Chapter 6 Water Base Dilling Fluids

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WELL AREA OPERATIONS DRILLING SUPERVISOR TRAINING COURSE

INHIBITING WATER BASED MUDS

Cod.: RPWA2021A

Date : 01/03/2005

Rev: 00

Page: 58

Code RPWA2021A Rev. 00 – Date 01/03/2005 Page 2 of 58

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INDEX

1.0

INTRODUCTION

5

2.0

LIME BASE MUDS

5

2.1

6

3.0

4.0

5.0

FLUIDS TREATED WITH LIME

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

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

SALT BASED MUD

21

5.1

SATURATED SALT MUDS

22

5.1.1 5.1.2 5.1.3 5.1.4 5.1.5

23 25 25 26 26

Main additives of saturated salt muds Typical property of SS (saturated salt) muds Conversion system/maintenance Maintenance Advantages and disadvantages of SS (Saturated Salt) muds

Well Area Operations


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5.1.6 5.2

5.3

6.0

26

SEAWATER MUDS

27

5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.2.6

27 30 30 30 31 31

Main additives of SW-LS (salt water) muds Typical properties of AS-LS fluids Conversion system Maintenance Advantages and disadvantages of AS-LS mud Problems and contamination (AS/LS) – Salt water Muds

BRACKISH WATER MUDS

32

5.3.1 5.3.2 5.3.3 5.3.4 5.3.5

32 34 34 34 35

Main additives Conversion system Maintenance Advantages and disadvantages of brackish water muds Problems and contamination of brackish water muds

POTASSIUM MUDS (FW/SW-KC)

36

6.1

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

38

6.1.1

38

6.2

6.3

6.4

7.0

Problems and contamination of SS (Saturated Salt) muds

Main additives for FW/SW-KC mud

KCL - POLIMERS

41

6.2.1 6.2.2 6.2.3

41 42 43

Preparation Maintenance Problems

KOH-LIGNITE (SYSTEM)

44

6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6

44 45 45 46 46 46

Main additives of KOH-lignite Muds Typical properties of KOH-lignite muds Conversion Maintenance Advantages/disadvantages of KOH-lignite muds Problems and contamination of KOH-lignite muds

KOH-LIME MUD

48

6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6

48 49 49 50 50 51

Main additives of KOH-lime mud Typical properties of KOH-lime mud Conversion system Maintenance Advantages/disadvantages of KOH-lime mud Problems and contamination of KOH – lime muds.

POLYMER FLUIDS

52

7.1

INTRODUCTION

52

7.2

NON-DISPERSED POLYMER MUDS

52

7.2.1

53

PAC/CMC low solids muds



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7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.2.7 7.2.8 7.2.9

Main additives of PAC/CMC low solids muds Typical properties of PAC/CMC low solids muds Conversion system/maintenance PHPA (partially hydrolysed polyacrylamide) low solids muds Main additives of PHPA low solids muds Typical properties of PHPA low solids mud Advantages/disadvantages of non-dispersed polymer muds Contamination of non-dispersed polymer muds


53 54 54 55 55 56 57 58


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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:

2.0

lime base muds

salt base muds

potassium base muds

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 with 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 directly 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 (Pf) and lime content, as classified in table 1. Table 1 - Classification of lime muds based on alkalinity values

Alkalinity

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 additives of lime muds Additive

Concentration, kg/m3

Bentonite

60 – 80

Lignosulfonate

0.5- 1.5

Deflocculant

Lime Ca(OH)2

5 – 30

Inhibitor, alkalinity control

for pH 10.5 - 12.5

pH control

0.5 – 1.2

Fluid loss control

*Starch

0.6 – 1.2

Fluid loss control

PAC

0.5 - 3

Fluid loss control

Caustic soda or potassium hydroxide Lignite

*Requires treatment with a biocide.


