Chapter 6 Water Base Dilling Fluids

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Cod.: RPWA2021A Date : 01/03/2005 Rev: 00 Page: 58

G R O U P

Agip KCO

WELL AREA OPERATIONSDRILLING SUPERVISOR TRAINING COURSE

INHIBITING WATER BASEDMUDS



Agip KCO Code RPWA2021A Rev. 00 – Date 01/03/2005

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Well Area Operations

INDEX

1.0 INTRODUCTION 5 2.0 LIME BASE MUDS 5

2.1 FLUIDS TREATED WITH LIME 6

3.0 LIME MUD (FW/SW-LI) 9 3.1 MAIN ADDITIVES OF LIME MUD 9 3.2 TYPICAL PROPERTIES OF LIME BASE MUD 11 3.3 CONVERSION AND MAINTENANCE 11 3.4 MAINTENANCE 13 3.5 ADVANTAGES AND DISADVANTAGES OF LIME MUDS 14 3.6 LIME MUDS - PROBLEMS AND CONTAMINATION 14

4.0 GYPSUM MUD (FW-GY) 16 4.1 MAIN ADDITIVES OF THE GYPSUM MUD 16 4.2 TYPICAL PROPERTIES OF GYP MUD 18 4.3 CONVERSION METHOD / MAINTENANCE 18 4.4 MAINTENANCE 19 4.5 ADVANTAGES/DISADVANTAGES OF GYP MUDS 19 4.6 GYP MUDS - PROBLEMS AND CONTAMINATION 20

5.0 SALT BASED MUD 21 5.1 SATURATED SALT MUDS 22

5.1.1 Main additives of saturated salt muds 23 5.1.2 Typical property of SS (saturated salt) muds 25 5.1.3 Conversion system/maintenance 25 5.1.4 Maintenance 26 5.1.5

Advantages and disadvantages of SS (Saturated Salt) muds 26




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5.1.6 Problems and contamination of SS (Saturated Salt) muds 26 5.2 SEAWATER MUDS 27

5.2.1 Main additives of SW-LS (salt water) muds 27 5.2.2 Typical properties of AS-LS fluids 30 5.2.3 Conversion system 30 5.2.4 Maintenance 30 5.2.5 Advantages and disadvantages of AS-LS mud 31 5.2.6 Problems and contamination (AS/LS) – Salt water Muds 31

5.3 BRACKISH WATER MUDS 32 5.3.1 Main additives 32 5.3.2 Conversion system 34 5.3.3 Maintenance 34 5.3.4 Advantages and disadvantages of brackish water muds 34 5.3.5 Problems and contamination of brackish water muds 35

6.0 POTASSIUM MUDS (FW/SW-KC) 36 6.1 KCL-POLYMERS (KCL-PHPA) = FW/SW-KC 38

6.1.1 Main additives for FW/SW-KC mud 38 6.2 KCL - POLIMERS 41

6.2.1 Preparation 41 6.2.2

Maintenance 42

6.2.3 Problems 43

6.3 KOH-LIGNITE (SYSTEM) 44 6.3.1 Main additives of KOH-lignite Muds 44 6.3.2 Typical properties of KOH-lignite muds 45 6.3.3 Conversion 45 6.3.4 Maintenance 46 6.3.5 Advantages/disadvantages of KOH-lignite muds 46 6.3.6 Problems and contamination of KOH-lignite muds 46

6.4 KOH-LIME MUD 48 6.4.1 Main additives of KOH-lime mud 48 6.4.2 Typical properties of KOH-lime mud 49 6.4.3 Conversion system 49 6.4.4 Maintenance 50 6.4.5 Advantages/disadvantages of KOH-lime mud 50 6.4.6 Problems and contamination of KOH – lime muds. 51

7.0 POLYMER FLUIDS 52 7.1 INTRODUCTION 52 7.2 NON-DISPERSED POLYMER MUDS 52

7.2.1 PAC/CMC low solids muds 53






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1.0 INTRODUCTION

Inhibiting fluids are fluids which do not induce considerable alterations in drilled formations.

These fluids are mainly used to drill clay or shale formations and are also chosen for areas

where contamination problems are expected. Even if major salt, anhydrite or cement levels are

present, a suitable inhibiting fluid can be used to drill them. Salt base inhibiting muds contain

sodium chloride (NaCl) to achieve an inhibiting effect; lime base muds lime (Ca(OH)2 or gypsum

(CaSO4.2H2O), and potassium base muds use potassium carbonate (K2CO3) and other

potassium base additives.

Inhibiting fluids are classified as follows:

lime base muds

salt base muds

potassium base muds

2.0 LIME BASE MUDS

These muds are mainly used to drill highly reactive shale intervals, and their inhibiting effect on

reducing hydration and/or dispersion capacities of shales is greater compared to sodium base

muds. The muds can tolerate solids well, but major contamination from drilled solids (low gravity)

make the rheological/viscosimetric properties instable. The muds have a good resistance to

contamination. In fact contamination from Ca++ e Mg++ ions or chlorine (Cl-) ions does not affect

the characteristics of these fluids; the fluids can be used with a maximum concentration of

chlorine ions of approximately 100.000 mg/l. When the bottomhole temperature (BHT) exceeds

300°F (150°C) lime base muds and particularly muds containing lime, are not used because of

possible gelation (solidification) problems. Gyp mud with an acceptable content of low gravity

solids can be used for temperatures up to 350°F (175°C). The main lime base muds are:

Lime Muds - Ca(OH)3 FW/SW-LI

Gyp Muds – CaSO4.2H2O FW/SW-GY




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2.1 Fluids treated wi th lime

When calcium ions, plus water, are added to a clay system, the Ca++ cation, which has a

higher bonding energy, replaces the Na+ cation in the clay, converting it to a lime base

clay. Figure 1 shows the amount of calcium adsorbed by Wyoming bentonite and native

clay. This cation exchange leads to the partial dehydration of hydrated clay particles,

reducing the section of adsorbed water around the clay particles. (Fig. 2). This in turn

decreases the amount of adsorbed water, bringing clay particles closer to each other, as

in flocculation. Flocculation causes the yield point and gel strengths to increase, unless a

thinner is used.

Figure 1:Absorption of calcium by clays.

Flocs will continue to decrease until precipitation/decantation. If thinner (deflocculant) is

added, the clay particles will still have a reduced adsorbed water section, but the flocs

will disperse. This phenomenon occurs in the case of calcium contamination in drilling

operations, when treatments are added, or when a mud is converted to a lime based

system (break over), i.e. from a lignosulfonate system to gyp or lime fluids.




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Figure 2: Decrease in water hydration in sodium-rich clays during exchange.

The concentration of reactive solids (clays and bentonites) increases viscosity (viscosity

peak in Fig. 3). Before converting the system to a lime base mud, or before drilling

formations containing calcium (anhydrites), reactive solids should sometimes be reduced

by dilution and viscosity can be maintained by adding polymers. Lime base systems

provide soluble calcium and some insoluble, suspended calcium as a reserve

Figure 3: Effect of the concentration of solids on viscosity, with added calcium.

The dissolved calcium has various functions; it minimises the hydration property of clays,

it guarantees more uniform shale borehole sections (cavings) and a minimum dispersion

of shale cuttings in the mud.

These functions are achieved by cation exchange between the mud (Ca++) and native

clays (Na+). The mud is perfectly compatible for inducing calcium formation (anhydrites =

CaSO4); it precipitates the CO3 ions from CO2 contamination. Calcium solubility is




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inversely proportional to the pH of the mud; calcium is practically insoluble with pH values

above 12.5, but easily soluble with low pH values (Fig. 4: Curve A - with the sole addition

of Ca(OH)2, the pH will not exceed 12.4; Curve B with the addition of Ca (OH)2+NaOH

the pH will reach 13.2 and the Ca++ will quickly decrease). Sometimes the calcium as

Ca(OH)2 acts as a buffer solution for the pH, when acid gases such as CO2 or H2S

(hydrogen sulphide) are present.

Figure 4: Calcium solubility is also di rectly related to salinity (CI concentration).

Calcium which is soluble in seawater often amounts to approximately 1200 mg/l and will

increase as salinity increases (Fig. 5). Figure 5 shows the trend of calcium solubility

(added gypsum), in relation to the increase in the salt concentration.

Figure 5.




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3.0 LIME MUD (FW/SW-LI)

Lime mud can be used when an inhibiting system is necessary and when temperatures do not

exceed 300-325°F (140 – 160°C). These systems are particularly useful because they can

tolerate solids incorporation well. The muds have a very wide-ranging filtrate alkalinity (P f ) and

lime content, as classified in table 1.

Table 1 - Classif ication of lime muds based on alkalinity values

Alkal inity Low Lime Intermediate High Lime

Pf 0.8 - 2 2 - 5 5 - 10

Pm 4 - 9 9 - 15 15 - 25

pH values of lime muds vary from 10.5 to 12.5; soluble calcium ranges from 120/400 mg/l and is

controlled by the mud filtrate alkalinity (Pf ). When the filtrate alkalinity increases, less calcium is

dissolved. Caustic soda or potassium hydroxide considerably increase the pH and restrict lime

solubility. If the soluble calcium content is not kept within certain values (120-400 mg/l) problems

relating to high viscosity and gel strengths (including break over) may occur. The salinity

threshold for this type of mud is 40000 – 50000 mg/l (Cl

-

)

3.1 Main additives of lime mud

Lime muds usually contain bentonite (including native clays), caustic soda, organic

thinners, lime = Ca(OH)2 and a fluid loss control additive. Table 2.

Table 2 - Main addit ives of l ime muds

Addi tive Concentration, kg/m3 Function

Bentonite 60 – 80 Viscosity, fluid loss controlLignosulfonate 0.5- 1.5 Deflocculant

Lime Ca(OH)2 5 – 30 Inhibitor, alkalinity control

Caustic soda orpotassium hydroxide

for pH 10.5 - 12.5 pH control

Lignite 0.5 – 1.2 Fluid loss control

*Starch 0.6 – 1.2 Fluid loss control

PAC 0.5 - 3 Fluid loss control

*Requires treatment with a biocide.




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Bentonite – Bentonite is added to adjust viscosity and partially control fluid loss. As

sodium is replaced by calcium during the system conversion stages (Broken Over),

bentonite must be previously hydrated in freshwater before being added to the circulating

system.

Lignosulfonate– Lignosulfonate is used as a thinner (to reduce the yield point and gels)

and to control fluid loss. In areas where chrome lignosulfonate may not be used for

environmental reasons, calcium lignosulfonates are used, without affecting performance.

Lime – Ca(OH)2 Lime is added to increase the Pm. Excess lime must range from 5 - 10

kg/m3. This excess (Pm) is the measurement of available alkalinity to be dissolved, when

Ca++ and OH- ions are depleted while drilling (e.g. eliminated by the shale shaker along

with the drill cuttings).

Caustic soda or potassium hydroxide - Caustic soda or potassium hydroxide are used

to check the Pf (filtrate alkalinity); this controls the solubility of lime and stabilises

rheological properties. (Fig. 3).

Lignite – Lignite is used to control fluid loss; however it forms soluble calcium if calcium

salt is present (humic acid precipitate). Lignite degrades at high temperatures and

produces carbonates.

