Designing & Managing Drilling Fluid

8/10/2019 Designing & Managing Drilling Fluid

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What Influences the Choice of Fluid?

Among the many factors to consider whenchoosing a drilling fluid are the wellsdesign, anticipated formation pressures androck mechanics, formation chemistry, theneed to limit damage to the producing for-mation, temperature, environmental regula-tions, logistics, and economics (see CriticalDecisions, next page).

To meet these design factors, drilling flu-ids offer a complex array of interrelatedproperties. Five basic properties are usuallydefined by the well program and monitored

during drilling: rheology, density, fluid loss,solids content and chemical properties (seeBasic Mud Properties and Ingredients,page 36).3

For any type of drilling fluid, all five prop-erties may, to some extent, be manipulatedusing additives. However, the resultingchemical properties of a fluid dependlargely on the type of mud chosen. And thischoice rests on the type of well, the natureof the formations to be drilled and the envi-ronmental circumstances of the well.

33April 1994

Ben BloysARCO Exploration andProduction TechnologyPlano, Texas, USA

Neal DavisChevron Petroleum Technology CompanyHouston, Texas, USA

Brad SmolenBP Exploration Inc.Houston, Texas, USA

Louise BaileyOtto HouwenPaul Reid

John SherwoodCambridge, England

Lindsay FraserHouston, Texas, USA

Mike HodderMontrouge, France

In this article MSM (Mud Solids Monitor) and FMP (Fluid

Monitoring Package) are marks of Schlumberger.Fann 35 is a mark of Baroid Corporation.

For help in preparation of this article, thanks to JohnAstleford, Schlumberger Dowell, Bottesford, England;Thom Geehan, Schlumberger Dowell, Houston, Texas,USA, Alan McKee and Doug Oakley, SchlumbergerDowell, St. Austell, England; Eric Puskar, SchlumbergerDowell, Clamart, France.

1. For a comprehensive review of the role of drillingfluids:

Darley HCH and Gray GR: Composition and Proper-ties of Drilling and Completion Fluids, 5th ed. Hous-ton, Texas, USA: Gulf Publishing Co.,1988.

2. Geehan T, Helland B, Thorbjrnsen K, Maddin C,

McIntire B, Shepherd B and Page W: Reducing theOilfields Environmental Footprint, Oilfield Review 2,no. 4 (October 1990): 53-63.

Minton RC, McKelvie DS, Caudle DD, Ayres RC Jr.,Smith JP, Cline JT, Duff A, Blanchard JR and Read AD:The Physical and Biological Impact of Processed OilDrill Cuttings: E&P Forum Joint Study, paper SPE26750, presented at the Offshore Europe Conference,Aberdeen, Scotland, September 7-10, 1993.

3. For a full description of these properties and theirmeasurement:

Geehan T and McKee A: Drilling Mud: Monitoringand Managing It, Oilfield Review1, no. 2 (July1989): 41-52.

Gone are the days when drilling fluidor mud as it is commonly calledcomprised only clay and water.

Today, the drilling engineer designing a mud program chooses from a comprehensive catalog of ingredients.

The aim is to select an environmentally acceptable fluid that suits the well and the formation being drilled, to

understand the muds limitations, and then to manage operations efficiently within those limitations.

Designing and Managing

Drilling Fluid

There are good reasons to improve drillingfluid performance and management, notleast of which is economics. Mud may rep-resents 5% to 15% of drilling costs but maycause 100% of drilling problems. Drillingfluids play sophisticated roles in the drillingprocess: stabilizing the wellbore withoutdamaging the formation, keeping formationfluids at bay, clearing cuttings from the bitface, and lubricating the bit and drillstring,to name a few.1 High-angle wells, high tem-peratures and long, horizontal sectionsthrough pay zones make even more rigor-

ous demands on drilling fluids.Furthermore, increasing environmental

concerns have limited the use of some ofthe most effective drilling fluids and addi-tives.2 At the same time, as part of the indus-trys drive for improved cost-effectiveness,drilling fluid performance has come underever closer scrutiny.

This article looks at the factors influencingfluid choice, detailing two new types ofmud. Then it will discuss fluid managementduring drilling.


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4. Boll GM, Wong S-W, Davidson CJ and Woodland DC:Borehole Stability in Shales, paper SPE 24975, pre-sented at the SPE European Petroleum Conference,Cannes, France, November 16-18, 1992.

5. Allen D, Auzerais F, Dussan E, Goode P, RamakrishnanTS, Schwartz L, Wilkinson D, Fordham E, Hammond Pand Williams R: Invasion Revisited, Oilfield Review3, no. 3 (July 1991): 10-23.

6. Bailey L, Reid PI and Sherwood JD: Mechanisms andSolutions for Chemical Inhibition of Shale Swelling andFailure, presented at the Royal Society of Chemistry5th International Symposium, Chemistry in the OilIndustry, Ambleside, Cumbria, UK, April 12-14, 1994.

Steiger RP: Fundamentals and Use of Potassium/Poly-mer Drilling Fluids to Minimize Drilling and Comple-tion Problems Associated With Hydratable Clays,Journal of Petroleum Technology34 (August 1982):1661-1670.

OBrien DE and Chenevert ME: Stabilizing SensitiveShales With Inhibited, Potassium-Based Drilling Flu-ids, paper SPE 4232,Journal of Petroleum Technology25 (1973): 1089.

7. Chemical potential can be though of as an increase inthe internal energy of the system when one mole ofsubstance is added to an infinitely large quantity of themixture so as not to change its overall composition.

For more information about thermodynamic potentials:

Fletcher P: Chemical Thermodynamics for Earth Scien-tists. Harlow, Essex, England: Longman Scientific &Technical, 1993.