Function Viscosity, fluid loss control


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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)) Low Lime 1.20 High Lime 1.20 3.3

Plastic viscosity (cPs)

Yield Point (g/100 cm2)

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

Pm cm3 H2SO4 N/50

Pf cm3 H2SO4 N/50

15 - 18

3–5

0-1 0-2

5 - 10

15 - 18

3-5

0 - 1 0- 2

12 – 18

pH

Excess Lime (kg/m3)

API fluid loss (cm3/30 min)

1-2

10.5 – 12.5

3-6

6 - 12

5 - 10

12.0 – 12.5

15 - 45

6 - 12

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

Table 4 - Filtrate alkalinity determined by adding caustic soda (NaOH) kg/m3

Pf, cm3 H2SO4 - N/50

1

2.8

1.0

2

5.6

3.0

3

7.8

5.0

4

11.3

7.0

NaOH, lb/bbl

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 of lime 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).



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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 Concentration,

Additive

lb/bbl

Caustic soda/potassium hydroxide

3.4

Kg/m3

2–3

5–8

Lime

4–8

10 – 20

Thinner

2–5

5 - 15

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 Pf 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 Advantages Low viscosity and gel streghts

Disadvantages 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 sands

3.6

High pH may pose risks to safety

Can tolerate cement, anhydrite and salt

Stabilises the borehole (more uniform

(C- 50,000 mg/l)

boreholes)

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 lime mud contaminants Contaminants Large number of solids

Indicators Increase in solids in the mud still, plastic viscosity, 10-minute gels, MBT. Increase in chlorides, Marsh viscosity, yield point, 10” / 10’ gel, fluid loss. Decrease in the Pm, Pf and pH.

Salt/saltwater

Increase in MF, 10-min gels. Carbonates/invasion Decrease in the PM and pH. If of CO2. The problem CO2 invasion continues, normal is not excessive treatments with lime will increase fine solids (CaCO3).

Poor quality products

Foaming

High temperature gelation

Treatments Dilute more. Improve solids removal. Use centrifuges. Increase the density (if the level increases), dilute with service water. Add thinner and caustic soda to control rheology, then treat with starch or PAC to keep fluid loss under control. If a large amount of salt is present, convert to a saturated salt system, or replace with oil base mud. Add lime to control the Pm and KOH to control the PF. Keep the solids content at optimal values (low)

Different packaging. Increase in treatment amounts. Anomalous trend of mud properties.

Check dispatch and supplier documents. Take samples and analyse. Work together with the supplier rather than change supplier.

Foaming in the pits. Trapped air. Decrease pump pressure, if possible

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

Pressure kick-up to restart circulation after trips. Very viscous bottomhole cushion. High viscosity even at the flow line.

Reduce low gravity solids. Add lignosulfonate if the bottomhole temperature (BHT)<300°F (150°C) Add polymer deflocculants when temperatures 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 Additives Bentonite Lignosulfonate

Concentration, Kg/m3 60 - 70 10 - 25

Function Viscosity, fluid loss control Deflocculant

Gypsum

10 - 25

Inhibition, alkalinity control

Caustic soda Potassium hydroxide Tannin sulfonate Starch PAC

pH 9.5 - 11.0 PH 9.5 - 11 5 – 10 5 – 18 0.7 – 4.5

Alkalinity control, inhibition Inhibition Deflocculant Fluid loss control 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

Plastic Density viscosity (kg/l) (cPs)

4.3


10 sec/10 min gels (g/100 cm2

Excess gypsum( kg/m3)

PF

pH 9.5 – 11.0 11.0 – 12.0

1.08

12 - 15

3– 5

1-2

4- 6

30- 40

0.2 – 2.7

1.44

15 - 20

1- 7.5

0– 2.5

17.5

30- 40

2-3

Ca++ (mg/l)


6001200

8 - 12

200600

6-8

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/m3). 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 Pf 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 Advantages Low viscosity and gel strengths. Tolerant to solids.

Disadvantages High pressure value (head losses) during conversion, which may cause borehole damage. Gelation at temperatures above 300°F (150°c)

Easy to weight up to 2.16 kg/l. Inhibits the hydration of shales and sandy shales. 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

High content of LG solids

Mud still values, PV, 10 minute gels, MBT Increase in chlorides, viscosity, yield point, 10”/10’ gel and fluid loss. Decrease in the Pm, Pf and pH. Increase in density if water is produced from the well

salt/saltwater

carbonates/CO2 (no problems in lime muds)

Poor foam product quality.

Gelation at certain temperatures.