Starch – Starch is used to control fluid loss up to temperatures of 250°F (120°C). The

high alkalinity (pH) of mud may cause fermentation, so a biocide is essential.

Polyanionic cellulose (PAC) – PAC is always used to control fluid loss, up to Ca++ ion

concentrations of 400 mg/l. PAC can also increase viscosity and encapsulate drilled

solids (inhibiting mud dispersion). Regular viscosity PAC is used for muds with a density

up to 1.40 – 1.50 kg/l, while a low viscosity PAC is better for higher densities (to avoid

excessive increases in viscosity).

Other additives - Gilsonite, asphalts and cellulose fibres are used to prevent mud

invasion (possible damage) in permeable and possible producing formations; they also

stabilise the borehole wall.




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3.2 Typical properties of lime base mud

Lime muds have low viscosities and gels (thixotropy) and are rheologically stable

compared to non inhibiting muds (with a low pH such as service water = FW/SW-LS) if

contaminated by gypsum, anhydrite, cement or carbonates. Table 3 lists the average

characteristics of low and high lime muds with a density of 1.20 kg/l.

Table 3 - Typical Properties of a Low and High Lime Mud

Density

(kg/l))

Plastic

viscosity(cPs)

YieldPoint

(g/100cm 2)

Gels 10sec/10 min

(g/100cm 2)

Pmcm 3

H2SO4 N/50

Pf cm3

H2SO4 N/50 pH

Excess

Lime(kg/m3)

API f lu idloss

(cm 3/30min)

Low Lime1.20

15 - 18 3 – 5 0 - 1 0 - 2 5 - 10 1 - 2 10.5 – 12.5 3 - 6 6 - 12

High Lime1.20

15 - 18 3 - 5 0 - 1 0- 2 12 – 18 5 - 10 12.0 – 12.5 15 - 45 6 - 12

3.3 Conversion and maintenance

Before converting a FW/LS system to a FW/LI system or lime treated system (low –

lime), the bit should be changed (new bit downhole). The conversion often takes place

inside the casing, in a cased hole, while milling the plugs and cement, but can also be

carried out in an open hole, with due care and evaluations. Old mud and settled cuttings

should be removed as far as possible from the pits. The mud to convert should be fully

analysed to obtain information on actual conditions and pilot tests should be planned to

determine the volume of dilution water needed and amounts of chemical products

required for the conversion. The mud should be converted, before weighting, as 10% -

25% dilutions are needed before adding the lime (break over). The mud is usually

converted in one or more circulation stages. If the mud downhole to convert has already

been weighted and the density cannot be decreased, the mud can be converted at the

surface (in pits) to avoid risks and is then displaced in the well, in several stages if

necessary. Treatments with lignosulfonate thinner are also necessary during the break

over, to prevent excessive increases in viscosity (which may cause pump or borehole

stability problems). Mud treated with lime can be converted in a number of ways. Before

adding the lime, dilute with water to obtain a Marsh viscosity of 30 – 35 seconds/l. Water

can then be added before the chemicals, or also during the break over (when chemicals

and Ca(OH)2 are added). Agitate the mud in the circulation pits (without using mud guns,

unless the mud is in the circulation pit) and make sure treated mud does not mix with

untreated and reserve mud, as well as with mud in other pits. Add caustic soda first, then




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the right amount of lime and lignosulfonates at a constant rate - based on the circulating

volume and pump flow rate (200 m3 tot, Q= 2000 l/min, so full circulation 100 min).

Repeat these steps in two circulation stages. Caustic soda or potassium hydroxide

should be added from a chemical barrel, while lime and lignosulfonate can be added from

a mixer funnel. Add the caustic soda base during the first circulation stage, and the lime

and deflocculant in one or two subsequent stages.

Mud viscosity can increase considerably before break over and this basically depends on

the content of solids. If mud becomes too viscous, add more water or thinner, or both.

Adjust the Pm – Pf and lime excess after the break over has been reached. One rigsite

rule of thumb to determine the amount of caustic soda for a given Pf is given below

Note: 1 lb/bl = 2.82 kg/m3 of caustic soda is needed to increase the Pf by one unit, when

the Pf > 7. Approximately 20% more lime should be added during the conversion stage to

obtain an excess amount of lime at the end of the treatment. For example, if 8.0 lb/bl =

20 Kg/m3 of excess lime have been planned, the treatment should be for 10 lb/bl = 28

kg/m3. If barite is added to increase the density, 2-3 sacks of lime for every 100 sacks of

barite will be needed (1 sack = 50 pounds = 22.5 kg), to maintain the excess amount oflime required. The excess lime is calculated based on the following equation: ex. lime

(lb/bl) = 0.26 (Pm-Fw-Pf ).

where Pm = mud alkalinity

Pf = filtrate alkalinity

Fw = volumetric fraction of water (from the mud still).

Table 4 - Filtrate alkalinity determined by adding caustic soda (NaOH)

NaOH, lb/bbl kg/m3 Pf , cm3 H2SO4 - N/50

1 2.8 1.0

2 5.6 3.0

3 7.8 5.04 11.3 7.0




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The amount of chemical products needed for the conversion will vary depending on

circumstances. Table 5 has indicative values.

Note: KOH requires more chemicals than NaOH (1.6 times more) to obtain the same

alkalinity.

Table 5 - Treatment ranges for lime mud conversions

Addi tive Concentration,

lb/bbl Kg/m3

Caustic soda/potassium hydroxide 2 – 3 5 – 8

Lime 4 – 8 10 – 20

Thinner 2 – 5 5 - 15

3.4 Maintenance

To maintain a lime base mud or mud treated with lime and reduce fluid loss, starch

(without a biocide) or PAC can be used. However, lignite or lignosulfonate with small

amounts of prehydrated bentonite is cheaper. Additional prehydrated bentonite may also

be used if the viscosity is still low after checking fluid loss against the required value. If

the mud is too viscous when the fluid loss additive is added, deflocculant (thinner) should

be used. Add lime to check the Pm and NaOH or KOH to check the Pf . A good balanced

ratio should be 1:5:1; the excess lime and the P f should be more or less the same, while

the Pm should be 5 times higher. Lime base mud can tolerate solids wells, even though a

good solids control system should be planned. Mud is more thermally stable when the

content of low gravity solids is not too high. The concentration of lignosulfonate should be

sufficient to ensure good rheological properties. This will ensure a thick, elastic cake and

good control of filtration values (API and HP/HT). Preliminary pilot tests are

recommended to determine the correct amounts of additives.




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3.5 Advantages and disadvantages of lime muds

These muds have many advantages over normal muds. They keep viscosity values and

gel strengths down, and have a good tolerance to low gravity solids contamination. Table

6 summarises these properties.

Table 6 – Advantages and Disadvanges of lime muds

Advan tages Disadvantages

Low viscosity and gel streghts

High head losses during conversation, may

cause borehole damage

High tolerance to solids

May be weighted up to 2.16 Kg/l

Inhibits the hydration of shales and shaly

sandsHigh pH may pose risks to safety

Can tolerate cement, anhydrite and salt

(C- 50,000 mg/l)

Stabilises the borehole (more uniform

boreholes)

3.6 Lime muds - problems and contamination

Chlorides and high temperatures are the most critical contaminating factors for these

muds. Table 7 lists the most common contaminants, contamination indicators and

treatment strategies.




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Table 7 - Treatments for l ime mud contaminants

Contaminants Indicators Treatments

Large number ofsolids

Increase in solids in the mud still,plastic viscosity, 10-minute gels,MBT.

Dilute more. Improve solids removal.Use centrifuges.

Salt/saltwater

Increase in chlorides, Marshviscosity, yield point, 10” / 10’ gel,fluid loss. Decrease in the Pm, Pf and pH.

Increase the density (if the levelincreases), dilute with service water.

Add thinner and caustic soda tocontrol rheology, then treat withstarch or PAC to keep fluid loss

under control. If a large amount ofsalt is present, convert to a saturatedsalt system, or replace with oil basemud.

Increase in MF, 10-min gels.Carbonates/invasionof CO2. The problemis not excessive

Decrease in the PM and pH. IfCO2 invasion continues, normaltreatments with lime will increasefine solids (CaCO3).

Add lime to control the Pm and KOHto control the PF.Keep the solids content at optimalvalues (low)

Poor qualityproducts

Different packaging. Increase intreatment amounts. Anomaloustrend of mud properties.

Check dispatch and supplierdocuments.Take samples and analyse. Worktogether with the supplier rather thanchange supplier.

Foaming in the pits. Trapped air.Decrease pump pressure, ifpossible

Foaming

Add a non-toxic defoamer.Identify the cause of foaming andeliminate.

High temperaturegelation

Pressure kick-up to restartcirculation after trips. Veryviscous bottomhole cushion. Highviscosity even at the flow line.

Reduce low gravity solids. Addlignosulfonate if the bottomholetemperature

(BHT)<300°F (150°C)

Add polymer deflocculants whentemperatures are above 150°C.




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4.0 GYPSUM MUD (FW-GY)

Gypsum mud was used to drill large thickness of anhydrites (CaSO4). However, the lack of a

high performance fluidizer, limited its use as low density mud, (which normally require high

viscosities and high Gels at 10” and 10’), until the CROME lignosulfonate, used as high

performance fluidizer, appeared.

Gypsum mud is less sensible to solidification due to high temperatures of calcium mud because

of the lower alkalinity value.

If the Pf is kept low (0,1 – 0,4) gypsum-based mud can tolerate temperatures up to 350°F

(180°C). This mud has also a higher level of soluble calcium. The pH range is = 9,5 – 12,5 but it

is preferable to maintain it from 9,5 to 11 so that the hardness remains higher and as a

consequence, the system is more inhibiting. The level of the Ca++ ions is kept 200 – 1200 mg/l.

A gypsum-based mud can tolerate chloride increasing until 100000 mg/l. The maximum

temperature for these muds is 350°F (180°C) and it will depend on the content of Low Gravity

solids (low specific weight).

4.1 Main additives of the gypsum mud

The main additives are similar to the ones in lime-based muds. However, the

concentrations of deflocculants and filtrate reducers are higher. Table 8 list the main

additives in these types of muds.

Table 8 - Main additives for gyp mud

Addi tives Concentration, Kg/m3 Function

Bentonite 60 - 70 Viscosity, fluid loss control

Lignosulfonate 10 - 25 Deflocculant

Gypsum 10 - 25 Inhibition, alkalinity control

Caustic soda pH 9.5 - 11.0 Alkalinity control, inhibition

Potassium hydroxide PH 9.5 - 11 Inhibition

Tannin sulfonate 5 – 10 Deflocculant

Starch 5 – 18 Fluid loss control

PAC 0.7 – 4.5 Viscosity, fluid loss control

Barite Planned density Weighting material




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Bentonite – Prehydrated bentonite is used to increase viscosity and control fluid loss.

Gyp – Provides Ca++

ions to convert formation shales from soda to lime shales for inhibition; a

200 – 1200 g/l level is maintained.

Lignosulfonate – Mixed-metal lignosulfonate has an effective thinning action and also provides

good filtration control, because it disperses clay particles.