8. Mody FK and Hale AH: A Borehole Stability Model to

Couple Mechanics and Chemistry of Drilling FluidShale Interactions, paper SPE 25728, presented at theIADC/SPE Drilling Conference, Amsterdam, TheNetherlands, February 23-25, 1993.

Chenevert ME: Shale Control With Balanced ActivityOil-Continuous Muds,Journal of Petroleum Technol-ogy22 (1970): 1309-1319.

In WBM, there have been many efforts toprotect a water-sensitive formation from

mud filtrate. One technique is to introducea buffer in the form of blocking and plaster-ing agents, ranging from starches and cellu-loses, through polyacrylamides to asphaltsand gilsonites. Total control cannot beachieved in this way so specific inhibitingcationschiefly potassium [K+] and cal-cium [Ca2+] ionsare traditionally addedto the base water to inhibit the clay fromdispersingto stop it from breaking upwhen attacked by aqueous solution. This is

drilled, although often still at a high costand with considerable difficulty. Since then,there have been numerous variations on thistheme as well as other types of WBM aimedat inhibiting shale.

However, in the 1970s, the industryturned increasingly towards oil-base mud(OBM) as a means of controlling reactiveshale. Today, OBM not only provides excel-

lent wellbore stability but also good lubrica-tion, temperature stability, a reduced risk ofdifferential sticking and low formation dam-age potential. OBM has been invaluable inthe economic development of many oil andgas reserves.

The use of OBM would probably havecontinued to expand through the late 1980sand into the 1990s but for the realizationthat, even with low-toxicity mineral base-oil, the disposal of OBM cuttings can have alasting environmental impact. In many areasthis awareness led to legislation prohibitingor limiting the discharge of these wastes.

This, in turn, has stimulated intense activityto find environmentally acceptable alterna-tives and has boosted WBM research.

To develop alternative nontoxic muds thatmatch the performance of OBM requires anunderstanding of the reactions that occurbetween complex, often poorly character-ized mud systems and equally complex,highly variable shale formations.

Requisites for a Successful Drilling Fluid

Most OBM is an invert emulsion comprisingdroplets of aqueous fluid surrounded by oil,which forms the continuous phase. A layer

of surfactant on the surface of the waterdroplet acts like a semipermeable mem-brane, separating the aqueous solution inthe mud from the formation and its water.Water will pass through this membrane fromthe solution with the lowest concentrationof a salt to the one with the highestosmo-sis (right).

A key method of maintaining shale stabil-ity using OBM is to ensure that the ionicconcentration of the salts in the aqueousinternalphase of the mud is sufficientlyhigh, so that the chemical potential of thewater in the mud is equal to or lower than

that of the formation water in the shale.7When both solutions have the same chemi-cal potential, water will not move, leavingthe shale unchanged. If the water in theinternal phase of the mud has a lowerchemical potential than the fluid in the for-mation, water will travel from the shale tothe mud, drying out the rock. Unless dehy-dration is excessive, this drying out usuallyleaves the wellbore in good condition.8

35April 1994

Water migration Base oil

SurfactantFormation

(low salinity

water)

Water and

salts (high

salinity)

n Shale instability. In this example, Pierre shale has been exposed to a mud comprisingfresh water and bentonite gel. Because this fluid contains no inhibitors, water hasentered the shale causing it to swell and weakening the formation. Continuous flow ofmud has eroded the borehole leaving an enlarged hole that would be hard to log andcomplete. This simulation was carried out using the small wellbore simulator at Schlum-berger Cambridge Research, Cambridge, England.

n How oil-base muds semipermeable mem-brane works. OBM comprises droplets ofaqueous fluid surrounded by oil. A layer ofsurfactant on the surface of each waterdroplet acts like a semipermeable mem-brane, separating the aqueous solution inthe mud from the formation and its water.Water passes through this membrane fromthe solution with the lowest concentrationof salt to the one with the highest.


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Basic Mud Properties

Five basic properties are usually defined by the

well program and monitored during drilling:1

RheologyA high viscosity fluid is desirable to

carry cuttings to surface and suspend weighting

agents in the mud (such as barite). However, if

viscosity is too high, friction may impede the cir-

culation of the mud causing excessive pump

pressure, decrease the drilling rate, and hamper

the solids removal equipment. The flow regime

of the mud in the annulus is also affected by vis-

cosity.2 Measurements made on the rig include

funnel viscosity using a Marsh funnelan orifice

viscometerand plastic viscosity, yield point

and gel strength using a Fann 35 viscometer or

equivalent.

DensitySufficient hydrostatic pressure is

required to prevent the borehole wall from caving

in and to keep formation fluid from entering the

wellbore. The higher the density of the mud com-

pared to the density of the cuttings, the easier it

is to clean the holethe cuttings will be less

inclined to fall through the mud. If the mud

weight is too high, rate of drilling decreases, the

chances of differential sticking and accidentally

fracturing the well increase, and the mud cost

will be higher. The most common weighting

agent employed is barite. Density is measured on

the rig using a mud balance.

Fluid lossThe aim is to create a low-permeabil-

ity filter cake to seal between the wellbore and

the formation. Control of fluid loss restricts the

invasion of the formation by filtrate and

minimizes the thickness of filter cake that builds

up on the borehole wall, reducing formation dam-

age and the chances of differential sticking.

Static fluid loss is measured on the rig using a

standard cell that forces mud through a screen,

and also using a high-temperature, high-pressure

test cell.

Solids contentSolids are usually classified as

high gravity (HGS)barite and other weighting

agentsor low gravity (LGS)clays, polymers

and bridging materials deliberately put in the

mud, plus drilled solids from dispersed cuttings

and ground rock. The amount and type of solids in

the mud affect a number of mud properties. A high

solids content, particularly LGS, will increase

plastic viscosity and gel strength. High-solidsmuds have much thicker filter cakes and slower

drilling rates. Large particles of sand in the mud

cause abrasion on pump parts, tubulars, measure-

ment-while-drilling equipment and downhole

motors. Measurement of total solids is tradition-

ally carried out using a retortwhich distils off the

liquid allowing it to be measured, leaving the

residual solids.