Increase in Mf, 10-min gel. Decrease in the Pf and pH. Released CO2 requires Ca(OH)2 treatments which lead to an increase in fine solids (CaCO3). Different product packaging. Poorer product performance. Mud characteristics not uniform. Foaming in the pits, air trapped in the mud, pressure pump is not uniform

Pressure kick-up at the pump (when circulation starts again). Very viscous bottomhole cushions and high viscosity at the flow line


Treatments Dilute more Improve solids disposal. Centrifuge Treat with deflocculant and soda for the rheology and with starch or PAC for fluid loss. Convert to a saturated salt mud or oil base mud if major salt levels are present. Add lime for Pm and KOH for Pf control. Minimise solids content

Supplier documents. Take samples and analyse. Work with the supplier to pinpoint the problem (do not change the supplier). Treat with (non-toxic) defoamer. Identify the source of foaming and take action Reduce low gravity solids. Treat with lignosulfonate BHT<300°F. Treat with polymer deflocculants when temperatures are above 300°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



Agip KCO

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

Prehydrated bentonite

30 - 70

Viscosity, fluid loss control

Starch

10 - 20

Fluid loss control

pH 9.0 - 11.0

Alkalinity control

Caustic soda Soda Ash (Na2CO3) PAC

3-8 0.7 - 4.2

Salt (NaCl)

350

Function

Ca++ removal Fluid loss control, viscosity 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



Agip KCO

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. Attapulgite – 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 accurately determined 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.



Agip KCO

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 of SS (Saturated Salt) muds

5.1.3


Density (Kg/l)


Yield Point (g/100/ cm2)

1.26

8 - 12

6-8

3 -4

4-6

8 - 12

1.56

15 - 20

7-9

4-5

5-7

6-8

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

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 of SS (Saturated Salt) muds Advantages Inhibiting agent (shales) Resistant to cement, anhydrite, salt and saltwater contamination. Low solids content, dissolved salt increases density. Good borehole cleaning properties (cuttings lifting) Stabilising effect on an open hole.

5.1.6

Disadvantages Fluid loss control is more difficult (starch or polymers) Tends to foam and trap air. Corrosive with salinity below saturation. Maximum temperature = 280°F (140°C).

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

5.2

Indicators

High solids content

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

Salt/saltwater

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


Material performance not up to standard. Different packaging.

Treatments Dilute, centrifuge and improve solids removal Increase the density due to the ingress of saltwater. Add salt to saturate, starch to control fluid loss and prehydrated bentonite or attapulgite for viscosity. Request data on the packaging process. Take samples and tests for efficiency + carry out complete analyses.

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 Additives

Concentration kg/m3


40 - 70

Function

9 – 18

Viscosity, fluid loss control Alkalinity/corrosion control Fluid loss control

PAC

1.5 – 3.0

Fluid loss control

Lignosulfonate

9 – 18

Deflocculant

Lignite

5 - 10

HP/HT fluid loss control

Caustic soda/potassium hydroxide Starch

1.5 – 4.5

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 when freshwater is not available. Attapulgite – 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,



Agip KCO

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.



Agip KCO

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)



1.10

16- 18

5-7

1-2

3-4

25,000 – 300,000

API fluid loss (cm3/30 min) 8 - 12

1.45

22 - 24

6-8

1-2

3-4

25,000 – 300,000

6-8

5.2.3

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

Chlorides (NaCl) mg/l

Conversion system 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.



Agip KCO

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 of a 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

Contaminants High solids content

Indicators Increase in the % of solids, PV, YP, gel, viscous mud cushions from the bottomhole. Increase in YP, fluid loss and chlorides. Decrease in density and pH in the case of formation water

Treatments Dilute considerably, use centrifuges and other equipment to remove solids Increase the density if the water invades the formations. Treat with a thinner for rheology. Control fluid loss with starch or PAC.

Poor quality product

Increase in amounts required; packaging different from previous supplies.

Find out about the manufacturing process. Take samples and analyse the product.

Cement

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

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, wrong rheology. Very viscous bottomhole cushions.

