calcium based mud systems (swaco)

The concen-tration ofreactivesolids in the drillingfluid determinesthe viscosityincreaseencounteredwhen calciumis added tothe system.

Water-Base SystemsCHAPTER

10

Water-Base Systems 10.6 Revision No: A-1 / Revision Date: 022801

When calcium is added to a clay-waterslurry, a base exchange occurs as the cal-cium (Ca2+) cation, which has higherbonding energy, replaces the sodium(Na+) cation on the clays, convertingthem to calcium-base clays. Figure 4shows the amount of calcium adsorbedby Wyoming bentonite and nativeclays. This cation exchange results inpartial dehydration of the hydrated clayparticles, reducing the size of the waterenvelope around the clay particles (seeFigure 5). The reduction in the size of the water envelope allows the clayparticles to come into contact withone another, resulting in flocculation.Flocculation causes an increase in theyield point and gel strengths. If a defloc-culant is not used, the size of the flocsof clay eventually will increase and mayprecipitate out, resulting in a gradualdecrease in the plastic viscosity.

If a deflocculant is used, then theclays will still have the reduced waterenvelope, but the flocs of clay will be dispersed.

This phenomenon occurs when cal-cium contamination occurs while drill-ing then is subsequently treated, orwhen a fluid is converted (brokenover) to a calcium-base drilling fluidsuch as a SPERSENEE/gyp or a SPERSENEE/lime system.

The concentration of reactive solids inthe drilling fluid determines the viscos-ity increase (viscosity hump) encoun-tered when calcium is added to thesystem (see Figure 6). Therefore, prior toconverting to a calcium-base system, orbefore drilling into formations that con-tain calcium (such as anhydrite), thereactive solids content of the drillingfluid should be reduced by dilutionwhile the viscosity is maintained withadditions of polymers.

Calcium systems provide solubleand reserve calcium in a drilling fluid.

Soluble calcium performs several func-tions. It provides wellbore inhibitionby minimizing the hydration of drillsolids and exposed shales throughbase exchange into calcium-basedclays. It makes a drilling fluid compati-ble with formations that contain high

Calcium-Treated Drilling Fluids

Figure 4: Adsorption of calcium by clays.

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14

12

10

8

6

4

2

00 500 1,000 1,500 2,000

Filtrate calcium (mg/l)

Cal

ciu

m a

dsor

bed

(mg/

g) Wyoming bentonite

Native clay

Figure 5: Reduction in water of hydration for sodiumclay during base exchange with calcium.

Na+

Na+

Na+

Ca2+

Water of hydration(envelope of water)

+ Ca2+

Flocculationcauses anincrease inthe yieldpoint and gelstrengths.

Ca2+

Figure 6: Effect of solids concentration on viscosity with calcium additions.

100

80

60

40

20

00 100 200 300 400 500 600 700 800

Filtrate calcium (mg/l)

Vis

cosi

ty (

cP)

High solids

Low solids

Water-Base Systems


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(CALCIUM-TREATED DRILLINGFLUIDS CONTINUED)concentrations of calcium, such asanhydrite. It precipitates carbonateions (CO32

) which result from carbondioxide (CO2) contamination.

The solubility of calcium is inverselyproportional to the pH of the drillingfluid. It is nearly insoluble at a pH above12.5, but is very soluble at a low pH.This is illustrated in Figure 7 where, onLine A (when only lime is added), thepH does not increase above 12.5, but on Line B (with added caustic), the pHincreases above 12.5 and the soluble calcium decreases rapidly. Therefore, calcium as lime (Ca(OH)2) helps tobuffer the pH when acid gases such as CO2 or hydrogen sulfide (H2S) are encountered.

Calcium solubility is also directlyrelated to salinity or chloride (Cl) con-centration. The soluble calcium in sea-water is often around 1,200 mg/l andwill increase as the salinity is increased,as shown in Figure 8. Figure 8 showsthe soluble calcium from gyp addedto increasing concentrations of salt.

SPERSENEE/GYP SYSTEMThe SPERSENEE/gyp (gypsum) system isdesigned to drill anhydrite (CaSO4)and/or provide inhibition while drillingwater-sensitive shales by using gypsum(CaSO42H2O) as the source of calcium.To maintain a sufficient amount of solu-ble calcium, the pH of the SPERSENEE/gypsystem should be kept low (9 to 10.5).The normal concentration of solublecalcium in this system is in the 600 to1,200 mg/l range. Since the solubility of calcium is affected by pH and salin-ity, the actual level will depend onthese properties.