Chemical propertiesThe chemical properties of

the drilling fluid are central to performance and

hole stability. Properties that must be anticipatedinclude the dispersion of formation clays or disso-

lution of salt formations; the performance of other

mud productsfor example, polymers are

affected by pH and calcium; and corrosion in the

well (see Corrosion in the Oil Industry, page 4).

Measurement rigside usually relies on simple

chemical analysis to determine pH, Ca2+, total

hardness, concentrations of Cl- and sometimes K+.

Mud Ingredients

WaterIn water-base mud (WBM) this is the

largest component. It may be used in its natural

state, or salts may be added to change filtrate

reactivity with the formation. Water hardness is

usually eliminated through treatment and alkalin-

ity is often controlled.

Weighting agentsThese are added to control for-

mation fluid pressure. The most common is barite.

ClayMost commonly, bentonite is used to pro-

vide viscosity and create a filter cake on the bore-

hole wall to control fluid loss. Clay is frequently

replaced by organic colloids such as biopolymers,

cellulose polymers or starch.

PolymersThese are used to reduce filtration,

stabilize clays, flocculate drilled solids and

increase cuttings-carrying capacity. Cellulosic,

polyacrylic and natural gum polymers are used in

low-solids mud to help maintain hole stability and

minimize dispersion of the drill cuttings. Long-

chain polymers are adsorbed onto the cuttings,

thereby preventing disintegration and dispersion.

36 Oilfield Review

Basic Mud Properties and Ingredients

1. For a complete description of the traditional mud check

techniques:

Geehan T and McKee A: Drilling Mud: Monitoring and

Managing It, Oilfield Review1, no. 2 (July 1989): 41-52.

2. Plastic viscosity (PV) and yield point (YP) are related

parameters and follow common oilfield conventions

based on the Bingham rheological model. PV is largely

dependent on the type of mud and its solids content. The

lower the PV, the faster the drilling penetration rate. How-

ever, this is limited by the YP, which is a direct measure of

the fluids cuttings-carrying efficiency.

For details of rheology:

Bittleston S and Guillot D: Mud Removal: Research

Improves Traditional Cementing Guidelines, Oilfield

Review3, no. 2 (April 1991): 44-54.

ThinnersThese are added to the mud to reduce

its resistance to flow and to stifle gel develop-

ment. They are typically plant tannins, polyphos-

phates, lignitic materials, lignosulfonates or syn-

thetic polymers.

SurfactantsThese agents serve as emulsifiers,

foamers and defoamers, wetting agents, deter-

gents, lubricators and corrosion inhibitors.

Inorganic chemical sA wide variety of inorganic

chemicals is added to mud to carry out various

functions. For example, calcium hydroxide is

used in lime mud and calcium chloride in OBM;

sodium hydroxide and potassium hydroxide

(caustic soda and caustic potash) are used to

increase mud pH and solubilize lignite; sodium

carbonate (soda ash) to remove hardness, sodium

chloride for inhibition and sodium chloride has

many usessuch as increasing salinity, increas-

ing density, preventing hydrate formation and pro-

viding inhibition.

Bridging materialsCalcium carbonate, cellulose

fibers, asphalts and gilsonites are added to build

up a filter cake on the fractured borehole and help

prevent filtrate loss.

Lost circulation materia lsThese are used to

block large openings in the wellbore. Theseinclude walnut shells, mica and mud pills con-

taining high concentrations of xanthum and modi-

fied cellulose.

Specialized chemicalsScavengers of oxygen,

carbon dioxide or hydrogen sulfide are sometimes

required, as are biocides and corrosion inhibitors.


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achieved by providing cation exchange withthe clays in the shalethe K+ or Ca2+ com-monly replace the sodium ion [Na+] associ-ated with the clay in the shale, creating amore stable rock that is better able to resisthydration. Hence KCl-PHPA fluids.9

The movement of WBM filtrate from thewellbore into the surrounding shale is con-trolled by the difference between the chemi-

cal potentials of the various species in themud, and the corresponding chemicalpotentials within the formation. Chemicalpotential depends both on the muds hydro-static pressure in the wellbore and on itschemical composition.10

To design an effective WBM, it is neces-sary to know the relative importance of muddifferential pressure versus chemical con-centration and composition, and how thisrelates to the type of mud and formation.For example, if the rock is chemically inertto WBM filtrate (as is the case with sand-stone), then invasion is controlled solely by

the differences between the welIbore pres-sure and the pore pressure within the rock.But for shale, opinion varies. Some experi-menters suggest that the shale itself can actas a semipermeable membrane, making thechemical components the key determinant.

Researchers at Schlumberger CambridgeResearch tested Pierre shale and found thatit behaves as an imperfect ion exclusionmembrane and that the role of chemicaldifferences between wellbore fluid andpore fluid is less significant than the differ-ence in pressure between the mud and theformation.11 This result is an oversimplifica-

tion since it does not consider what hap-pens after fluid invades the formation rais-ing its pore pressure. However, it doessuggest that mud weight should be kept aslow as well safety and mechanical welIborestability considerations allow.12 These andother results are now being used to designmore effective WBM systems and evaluatethose that are already available (see Strate-gies for Improving WBM Shale Inhibition,page 39).

A number of relatively new types of mudsystems have been introduced. For example,one route is to substitute the oil phase in

OBM with synthetic chemicals. In this way,the excellent characteristics of OBM may bereproduced with a more rapidly biodegradedcontinuous phase than was available before.