Caustic soda/potassium hydroxide - Used to control calcium solubility and stabilise mud

properties.

Tannin sulfonate – An effective thinner for gyp muds. It can be used either as a primary or

secondary deflocculant.

Starch – For fluid loss control. Add a biocide to prevent product fermentation.

Polyanionic cellulose (PAC) – Provides additional fluid loss control. Use LV (low viscosity)

PAC when the yield point should not be increased.

Other additives - Gilsonite, Asphalt, DMS, and cellulose materials for additional actions to

stabilise the borehole.




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4.2 Typical properties of gyp mud

This mud has a higher yield point, gel strength and soluble calcium values compared to

previous lime base muds. Table 9 lists its properties.

Table 9 - Typical properties of gyp mud

Density(kg/l)

Plasticviscosity

(cPs)

YieldPoint(g/100cm2)

10 sec/10min gels

(g/100 cm2

Excessgypsum(

kg/m3) PF pH

Ca++ (mg/l)

API f lu idloss

(cm 3/30min)

1.08 12 - 15 3– 5 1-2 4- 6 30- 400.2 –2.7

9.5 –11.0

600-1200 8 - 12

1.44 15 - 20 1- 7.50 – 2.5

1-7.5

30- 40 2 - 311.0 –

12.0

200-600

6 - 8

4.3 Conversion method / maintenance

The procedure for converting to a gyp mud is similar to that of lime base mud.

CaSO42H2O is used as the calcium source instead of Ca(OH)2. Any water base mud can

be converted to a gyp system. If a lime mud or a mud with a high pH has to be converted,

more water is needed to reduce the solids, while more gypsum is needed to control

alkalinity. In this case, caustic soda is not required. A typical break over, starting from a

slightly treated freshwater mud, can be achieved by first reducing the Marsh viscosity to

30 – 35 sec/qt (qt = ¼ of an American gallon), using service water. The amount of water

will depend on the solids content and previous chemicals used. Add 4 to 8 lb/bbl of

gypsum through the mixer funnel in one or two circulation stages (10 – 20 kg/m 3). At the

same time, add 3 to 6 lb/bbl (8 - 16 kg/m3) of lignosulfonate to control excessive

increases in viscosity (rheology). Caustic soda, lime or both products may be added

(using a chemical barrel), when dosing the gypsum, to keep the pH at 9.5 - 11 (the P f is

usually 0.2 - 0.7 cc of H2SO4 N/50). Caustic soda minimises the viscosity trend during

conversion. The amount needed depends on the planned Pf and the pH of the previous

mud. A break over of 1.5 – 4.5 kg/m3 caustic soda is generally required. If starch is used

to control fluid loss, a biocide should also be added. Foaming may occur during or after

conversion, but this is generally a surface phenomenon and does not cause drilling

problems (though it does affect the pumps). Mechanical action which helps to trap air in

the mud, such as putting surface guns in the mud, should be avoided. If foam is

excessive, use suitable defoamers.




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4.4 Maintenance

This type of mud is easy to maintain. Prehydrated bentonite must be added because the

mud is very hard (Ca++ ions). The system needs an additional polymer treatment to

obtain filtrates with an API value of 8 c.c. or less. Rheology is controlled by adding

lignosulfonate; and alkalinity is controlled using NaOH or KOH. Treatments with

additives, while drilling, depends on the volume drilled, the volume of water added and

density value to maintain.

4.5 Advantages/Disadvantages of gyp muds

These muds have a number of properties similar to lime muds and have a greater

resistance to high temperatures. Table 10 lists the advantages and disadvantages.

Table 10 - Advantages/Disadvantages of gyp mud


Low viscosity and gel strengths.High pressure value (head losses) duringconversion, which may cause boreholedamage.

Tolerant to solids.

Gelation at temperatures above 300°F

(150°c)

Easy to weight up to 2.16 kg/l.

Inhibits the hydration of shales and sandyshales.

Can tolerate cement, anhydrite and salt(up to 50000 mg/l Cl-) contamination

Stabilises the borehole (uniform section)




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4.6 Gyp muds - problems and contamination

Gyp muds can tolerate contamination quite well. Salt and cement do not have any effect

on viscosity and treatments with deflocculants are usually highly effective. Table 11 lists

the most common contaminants, contamination indicators and main strategies to adopt.

This table also includes suggestions for poor quality materials and foaming.

Table 11 - Contaminant/treatments Gyp mud treatments

Contaminant Indicators Treatments

High content of LG solids Mud still values, PV, 10 minutegels, MBT

Dilute more Improve solidsdisposal. Centrifuge

salt/saltwater Increase in chlorides, viscosity,yield point, 10”/10’ gel and fluidloss. Decrease in the Pm, Pf andpH. Increase in density if wateris produced from the well

Treat with deflocculant andsoda for the rheology and withstarch or PAC for fluid loss.Convert to a saturated salt mudor oil base mud if major saltlevels are present.

carbonates/CO2 (noproblems in lime muds)

Increase in Mf , 10-min gel.Decrease in the Pf and pH.

Released CO2 requires Ca(OH)2 treatments which lead to anincrease in fine solids (CaCO3).

Add lime for Pm and KOH for Pf control. Minimise solids content

Poor foam productquality.

Different product packaging.Poorer product performance.Mud characteristics not uniform.Foaming in the pits, air trappedin the mud, pressure pump isnot uniform

Supplier documents. Takesamples and analyse. Work withthe supplier to pinpoint theproblem (do not change thesupplier).Treat with (non-toxic) defoamer.Identify the source of foamingand take action

Gelation at certaintemperatures. Pressure kick-up at the pump(when circulation starts again).Very viscous bottomholecushions and high viscosity atthe flow line

Reduce low gravity solids. Treatwith lignosulfonate BHT<300°F.Treat with polymer deflocculantswhen temperatures are above300°F (150°C).




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5.0 SALT BASED MUD

Salt based-muds contain mainly sodium chloride in variable quantities from 10000 mg/l to

saturation 315000 mg/l of NaCl However, with reference to the Cl- ions contamination which is

colorimetricly titrated, the total chlorides (Calcium, Magnesium, Sodium, Potassium etc..) are

reported in the Drilling Mud Report. For example, 10000 mg/l NaCl stoichiometrically

corresponds to 6000 mg/l of Cl-.

Other terms which can cause confusion are parts per million (ppm) and milligrams per liter (mg/l)

which refer to weight/Volume measured.

Routine titrations developed on the site, are linked to weight per volume =milligrams/litre (mg/l)

or grams/litre (g/l). The effect of salt on drilling mud depends on the pre-existent salt content

and the type and quantity of solids (shale, sand, limestone/chalkstone, barite etc.) Salt is a

contaminant in fresh water muds. Even if in small quantities, it can cause increase in viscosity,

gel strengths and filtration problems. A salt concentration which exceeds 10000 mg/l can create

problems for the control of the mud characteristics. We have salt-based muds when the sodium

chloride exceeds 10000 mg/l (10 g/l) of NaCl. In the following table 3 main types of fluids are

reported.

Salt Saturated Mud, can be prepared or can gradually transform to drill salt levels (danger of

landslide and/or cavings).

Sea water muds they are often linked to the use of sea-water or the drilling of small saline

levels.

Brackish Water muds depend on the type of water available.

Classification of salt muds NaCl, mg/l

Saturated salt (NaCl) 315,000

Seawater 25,000-315,000

Brackish water 10,000-25,000

Note: Seawater or brackish water is used in muds, in offshore or coastal drilling, as it is so

readily available. When using a seawater base mud, the water and mud components must be




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fully analysed. Shales are not hydrated easily when salt or brackish water is used. The pH of

seawater is buffered to prevent variations, using a solubility equilibrium with atmospheric CO2

and limestone sediments (CaCO3). The addition of alkaline materials (NaOH, KOH, Ca(OH)2)

will increase the pH and atmospheric CO2 will be adsorbed by the seawater, stabilising the pH.

As excess carbonates may harm mud properties, the system should have an insoluble lime

excess, to precipitate soluble carbonates. Lime prevents an increase in soluble carbonates and

buffers the pH in the planned range. A seawater mud can be defined as a system with a low lime

content (see lime base muds).

5.1 Saturated salt muds

These muds are used to drill thick halite intervals, to prevent borehole cavings and

collapse, as well as reduce shale and clay dispersion. High viscosities are not frequent,

however low gravity solids should (as usual) be minimised, using mechanical equipment

and/or diluting with saturated saltwater. SS (saturated salt) water contains approximately

13% dissolved solids (salt), so the percentage volume of the mud still should be

multiplied by 1.13 and subtracted from 100 to determine the real value of solids in the

mud (solids + salt). The volume of various salinities can be calculated in the same way

(the salt during and after distillation remains in the solid residue). The Cl content in SS

mud is 192000 mg/l (315000 mg/l of NaCl). The well temperature increases as the depth

increases, and likewise salt solubility increases with depth, so a mud is saturated with

salt at the surface but not at bottomhole. This can cause wash outs in saline sections,

because the mud is more soluble at bottomhole. pH control varies a great deal and is not

a fundamental function of the system.

Many low solid muds with attapulgite and starch are formulated without caustic soda. In

other areas, it is common practice to keep the pH from 11 to 11.5 by adding NaOH. SS

mud needs more soda to maintain a high pH. Maintaining the pH at 11 to 11.5 has

numerous benefits:

Thinners are more effective

Corrosion is reduced

Lower amounts of additives are needed to decrease fluid loss, when solubility is

reduced by Ca++ and Mg++

Foaming is minimised




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Mud is usually more stable

SS mud normally contains soluble calcium from the drilled formation and type of water

used. The sodium in salt also provides further Ca++, when it replaces this mineral in

drilled shales. The presence of Ca++ ions does not usually have an effect on mud, except

when the pH goes up to 12, making it harder to control fluid loss. Foaming will also occur,

though this is not a major problem if it is on the surface. The intensity of foaming can be

reduced by adding Ca(OH)2 to increase the Pm, and a defoamer may be necessary. If

Mg++ sensitive additives are not used, a SS mud is less sensitive to foaming, maintaining

a pH from 9.0 to 9.5. The temperature threshold is 250°F (120°C) and temperatures

around this limit make fluid loss control more difficult. The hardness of Ca++ e Mg++ does

not affect fluid loss control when starch is used, while the hardness value should be kept

below 400 mg/l when using PAC.

5.1.1 Main additives of saturated salt muds

Saturated salt muds are not usually expensive and contain few additives. This

system is not complex, as few additives are effective in these kinds of mud.

Table 12 lists these additives, their function and concentrations.

Table 12 - Main additives of saturated salt muds

Additive Concentration, kg/m3 Function

Prehydrated bentonite 30 - 70 Viscosity, fluid loss control

Starch 10 - 20 Fluid loss control

Caustic soda pH 9.0 - 11.0 Alkalinity control

Soda Ash (Na2CO3) 3 - 8 Ca

++

removal

PAC 0.7 - 4.2 Fluid loss control, viscosity

Salt (NaCl) 350 Inhibition Weighting

Bentonite – Viscosity can be controlled with bentonite prehydrated in service

water. Positive sodium ions (from salt) act on the hydrated bentonite, causing

flocculation and producing viscosity, even with a minimum clay percentage. The

considerable effect of the Na+ ions over time on the surface of the clay particles

causes adsorbed water to be released, producing free water and a rapid drop in




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viscosity (break over). This decrease in viscosity can be slowed down by adding

prehydrated bentonite treated with caustic soda and lignosulfonate. Prehydrated

bentonite must be continually added to maintain viscosity at the required value.