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

Salt/saltwater



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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 additives 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 additives of brackish water muds Additives

Concentration, Kg/m3


40 - 70

Caustic soda / potassium hydroxide Starch

Function Viscosity and fluid loss control

1.5 – 4.5

Pf and corrosion control

9 – 18

Fluid loss control

PAC

1.5 – 3

Fluid loss control

Lignosulfonate

9 – 18

Deflocculant

Lignite

6 - 10


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. Attapulgite – 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. Lignite (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)



1.10

16

4-5

1-2

2-5

10.5 – 11

API fluid Chlorides loss mg/l (cm3/30 min) 10,000 - 25,000 6 - 10

1.45

22

6-8

1 - 1,5

2-4

10.5 - 11

10,000 - 25,000

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


pH

6-8


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5.3.2

Conversion system 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 Advantages

Disadvantages

Moderate inhibitor

Chemical products increase (because they are less effective).

Less freshwater needed (brackish water 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

Contaminants

High solid content

Salt/saltwater (flow)

Indicators

Treatments

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

Dilute considerably, centrifuge constantly and improve effectiveness with super screens, desanders, mud cleaners, etc.

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

Increase in frequency of treatments. Different packaging.

Check the manufacturing stage with the producer, take samples and analyse loose products. Check tankers transporting products. Check loose products delivered by ship (water/diesel fuel, barite/cement)

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

Treat with bicarbonate (NaHCO3) or SAPP to stop cement contamination. Use lignosulfonate, starch and PAC to control rheology (PV,YP, gel) and fluid loss.

Increase in gels, YP, unreliable viscosity meter values. Bottomhole mud cushions very viscous after trips.

Increase the pH to 10.7 to convert bicarbonates to carbonates. Treat with lime or gypsum to precipitate the carbonates.

Poor quality product

Cement

Carbonates

Weight if the well is producing! Control the rheology with thinners and reduce fluid loss with starch or PAC.



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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 and produces 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.



Agip KCO

Table 24 - Main additives for FW/SW-KC mud Additive Prehydrated bentonite

Concentration, Kg/m3 15 - 45

Function Viscosity and fluid loss control

Potassium chloride

15 - 170

K+ inhibiting source

Potassium hydroxide

0.7 – 2

Provides K+ ions and controls alkalinity

Starch

8 – 16

Fluid loss control

PAC

1.5 – 3

Fluid loss control

Lignosulfonate

8 - 16-

Thinner

Lignite

5 - 10


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. Additive-free bentonite (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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Xanthan Gum – Biopolymers such as XC or XCD are used to make KCl/polymer muds viscous, in addition to or instead of prehydrated bentonite. Even though this system may record high yield point values, it cannot keep barite adequately suspended and in this case small amounts of xanthan gum are added. Polyanionic cellulose/carboxymethylcellulose (PAC/CMC) – These additives are used to control fluid loss. When the KCl concentration is below 50,000 mg/l, either special or regular grade CMCs are used instead of PAC. High viscosity CMC is not normally chosen, because of its deflocculating capacity. Pilot tests should be carried out before using the treatment downhole. Starch – Starch controls fluid loss in KCl muds; starch which has been pre-treated with a biocide should preferably be used. Modified starches are normally produced from potatoes rather than corn and are thermally stable at 250°F (120°C). Pre-gelatinised corn starch can also be used, though it is less thermally resistant and may be affected by bacterial degradation. PHPA – Partially hydrolysed polyacrylamide is mainly used to incorporate solids and inhibit the system. The purity grade of PHPA varies a great deal from supplier to supplier, and so the value of each supply should be analysed to prevent under- or over-treatment. PHPA is sensitive to Ca++ e Mg++; the hardness should be kept below 400 mg/l. Gilsonite – Gilsonite (powder asphalt) is used to cover over (and stabilise) microfractures and to plug depleted sands (depleted production levels). Operating parameters – A low concentration of polymers in FW-KC muds is a common cause of problems. If polymers are insufficient, shale cuttings will be dispersed. This leads to considerable increases in viscosity, making it hard to add polymers. Table 25 lists the main properties of FW-KC muds.



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6.2

KCL - Polimers

Table 25 - Typical properties of FW/SW-KC muds Density (kg/l)



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

6.2.1

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


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.