Typical synthetic base chemicals includeesters, ethers, polyalphaolefins, linear olefinsand linear alkyl benzenes. One of the chiefdisadvantages of these systems is that theytend to be relatively expensive compared toconventional OBM. However, such systemscan still be cost-effective options compared

those laid down in Norway, the UK, TheNetherlands, Denmark and the USA.

Glycols in mud were proposed as lubri-cants and shale inhibitors as early as the1960s. But it was not until the late 1980sthat the materials became widely consid-ered. Properly engineered polyol muds arerobust, highly inhibitive and often cost-effective. Compared with other WBM sys-

tems, low volumes are typically required.Polyols have a number of different effects,such as lubricating the drillstring, opposingbit balling (where clays adhere to the bit)and improving fluid loss. Today, it is theirshale-inhibiting properties that attract mostattention. For example, tests carried out byBP show that the addition of 3 to 5% by vol-ume of polyglycol to a KCl-PHPA mud dra-matically improves shale stabilization(below). However, a significant gap still

9. Hale AH and Mody FK: Partially Hydrolyzed Poly-acrylamide (PHPA) Mud Systems for Gulf of MexicoDeepwater Prospects, paper SPE 25180, presentedat the SPE International Symposium on OilfieldChemistry, New Orleans, Louisiana, USA, March 2-5, 1993.

Bol GM, The Effect of Various Polymers and Saltson Borehole and Cutting Stability in Water-Base

Shale Drilling Fluids, paper SPE 14802, presentedat the SPE/IADC Drilling Conference, Dallas, Texas,USA, February 10-12, 1986.

10. Sherwood JD and Bailey L: Swelling of ShaleAround a Cylindrical Wellbore, Proceedings of theRoyal Society444, London, England (1994): 161-184.

Hale AH, Mody FK and Salisbury DP: ExperimentalInvestigation of the Influence of Chemical Potentialon Wellbore Stability, paper SPE 23885, presentedat the SPE/IADC Drilling Conference, New Orleans,Louisiana, USA, February 18-21, 1992.

11. See Bailey et al, reference 6.

12. For details of how mud weight affects mechanicalstability:

Addis T, Last N, Boulter D, Roca-Ramisa L andPlumb D: The Quest For Borehole Stability in theCusiana Field, Colombia, Oilfield Review 5, no.2/3 (April/July 1993): 33-43.

Steiger RP and Leung PK: Predictions of WellboreStability in Shale Formations at Great Depth, Maury

V and Fourmaintraux D (eds): Rock at Great Depth.Rotterdam, The Netherlands: A.A. Balkema (1990):1209-1218.

13. Reid PI, Elliott GP, Minton RC, Chambers BD andBurt DA: Reduced Environmental Impact andImproved Drilling Performance With Water-BasedMuds Containing Glycols, paper SPE 25989, pre-sented at the SPE/EPA Exploration and ProductionEnvironmental Conference, San Antonio, Texas,USA, March 7-10, 1993.

Downs JC, van Oort E, Redman DI, Ripley D andRothmann B: TAME: A New Concept in Water-Based Drilling Fluids for Shales, paper SPE 26999,presented at the Offshore Europe Conference,Aberdeen, Scotland, September 7-10, 1993.

to WBMparticularly where OBM wouldhave been used prior to the introduction ofnew environmental constraints.

The State of the WBM Art

This article will now concentrate onadvances in WBM technology by looking attwo distinct directions of development: theuse of polyols for shale inhibition and the

introduction of mixed-metal hydroxides toimprove hole cleaning and help reduce for-mation damage.

Polyol mudsPolyol is the generic namefor a wide class of chemicalsincludingglycerol, polyglycerol or glycols such aspropylene glycolthat are usually used inconjunction with an encapsulating polymer(PHPA) and an inhibitive brine phase (KCl).13

These materials are nontoxic and pass thecurrent environmental protocols, including

37April 1994

n

Improving inhibition with addition of polyglycol. This chart shows the recovery of cut-tings comprising Tertiary shaleLondon Clay that contains about 20% smectitethathave been exposed to different muds in an aggressive dispersion test. This test is anindication of a muds shale stabilizing qualities rather than a simulation of downholeconditions. The weight of the cuttings before treatment is compared to the weight after-wards. Recovery increased from about 40% to 80% with the addition of polyglycol to aKCl-PHPA mud. Conventional seawater-polymer mud yields about 10%, while OBMshowed almost 100% recovery.

KCL @ 25 lbm/bbl

PHPA @ 0.75 lbm/bbl

KCI/PHPA

20

0

Seawater/polymer mud

Shalerecovery,wt%

3% Polyol additive

40

60

80

100

OBM


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remains between the performance of polyolmuds and that of OBM.

Field experience using polyol muds hasshown improved wellbore stability andyielded cuttings that are harder and drierthan those usually associated with WBM.This hardness reduces breakdown of cut-tings and makes solids control more effi-cient. Therefore, mud dilution rates tend tobe lower with polyol muds compared withother WBM systems (for an explanation of

solids control and dilution, see mud man-agement, page 39).As yet, no complete explanation of how

polyols inhibit shale reactivity has beenadvanced, but there are some clues:Most polyols function best in combination

with a specific inhibitive salt, such aspotassium, rather than nonspecific highsalinity.

Polyol is not depleted rapidly from themud even when reactive shales aredrilled.

Many polyols work effectively at concen-trations as low as 3%, which is too low tosignificantly change the water activity ofthe base fluid.

Polyols that are insoluble in water are sig-nificantly less inhibitive than those thatare fully soluble.

No direct link exists between the perfor-

mance of a polyol as a shale inhibitor andits ability to reduce fluid loss.Many of these clues eliminate theories thattry to explain how polyols inhibit shales.Perhaps the most likely hypothesisalthough so far there is no direct experimen-tal evidence supporting itis that polyolsact as a structure breaker, disrupting theordering of water on the clay surface thatwould otherwise cause swelling and disper-sion. This mechanism does not require theglycol to be strongly adsorbed onto theshale, which is consistent with the lowdepletion rates seen in the field.