Small amounts of additives (starch and PAC) should be used to control filtration

and the size and distribution of flocculated particles will help control this

parameter. Attapulgite (salt gel) can be used instead of bentonite to make the

mud viscous when freshwater is not available.

At tapulgi te – This is a type of clay which produces viscosity (yield) when it is

agitated using a funnel with a high pump pressure (shear) rather than being

hydrated. It is commonly used to make fluids viscous and is not affected by salt

or hardness. Because of its specular form, attapulgite does not control filtration.

The standard concentration for this product is 30 – 60 kg/m3.

Starch – This is the most common additive for controlling fluid loss. It is not

affected by high hardness levels (2000-3000 mg/l) and does not affect rheology

in particular, at least until drilled incorporated solids are at an acceptable level.

Starch is thermally stable up to 250°F (120°C). It does not usually ferment until

the system is salt saturated or the pH goes above 11.5 (hydrolysis).

Caustic soda – This is used to check alkalinity; the pH of 9.0 – 11.0 minimises

the corrosive effect on the drill string and casing, and prevents starch

fermenting. SS muds need large amounts of caustic soda to keep the pH high,

as the sodium ion and clay exchange releases hydrogen ions that lower the pH.

Soda Ash – (Na2CO3) - Soda ash is added to precipitate Ca++ and Mg++, and

make sensitive additives such as PAC more effective. The soda ash is added in

relation to the amount of soluble calcium in the system: Na2CO3 + CaSO4 =

Na2SO4 + CaCO3 (precipitates). The total hardness must be accuratelydetermined to prevent excess soda ash. Large amounts of soda ash in the mud

lead to high gel strengths. (10” – 10’ gel). Adding soda ash is counterproductive

when using hard brines.

Polyanionic cellulose (PAC) - Viscosity and fluid loss control. PAC is more

effective when the content of low density solids is below 6 vol. % and the

hardness is less than 400 mg/l. LV (low viscosity) PAC is an excellent solution

for fluid loss control without increasing viscosity.




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5.1.2 Typical property of SS (saturated salt) muds

These systems have high yield points and gels. Although high, 10”/10’ gels are

normally fragile. Table 13 lists the properties for a non weighted and weighted

mud.

Table 13 - Typical properties o f SS (Saturated Salt) muds

Density(Kg/l)

Plasticviscosity

(cPs)

Yield Point(g/100/cm 2)

10 sec/10 min gels(g/100/ cm2)

API flu idloss

(cm3/30min)

1.26 8 - 12 6 - 8 3 -4 4 - 6 8 - 12

1.56 15 - 20 7 - 9 4 - 5 5 - 7 6 - 8

5.1.3 Conversion system/maintenance

Freshwater muds are normally used down to the top of the salt section; the

muds must then be converted before drilling this level. Some operators drill salt

levels using freshwater muds, saturating the mud with the drilled salt; this is a

serious mistake as it can cause major caving and various other operating

problems (for example a circulating mud volume of 200 m3

which should

dissolve formation salt and increase up to 30000 mg/l, corresponds to 6000 kg

of dissolved salt). Problems do not usually occur during the conversion stage

(conversion recommended in a cased hole); at least two circulation stages

should take place before converting the system. The outlet of the mixer funnel

should not be close to the inlet, to prevent incorporation caused by foam and air

(pump problems). When converting a freshwater mud to a saturated salt mud,

as much old mud as possible should be used; this provides viscosity and density

and will reduce the amount of attapulgite and prehydrated bentonite needed (as

well as reduce mud disposal costs).

Adding salt to a freshwater mud leads to significant flocculation and high

viscosity values; 30 – 50% of freshwater is nearly always needed in the old mud,

depending on the concentration of solids to add (via the mixer funnel).

Approximately 350 kg/m3 of NaCl are needed to saturate freshwater (add from

the mixer funnel) and this increases the water volume by 15%. The density of




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SS mud will range from 1.20 – 1.25 kg/l without adding weighting material.

Starch is normally used as a fluid loss additive - added through the mixer funnel

– and as an additional thickener. 10 – 15 kg/m3 of starch will produce a 6 c.c.

(30’) fluid loss. If a greater density is required, barite (BaSO4) is usually selected.

When weighting the mud, lignosulfonate and water to wet the barite should be

added, to avoid increasing the viscosity and gels. Deflocculant is normally added

to the caustic soda (to dissolve the mud more effectively).

5.1.4 Maintenance

SS mud is often treated while drilling with water and small amounts of thickener

and fluid loss additive. Viscosity can be increased by opting for prehydrated

bentonite or attapulgite, and decreased with saturated saltwater.

Lignosulfonates can be used as deflocculants, but polymer thinners have proven

to be more effective at high temperatures and do not require caustic soda.

5.1.5 Advantages and disadvantages of SS (Saturated Salt) muds

SS mud is fairly easy to maintain and is highly resistant to contamination.

Table 14 lists some of its properties.

Table 14 - Advantages and Disadvantages o f SS (Saturated Salt) muds


Inhibiting agent (shales)Fluid loss control is more difficult (starch orpolymers)

Resistant to cement, anhydrite,salt and saltwater contamination.

Tends to foam and trap air.

Low solids content, dissolved salt

increases density.Corrosive with salinity below saturation.

Good borehole cleaningproperties (cuttings lifting)

Maximum temperature = 280°F (140°C).

Stabilising effect on an open hole.

5.1.6 Problems and contamination of SS (Saturated Salt) muds

The system is usually very resistant. Table 15 outlines some aspects requiring

treatment strategies.




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Table 15 - Contaminant /treatments for SS muds

Contaminant Indicators Treatments

High solidscontent

Increase in PV, YP, gels,viscosity, fluid loss, MBT.Foam, trapped air

Dilute, centrifuge andimprove solids removal

Salt/saltwater

Decrease in PV, YP, gels.Increase in fluid loss.Possible decrease inchlorides and density.

Increase the density due tothe ingress of saltwater. Addsalt to saturate, starch tocontrol fluid loss andprehydrated bentonite orattapulgite for viscosity.


Material performance not upto standard. Differentpackaging.

Request data on thepackaging process. Takesamples and tests forefficiency + carry outcomplete analyses.

5.2 Seawater muds

These muds are often formulated from freshwater or FW-GE muds, which have few

solids, low densities, a minimum quantity of chemical additives, low viscosities and a high

filtrate content (spud mud). SW-LS mud can be specifically prepared to drill troublesome

shales; it is also used as an inhibiting mud to decrease the dispersion of drilled solids and

control viscosity increases. The salinity (NaCl) varies from 25000 mg/l up to saturation.

Specially manufactured brines are used in workover and/or completion operations.

Seawater is often used to make up and maintain these muds. The hydration properties of

clays or shales are partially reduced. The muds are also used as fluids with a low solids

content, to control low pressures (depletion) in workovers or completions. Seawater is

often employed in offshore operations, obviously because it is so readily available and

typically has an NaCl content of 35000 mg/l and total hardness of 1500 – 2500 mg/l. The

maximum operating temperature for seawater is 280 – 300°F (approximately 150°C), and

depends mainly on the clay content.

5.2.1 Main additives of SW-LS (salt water) muds

These muds are more complex than SS muds, and their chloride variation

makes it harder to select additives. Table 16 lists these materials and their

properties:




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Table 16 - Main additives of SW-LS (saltwater) muds

Addi tives Concentration

kg/m3

Function

Prehydrated bentonite 40 - 70Viscosity, fluid loss

control

Caustic soda/potassiumhydroxide

1.5 – 4.5 Alkalinity/corrosion

control


PAC 1.5 – 3.0 Fluid loss control

Lignosulfonate 9 – 18 Deflocculant

Lignite 5 - 10 HP/HT fluid loss control

Bentonite – Prehydrated bentonite controls viscosity fairly well. The Na+

ions

(from salt) act as flocculants on the hydrated clays, producing viscosity with a

minimum amount of added clay. The action of the Na+ ions on hydrated

bentonite drive back the hydration water from the shale levels, producing free

water and decreases in viscosity. This process can be stopped by adding

prehydrated bentonite treated with caustic soda and lignosulfonate. Adding

prehydrated bentonite is often necessary to maintain the right viscosity, and fluid

loss additive should also be added. Attapulgite can be used as a thickener whenfreshwater is not available.

At tapulgi te – Attapulgite is added to increase viscosity, however prehydrated

bentonite or polymers are preferable. Attapulgite is not affected by chlorides or

hardness. It does not reduce fluid loss and its normal concentration is 30 – 60

kg/m3. Local environmental regulations prohibit the use of this material in some

areas.

Starch – Starch is used for fluid loss control; 9 kg/m3 can approximately produce

an API of 12 - 15 c.c./30 min and 20 kg/m3. A biocide should be added before

treating with starch, keeping to the recommended concentration. Starches

incorporating biocides are available on the market (these are more expensive).

Caustic soda or potassium hydroxide - NaOH and KOH – These are used to

control the pH and alkalinity and to offset corrosion. Keeping the pH from 9 to 11

considerably improves the effectiveness of lignosulfonates.

Polyanionic cellulose/PAC – PAC is used to control filtration. Increases in

viscosity and 10”/10’ gels occur when the product is added in the sump pit,




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however normal values are restored after bottomhole circulation. Hardness

should be kept below 400 mg/l. Low viscosity PAC is the most reliable mud

product to use if only filtration control is required.

Lignosulfonate – This chemical is the most effective thinner for SW – LS mud

and helps to control fluid loss.

Lignite – Lignite is used to improve HP/HT fluid loss, but is not ideal in this kind

of mud. The lignite should preferably be dissolved beforehand with freshwater,

and have a pH of 10.5 - 11

Soda Ash – Soda ash is used to keep the Ca++ content below 400 mg/l (this

optimises the performance of many fluid loss additives).

Corrosion inhibitor - Corrosion is very severe compared with freshwater or

saturated salt muds; keeping the pH at high values is usually sufficient, however

corrosion inhibitors such as film-forming amines are often used.

Biocides – Biocides are used to prevent starch and PAC fermenting. Many

different types are available on the market and tests have shown that

isothiazoline base biocides are the most effective. Biocides are not necessary if

the pH is above 11.5.

Defoamers – Defoamers are necessary (pilot tests are recommended). Another

widely used AS – LS system is “seawater-lime spud mud”, with prehydrated

bentonite, hydrated lime and seawater; starch or PAC (regular or low viscosity)

can be used to control fluid loss; if both substances are used, the ratio should be

5:1. (sacks: unit of measurement corresponding to 50 pounds). The mud base

comprises 90 – 120 kg/m3 of prehydrated bentonite (freshwater), with added

seawater and lime (3-12 kg/m3) to control the viscosity. The Ca++ ions in the lime

replace the sodium and inhibit formation shale hydration. Lime reduces bit and

stabiliser balling. If bit balling does occur, increase the Pm (mud alkalinity) to 5

c.c. or more with hydrated lime, to try and clean the bit and stabilisers. When

drilling gumbo shales, the pH must be kept between 9 and 10. If shales are

troublesome (highly dispersive), KOH should be used instead of Ca(OH)2 NaOH

must not be used.




