SOCIETYOF PETROLEUMENGINEERSOF AIME PAPER

6200 North CentralExpressway NUMBER SPE 3504Dallas,Texas 75206

THIS IS A PREPRINT --- SUBJECTTO CORRECTION

Effects of Drilling Fluids on Gas wellProductivity

By

Norbert E. Methven, Member AIME, Oil Base, Inc.

@ Copyright 1971American Institute of Mining, Metallurgical, and Petroleum Engineers, inc.

This mQer was DreDaredfor the &th Annual Fall Meeting of the Societyof PetroleumEngineersof AIME, t: ~e held ;n fiewOrleans,La., Oct. 3-6, 1971. Permissionto copy is restrictedto anabstractof not more than 300 words. Illustrationsmay not be copied. The abstractshould containconspicuousacknowledgmentof where and by whom the paper is presented. Publicationelsewhereafterpublicationin the JOURNALOF PETROLEUMTECHNOLOGYor the SOCIETYOF PETROLEUMENGINEERSJOURNAL isusually grantedupon requestto the Editor of the appropriatejournalprovidedagreementto giveproper credit is made.

Discussionof this paper is invited. Three copiesof any discussionshouldbe sent to theSocietyof PetroleumEngineersoffice. Such discussionmay be presentedat the above meetingand,with the paper, may be consideredfor publicationin one of the two SPE magazines.

ABSTRACT

This paper definesa method ofincreas-ing gas recovery and reserves by ~i~~unlz-

.-._ ---

ing natural gas well deliverabilities.Thismethod involvesprevention of formation dam-age while drilling,completing, and workingover gas wells.

Included is the definitionofthe forma-tiondamage mechanisms attributabletodrill-ing,completing, and working-over operations.

Formation damage, the principledeterrenttomaximum productivityand recovery, involvesthephysicochemical relationshipsof fluidsys-tems with the formation. These relationshipsare herein defined and then specifically ex-tended toincludethe effects on gas reservoirs.Of particular importance is a treatiseon theeffects on the relative permeability to gasfrom hydrocarbon invasion. This treatise in-cludes a means to evaluate the vaporization-equilibrium constants of the invading com-plex hydrocarbons. “--- “lllese prmcipks h“..~la vethen been used to develop an oil base fluidsystem that precludes formation damage and

References and illustrations at end of paper.

that has a filtrate that will be vaporized byproducedgas topreclude anyreductioninrel-ative permeability to gas.

Also presented are computed data, lab-oratory data, and field data from wells drilledand completed with various fluid systems.These data include core data, return perme-ability data, back-pressure tests, well pro-ductility histories, andpressure buildup sur-veys. These data show substantially increasedinitial and ultimate income from gas wellswhen non-damaged completions are effected.

INTRODUCTION

The current and forecasted naturai gasshortages increase the importance of maxi-mizingthe initial productivity and ultimate re-covery from gas wells. A frequent impedi-mentto maximum wellproductivityandhydro-carbon recovery isformationdamage thatmayoccurduringthe drilling or completion opera-+innc..”..”.

Thtanwincinal f~~rn.at~on damage mech-A..- F. ...-.=-.anisms are attributable to the physiochemic-al reactions of the drilling and completion

2 EFFECTS OF DRILLING MUDS

fluids with the formation. These damage mech-anisms can be defined by delineating availablefluid systems and investigating their effectson the formation and on the flow of’ formationfluids from reservoirs. Damage preventativemethods can then be devised from the knowl-edge of fluid systems and their physiochemic-al reaction with formations.

DRILLING FLUIDS

In order to define the physiochemicalreactions of drilling fluids with the formation,it is in order to summarize the principal in-gredients of drilling fluids that are availablefor use in drilling and completing. The prin-cipal fluid systems may be defined as follows:water Base = clay + Water + Oil. + Chemicals + cuttings + Wetght (1 I

Invert Emul. ion = (clay + Water) 0,1 + Chenw.ls + cuttings + Wetght (2)

Oil Base = Asphalt + (water.) Oil + Chemicals + Cuttings + Weight (3)

These expressions for the basic fluid systemsare’ generalized expansions of the simplified“three-part system”. 1 That is, (a) the colloi-dal or reactive portion, (b) the liquid phase,and (c) the inert fraction.

The function of the parts of drillin fluidshas been well defined in the literature F-4 andtherefore will not be repeated here. However,the physical and chemical effects of these in-gredients on the drilled formations must bedeveloped in order to assess the formationdamage mechanisms caused by the variousfluid systems.

FORMATION DAMAGE

Avoidable formation damage mechanismsare those related to fluids used during drill-2.--.-2 --mg dJ3U wfi@~~iii~ processes. These da.nmgemechanisms are physiochemical and capil-lary changes caused by the reaction of the drill-ing fluids as they invade and/or react with theformation.

Drilling (and completion) fluids maycause at least six different damage mechan-isms:

~. ~we~}@ ap~d U.rr+7.9+inn of.LJ “. s.... =..