Mixed-metal hydroxide (MMH) mudMMH mud has a low environmental impactand has been used extensively around theworld in many situations: horizontal andshort-radius wells, unconsolidated ordepleted sandstone, high-temperature,unstable shales, and wells with severe lostcirculation. Its principal benefit is excellenthole-cleaning properties.14

Many new mud systemsincludingpolyol mudsare extensions of existing flu-

ids, with perhaps a few improved chemicalsadded. However, MMH mud is a completedeparture from existing technology. It isbased on an insoluble, inorganic, crystallinecompound containing two or more metalsin a hydroxide latticeusually mixed alu-minum/magnesium hydroxide, which isoxygen-deficient. When added to prehy-drated bentonite, the positively charged

38 Oilfield Review

n Comparison of the rheologies of MMH and conventional PHPA mud. For MMH, the relatively high 3-and 6-rpm readings and low 300- and 600-rpm readings result in a flat rheology profile that is quite dif-ferent from that of conventional PHPA mud. With use of a Huxley-Bertram rheometer to measure therheologies at 190F [88C] and 2500 psi, the MMH shows a relatively high shear-stress intercept and anearly linear rheologic profile. This contrasts with the downward curve of the PHPA mud. [Adapted fromSparling DP and Williamson D: Mixed Metal Hydroxide Mud Improves Drilling in Unstable Shales, Oil & Gas Journal 89

(June 10, 1991): 29.]

50

20

40

10

30

00 3 6 100 200 300 600

Fann rheometer speed, rpm

MMHPHPA, Partially hydrolized polyacrylamide

D

ialreading

Rheology Profile

28

16

24

12

20

00 200 400 600 800 1000

Shear rate, sec1

MMHPHPA

S

hearstress

Rheology Profile at 190F and 2500 psi

4

8


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MMH particles interact with the negativelycharged clays forming a strong complex thatbehaves like an elastic solid when at rest.

This gives the fluid its unusual rheology:an exceptionally low plastic viscosity-yieldpoint ratio. Conventional muds with highgel strength usually require high energy toinitiate circulation, generating pressuresurges in the annulus once flow has been

established. Although MMH has great gelstrength at rest, the structure is easily bro-ken. So it can be transformed into a low-vis-cosity fluid that does not induce significantfriction losses during circulation and givesgood hole cleaning at low pump rates evenin high-angle wells (previous page). Yetwithin microseconds of the pumps beingturned off, high gel strength develops, pre-venting solids from settling.

There are some indications that MMHalso provides chemical shale inhibition.

This effect is difficult to demonstrate in thelaboratory, but there is evidence that a staticlayer of mud forms adjacent to the rock faceand helps prevent mechanical damage tothe formation caused by fast-flowing mudand cuttings, controlling washouts.15

MMH is a special fluid sensitive to manytraditional mud additives and some drillingcontaminants. It therefore benefits from the

careful management that is vital for all typesof drilling fluid.

Mud ManagementKeeping the Fluid

in Shape

Selecting a reliable chemical formulation forthe drilling fluid so that it exhibits therequired properties is one part of the job.Maintaining these properties during drillingis another.

Circulation of drilling fluid may be con-sidered a chemical process with the well-

bore acting as a reactor vessel. In this reac-tor, the composition of the drilling fluid willbe changed dynamically by such factors asfiltration at the wellbore and evaporation atsurface; solids will be added and takenaway by the drilling process and the solids-control equipment; chemicals will be lost as

39April 1994

Strategies for Improving WBM Shale Inhibition

Researchers at Schlumberger Cambridge Research, Cambridge, England, have proposed a number of

strategies for developing mud formulations with improved shale inhibition.1

Preventing Filtrate Access

Creation of a semipermeable membraneIf an

effective membrane can be produced on the sur-

face of the shale by adding suitable surfactants to

WBM, then water ingress could be controlled

using chemical activity as in OBM. This effect

was obtained, to some degree, with the direct-

emulsion WBM used occasionally in the 1980s.

The challenge is to identify effective surface

active molecules that are environmentally

acceptable, do not unduly affect other mud prop-

erties and, ideally, show low depletion rates.

Provision of fluid-loss controlConventional

fluid-loss control polymers produce mud filter

cakes that are typically one or two orders of mag-

nitude higher in permeability than shales. Even if

fractures are present, such polymers may be

effective at plugging these relatively large holes,

but filter cakes are otherwise unlikely to form on

shale. If they did, the shalethe less permeableof the two solid phaseswould still control the

rate of fluid transport. Given the small dimen-

sions of pores in shaleson the order of

nanometersfluid loss control is likely to be best

achieved either by chemical reactions that greatly

reduce, or even eliminate, permeability or by

molecules small enough to block pore throats.

Increasing the viscosity of the filtrate

By increasing the viscosity of the filtrate (using for

example, silicates or glycols) the rate of ingress

is reduced. However, this slowing may not be

sufficient to control wellbore stability and the

mud may have an infeasibly high plastic viscosity.

Minimizing Subsequent Swelling

If invasion of a WBM filtrate cannot be avoided,

appropriate design of the filtrate chemistry may be

used to minimize the swelling response of the

shale. However, even if swelling is effectively

inhibited, filtrate invasion of the shale will

increase the pore pressure and add to possible

mechanical failure of the rock.

Control of ionic strengthThe salinity of the fil-trate should be at least as high as that of the pore

fluid it replaces.

Choice of inhibiting ionCations such as potas-

sium should be incorporated into the formulation.

