Khalda and Kahraman fields (figures 10 and 11). The data set of moment magnitudes calculated for these events does not exceed  −0.3 Mw, with only two categories of events identified (−0.3 to −0.4 Mw and

less than −0.4 Mw; figures 10 and 11). Generally the Khalda site reported more events compared to Kahraman and, in both cases almost all recordable events fall within the Upper Bahariya Formation, where the injection took place. In map view (figures 10 and 11), event locations are scattered with a

Figure 9. A plot of number of microseismic events versus well-head pressure at the Khalda site.

0 1 2 3 4 5

5000

4000

3000

2000

1000

0

6000

Sur

face

Pre

ssur

e (p

si)

1

2

3

4

5

Start EndTime (hours)

Num

ber

of E

vent

s

Well-head pressure

Figure 10. Location of microseismic events in cross sectional view (top) and map view (bottom) for Khalda site.

0 -50 -10050100150200250

1860

1890

1920

1950

1980

1830

1800

Less than -4.0 M -3.0 M - -4.0 M

Looking NE

UpperBahariya

Lower Bahariya

Distance (meter)

Dep

th (m

eter

)

Khalda

Distance (meter)

Dist

ance

(met

er)

Looking NE

050100150200250300350

50

100

150

200

0

50

Khalda

Figure 11. Location of microseismic events in cross sectional view (top) and map view (bottom) for Kahraman site.

Lower Bahariya

UpperBahariya

0 50 100 15050100150

1890

1920

1950

1980

2010

2040

Looking NNW

0

50

100

150

200

250

300

350

0 5050 100100150200

Looking North

Distance (meter)

Dept

h (m

eter

)

Distance (meter)

Dist

ance

(met

er)

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relatively narrow elongation in the E–W direction at both the Kahraman and Khalda sites, the known fracture direction in the study area. The events located to the north of the dominant fracture direction at the Khalda site (figure 10) are attributed to a small lower perforation interval that did not enable deep lat-eral penetration and fracture propagation. The location pattern of microseismicity indicates that the events in proximity to the treatment well are located at approximately 500 m from the nearest geophone (geophone 6 in Khalda and geophone 5 in Kahraman). At the Khalda site, only 45% of the total recorded P wave was strong enough to produce an acceptable azimuth hodogram. This indicates that some events located within the distant wing of the induced fracture (~350 m from the moni-toring well) are not included. The hodogram at the Khalda site and the general trend in orientation of the recorded events at Khalda and Kahraman indicate that the developed fracture azimuth is oriented N85°E from the treatment well (figures 10 and 11), that comparatively aligns with the proposed specifi-cations of fracture model design. The microseismicity zone around the Khalda site (figure 10) is moderately dense com-pared to Kahraman (figure 11), representing better imaging of the fracture system at the Khalda site. The seismic zone spread over 45 m height and 100 m length during the mini-frac test at the Khalda well and expanded to 58 m height and 320 m length through the mainfrac treatment (figure 10). At

Kahraman the microseismicity was enclosed within a zone of 25 m height and 250 m length during the mainfrac operation (figure 11). Small magnitude events could be detected within 25 m outside the identified zones, but were ambiguous enough to be excluded from seismic zone delineation. In both cases the seismic zone was concentrated within 20 m over most of its length and is generally denser close to the perforation shots.

5. Discussion

Due to a considerable anisotropy within the reservoir, the arrival time of horizontally polarized shear waves (SH) is repeatedly smaller than the vertically polarized shear waves (SV) (figure 6). This indicates a dominant horizontal anisot-ropy that can most likely be attributed to fine interbedding of laminated sand and shale. Microseismic events recorded in zones within the Upper Bahariya horizon showed a rela-tively low frequency (200–350 Hz). This reflects a moderate level of signal-to-noise ratio for these data, while attenuation along the ray path considerably affects the waves recorded within the Lower Abu Rawash and Lower Bahariya zones. The calculated S/P amplitude ratios and the first motions of both compressional and dilatational energy indicate a small to intermediate component of shear slip during failure. On pro-gressively moving away from the monitoring wells, smaller

Figure 12. Sequence of events relative to the start of the mainfrac in cross sectional view for the Khalda (top) and Kahraman (bottom) sites.

0 50 10050100150200250

1860

1890

1920

1950

1980

1830

1800Looking NNE

UpperBahariya

Lower Bahariya

Distance (meter)

Dep

th (m

eter

)

Khalda

Early Late

Sequence of events

Lower Bahariya

UpperBahariya

0 50 100 15050100150

1890

1920

1950

1980

2010

2040

Looking NNW

Distance (meter)

Dep

th (m

eter

)

Kahraman

Figure 13. Cross sectional view of confidence level calculated for locatable events in Khalda (top) and Kahraman (bottom) sites.

0 50 10050100150200250

1860

1890

1920

1950

1980

1830

1800Looking NE

UpperBahariya

Lower Bahariya

Distance (meter)

Dep

th (m

eter

)

Khalda

High Low

Confidence in events

Lower Bahariya

UpperBahariya

0 50 100 15050100150

1890

1920

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2040

Looking NNW

Distance (meter)

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event signals become generally indistinguishable and the signal-to-noise ratio becomes a maximum.