In Situ Clays

B. Dispersion of In Situ Clays

C. Non - Gauge or UnstableWellbores

Y GAS WELL PRODUCTIVITY SPE 3504

D. Capillary Phenomena or WaterBlocking

E. Migration of Clays from FluidFormulations

F. Reduction of Relative Perme-ability to Gas by Oil

A. Swelling and Hydration of III Situ

*“ All clays absorb water molecules onthe surfaces and edges of individual particles.In addition, montmorillonites, mixed- layerclays, and certain illites can hold largeamounts of inter-layer water. Thus, whenaqueous fluids are used in drilling or comple -tion operations, clay swelling can occur.These swollen clays plug flow channels andimpair well productivity.

B. Dispersion of In Situ Clays. Clay-size particles are bonded by electrostaticforces and are in equilibrium in a virgin res-ervoir. However, when water or brine in-vades the formation, the cation concentrationof the clays may be altered, causing a changein electrostatic forces and subsequent disper-sion of the clay-size particles. When dis-persed, these particles will then cause wellproductivity impairment. Gray and Rex5 showthat clay migration can cause severe perme -ability reduction, even in clean sandstones.They point out that this type of damage candrop an otherwise permeable sandstone to lessthan l% of its original permeability.

c. Non-Gauge Wellbores. Wellboresare frequently out of gauge in shaley or siltyzones because of the reaction of aqueous fluidon the clays within the formation. These re-actions are similar to those in paragraph Aabove and have been defined in References 4,6, and 7. The two swelling mechanisms ofshaies (occurs in productive zorms also) arereferred to in l?afsmmee G ~S SUZ-f~Ce hydra-A“”.”. ----tion and osmotic swelling. The net effect ofeven minor swelling potentials at the wellboreis an increase in shale volume. Since theshale is otherwise confined, it will relieve it-self by collapse or spalling into the hole, re-sulting in unstable and non-gauge wellbores.Shales can also be dehydrated by an osmoticor diffusion process; however, such dehydra-tion means a reduction in mass. This reduc-tion in mass is then reflected as an alterationand destruction of the bulk strength of the shalebed. Subsequently, spalling will occur, again

PE 3504 N. E. METHVEN 3

resulting in co’ ‘apse of the wellbore. Suchswelling or shrinking can be partially inhib -ited by, say, coating the wellbore with as-phalt. 4’6 However, it cannot be completelyeliminated in ‘~ater base mud systems.

Non-gauge wellbores probably accountfor the majority of the primary cement jobfailures and s ~bsequent squeeze job costs.Also, it is likl?ly that errant cement froma“t,~n-a inh= n=IIQne nrndtln+ivs interval ~~rn.-‘WU=Q-- JVU” e--””” y. w--u.. . -

. . ..-. . --

age.

D. Capillary Phenomena or WaterBlocking. A capillary phenomena, referredto as water blocking, is caused by water satu-ration changes within the reservoir. For ex-ample, when aqueous fluids from mud filtrateor completion fluids are introduced into a hy-drocarbon-bearing reservoir, the water satu-ration necessarily increases. This increasedwater saturation causes a reduction in relativepermeability to gas (or oil). This reductionin permeability may be partial, or it canamount to total plugging when a water globuleplugs a pore channel between sand grains.Water blocking is often not reversible, or atbest, requires long periods of time, some-times years, especially in gas wells, for even-.-....4:. 1 -- —-... 1 l’ ----- +Le ..e”a . . ..n.-pcu LLaL 1 cllluvaL lL UL1l LIJC L C=ci VU* I .

E. Migration of Clays from Fluid For-mulations. Clays are often included in vari-ous fluid systems to provide rheology and fluid10Ss properties. The clays are usually bento-nite (montmorillonite) and are in a colloidal-size range (i. e. , approximately 0.01 to 2 mi-crons). These clay colloids may be carriedinto the formation with the filtrate from thefluid system. These particles can then beflocculated and permanently deposited in theformation when the chemical properties of thefiltrate are altered by mixing with the connatewater. Subsequently, the flocculated clay par-ticles will impede the flow of reservoir fluidsand aggravate any pore plugging by swollenclays previously described. Laboratory testsindicate that such migration can occur through24-inch long cores. g

F. Reduction of Relative Permeabilityto Gas bv Oil. A Dotential damage mechanism

“—-.

is the reduction in relative permeability to gasby oil invasion. This is a condition somewhatsimilar to Mechanism D above, in that it in-volves an increase in liquid saturation in the

reservoir. Such an increase will cause a re-duction in relative permeability to gas. Thisis of particular significance in a dry gas res-ervoir (two phases: gas and water), wherethe relative permeability to gas could be re-duced to about 30?70,even after removal by theflow process of the mobile portion of the in-vaded third phase. These principles will befurther developed in a later section of thispaper.