These will replace ions such as sodium found in

most shales to produce less hydrated clays with

significantly reduced swelling potential. Any

inhibitors added to the mud should have sufficient

14. Fraser L and Enriquez F: Mixed Metal HydroxideFluids Research Widens Applications, Petroleum

Engineer International 63 (June 1992): 43-45.Fraser LJ and Haydel S: Mixed Metal HydroxideMud Application in Horizontal WellsCase StudiesUnder Diverse Drilling Conditions, presented at the5th International Conference on Horizontal WellTechnology, Houston, Texas, USA, November 9-11,1993.

15. Fraser LJ: Unique Characteristics of Mixed MetalHydroxide Fluids Provide Gauge Hole in DiverseTypes of Formation, paper SPE 22379, presented atthe SPE International Meeting on Petroleum Engi-neering, Beijing, China, March 24-27, 1992.

Lavoix F and Lewis M: Mixed Metal HydroxideDrilling Fluid Minimizes Well Bore Washouts, Oil& Gas Journal 90 (September 28, 1992): 87-90.

1. Bailey L, Reid PI and Sherwood JD: Mechanisms and

Solutions for Chemical Inhibition of Shale Swelling and

Failure, presented at the Royal Society of Chemistry 5th

International Symposium, Chemistry in the Oil Industry,Ambleside, Cumbria, UK, April 12-14, 1994.

concentration to remain effective as the filtrate

travels through the shale.

Although potassium ions reduce clay swelling,

they rarely eliminate it. Recently, there have

been attempts to find more effective cationsfor

example, aluminium complexes or low molecular

weight, cationic polymers.

Use of ceme nting agentsAn alternative

approach may be to use mud additives that react

with the clay minerals and/or pore fluids present

in shales to produce cements that strengthen the

rock and prevent failure. In field trials, silicate

and phosphate salts have demonstrated the

potential to cement the formation, although some

drilling difficulties unrelated to welIbore stability

have been reportedfor example, hole cleaning.


8/11

they adhere to the borehole wall and to cut-tings, and they will be added routinely atsurface; formation fluids will contaminatethe mud, perhaps causing flocculation orloss of viscosity, and oxygen may becomeentrained. Temperature, pressure and possi-ble bacterial action may also have signifi-cant effects.

Under these circumstances effective man-

agement is not trivial. Nevertheless, basicprocess control techniques have beenapplied rigside for some years to aid in theselection and maintenance of the fluid for-mulation and to optimize the solids-controlequipmentsuch as shale shakers and cen-trifuges (next page).16 This approach is oftenlinked to incentive contracts, where savingsin mud costs are shared between contractorand operator, and has led to remarkablesavings in mud costs.

For example, with a systems approach todrilling fluid management for 16 wells off-shore Dubai, mud costs were cut in half and

reduced as a proportion of total drillingcosts from 6% to 3%. At the same time,hole condition remained the same or bet-terthis was assessed by looking at holediameter, time to run casing and mud usageper foot of well drilled.17

Such an approach is based on threepremises: More frequent and more precise measure-

ments, for example five mud checks perday and the introduction of advancedmeasurement techniques (more aboutthese later)

Efficient data management using mass

balance techniqueswhich track thevolumes of chemicals, hole and cut-tingsand computerized data storageand acquisition

Integration of the management of thesolids control equipment with that of thedrilling fluids.

Solids-control efficiencythe percentageof drilled solids removed versus the totalamount drilledis central to drilling effi-ciency and is a function of the surfaceequipment, drilling parameters and mudproperties. For example, muds that have alower tendency to hydrate or dispersedrilled cuttings generally give higher solids-control efficiency.

The significance of solids control is thatpenetration rate is closely linked to the vol-ume of solids in the fluid. The greater theamount of solids, the slower the rate ofdrilling (below). Mud solids are dividedinto two categories: high-gravity solids(HGS) comprising the weighting agent, usu-ally barite; and low-gravity solids (LGS)made up from clays, polymers and bridgingmaterials deliberately put in the mud, plusdrilled solids from dispersed cuttings andground rock.

The volume of HGS should be maxi-mized, so that the total volume of solids in

the mud is minimized, while still achievingthe density required to control formationpressures. Therefore, drilled solids must beremoved by the solids-control equipment.However, some solids become dispersed asfine particles that cannot be removed effec-

tively. In this case, the fluid must be dilutedwith fresh mud containing no drilled solids.

But desirable properties are not alwaysoptimum ones. For instance, zero drilledsolids at the bit is desirable. However,achieving zero drilled solids would increasemud costs dramatically.18 It is the job ofmud management to plot the optimumcourse. To do this successfully requires

accurate and regular input data.Traditional field practice is to measuremud density and viscosity (using a Marshfunnel) about every 30 minutes at both thereturn line and the suction pit. Other prop-ertiessuch as rheology, mud solids, fluidloss, oil/water ratio (for OBM), pH, cation-exchange capacity, and titrations for chlo-ride and calciumare measured onceevery 8 or 12 hours (depending on drillingconditions) using 1-liter samples taken fromthe flowline or the active pit. These deter-minations are then used as a basis for mudtreatment until the next set of measure-

ments is made.To gain better control over the mud sys-tem, a more meaningful monitoring strategymay be required. Simply increasing the fre-quency of traditional measuring techniquesto at least five times a day and making sam-pling more representative of the whole mudsystem has improved control and signifi-cantly reduced the amount of chemicalsused to drill a well.19 However, new typesof measurement are now available. Twonew monitoring systems developed byDowell are the MSM mud solids monitorand the FMP fluids monitoring package.

Mud Solids MonitorA common indica-tor describing the solids content in the mudis the LGS-HGS volume ratio. This is tradi-tionally measured using the retort, a tech-nique that requires good operator skills,takes at least 45 minutes and often has anerror margin of more than 15%.