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Table 26 - Typical concentrations for FW/SW-KC muds (KCl-Polymer Muds) CONCENTRATION, Kg/m3 Density Water Caustic Gum Reg Active Bentonite KCl Xanthan PAC Barite kg/l *(l) potash LV Starch PHPA 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: Litres of water per cubic metre of 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 are adsorbed 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

Contaminants High solids content

Indicators Increase in solids, PV, YP, gels. Viscous bottomhole cushions after trips. Increase in Pm, Pf, pH, YP, fluid loss and Marsh viscosity

Cement


Saltwater/salt

Different product packaging. More product used. Increase in chlorides, Marsh viscosity, YP, gels and fluid loss Increase in Ca++, YP, gels, fluid loss. Decrease in pH, Pm, Pf.

Gypsum/anhydrite

Carbonates

Increase in Mf, YP, gels. Decrease in pH, Pm, Pf. Viscous bottomhole cushions, and high viscosities also at the flow line.


Treatments Dilute considerably and improve the performance of solids control equipment. Dilute less. Treat with bicarbonate and/or SAPP. When the rheology stabilises, treat with starch or PAC to reduce fluid loss. Product documents from the supplier (quality history). Take samples and analyse. Increase density if the well is producing saltwater. Convert to a saturated salt mud, if major salt levels are present Treat with SAPP, soda ash or potassium carbonate. Use a thinner as necessary. GGT analysis. Increase the pH to >10,7 with KOH. Add lime and make sure solids values are within acceptable limits.


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6.3

KOH-lignite (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-lignite 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 Additives

Concentrations, kg/m3

Bentonite

45 - 75

Lignite Potassium hydroxide

15 - 23

Viscosity and fluid loss control. Thinner and fluid loss control.

1,5 – 4,5

Alkalinity and K+ control.

PAC/CMC

1,5 – 3

Fluid loss and viscosity control.

Barite

As necessary, depending on density

Weighting material.

Function

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-lignite 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

6.3.3

Density (kg/l)


Yield point (g/100 cm2)

1.08

12 - 14

4-6

1–2

1.44

16 - 20

5–9

1-3


pH


2–4

10.0

10 - 12

3-5

10.0

6–8

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-lignite 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 Advantages

Disadvantages

Inhibiting system

Intolerant to contaminants such as salt, Ca++, cement, carbonates and anhydrite.

Cheap. Fluid loss control with lignite and bentonite.

The content of low gravity shale solids must be minimised

Simple, with a small range of products Thermally stable up to 400°F(240°C).

6.3.6

Problems and contamination of KOH-lignite 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 Contaminants High solids content

Indicators Increase in solids, PV, YP, and gels. Viscous bottomhole cushions. High consumption of additives.

Treatments Dilute and centrifuge, improve solids removal.

Increase in Pm, Pf, pH, YP, gel and fluid loss.

Deflocculate with bicarbonate and/or SAPP. Dilute with water. Increase the Pf to limit Ca++. Make up fluid to reduce rheological properties. Convert to a FW-LI mud if necessary. Manufacturing documents. Take samples and analyse. Run a pilot test on good quality material (comparison) Increase the density (kill flow). Treat with water and thinner to control rheology, then add PAC/CMC for fluid loss. Convert to a SS mud when the salt content is high. Increase the pH with KOH to reduce Ca++. Treat with Bicarbonate and soda ash. Add thinner or convert to a FW-GY system. GGT analysis. Increase the pH to above 10.7 with KOH. Add lime and/or gypsum to precipitate carbonates (avoid over-treatment). Always keep the content of low gravity cuttings within an acceptable level.

Cement

Increase in treatments (amounts) Poor quality product

Saltwater/salt

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

Change in the drilling rate (metres/hour). Increase in Anhydrite/Gypsum Ca++ , decrease in the pH, Pm and Pf.

Carbonates

High temperature gelation

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

High pressure needed at the pump to restart circulation. Viscous cushions from the bottomhole, after trips.