When converting an existinguntreated or lightly treated system to a SPERSENEE/gyp system, the MBT andlow-gravity solids content should bereduced to minimize the break-overviscosity hump. Then, about 8 lb/bbl

gyp, 8 lb/bbl SPERSENEE and 2 lb/bblcaustic soda should be added simultane-ously over one or two circulations.After the initial conversion, propertiessuch as fluid loss, pH and alkalinityshould be refined by the additions ofthe proper materials. Materials thathave a low hardness tolerance shouldnot be used in this system. Since solu-ble calcium increases the hardness of the water phase, treatments withabout 2 lb/bbl SURFAK-ME are beneficialfor reducing the surface tension of thewater phase, and improving the per-formance of the chemical additives.

In addition to the maintenance procedures previously described, theexcess gyp test should be used to

Figure 8: Solubility of calcium vs. chlorides.

2.0

1.6

1.2

0.8

0.4

00 50 100 150 200

Chlorides (mg/l x 1,000)Solu

ble

calc

ium

(m

g/l

x 1,

000)

Figure 7: Line A - soluble calcium vs. lime concentration; Line B - Soluble calcium of 4 lb/bbl

of lime added to caustic solutions.

1,000

900

800

700

600

500

400

300

200

100

00 1 2 3 4 5 6Caustic soda or lime concentration (lb/bbl)

Cal

ciu

m (

mg/

l)

pH 12.4Line A

pH 12.2

pH 12

pH 12.4

pH 12.9pH 13.2Line B

Materialsthat have alow hardnesstoleranceshould notbe used inthis system.


10


(SPERSENEE/GYP SYSTEM CONTINUED)

monitor the concentration of excessgyp in the system. Mass-balance equa-tions cannot accurately monitor excessgyp, because gyp is removed from thesystem on drilled solids due to baseexchange.

Excess gyp procedureThe excess gyp content can be deter-mined by measuring the whole mudVersenate total hardness (Vt) and thetotal hardness of the filtrate (Vf) usingthis procedure and the calculationwhich follows:

Procedure to determine the gyp con-tent (see API RP13B-1, Appendix A.8):1. Add 5 ml whole mud to 245 ml

distilled water.2. Stir for 30 min at room temperature

or 15 min at 150F.3. Filter the solution with the API fil-

ter press. Discard the first cloudyportion of the filtrate. Collect theclear filtrate.

4. Pipette 10 ml of the collected clearfiltrate into a titration dish and add1 ml strong buffer and 4 to 6 dropsCalmagite Indicator.

5. Titrate with Standard Versenate to ablue or blue-green end point, recordthe number of ml of StandardVersenate as Vt.

6. To 1 ml of mud filtrate from thestandard API filtrate test, add 1 mlstrong buffer and 4 to 6 dropsCalmagite Indicator, titrate withStandard Versenate from wine-redto blue, record the number of ml of Standard Versenate as Vf.

Total calcium sulfate (lb/bbl) = 2.38 x Vt

Excess calcium sulfate (lb/bbl) = 2.38 x Vt - (0.48 x Vf x Fw)

Where:Fw = Water fraction from retort

NOTE: A simplified field methodtitrates 1 ml of whole mud in 150 to 350 ml distilled water in a quart jar,using 2 to 3 ml strong buffer and 1 to 2 ml Calmagite Indicator. Record the mlof Standard Versenate as the Vm. Thecolor change may be hard to see due to

Typical Properties

Density (lb/gal) 10 - 18

Funnel viscosity (sec/qt) (3.5 x mud weight)

Plastic viscosity (cP) See Figure 1

Yield point (lb/100 ft2) See Figure 1

Initial gel (lb/100 ft2) 1 - 5

10-min gel (lb/100 ft2) 1 - 10

pH 9.0 - 10.5

Pm (cm3 0.02N H2SO4) 0.5 - 2.5

Pf (cm3 0.02N H2SO4) 0.2 - 1.6

Calcium (mg/l) 600 - 1,200

Chlorides (mg/l) 0 - 20,000

Fluid loss (cm3/30 min) As needed

Low-gravity solids (%)* 4.5 - 7

MBT (lb/bbl) See Figure 1

Excess gyp (lb/bbl) 3 - 12

*See Figures 2 and 3.

Typical Products Primary Function

M-I BART Increase density

M-I GELT (prehydrated) Viscosity and fluid-loss control

Caustic soda Increase pH and PfGyp Calcium source

SPERSENEE Thinner

TANNATHINT Fluid-loss control

POLYPACT R API fluid-loss control

RESINEXT HTHP fluid-loss control

SURFAK-ME Surface-acting agent

ConcentrationMaterial (lb/bbl)

M-I BART or FER-OXT 0 - 550

M-I GELT 7.5 - 25

Caustic soda 0.2 - 1.5

Gyp 8 - 12

SPERSENEE 5 - 15

TANNATHINT 2.5 - 10

POLYPACT R 0 - 2

RESINEXT 3 - 6

SURFAK-ME 0 - 2

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the dark brown color of the lignosulfonateand lignite. This color change may appearto be from the original color of the solutionwith a red tint to only a slight green or blue-green tint. The rule-of-thumb calculation forthis procedure is:

Excess gyp (lb/bbl) = (VmVf) 2

SPERSENEE/LIME SYSTEMGenerally, SPERSENEE/lime systems areused to reduce the effects of acidgases such as CO2 or H2S and/or toreduce the hydration of formationclays. SPERSENEE/lime systems use lime(Ca(OH)2) as their source of calcium.Since lime has a high pH (12.4), thepH of the system will be high. ThepH of the system depends on the con-centration of lime and caustic soda(NaOH). Lime muds maintain a con-centration of excess lime which is notin solution, since the solubility oflime is an inverse function of pH.Therefore, this excess (reserve) limegoes into solution only as the pH ofthe system is reduced by reactionswith acidic contaminants incorpo-rated into the system during drillingoperations. This results in the excesslime having a buffering effect on thepH, which provides greater stability tothe system.

Lime muds are subdivided into low-,medium- and high-lime categoriesaccording to the amount of excesslime that they contain. This level ofexcess lime is chosen based on the antic-ipated severity of contamination andon local practice. Typical alkalinitiesand excess lime concentrations for the low-, medium- and high-lime cate-gories are shown below. These systemsare more stable if the Pf is kept roughlyequal to the excess lime content(lb/bbl). Lime muds generally are not used when mud densities are

below 10 lb/gal because it is difficult tomaintain rheological properties sufficientto clean the wellbore. Temperatures inexcess of 300F (149C) may causesevere gelation or cementation ofmedium- and high-lime drilling fluids.This severe gelation, or cementation,is caused by high alkalinity, high con-centrations of reactive solids and hightemperature which combine to formalumino-silica cement.

When converting an existinguntreated or lightly treated system to a SPERSENEE/lime system, the MBT and low-gravity solids content should be reduced to minimize the break-overviscosity hump. Then a treatment of 1to 10 lb/bbl lime, 2 to 12 lb/bbl SPERSENEEand 2 lb/bbl caustic soda should beadded simultaneously during one ortwo circulations. After the initial conver-sion, properties such as fluid loss, pHand alkalinity should be refined by theadditions of the proper materials.

In addition to the maintenance proce-dures previously described, the excesslime should be calculated as often asrequired to monitor the concentrationof excess lime in the system. Mass-balance equations cannot accuratelymonitor excess lime, because lime isremoved from the system on drilledclays as the result of base exchange.The equation for calculating excesslime is:

Excess lime (lb/bbl) = 0.26 (Pm - PfFw)

Generally,SPERSENEE/limesystems areused toreduce theeffects ofacid gases

Alkalinities

Low-lime Pf (cm3 0.02N H2SO4) 0.5 - 1

Pm (cm3 0.02N H2SO4) 2.4 - 4.8

Excess lime (lb/bbl) 0.5 - 1

Medium-lime Pf (cm3 0.02N H2SO4) 1 - 4

Pm (cm3 0.02N H2SO4) 4.8 - 19

Excess lime (lb/bbl) 1 - 4

High-lime Pf (cm3 0.02N H2SO4) 4 - 10

Pm (cm3 0.02N H2SO4) 19 - 46

Excess lime (lb/bbl) 4 - 9.4

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Typical Properties

Density (lb/gal) 10 - 16

Funnel viscosity (sec/qt) (3.5 mud weight)

Plastic viscosity (cP) See Figure 1

Yield point (lb/100 ft2) See Figure 1

Initial gel (lb/100 ft2) 1 - 5

10-min gel (lb/100 ft2) 1 - 10

pH 11.5 - 13.5

Calcium (mg/l) 40 - 200

Chlorides (mg/l) (freshwater) 0 - 5,000

Chlorides (mg/l) (seawater) 20,000

Low-gravity solids (%)* 4.5 - 7

MBT (lb/bbl) See Figure 1

Excess lime (lb/bbl) 1 - 10

*See Figures 2 and 3.

Typical Products Primary Function

M-I BART Increase density

M-I GELT (prehydrated) Viscosity and fluid-loss control

Caustic soda Increase PfLime Excess lime and

increase PmSPERSENEE Fluid loss and thinner

TANNATHINT Fluid-loss control

XP-20KE HTHP thinner and fluid-loss control

POLYPACT R Viscosity and API fluid-loss control

MY-LO-JELE Fluid-loss control

POLY-SALE Fluid-loss control

RESINEXT HTHP fluid-loss control

ConcentrationMaterial (lb/bbl)

M-I BART or FER-OXT 0 - 550

M-I GELT 15 - 30

Caustic soda 0.5 - 1.5

Lime 0.5 - 10

SPERSENEE 2 - 15

XP-20KE or TANNATHINT 3 - 8

RESINEXT 0 - 6

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(SPERSENEE/LIME SYSTEM CONTINUED)

calcium based mud systems (swaco)

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