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5.2.2 Typical properties of AS-LS fluids

These fluids have a salinity of 25000 mg/l +, as shown below:

Table 17 - Typical properties of AS-LS muds (Salt water Muds)

Density(kg/l)

Plasticviscosity

(cPs)

YieldPoint(g/100cm 2)

10 sec/10 mingels (g/100 cm2)

Chlorides (NaCl)mg/l

API fluidloss

(cm3/30min)

1.10 16- 18 5 - 7 1 - 2 3 - 4 25,000 – 300,000 8 - 12

1.45 22 - 24 6 - 8 1 - 2 3 - 4 25,000 – 300,000 6 - 8

5.2.3 Conversion sys tem

AS-LS muds usually have the same problems as converting and using saturated

salt muds. Many problems with this kind of mud are related to the VERY HIGH

hardness of seawater. Carbonate magnesium ions are fairly soluble, but as soda

ash is used to reduce the total hardness (Ca++ e Mg++) of seawater, the ions are

not very effective. Magnesium is insoluble at a pH of 10, so NaOH can be

effective at removing it. Additional lime treatments provide the necessary

content of OH ions. Soda ash is used to precipitate Ca++ ions and obtain better

mud characteristics. The Ca++ ions do not have a severe contaminating effect,

but should be kept below 400 mg/l. A few simple guidelines should be followed

when converting to an AS-LS mud. Firstly, the solids content must be reduced to

acceptable values. If the content is too high, super screens, centrifuges,

desanders and desilters should be used, as well as available water for dilution. If

viscosity is too low, add prehydrated bentonite and treat with lignosulfonate and

caustic soda. After treating the rheological properties, PAC is added to control

fluid loss; opt for a biocide if using starch and when the pH is below 11.5.

5.2.4 Maintenance

In this system the content of solids should be kept within planned limits

(depending on the density); the muds can tolerate the incorporation of solids

fairly well, but are more cost-effective when they contains less than 6 vol. % of

low gravity solids. Prehydrated bentonite is added depending on MBT (methyl

blue test) results. The clay content should be reduced in proportion to the

increase in density to prevent bottomhole gelation problems.




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5.2.5 Advantages and disadvantages of AS-LS mud

Table 18 lists the advantages and disadvantages of this type of fluid

Table 18 - Advantages and Disadvantages of AS-LS mud

Advantages Disadvantages

Inhibiting material (formation shale)Increase in additives used, because ofa poorer performance

Less freshwater used Filtration control difficult

Fewer negative effects of anhydrite, cement,salt and formation saltwater contaminants

Prehydrated bentonite necessary

5.2.6 Problems and contamination (AS/LS) – Salt water Muds

Contamination is more frequent than SS muds, because AS/LS mud has more

additives and the salinity range and hardness affect fluid performance.

Treatment strategies are listed in table 19.

Table 19 - Contaminant / Treatments


High solidscontent

Increase in the % of solids, PV,YP, gel, viscous mud cushionsfrom the bottomhole.

Dilute considerably, use centrifugesand other equipment to removesolids

Salt/saltwater Increase in YP, fluid loss andchlorides. Decrease in densityand pH in the case of formationwater

Increase the density if the waterinvades the formations. Treat with athinner for rheology. Control fluidloss with starch or PAC.

Poor qualityproduct

Increase in amounts required;packaging different fromprevious supplies.

Find out about the manufacturingprocess. Take samples and analysethe product.

Cement Increase in PV, YP, pH, Pm, Pf ,gel and fluid loss. Increase inCa++.

Add sodium bicarbonate or SAPP.Dilute with water (fresh or seawater).Treat with thinner, starch or PAC(rheology and fluid loss).

Carbonates Increase in gels, YP, wrongrheology. Very viscousbottomhole cushions.

Increase the pH to 10.7 or higher, toconvert bicarbonate into carbonate;treat with lime or gypsum to removethe carbonates.




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5.3 Brackish water muds

Brackish waters are used to make up mud in many areas, for cost reasons or because

freshwater is not readily available. Brackish waters have a salinity (NaCl) ranging from

10000 to 15000 mg/l and are used in areas are close to the sea and/or in marshy zones.

5.3.1 Main addi tives

These are basically the same as mud and seawater and are easier to use (lower

salinity). As brackish water contains bacteria and organic products, more

chemicals are consumed (due to bacterial degradation).

Table 20 - Main addit ives of brackish water muds

Addi ti ves Concentration, Kg/m3 Function

Prehydrated bentonite 40 - 70Viscosity and fluid loss

control

Caustic soda / potassiumhydroxide

1.5 – 4.5 Pf and corrosion control


PAC 1.5 – 3 Fluid loss control

Lignosulfonate 9 – 18 Deflocculant


Bentonite – Bentonite is used to control viscosity and fluid loss; as usual, it

must be prehydrated in freshwater (to optimise performance). The high content

of Na+ ions means that the prehydrated clay particles release adsorbed water

(free water) and viscosity decreases rapidly (break over). This quick decrease in

viscosity can be controlled by adding prehydrated bentonite, lignosulfonate and

caustic soda. Prehydrated bentonite should be added at a continual rate to

control viscosity. Attapulgite can be used instead as a thickener, when

freshwater for bentonite is not available.

At tapulgi te – Unlike prehydrated bentonite, Attapulgite controls viscosity, but

not fluid loss. Attapulgite is not affected by increases in chloride or water

hardness. Because of its brush-heap structure, it cannot control fluid loss. A

concentration of 30 – 60 kg/m3 is normally used.

Caustic soda – Caustic soda is used to keep a pH of 9 – 11 in the muds.




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Starch – Starch controls fluid loss. A biocide must be added before starch

treatments, and planned concentration values should be maintained.

Polyanionic cellulose (PAC) – PAC controls fluid loss, but a Ca++ e Mg++

hardness below 400 mg/l is required.

Lignosulfonate- (LS.) Lignosulfonate is the best thinner for these muds and

helps to control fluid loss.

Lign ite (xp-20, cc-16) – Lignite is used to control HP/HT (High Pressure - High

Temperature) fluid loss, but is not effective as a thinner, depending on the type

of brackish water (chloride content and hardness).

Soda ash – Soda ash is used to precipitate Ca++ in brackish water. This

treatment improves the hydration properties of clays and makes fluid loss

additives more effective.

Corrosion inhibitor – Corrosion in brackish water muds, compared to FW-LS

muds, is greater, but if pH values are high (see above) good results can be

achieved. An oxygen scavenger can also be used. Lignite and lignosulfonate will

also act as an oxygen scavenger if added in sufficient amounts.

Typical properties of brackish water muds:

Table 21 - Typical properties of brackish water muds

Density(kg/l)

Plasticviscosity

(cPs)

YieldPoint(g/100cm 2)

10 sec/10 mingels (g/100

cm2) pH

Chloridesmg/l

APIfluidloss

(cm 3/30min)

1.10 16 4 - 5 1 - 2 2 - 5 10.5 – 11 10,000 - 25,000 6 - 10

1.45 22 6 - 8 1 - 1,5 2 - 4 10.5 - 11 10,000 - 25,000 6 - 8




Page 34 of 58



These systems are not converted, but a “new” mud is made up, as only brackish

water is available (freshwater not readily available).

5.3.3 Maintenance

Control the content of solids and keep to planned values. These muds can

tolerate drilled solids quite well, but the concentration of low gravity solids (and

shales) must be below 6 vol. % (mud still). Analyse methyl blue testing (in clay

and bentonite) to evaluate if prehydrated bentonite should be added (if

freshwater is available). As usual, the clay content must be decreased when the

weight has to be increased (use wettability water to minimise bottomhole

gelation. For barite, for example, this is equal to 200-300 litres/ton).

5.3.4 Advantages and disadvantages of brackish water muds

Table 22 lists the characteristics/properties of brackish water muds compared to

freshwater muds.

Table 22 - Advantages and disadvantages of brackish water muds


Moderate inhibitorChemical products increase (becausethey are less effective).

Less freshwater needed (brackishwater is used instead).

Prehydrated bentonite (in freshwater)needed.




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5.3.5 Problems and contamination of brackish water muds

Contamination and problems are basically the same as seawater systems.

Table 23 summarises problems and contamination.

Table 23 - Contaminants / Treatments of brackish water muds


High solid content

Increase in the % of solids, PV, YP,gel. Very viscous bottomholecushions after trips (even 10-20

hours).

Dilute considerably, centrifugeconstantly and improveeffectiveness with super

screens, desanders, mudcleaners, etc.

Salt/saltwater(flow)

Increase in YP, fluid loss andchlorides. Decrease in density(production of formation water)

Weight if the well is producing!Control the rheology withthinners and reduce fluid losswith starch or PAC.

Poor quality

product

Increase in frequency oftreatments. Different packaging.

Check the manufacturing stagewith the producer, takesamples and analyse looseproducts. Check tankers

transporting products. Checkloose products delivered byship (water/diesel fuel,barite/cement)

Cement

Increase in PV, YP, pH, Pm, Pf , andfluid loss. Possible increase in Ca++

Treat with bicarbonate(NaHCO3) or SAPP to stopcement contamination. Uselignosulfonate, starch and PACto control rheology (PV,YP,gel) and fluid loss.

Carbonates

Increase in gels, YP, unreliableviscosity meter values. Bottomholemud cushions very viscous aftertrips.

Increase the pH to 10.7 toconvert bicarbonates tocarbonates. Treat with lime orgypsum to precipitate thecarbonates.




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6.0 POTASSIUM MUDS (FW/SW-KC)

Potassium-based muds are employed in those areas where inhibition is required in order to limit

the chemical alteration (hydratability) of the clays layers (borehole restriction, caving and

landslides) – AGIP codifies/defines other two types of muds treated with potassium:

• FW-PK: AGPAK Mud with KCMC and KOH;

• FW/SW-MR: It uses mainly KOH, Ca(OH)2, MOR-REX as additives.

The potassium performance is based on the transformation of the “Sensible” clay layer from

sodium to potassium base (SMECTITE). K+ ions compared to Ca++ or other inhibited ions. K +

ions concentrate especially on the surfaces of clay particles reducing the hydration of clays very

much. The best performance of FW/SW-KC muds is on clays with high percentages of Smectite

or thin clay levels (interlayered) in the total section of clay. Superficial clays with large quantities

of Montmorillonite also always hydrate in potassium-based fluids. As a consequence, the high

costs of FW/SW-KC muds are not justified. The interaction between potassium and clay particles

is caused by two effects:

• The size of the ions;

• The energy of hydratability.