Reduce LG solids and MBT. Use heat-stable thinners. Analyse for carbonate 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 additives of KOH-lime mud Additives

Concentration, Kg/m3

Function Viscosity and fluid loss control Thinner and fluid loss control

Bentonite

45 - 75

Lignosulfonate

12 - 24

Lime Potassium hydroxide (KOH) Tannin sulfonate

12 - 30

High pH and Ca++ source

6-9

Pf control and K+ source

6-9

Deflocculant

3-6

Fluid loss control

PAC /Starch 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. Polyanionic cellulose (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

6.4.3

Density (kg/l)


Yield Point (g/100 m2)

1.08

10 - 12

4–6

2–3

3-5

6-9

1.44

16 - 18

8 – 10

2-3

3–6

4-6



Conversion system 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/m3 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 Advantages Inhibiting agent (Ca

++

+

K)

Disadvantages Fluid is not dispersed.

Tolerates solids

Decreases the ROP in hard formations.

Can tolerate anhydrite, cement, carbonate and salt contamination

Complex system, with many additives. 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 Contaminants

High solids content

Indicators Increase in the % of solids, PV, YP and 10’ gel Increase in chlorides, viscosity, YP, gel and fluid loss. Decrease in the Pm, Pf, pH and density (saltwater)

Salt and saltwater

Carbonates / CO2

Poor product quality

Temperature – induced gelation

Foaming

Increase in Mf, 10’ gels. Rheology hard to control. Decrease in the Pm and pH Different product packaging. Increase in the amount of products needed to achieve the same results. Unreliable mud properties. Viscous bottomhole cushions after trips. Viscous mud at the flow line (not at the sump pit). Pressure increases at the pump, after stopping.

Foaming in the pits and at the shale shaker. Mud which tends to incorporate air, pressure drop at the pump.


Treatments Dilute considerably, centrifuge and improve solids removal Increase the density by killing the flow. Dilute with freshwater. Treat with thinner and KOH for rheology, and with PAC or starch for fluid loss. If the salt content is very high, convert to a saturated salt system or replace with an oil base mud Add Ca(OH)2 for the Pm and KOH for the Pf. Keep shale solids below programmed values. Supplier documents on manufacturing methods and quality control. Take samples and analyse, comparing with normal products, as necessary. Reduce LG solids. Increase the concentration of lignosulfonate if the temperature if below 160°C. If values are higher, use deflocculant for high temperatures. Treat with toxic-free defoamer. Identify the cause of the problem and eliminate.


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7.0

POLYMER FLUIDS 7.1

Introduction Polymer fluids contain polymers used for very different purposes – to thicken muds, to control fluid loss, to deflocculate the mud, and stabilise it at high temperatures. The fluids usually have lower amounts of bentonite, to facilitate thickening. High molecular weight polymers such as PHPA, PAC and XC Polymer are main thickeners. Because of their low bentonite or shale content, these muds are less sensitive (rheology and fluid loss) to the effects of traditional contaminants such as shales. The polymers (muds) reduce the dispersion of cuttings and stabilise the borehole walls, because of their encapsulating capacity. These fluids usually contain less than 5 vol. % of low gravity solids and can be classified into two main categories:

7.2

Non-dispersed polymer muds ;

Deflocculated polymer muds for high temperatures.

Non-dispersed polymer muds Service or saltwater cannot be used as a drilling fluid in many areas, because of its effect on the formation (instability) and insufficient viscosity for lifting cuttings and cleaning the borehole. In this context, non-dispersed polymer muds can simulate the drilling characteristics of water. These muds are most effective in areas with hard formations and low ROPs. They contain less than 5 vol. % of LG solids and can be defined as low solid non-dispersed (LSND) muds. LSND muds are not effective in areas with considerable reactive shale sections, as they do not tolerate solids contamination. Salt, saltwater, gypsum/anhydrite and cement contamination also severely restricts the use of this mud. Most LSND muds comprise water with varying amounts of bentonite and polymer. Polymers are added for viscosity, by flocculating and thickening the aqueous phase, and are also used for fluid loss control. Some LSND mud systems include:

PAC/CMC low solids muds

PHPA low solids muds



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7.2.1

PAC/CMC low solids muds These systems use cellulose base polymers and namely sodium polyacrylate, and should have a hardness below 400 mg/l, compared to 250 mg/l when using acrylate base polymers. Cellulose polymers can work with a chloride level up to saturation, while sodium acrylate polymers do not work with a chloride level above 5000 mg/l. The wide range of PAC/CMC thickeners enables a high level of flexibility in mud treatments.