The Dowell MSM system takes the placeof the retort. Without complicated samplepreparation, it offers a 10-minute test withan accuracy of more than 95%. The basicmeasurement uses X-ray fluorescence (XRF).A standard software package uses the bar-ium fluorescence and backscattering inten-

sity from XRF spectra, together with the fluiddensity to predict the concentrations of bar-ium and water. From these primary outputsthe LGS concentration is also determined.As an off-line measurement, XRF has the

40 Oilfield Review

n Mud solids versus rate of penetration.The greater the quantity of solids in themud, the slower the rate of drilling.

12

00

Solids content volume, %

Drilling

rate,

ft/hr

8

4 8 12 16

4


9/11

n Cleaning the mud. The cuttings-removalperformance of solids-control equipmentdepends on many factors, including thesize of the mesh for the shale shakerscreen, flow rate and density of the drillingfluid, and the size of the cuttings. Decidinghow to use the surface equipment alsodepends in part on the type of mud run.

With the shale shakers, the aim is to

choose a screen mesh size that sieves outas much of the drilled solids as possible,leaving barite, which is finer, in the sys-tem. However, the finer the screen, thelower the throughput of mud and the moreshale shaker capacity required. In thiscase, the choice is either to install an extra

shale shaker or to fit a wider mesh screenallowing more of the solids to remain inthe fluid that must then be diluted withnew, clean mud.

Centrifuges may be used to controlfines. For a low-density mud containingmostly drilled solids, the aim is to stripaway as much of the solids as possible.However, if the mud is weighted, fines-

control strategy depends on the liquidphase. If the liquid phase is relativelycheap (for example, a seawater-lignosul-fonate mud), the barite is the most valu-able part of the fluid. In this case, the cen-trifuge is used to remove all the baritewhile the rest of the fluid may be dis-

posed of. However, if the liquid phase isalso valuable (such as in OBM, KCl-PHPAor glycol muds), both phases are worthkeeping. In this case, two centrifugesmay be used. First, to remove the barite,which may be reused. Then, the remain-ing larger solidsassumed to be drilledsolidsmay be removed and disposed ofand the liquid returned to the active sys-

tem. Clearly, treating mud with the cen-trifuge is a lengthy process and cen-trifuges can typically handle only about15% of the active system.

41April 1994

16. The MUDSCOPE service was originally developedby Sedco Forex, but has subsequently been offeredby Dowell IDF Fluid Services.

Geehan T, Dudleson WJ, Boyington WH, Gilmour Aand McKee JDA: Incentive Approach to Drill FluidsManagement: An Experience in Central North Sea,paper SPE18639, presented at the SPE/IADC DrillingConference, New Orleans, Louisiana, USA, Febru-ary 28-March 3, 1989.

17. Moore DJ, Forbes DM and Spring CR: A SystemsApproach to Drilling Fluids Management ImprovesDrilling Efficiency: A Case Study on the NN Platformin the Arabian Gulf, paper SPE 25646, presented atthe SPE Middle East Oil Technical Conference andExhibition, Bahrain, April 3-6, 1993.

18. Beasley RD and Dear SF: A Process EngineeringApproach to Drilling Fluids Management, paperSPE 19532, presented at the 64th SPE Annual Tech-nical Conference and Exhibition, San Antonio,Texas, USA, October 8-11, 1989.

19. Geehan T, Forbes DM and Moore DJ: Control ofChemical Usage in Drilling Fluid Formulations toMinimize Discharge to the Environment, paper SPE23374, presented at the First International Confer-

ence on Health, Safety and Environment, TheHague, The Netherlands, November 10-14, 1991.

Mud from hole

Down hole

Shale shaker

Solids to waste

Barite

(HGS)

New mud

Centrifuge 2

LGS discharge

Degasser

Mud pump

Centrifuge 1

Mud


10/11

advantages of more frequent measurement,greater precision and less dependence onoperator skills (right).20

These data provide the basis for informedmud management decisions. For example,

using the MSM package offshore Congo,inflows and outflows through the desanderand desilter were monitored. From thesemeasurements, the amount of barite andLGS being dumped on an average day wascalculated. The MSM package showed thatthe desander and desilter were removing alot of valuable barite and not enough of theunwanted LGS.

Analysis of the MSM data showed that ineliminating 11.5 tons [10,430 kg] of LGS perdaythe capacity of the desander anddesiltersome 45 cubic meters [1590 ft3] ofmud were lost, requiring a maintenance

treatment including 41.65 tons [37,800 kg]of barite. Based purely on the cost of thebarite, it was found to be more cost-effec-tive to dispose of 60 cubic meters [2120 ft3]of mud and dilute the remaining systemwith new mud requiring only 23.25 tons[22,900 kg] of barite, saving $3339 per day.These findings may vary if mud componentcosts are included in the analysismanyinhibitive muds have high-value liquidphasesand if the environmental impact ofdumping the mud is considered.

Fluid Monitoring PackageAt the heart ofthe system is an in-line skid that continu-

ously monitors the rheology, density, pH,temperature and electrical conductivity ofthe mud (above). Data are stored on harddisk and may be viewed on screen in real ordeferred time and on hard copy. Data corre-late with data obtained using standard rigequipment, but of course they are continu-ously delivered.

For example, rheology is measured usingthree pipe rheometers. Each of these coiledpipes has a different length and diameterand therefore exerts different shear on the

sample of mud as it passes at a known ratethrough the pipe. Pressure drop on enteringand leaving each pipe may then be equatedto shear stress. So that data are presented ina form that is comparable to traditionalinformation, shear rate and shear stress areconverted to equivalent Fann 35 viscometerreadings (next page, left). From these, plas-tic viscosity and yield-point readings maybe derived. However, while mud rheologyis traditionally measured at constant temper-ature, the FMP continuous measurement is

made as the mud temperature fluctuatesduring drilling.