The K+ diameter is 2,66 A° (angstrom) very near to the available distance of 2,8 A° in the gaps of

the clay structure. A cation slightly smaller than 2,8 A° is preferable for the crystalline

compaction. When there is montmorillonite, the potassium replaces sodium and calcium andproduces a structure more stable and less hydratable. When illites are present, the potassium

replaces each exchangeable cation (impurities) in the structure. The potential as a further

exchange-base is reduced, after the replacement with the K+ and clays are more stable. On the

particles (thin layers) of the clays, the K+ operates both on illite and montmorillonite and reduces

the quantity of hydration water which exsists in origin. Sometimes the K+ cations stabilize clays

with high percentage of illite or Illite/Smectite.

The best performance of the K+ cation is on clays with high quantities of Illites stratified in the

whole clay section.




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This is true only when the clay is not extremely “compact” with a matrix which contains several

microfaults. In these cases, a small percentage of the overall hydratability potential can be

sufficient to cause problems during the drilling. The filtrate invasion in the microfaults helps the

acceleration of clays swelling. Also a reduction of 80% in the hydration could not be sufficient to

stabilize the drilled formation. These types of clays (argillites) have been successfully drilled by

means of potassium-based muds with Asphaltites or Asphaltenes. These muds have been used

to drill strong illite clays. In theory, this type of clays should be analyzed and studied before

planning a programme.

Clays containing considerable percentages of montmorillonnite will swell to some extent with

potassium-based mud. The degree of inhibition required by these clays cannot be sufficient to

justify the cost of K+ muds. In particular when this type of clays is met at low depth (GUMBO).

With clays, large quantities of potassium are necessary for the ionic exchange especially in deep

borehole section (for instance 15” -23”) and high drilling rate. The testing of the clays to be

drilled should be done to decide to which extent the inhibition degree justifies the costs. If the

cutting s dispersion instead of the erosion of the borehole wall is the most important factor, these

K+ mud can reduce the problem significantly.

However, the advantage of the laboratory test before the use of this inhibiting mud in an area

with clay problems, must not be overestimated. As cores of clay strata are available from an Off

Set well it will be necessary to develop a whole series of laboratory test, x-ray analysis,

isothermic absorption, hydratability and dispersibility. If cores are not available, cuttings from a

previous well of that area can be used to develop these laboratory works. Without any kind of

material, an estimation of equivalent clay can be done by: depth of the clay section, geological

correlation and available electrical logs.

Exchange reactions with cations in clay layers, cuttings, borehole surfaces and bentonite used to

prepare the mud reduces the K+ content during the drilling. Therefore, an adequate excess of

potassium in the system must be maintained to constantly guarantee an efficient degree of

inhibition.

The theory of the ionic inhibition of the several types of muds (FW-LI, FW-GY) is essentially the

same. However, the selection of a particular mud to be chosen, depends on these factors such

as: preference of the operators, planned density of the mud, types of formations to be drilled

(exploration or development well) maximum temperatures, filtrate required, rig equipment,

availability of the equipment to control the solids drilled. The importance of an appropriate

control of the solids must not be underestimated. If the level of K+ drops under the programmed




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level, all clays will hydrate causing problems of mud maintenance and borehole stability. With

insufficient and wrongly timed treatments the advantages of K+ can be compromised. There are

three types of potassium-based muds.

• KCl-Polymers (KCl-PHPA)

• KOH-Lignite

• KOH-Lime

6.1 KCL-POLYMERS (KCL-PHPA) = FW/SW-KC

These muds have been developed for the borehole stability. They limit the dispersion of

“cuttings” in the mud. When correctly formulated, advantages such as minor damage of

the “mineralized” formations and permeability encourage the use of this fluid. FW/SW-KC

mud uses the KCL (potassium chloride) at extremely variable concentrations 3% to 15%

in weight and a wide range of polymers as well. For a cheaper system, it is necessary to

maintain a low content of solids and the availability of efficient solids control equipment

(centrifuges, desilter, mud cleaner, etc..).

6.1.1 Main additives for FW/SW-KC mud

Mud with KCl and Polymers with low KCl concentration (3-5% in weight) and low density

are easy to maintain when harder formations are drilled. When density increase is

required, the mud composition is more complicated and the maintenance more difficult.

The materials and their concentration are reported in table 24. PPG (Propylenic Glycole)

is not listed. However, its popularity as an inhibitor promoter is increasing. This low

molecular weight polymer, is used for concentrations up to 40-45 kg/m3.




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Table 24 - Main additives for FW/SW-KC mud

Addi ti ve Concentration, Kg/m3 Function

Prehydrated bentonite 15 - 45 Viscosity and fluid loss control

Potassium chloride 15 - 170 K+ inhibiting source

Potassium hydroxide 0.7 – 2Provides K+ ions and controls

alkalinity


PAC 1.5 – 3 Fluid loss control

Lignosulfonate 8 - 16- Thinner


Bentonite – Bentonite is prehydrated with freshwater and used to increase mud viscosity and

partially control fluid loss. Bentonite dehydrates if salinity levels are high (KCl= 10-15%), and

viscosity values drop, so it must be frequently re-hydrated.

Addi tive-free bentoni te (API standards) - When available, this type of bentonite is

recommended as it is more effective and smaller amounts of other additives are needed. On

average, 15 - 45 kg/m3

of prehydrated bentonite are required to control viscosity and the API

fluid loss. Dry bentonite (added from a mixer funnel) does not produce suitable ambient viscosity

with high salinity and hardness values, but small amounts (3-9 kg/m3) can increase the particle

solids distribution (P.S.D) and improve filtration values, particularly at temperatures of 225 -

275°F (107-135°C)

Potassium chloride (KCl) – Potassium chloride is used to inhibit shale hydration. The amount

of KCl needed for a given area – to develop an inhibiting action – is hard to determine. Older

formations with shales that are not easy to hydrate require 3.5% of KCl in weight, while more

recent shales which are easier to hydrate require up to 15% salt in weight. Other sources of K+

such as KNO3, K2CO3, or K4P2O7 can be used if environmental restrictions on chlorides apply.

Potassium hydroxide – Potassium hydroxide is added to adjust the pH value in KCl systems,

instead of caustic soda which has a destabilising effect with Na+

ions. The pH is usually kept

from 9.5 – 10.5, as higher pH values have a negative effect on polymer adsorption. In some

cases, such as coring, pH values of 7 – 8 are recommended (lower values damage the shaly

parts of the core).






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6.2 KCL - Polimers

6.2.1 Preparation

KCl-polymer Muds should be prepared without using old mud as follows:

• Treat service water with 0.7 kg/m3 of soda ash (Na2CO3) and 0.35 kg/m3

of KOH, to reduce calcium and magnesium. If service water contains

magnesium, potassium hydroxide is not necessary.

• Prehydrate bentonite in freshwater.

• When adding polymers, begin with the thickening polymer. If viscosity

increases too much (make up pump problems), treat with KCL, as the

salt will reduce the viscosity. Adjust the pH to 9.0 – 9.5. When viscosity

has been reduced, add the remaining polymers.

• Add barite and agitate the mud as necessary. Check the viscosity and

density at regular intervals during agitation, until viscosity values are

correct and stable. If decantation problems occur (barite), add polymer

thickeners and prehydrated bentonite.

Table 26 lists the typical concentrations of different density muds. As these

systems are made up NEW, one concentration is given for each mud.

Table 25 - Typical properties of FW/SW-KC muds

Density(kg/l)

Plasticviscosity

(cPs)

Yield Point(g/100 cm2)

10 sec/10 min gels(g/100cm2)

API f lu id loss(cm 3/30 min)

1.10 -1.20 12 - 25 5 - 10 3 - 4 4 - 10 10 - 12

1.20 -1.32 15 - 25 5 - 10 1 - 4 4 - 8 5 - 8

1.32 -1.44 15 - 35 3 - 8 1 - 4 2 - 8 3 - 6

1.44 -1.68 20 - 40 3 - 8 1 - 3 2 - 8 2 - 4

1.68 -1.92 25 - 45 3 - 8 1 - 3 2 - 6 2 - 4

1.92 -2.16 30 - 45 3 - 4 1 - 3 2 - 5 1 - 3




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Table 26 - Typical concentrations for FW/SW-KC muds (KCl-Polymer Muds)

CONCENTRATION, Kg/m3

Densitykg/l

Water*(l)

Bentonite Causticpotash

KCl XanthanGumLV

PACReg

Starch Barite

Ac ti vePHPA

1.0 937 40 0.7 100 2.8 4 12 0 2.8

1.4 812 40 0.7 90 2.8 4 12 350 2.8

1.68 750 36 1.4 85 2.2 4 12 670 2.8

1.92 718 28 1.4 80 1.5 3 9 830 2.8

2.16 687 20 1.4 75 1.5 3 9 1080 2.8

A: Li tres o f water per cubic metre o f mud

Note: The values in the table are only guidelines. Pilot tests on finalised

formulas should be carried out before making up the mud at the rig.

6.2.2 Maintenance

FW-KC (KCl-polymer) mud is maintained with a suitable polymer concentration

and by keeping low gravity solids below 6 vol. %. PAC and PHPA should be

added continually during drilling operations to keep mud in good conditions.

PHPA partially degrades as it flows through the choke bits (a very high rate of

100/150 m/sec and a maximum temperature), and a new product should be

added to maintain a suitable concentration to encapsulate and prevent the

hydration of shale cuttings. Monitoring the trend of cuttings and MBT analysis

(content in active clay) will indicate when extra PAC or more PHPA is needed.

Starch can be used to further control fluid loss. Both PHPA and PAC areadsorbed by solids (and by clay in particular), but only PHPA can inhibit clay

dispersion (a process known as encapsulation). As PHPA and PAC are

adsorbed by the cuttings and eliminated by shale shakers, centrifuges,

desanders and desilters, or because cuttings are still circulating and adsorbing

polymers from the mud, they need to be removed. Mud should be diluted more

and contain more PHPA and PAC to encapsulate solids, and this leads to high

costs. Like most other muds, low gravity solids should be kept below 6 vol. %.

To offset PHPA losses caused by adsorption and degradation through the bit




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(chokes), 2.80 kg/m3 of PHPA should be added for every 110 feet drilled (33.5

m).

6.2.3 Problems

Most problems are associated with a high solids content, cement contamination

and poor quality products. If the content of solids is high, solids removal

equipment should be quickly checked. Milling cement can have a major impact

on the properties of KCl-polymer mud. Mud viscosity will increase if the solids

content is high, whereas it will decrease if the solids content is low and the mud

contains reactive solids. Small amounts of cement can be treated with sodium

bicarbonate (NaHCO3); if there is more cement, sodium bicarbonate and lignite

should be added to increase the pH. Table 26 b lists various contaminants, their

effects on properties and treatments.

Table 26 (b) - Contaminants / Treatments for FW/SW-KC muds


High solidscontent

Increase in solids, PV, YP, gels.Viscous bottomhole cushions

after trips.

Dilute considerably andimprove the performance of

solids control equipment.

Cement

Increase in Pm, Pf , pH, YP, fluidloss and Marsh viscosity

Dilute less. Treat withbicarbonate and/or SAPP.When the rheology stabilises,treat with starch or PAC toreduce fluid loss.


Different product packaging.More product used.

Product documents from thesupplier (quality history). Takesamples and analyse.