7.2.2

Main additives of PAC/CMC low solids muds Table 35 lists the main components of the system. This mud (such as Ben-Ex) has few additives and is relatively easy to maintain, plus it is one of the cheapest in the LSND category. Table 35 - Main additives of PAC/CMC low solids muds Materials

Concentration Kg/m3

Bentonite

15 - 30

Caustic soda

pH 9.0 – 9.5

Function Viscosity and fluid loss control Alkalinity

Soda Ash

0.75 – 1.5

Removal of calcium

Regular PAC/CMC

1.5 – 4.5

Fluid loss control

Barite

Density to programme

Weighting material

Bentonite – Bentonite is used to increase viscosity and control fluid loss. It should preferably be prehydrated in freshwater for optimal performance. The polymer which is added afterwards will also be more effective. Caustic soda – Caustic soda is used to produce an alkaline environment, with a pH > 9.7; high pH values precipitate magnesium ions which could interfere with polymers. Premium type bentonite (untreated) reduces the amount of materials to use. Soda Ash – Soda ash is used to reduce make up water hardness. Polymers in LSND systems are more effective with a hardness below 250 mg/l.



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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 properties of PAC/CMC low solids muds

Densit y (Kg/l)



1.08

4-6

4-6

2-4

1.44

8 - 10

5-8

3-6

7.2.4

Chlorides mg/l


Hardnes s (mg/L)

pH

3-5

< 2000

10 – 12

< 200

9.0 -9.5

5-8

< 2000

6-8

< 200

9.0 - 9.5


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 hydrolysed polyacrylamide), 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 additives of PHPA low solids muds Materials

Concentration, Kg/l

Bentonite

3 - 40

Caustic soda/potassium hydroxide

pH 9.0 – 9.5

PHPA

2.85

SPA

0.75 – 1.5

Solids encapsulation, borehole stability, viscosity control Fluid loss control

Soda Ash

0.75 – 2.15

Precipitate Ca++ ions


Function Viscosity and fluid loss control Alkalinity control


Agip KCO

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 polyacrylate/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)



1.08

4–6

5–7

2–4

3–5

10 - 12

1.44

8 - 10

6 – 10

4-6

5–8

6–8





Agip KCO

7.2.8

Advantages/disadvantages of non-dispersed polymer muds Table 39 - Advantages/Disadvantages of non dispersed polymer muds Advantages

Disadvantages

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 a deflocculated/dispersed system

Requires more dilution than the deflocculated system.

Does not disperse solids (inhibited system).

Fluid loss control is expensive. More corrosive than the deflocculated system. Sensitive to contaminants. Carbonate contamination hard to treat. Weighting problems. Not very inhibiting.



Agip KCO

7.2.9

Contamination of non-dispersed polymer muds Table 40 - Contaminants / Treatments of non-dispersed polymer muds Contaminants

High solids content

Indicators Increase in the % of solid, PV, YP, gels, MBT. Viscous bottomhole cushions after trips. High viscosity at the flow line.

Treatments Dilute more and centrifuge. Improve solids control.

Increase in Ca++, YP, Gels Gypsum/anhydrite and filtration. Decrease in pH, Pm and Pf.

Control contamination (Ca++) with bicarbonate and/or SAPP. Dilute with freshwater. Increase the PF to limit solubility of Ca++. Deflocculant may be necessary or convert the mud to lime. Increase density (if the well is producing). Dilute with freshwater to reduce chlorides. Treat with deflocculant for contamination and PAC for fluid loss control. If necessary convert to an SS mud. Treat for Ca++ with soda ash, bicarbonate and/or SAPP. Add freshwater. Deflocculant may 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 KOH for the Pf. Minimise the shale content.

Poor product quality

Increase in treatment amounts. Different packaging. Poor results with standard treatments.

Documents of the supplier’s production process. Take samples and analyse. Pilot test to compare with reliable products.

Cement

Saltwater/salt

Increase in Marsh viscosity, pH, Pm, Pf, gels and filtration

Well producing, increase in the Marsh viscosity, YP, gels, filtration. Surface water separation. Decrease in pH, Pm, Pfand density.


Chapter 6 Water Base Dilling Fluids

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