In Upper Bahariya, hydraulic stimulation is regularly applied in the Khalda and Kahraman fields to increase the oil mobility by creating fractures in the tight reservoir (4–5 mD). Both minifrac and mainfrac induced events were mostly located in the Upper Bahariya, with the majority of locatable events contained within the upper part (figures 10 and 11). The three-dimensional distribution of microseismic events replicates the SRV, a part of the reservoir in which injection fluids are raised to a pressure sufficient to cause rock failure and slippage (Mayerhofer et al 2008). In consequence, micro-seismic imaging has been much used over the past decade to monitor reservoir stimulation, especially in shale gas res-ervoirs (Abdulaziz 2013). In the Khalda well the SRV was estimated to be 433 200 m3, with asymmetrical wings around the stimulation well. Relative to the monitoring well, the near wing (~200 m) was distinctly longer than the far wing (~120 m), probably due to the larger distance and/or lower seismic Q factor. Alternatively, at the Kahraman site, the stimulation imaging was marginally successful compared to the Khalda well, and the SRV was 247 000 m3 with asymmet-rical wings around the stimulation well. The near wing was reported as ~160 m long while the far wing was only ~90 m. The larger SRV around the Khalda well and the concordant fracture azimuth (N85°E) to the regional structure of the study area may indicate the presence of natural fractures.

During minifrac the recorded microseisms are mainly located around the treatment well, suggesting that all perfora-tions are open and releasing treatment fluid. The perforation shots are useful in testing the validity of the velocity model. This is accomplished using the recorded time for shots and the calculated velocity model to locate the perforation. As shown in figures 11 and 12 the fracture density, as a function of event density, is notably different in the two sites, with denser patterns recorded at the Khalda site compared to the limited number of locatable microseisms at the Kahraman site. Generally, the recorded events in both sites are relatively small magnitude microseisms. In addition, the far wing of the stimulated area in both has a notably lower number of identified events compared to the near wing (figures 11 and 12). Due to the small magnitude, the identification of p-wave signals in the records was extremely difficult and results in location error, particularly in the azimuth. Although locat-able microseisms during the minifrac were generally inad-equate to accurately identify the azimuth at the Khalda site, the locatable events in the mainfrac (figure 10) generally follow the known fracture trend. This was more obvious at the Kahraman site, where the E–W azimuth, assumed to be symmetrical about the treatment well, was inadequately iden-tified (figure 11).

Microseismicity starts shortly (usually less than an hour) after pressurizing the treatment hole, and continues for a period that may extend for days after well shut-in. During stimulation at both the Khalda and Kahraman sites the seismic zone started to trigger 30 minutes after pressurizing the pay zone, and continues to be active for an hour (figure 9). Using the arrival time record, the temporal sequence of events for

both sites (figure 12) was set up, and shows that location errors tend to increase as the width of the seismic zone increases. At the Khalda site stronger events were notably close to the treatment well and mostly concentrated at the early stage. In contrast, late records are mostly low magnitude events that were located at the outermost edges and close to the treat-ment well after pressure shut-in. This trend is encountered at the Kahraman site too but was less obvious, probably due to the limited number of located events (figure 12). Generally, a characterizing feature of the low magnitude events is the temporal and spatial clustering within the microseismic zone.

To evaluate the capability of the microseismic record to correctly locate the events, uncertainty analysis, that incor-porates several measurable parameters, is utilized. These parameters include the average arrival time residual error, signal-to-noise ratio, number of sensors picked, and hodogram angle variation for both P and S waves. A confidence number, ranging from 0 to 5, was assigned to each of these param-eters, that subsequently provide the overall confidence in the calculations of event location. For example, consider assigning a confidence number to the hodogram angle varia-tion; as formerly shown, the 7° azimuth error of the hodogram can be linearly fit with a confidence between 0 and 5, with a confidence value of 5 corresponding to an azimuth error of 0–1.4° and a confidence value of 0 for more than 7° azimuth error. Obviously, a confidence of zero value is excluded, as data points with azimuth error greater than 7° are typically excluded. Generally, an average value for the hodogram angle variation can be obtained using all the confidence values of the sensors. The confidence values of the other measurable parameters are similarly calculated, and a representative con-fidence value for this individual event confidence is calcu-lated using the arithmetic average of all their corresponding measurable parameter confidence values. Subsequently, the obtained values are normalized to be presented as low, inter-mediate, or high confidence level. Figure 13 demonstrates the overall confidence level calculated for each recorded event at the Khalda and Kahraman sites. Generally, all the calculated confidence values for all events fall within the average to low values (3–0). Low confidence events are generally concen-trated within the far wing relative to the recording system or below the perforation shots (figure 13). This is essentially attributed to the low signal-to-noise ratio, long distance between the event and recoding system, and overall small magnitude of the recorded events.

6. Conclusion

Microseismic monitoring has been applied in hydraulic frac-turing monitoring of the Upper Bahariya reservoir at two sites in the Khalda concession in the Western Desert of Egypt. With the microseismic technique the geometry of the stimulated part was described in some detail and the SRV was calculated around the two stimulated wells. These experiments helped understand important features of reservoir level through tem-poral and spatial changes in reservoir architecture associated with the hydraulic fracturing process.

J. Geophys. Eng. 11 (2014) 045013

A M Abdulaziz

10

Acknowledgments

The author would like to thank Khalda Petroleum Company for their permission to publish this paper. In addition, the hard work of the Pinnacle field crew who acquired the data-set and the geophysicists who processed these data is highly acknowledged.

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