Saltwater/salt

Increase in chlorides, Marshviscosity, YP, gels and fluid loss

Increase density if the well isproducing saltwater. Convert

to a saturated salt mud, ifmajor salt levels are present

Gypsum/anhydrite

Increase in Ca++, YP, gels, fluidloss. Decrease in pH, Pm, Pf .

Treat with SAPP, soda ash orpotassium carbonate. Use athinner as necessary.

Carbonates

Increase in Mf, YP, gels.Decrease in pH, Pm, Pf . Viscousbottomhole cushions, and highviscosities also at the flow line.

GGT analysis. Increase thepH to >10,7 with KOH. Addlime and make sure solidsvalues are within acceptablelimits.




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6.3 KOH-lign ite (system)

In areas where a high chloride (Cl) concentration can be problematic (electric logs,

environmental regulations on waste disposal and relative costs, etc.), a “KOH – Lignite”

system can be used instead. Potassium and lignite muds have inhibiting properties and

are flexible enough to be made up for drilling requirements. Polymers can be used to

control both viscosity and fluid loss. Lignosulfonates are used if and when extra thinning

action is required. The planned pH will be maintained with KOH and the addition of

potassium lignite for more potassium ions. KOH-lignite fluid is defined as a low pH,

partially inhibiting system. The pH is kept at around 10. This system cannot tolerate high

chloride and calcium levels. The maximum limit for chlorides is 5000 mg/l (cl), while the

maximum limit for Ca++

ions is 250 mg/l. KOH-lignite mud is stable up to 400°F (205°C).

6.3.1 Main additives of KOH-lign ite Muds

Table 27 lists the main additives of this system; the composition and use of the

mud is very similar to freshwater lignite base systems, replacing caustic soda

(NaOH) with potassium hydroxide (KOH) to control pH and alkalinity.

Table 27 - Main additives of KOH-lignite mud

Addi tives Concentrations, kg/m3 Function

Bentonite 45 - 75Viscosity and fluid loss

control.

Lignite 15 - 23 Thinner and fluid loss control.

Potassiumhydroxide

1,5 – 4,5 Alkalinity and K+ control.

PAC/CMC 1,5 – 3Fluid loss and viscosity

control.

Barite As necessary, depending

on density Weighting material.

Bentonite – Bentonite is used to control viscosity and fluid loss. It can be added

dry (through a mixer funnel) or prehydrated in a separate pit and added at

regular intervals to the circulating system.

Lignite – Lignite is used to reduce fluid loss and make the mud fluid. It is not a

strong deflocculant and is not very effective if there is a high, content of low

gravity solids.




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Potassium hydroxide (KOH) – Potassium hydroxide controls alkalinity and is

the primary source of K+ to inhibit clays.

Carboxymethylcellulose - HV/LV CMC - (with a sodium or potassium base) -

This is used to control fluid loss.

Polyanionic cellulose (PAC) – PAC is used to help control filtration and as a

secondary thickener.

Barite – Barite is used to weight mud.

6.3.2 Typical properties of KOH-lign ite muds

These muds have many similar properties to lignosulfonate/lignite (FW-CL)

muds. They are inhibiting to a certain degree, with KOH used instead of NaOH

to control the pH and alkalinity. Table 28 lists these properties.

Table 28 - Typical properties of KOH-lignite muds

Density(kg/l)

Plasticviscosity

(cPs)

Yield po int(g/100 cm2)

10 sec/10min gels

(g/100cm2

)

pH API flu id

loss (cm3/30

min) 1.08 12 - 14 4 - 6 1 – 2 2 – 4 10.0 10 - 12

1.44 16 - 20 5 – 9 1 - 3 3 - 5 10.0 6 – 8

6.3.3 Conversion

These muds are formulated as new muds, but can be converted from a spud

mud; in this second case, they should be diluted and pilot tests run. Convert the

mud by minimising solids (dilute or use centrifuges, mud cleaners, desanders or

desilters (freshwater)). If viscosity is too low, add bentonite (dry, if prehydrated).

Add lignite and KOH as well. Add PAC or CMC to control fluid loss; usually 0.75

– 1.5 kg/m3 will be sufficient. When necessary, use barite to weight the mud and

always take account of the wettability water of this material: 0.7 gallons of water

for every sack of barite (2.65 litres for every 22.5 kg of barite).




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6.3.4 Maintenance

Make sure solids are kept within acceptable limits; the negative effect of

carbonates is more marked when the shale solids content is higher. K+ ions

must be monitored and kept to the planned level. Ca++ and Cl-

ions must be kept

within acceptable limits to enable chemical products to be effective. KOH-lignite

muds can be weighted up to 2.16 kg/l, subject to checking the minimum level of

low gravity solids and in particular clay solids.

6.3.5 Advantages/disadvantages of KOH-lign ite muds

This system has average costs and its inhibiting properties are fairly easy to

maintain. Table 29 lists some of the advantages/disadvantages.

Table 29 - Advantages/Disadvantages of KOH-lignite muds


Inhibiting system Intolerant to contaminants such as salt,Ca++, cement, carbonates andanhydrite.

Cheap. Fluid loss control with ligniteand bentonite. The content of low gravity shale solidsmust be minimised

Simple, with a small range ofproducts

Thermally stable up to 400°F(240°C).

6.3.6 Problems and contamination of KOH-lign ite muds

These muds are treated in the same way as lignite lignosulfonate(FW-Cl) muds.

Table 30 lists contaminants, indicators and relative treatments.




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Table 30 - Contaminants / Treatments of KOH-lignite muds


High solidscontent

Increase in solids, PV, YP, and gels.Viscous bottomhole cushions. Highconsumption of additives.

Dilute and centrifuge, improvesolids removal.

Cement

Increase in Pm, Pf , pH, YP, gel and fluidloss.

Deflocculate with bicarbonateand/or SAPP. Dilute with water.Increase the Pf to limit Ca++

. Make

up fluid to reduce rheologicalproperties. Convert to a FW-LImud if necessary.

Poor qualityproduct

Increase in treatments (amounts) Manufacturing documents. Takesamples and analyse. Run a pilottest on good quality material(comparison)

Saltwater/salt

The well is producing fluids, increase inviscosity, chlorides, YP, fluid loss.Decrease in density.

Increase the density (kill flow).Treat with water and thinner tocontrol rheology, then addPAC/CMC for fluid loss. Convert toa SS mud when the salt content is

high.Change in the drilling rate (metres/hour).Increase in

Increase the pH with KOH toreduce Ca++. Treat with

Anhydrite/Gypsum Ca++ , decrease in the pH, Pm and Pf . Bicarbonate and soda ash. Addthinner or convert to a FW-GYsystem.

Carbonates

Increase in Mf , YP and gels. Unreliablerheological results. Decrease in the pH,Pm and Pf . Viscous cushions after trips.High viscosities at the flow line.

GGT analysis. Increase the pH toabove 10.7 with KOH. Add limeand/or gypsum to precipitatecarbonates (avoid over-treatment).

Always keep the content of lowgravity cuttings within anacceptable level.

High temperaturegelation

High pressure needed at the pump torestart circulation. Viscous cushionsfrom the bottomhole, after trips.

Reduce LG solids and MBT. Useheat-stable thinners. Analyse forcarbonate contamination (GGT).




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6.4 KOH-lime mud

KOH-lime mud is the same as the lime mud described in the previous section, but KOH is

used instead of NaOH to control alkalinity and limit Ca++ solubility. This mud provides two

types of ions: Ca++ and K+ which have an inhibiting effect on shales. This fluid also has

three levels of Ca(OH)2 content: low, intermediate and high, as in FW-LI muds. Fluid loss

is controlled using starch, CMC HV/LV or PAC; the pH is kept from 11 – 13. High lime

with a Pm of 17-20 and Pf of 5-6 is usually programmed. Soluble calcium ranges from

200 – 400 mg/l; the high pH value limits solubility a great deal. These muds tolerate

chlorides, (Cl-) = 1500 - 1700 mg/l, fairly well, however a high chloride content makes

them more expensive, as chemical additives are not so effective. The temperature

threshold is closely related to shale solids in the system; mud with a minimum

percentage of shale and bentonite can withstand temperatures up to 320°F (160°C).

6.4.1 Main additives of KOH-lime mud

Table 31 - Main addi tives of KOH-lime mud

Addi tives Concentration, Kg/m3 Function

Bentonite 45 - 75Viscosity and fluid losscontrol

Lignosulfonate 12 - 24Thinner and fluid loss control

Lime 12 - 30 High pH and Ca++ source

Potassium hydroxide(KOH)

6 - 9 Pf control and K+ source

Tannin sulfonate 6 - 9 Deflocculant

PAC /Starch 3 - 6 Fluid loss control

Barite In relation to the density Weighting material

Bentonite – Bentonite is used for viscosity and fluid loss control and must be

prehydrated. Fluid loss is achieved by the deflocculating effect of the

lignosulfonates on the bentonite.

Lignosulfonate – Lignosulfonate acts as a deflocculant to control rheology and

fluid loss to some extent.

Lime Ca(OH)2 – Lime controls the pH and provides Ca++

ions to control the Pm

and stabilise rheological properties.

Potassium hydroxide – Controls the alkalinity and provides K+

ions.




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Tannin sulfonate - (DESCO manufactured by M.I., or New-Thin by BHI): tannin

sulfonate is used as the deflocculant in lime muds.

Starch – Starch is used for fluid control. High concentrations of starch can

sometimes cause viscosity problems.

Polyanion ic cellu lose (PAC) – Additional fluid loss control.

Barite – Weighting material. When density increases, the bentonite content

must be reduced (dilution and/or lower percentage in new mud), to prevent

negative increases in rheology and temperature-induced gelation.

6.4.2 Typical properties of KOH-lime mud

KOH-lime mud has similar properties to lime mud. Table 32 lists the

characteristics of a light and weighted mud.

Table 32 - Typical properties of KOH-lime mud

Density(kg/l)

Plasticviscosity

(cPs)

Yield Point(g/100 m2)

10 sec/10 mingels (g/100cm2)

API fluidloss (cm3/30

min) 1.08 10 - 12 4 – 6 2 – 3 3 - 5 6 - 9

1.44 16 - 18 8 – 10 2 - 3 3 – 6 4 - 6


A freshwater spud mud can be converted to a KOH-lime mud. If the chloride

content is high though, this conversion is not cost-effective. Spud mud must

have a low density, low gels and low solids content. If the solids content is high,

the solids should be diluted and removed using solids control equipment. The

mud should be weighted, if applicable, after conversion. To convert the mud,

dilute from 10% to 25% before the break over. The mud is converted downhole,

in two circulation stages. The water (10-25%) is put in before adding chemical

products. Close all the mud guns, apart from the sump pit guns, to prevent lime

flocculating in the reserve mud in other pits. Add KOH, deflocculant and

hydrated lime together, to limit viscosity increases. Add KOH via the chemical

barrel, and deflocculant and lime from the mixer funnel. During the first

circulation stage, add half the lime and all the deflocculant and potassium

hydroxide, then add the remaining lime in the second stage. The “break-over




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hump” value will depend mainly on the solids concentration. If mud gets too

thick, add thinner, water or both. Adjust the Pm, Pf and excess lime after

reaching the break over. Then add fluid loss additives. Hydrated lime should be

added each time the density increases, to maintain the programme excess lime

content (low-int-high).