The FMP service is currently being field-tested in Europe and Africa. In one field trial

lasting five weeks, the FMP was tested ontwo wellsites for over 915 hours. The systemwas exposed to three different mud systemsformate, KCl-gypsum, and NaCl saturatedand a wide temperature range10C to79C [50F to 174F]. The tests showed thatthe hardware is capable of withstanding therugged demands of drilling, and yieldeduseful mud logs (next page, right).

42 Oilfield Review

n FMP Fluid Moni-toring Package sen-sor skid and controlrack. This is the first

prototype skidwhich was devel-oped in France, is acomplete packagecomprising a sensorskid, feed pump,control rack, work-station with monitor

and printer, and thesoftware.

n Comparison of thesolids content ofmuds using the tra-ditional retort andMSM measurements

In this example theretort measurementoverestimates thebarite content, whilethe MSM measure-ment indicates a rel-atively largeramount of drilledsolids. If decisionshad been based onthe retort measure-ment, necessaryremedial action forthe mud would nothave been carriedout and drilling effi-ciency would have

suffered.

25

20

15

10

5

0800 1000 1200 1400 1600 1800 2000 2200

Depth, m

Volum

e,

%

Measurement by Retort Method

25

20

15

10

5

0800 1000 1200 1400 1600 1800 2000 2200

Depth, m

Volume

,%

Measurement by MSM

LGS

Barite

LGS

Barite


11/11

Future DevelopmentsIt is still early days for these techniques, butsuch measurements, and others in develop-ment, will furnish the information requiredto help control a fully automated mud pro-cessing plant.21

Joint industry field trials are already underway to automate mud management. Theaim is to deliver a system with automatedsolids-control equipment, automated addi-tion of mud chemicals, continuous monitor-ing of key mud parameters, automated mudsystem valve control and tank lineup, andcentral monitoring of integrated process

control. A demonstration system has beeninstalled on the semisubmersible rig Sedco712, working in the UK sector of the NorthSea, to allow full-scale evaluation.22

However, it is clear that the driving forcefor automated mud processing, and otherfuture developments, must be more cost-effective drilling, improved employee healthand environmental compliance, andenhanced well performance. CF

43April 1994

20. Houwen OH, Sanders MW, Anderson DR, ProuvostL, Gilmour A and White DB: Measurement ofComposition of Drilling Mud by X-Ray Fluores-cence, paper SPE 25704, presented at theSPE/IADC Drilling Conference, Amsterdam, TheNetherlands, February 23-25, 1993.

21. Hall C, Fletcher P, Hughes TL, Jones TGJ, MaitlandGC and Geehan T: Mud Analysis and Control forDrilling, presented at the 4th European CommunitySymposium on Oil and Gas in a Wider Europe,Berlin, Germany, November 3-5, 1992.

Hughes TL, Jones TGJ and Geehan T: The ChemicalLogging of Drilling Fluids, paper SPE 23076, pre-sented at the Offshore Europe Conference,Aberdeen, Scotland, September 3-6, 1991.

Hughes TL, Jones TGJ, Tomkins PG, Gilmour A,Houwen OH and Sanders M: Chemical Monitoringof Mud Products on Drilled Cuttings, paper SPE23361, presented at the First International Confer-ence on Health, Safety and Environment, TheHague, The Netherlands, November 10-14, 1991.

22. The demonstration project is being undertaken bySedco Forex, Dowell, Thule Rigtech and MarineStructure Consultants (M.S.C.) bv. It is partiallyfunded by The Commission of European Communi-ties Thermie project, Shell UK Exploration and Pro-duction, Conoco (UK) Limited and BP InternationalLimited.

Murch DK, White DB, Prouvost LP, Michel GL andFord DH: Integrated Automation for a Mud Sys-tem, paper SPE 27447, presented at the SPE/IADCDrilling Conference, Dallas, Texas, USA, February15-18, 1994.

Minton RC and Bailey MG: An assessment of Sur-face Mud System Design Options for Minimising theHealth, Safety and Environmental Impact ConcernsAssociated With Drilling Fluids, paper SPE 23362,presented at the First International Conference onHealth, Safety and Environment, The Hague, TheNetherlands, November 10-14, 1991.

n Minute-by-minutemud information.This example of anFMP log from pilottests shows how themud parameterschange over 2 hoursand 20 minutes.The right columnincludes benchtests carried out tovalidate the FMP

measurements.

n Comparing plastic viscosity (PV) datagathered in the field from KCl mud usingthe FMP skid with that generated the tra-ditional way using a Fann 35 viscometer.

Take sample

pH skid 2

(PH)

Conductivity skid 2

Temperature skid 2

Flow rate skid 2

(GPM)

(DEGF)

(MS/C)

Pressurized mudbalance 15.26 ppg

FMP 15.23 ppg

Start add 50 kgbarite

Barite additionfinished

Take sample 2

Take sample 3

Fann PV/YP 37/28

Fann PV/YP 32/20

Pressurized mudbalance 15.91 ppg

FMP 15.9 ppg

FMP 15.91 ppgPressurized mudbalance 15.91

Bench pH 10.08

Bench pH 9.697

Bench temp. 22C

Add 2 kg of NaCl

Add 5 kg of NaCl

Bench conductivity1.0 mS (25C comp.)

Bench conductivity8.9 mS (25C comp.)

15:0

15:10

15:20

15:30

14:40

127

1000

1000

100

Yield point skid 2

Plastic viscosity skid 2

Density skid 2

(PPG)

(CP)

(LCF2) 500

500

1614

14:50

16:50

17:00

16:40

16:20

16:10

16:30

30

10

PV Fann 35

PVFMP

Comparison of PV ReadingsUsing FMP and Fann Viscometer

10 30 50

50

Documents

Designing & Managing Drilling Fluid