6.4.4 Maintenance

LG (low gravity) solids (MBT and mud still) must be continually monitored and

kept in their optimal range to maintain this mud. In many cases, this means

keeping volume of low gravity solids below 8 vol. % and drilled solids below 6

vol. %. PAC or CMC are used to control fluid loss, or lignite or lignosulfonate are

added along with prehydrated bentonite, as they are cheaper. If the viscosity is

too low, add prehydrated bentonite. Small amounts of PAC (0.35-0.7 Kg/m3) are

preferable for heavy muds. If the mud becomes too viscous, treat with more

thinner. Lignosulfonate, KOH and Ca(OH)2 when added in regular values, must

correspond to dilution values. Lignosulfonate should be kept at 1.2 – 1.5 Kg/m 3

and the concentration of low gravity solids to the minimum (ratio between the

efficiency of solids control equipment and the penetration rate). Pilot tests are

recommended to determine the optimum amounts of material for treatment.

6.4.5 Advantages/disadvantages of KOH-lime mud

KOH-lime mud has the same advantages as lime muds. Both systems have low

viscosity values and low gels, and can tolerate solids well.

Table 33 - Advantages/Disadvantages of KOH-lime muds


Inhibiting agent (Ca++ K+) Fluid is not dispersed.

Tolerates solidsDecreases the ROP in hardformations.

Can tolerate anhydrite, cement,carbonate and salt contamination

Complex system, with manyadditives.

Gels at high temperatures.

Bentonite must be prehydrated




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6.4.6 Problems and contamination of KOH – lime muds.

Table 34 - Contaminants / Treatment of KOH – lime muds


High solidscontent

Increase in the % of solids, PV,YP and 10’ gel

Dilute considerably,centrifuge and improve solidsremoval

Salt and saltwater

Increase in chlorides, viscosity,YP, gel and fluid loss. Decreasein the Pm, Pf , pH and density(saltwater)

Increase the density by killingthe flow. Dilute withfreshwater. Treat with thinnerand KOH for rheology, andwith PAC or starch for fluidloss. If the salt content is veryhigh, convert to a saturatedsalt system or replace with anoil base mud

Carbonates / CO2

Increase in Mf , 10’ gels.Rheology hard to control.Decrease in the Pm and pH

Add Ca(OH)2 for the Pm andKOH for the Pf . Keep shalesolids below programmedvalues.

Poor productquality

Different product packaging.Increase in the amount of

products needed to achieve thesame results. Unreliable mudproperties.

Supplier documents onmanufacturing methods and

quality control. Take samplesand analyse, comparing withnormal products, asnecessary.

Temperature – induced gelation

Viscous bottomhole cushionsafter trips. Viscous mud at theflow line (not at the sump pit).Pressure increases at thepump, after stopping.

Reduce LG solids. Increasethe concentration oflignosulfonate if thetemperature if below 160°C. Ifvalues are higher, usedeflocculant for hightemperatures.

Foaming

Foaming in the pits and at the

shale shaker. Mud which tendsto incorporate air, pressure dropat the pump.

Treat with toxic-free

defoamer. Identify the causeof the problem and eliminate.








Page 54 of 58


Polyanionic cellulose (PAC) – PAC is used to control viscosity and fluid loss.

PAC can tolerate salts extremely well, and is more effective with a hardness

below 400 mg/l.

Carboxymethylcellulose (CMC) – CMC is another cellulose base polymer

used for viscosity and fluid loss control. CMC is not very resistant to salt (max

50000mg/l of NaCl) compared to PAC, and is more effective with a hardness

below 250 mg/l. 4 types of CMC are available on the market: soda base,

potassium base, high viscosity and low viscosity CMC.

Barite – barite is the most widely used weighting material (and the least

expensive). If barite is added to increase density, the percentage of LG solids

should preferably be decreased to below 6 vol. %. Water and PAC/CMC should

be added together to avoid excessive rheological values.

7.2.3 Typical properties of PAC/CMC low solids muds

These muds are very similar to the BEN-EX system and to PHPA low solids

muds. Table 36 lists the main properties for 9 lb/gal and 12 lb/gal density muds.

Table 36 - Typical propert ies of PAC/CMC low so lids muds

Density (Kg/l)

Plasticviscosity

(cPs)

YieldPoint(g/100cm2)

10 sec/10min gels

(g/100cm2)

Chloridesmg/l

API fl uidloss

(cm 3/30min)

Hardness (mg/L)

pH

1.08 4 - 6 4 - 6 2 - 4 3 - 5 < 2000 10 – 12 < 200 9.0 -9.5

1.44 8 - 10 5 - 8 3 - 6 5 - 8 < 2000 6 - 8 < 200 9.0 - 9.5

7.2.4 Conversion system/maintenance

PAC/CMC low solids muds are usually made up as NEW, without re-using

old mud. The pits must first be cleaned, removing any settled solids. Service

or freshwater should preferably be used, with pre-treatments to reduce the

hardness (Ca++ and Mg++) to below 200 mg/l, before adding the polymers.

Gradually add the bentonite (30 – 40 kg/m3) and leave to mature for at least

24 hours if possible. Add PAC in relation to viscosity parameters and

programme filtration. Use regular or low viscosity PAC depending on the




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rheology trend. CMC can be used with PAC, to control viscosity and fluid

loss. Keep the pH at 9.0 – 9.5 with caustic soda or soda ash. Density can be

increased with barite, however this mud cannot be weighted above 1.55 Kg/l

(difficult to control rheological parameters at greater densities).

7.2.5 PHPA (partially hydrolysed polyacrylamide) low solids muds

These systems are used to inhibit shales. Acrylate/acrylamide polymers are

adsorbed on the surface of shale particles. As PHPA is a long-chain

molecule, it can effectively bond with a certain number of shale laminae,

producing viscosity with a minimum concentration of low gravity solids. As a

result, a PHPA low solids mud can be formulated to optimise the ROP and

borehole cleaning (lifting of cuttings). Moreover, this inhibiting system can be

improved by adding KCl and POLY (propylene glycol). These additives

produce viscosity, encapsulate solids and stabilise filtration. Small amounts of

bentonite should be added, when the mud is made up as new. PHPA is used

to thicken the fluid, when a minimum amount of bentonite is used to stabilise

the borehole walls.

The main component of this mud is long-chain PHPA (partially hydrolysedpolyacrylamide), with a high molecular weight. The system is sensitive to

chlorides, Ca++ and solids. Solids should be kept to a minimum with dilution

and mechanical separation, otherwise high viscosity values and gels will

develop.

7.2.6 Main additives of PHPA low solids muds

Table 37 - Main addit ives of PHPA low solids muds

Materials Concentration, Kg/l Function

Bentonite 3 - 40Viscosity and fluidloss control

Caustic soda/potassiumhydroxide

pH 9.0 – 9.5 Alkalinity control

PHPA 2.85Solids encapsulation,borehole stability,viscosity control

SPA 0.75 – 1.5 Fluid loss control

Soda Ash 0.75 – 2.15 Precipitate Ca++ ions




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Bentonite – Bentonite is used for viscosity and fluid loss control. As PHPA

encapsulates bentonite and limits its hydration properties, prehydrated bentonite

should be programmed (before adding PHPA).

Caustic soda/potassium hydroxide – (NaOH – KOH) these two substance are

used in moderation, to protect against corrosion and adjust the pH to a

maximum of 9.5, unless conditions require higher values. With a pH of 9.5, Ca++

and Mg++

precipitate in an insoluble form. Magnesium ions have very negative

effects on polymer performance and must be eliminated.

PHPA – PHPA is used to guarantee inhibition, with an encapsulating effect on

shale cuttings. The plugging of microfractures along the borehole walls also acts

as a further form of inhibition, preventing the hydration of shales and thus their

instability. PHPA is also a secondary thickener and can provide some fluid loss

control.

Sodium po lyacry late/SPA – SPA is used to control fluid loss; a hardness below

400 mg/l is required for an effective and cheap use of SPA.

Soda ash – Soda ash controls make up water hardness. This provides for a

better hydration of bentonite and more effective fluid loss control of SPA.

7.2.7 Typical properties of PHPA low solids mud

These muds have similar properties to PAC/CMC low solids mud. Table 38 lists

these muds with 9 lb/gal and 12 lb/gal densities.

Table 38 - Typical properties of PHPA low solids mud

Density(kg/l)

Plastic

viscosity(cPs)

Yield Point(g/100 cm2) 10 sec/10 mingels (g/100cm2) API f lu id loss(cm3/30 min)

1.08 4 – 6 5 – 7 2 – 4 3 – 5 10 - 12

1.44 8 - 10 6 – 10 4 - 6 5 – 8 6 – 8




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7.2.8 Advantages/disadvantages of non-dispersed polymer muds

Table 39 - Advantages/Disadvantages of non dispersed polymer muds


High ROPs (m/hour) in hard formations Limited use

Low head loss values Adsorption of polymers on shales is

irreversible.

Good borehole cleaning (lifting capacity) Not very stable at high temperatures.

Easy to maintain Not resistant to increase in solids.

Easily convertible to adeflocculated/dispersed system

Requires more dilution than thedeflocculated system.

Does not disperse solids (inhibitedsystem).

Fluid loss control is expensive.

More corrosive than thedeflocculated system.

Sensitive to contaminants.Carbonate contamination hard to

treat.

Weighting problems.

Not very inhibiting.




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7.2.9 Contamination of non-dispersed polymer muds

Table 40 - Contaminants / Treatments o f non-dispersed po lymer muds


High solidscontent

Increase in the % of solid,PV, YP, gels, MBT.Viscous bottomholecushions after trips. Highviscosity at the flow line.

Dilute more and centrifuge.Improve solids control.

CementIncrease in Marsh

viscosity, pH, Pm, Pf , gelsand filtration

Control contamination (Ca++)with bicarbonate and/orSAPP. Dilute withfreshwater. Increase the PF to limit solubility of Ca++.Deflocculant may benecessary or convert themud to lime.

Saltwater/salt

Well producing, increase inthe Marsh viscosity, YP,gels, filtration. Surfacewater separation.

Decrease in pH, Pm, Pfanddensity.

Increase density (if the wellis producing). Dilute withfreshwater to reducechlorides. Treat withdeflocculant forcontamination and PAC forfluid loss control. Ifnecessary convert to an SSmud.

Gypsum/anhydriteIncrease in Ca++, YP, Gelsand filtration. Decrease inpH, Pm and Pf .

Treat for Ca++ with soda ash,bicarbonate and/or SAPP.

Add freshwater. Deflocculantmay be required.

Carbonates / CO2

(Not very

problematic)

Increase in Mf , 10’ gels.Rheology hard to control.

Decrease in Pm, pH.

Add lime for the Pm and KOHfor the Pf. Minimise the shale

content.

Poor productquality

Increase in treatmentamounts. Differentpackaging. Poor resultswith standard treatments.

Documents of the supplier’sproduction process. Takesamples and analyse. Pilottest to compare with reliableproducts.