DISCUSSION

DAMAGE FROM WATER BASE MUD

Four of the above damage mechanisms(A, B, C, and D) are attributable to the pres-ence of water in the drilling or completionfluid system and in the filtrate. In order tominimize the effects of this filtrate, numer -ous efforts have been undertaken; i. e. , usingbrine or salt water systems, using chemicalsto inhibit or retard the water attack on claysand using low fluid loss systems. Unfortu-nately, these methods can sometimes reducebut cannot eliminate the effects of the waterfiltrate. For example, salt systems mustmatch the salinity, ions, and effects of stress.-a:cc . . . . ..-. .&...1- +La”- a..- “-l .-la- ha .#. h;a..a A 7.9Uulel-cll(.lal= - I,llcclc &all =!Cluullk UC aU.L4AGV=u. -

Another example is the use of low fluidloss systems to reduce the effects of invasion.The assumption in this case is that the fluidloss values, based on API fluid loss tests, areindicative of the amount of invasion that wouldbe experienced while drilling a well. Unfor-tunately, API fluid loss tests are conductedunder static conditions across a piece of fil-ter paper and are not representative of thedownhole dynamic conditions and of the for-mation. Generally, the dynamic fluid loss canbe effectively reduced when the API fluid lossis in the range of 20cc/ 30 min. or greater.However, when the API fluid loss is reducedbelow about 10CCS, the dynamic fluid loss rateis only slightly reduced. These principleshave been outlined in Reference 10. In essence,this reference indicates that you can reducethe dynamic fluid loss in the upper ranges andtherefore, you can run a better mud to reducemud cake, reduce sloughing, and get betterlogs. However, reductions below about 10CCSfor reducing formation damage may not helpmuch.

One other reason that water filtrate re-ductions don’t help much is that the invasionof water is seldom less than two feet, evenwith low water loss water base mud systems.If the invasion or formation damage is any-where near two feet or over, then the resultantreduction in productivity will be substantial.This can be appreciated by referring to Figure1. This figure presents a family of curvesof the well productivity versus depth of thedamage zone. The family of curves are forthe return permeability y in percent as may bedetermined from formation damage tests inthe laboratory. For example, if the returnpermeability is 1070 in the damage zone of twofeet, the well will produce about 35!J’oof whatit would have had there been no damage, or,

if there had been damage to four feet, the wellwould have produced about 32!L0of the virgincapability. Hence, it deem! t nmtter muchwhether the damage is two or four feet or be-yond, as a substantial amount of well produc-tivity and ultimate hydrocarbon recovery wouldbe lost in each case.

DAMAGE FROM INVERT EMULSION MUDS

Invert emulsion muds * incorporate theprincipal low- cost products of water base mud-- clay and water, the principle formationdamage culprits -- in their formulation. Theinvert systems generally have oil as the ex -ternal phas e and must have water in the inter-nal phase, as indicated in Formula (2).

The invert oil muds include clay** intheir formulation to provide rheology andfluid loss control at formation temperatures.These muds also include substantial quantitiesof water. These clays and the water can causethe damage mechanisms of A, B, C, D, and Eas defined in the previous section. This isparticularly true because water is frequently

* Invert emulsion muds are defined as oilmuds that require water in their formula-tion. An oil base mud is defined as an oilmud that does not require water or clay inits formulation.

‘~*These clays are oil-wetted montmorillonite,attapulgite, and / or other clays and aresometimes referred to as “Synthetic Organo -phillic Colloids”. The oil-wetting agent isoften a quaternary amine - a cationic sur -factant.

present in the filtrate from invert emulsionmuds at the higher reservoir temperatures ofmost present day gas well drilling. This hasbeen observed in laboratory tests of field mudsamples (see Figure 2) and probably in theperformance of wells.

Dependent upon the mixing oil and sur -factant properties, the invert muds can alsocause Mechanism F, Reduction of Relative pe~meab ilitv to Gas by Oil.

OIL BASE MUD

An oil base mud, Black MagicR, wasfirst developed in the early 1940’s to precludeformation damage. This oil base mud systemis characterized by Formula (3).

This ~y~tern uses oil as the externalliquid phase, asphalt rather than clay fortheological and fluid loss control, very littlewater, and provides a minimal fluid loss. In,.. -a -1~.~An mmti-n.v~d~ the for-mm way, ‘water &lu ~&=J~w ..-.mation. In addition, the thin filter sheath pro-vialed by this system is primarily a speciallymanufactured asphalt. This asphalt is com-pletely soluble in crude oill 1 or in aromaticsolvents such as Mix FixR or OB SolventR.Hence, formation damage Mechanisms Athrough E, as previously defined, can beavoided.

In addition, this oil base mud systemhas been further imporved to eliminate theeffects of Mechanism F. This has been ef-fected through manufactured asphalts withunique proprietary properties and throughselection of mixing oils that will not cause apersistent reduction in relative permeabilityto gas.

The oil used in this mud system and theresultant filtrate will be partially produced bya flow process, and the remainder will bevaporized within a few hours after a gas wellis placed on production. The flow and vapori -zation process can best be analyzed by re-viewing and developing the principles of rela-tive permeability, hydrocarbon phase dia -grams, and oil vaporization characteristics.

OIL FILTRATE IN GAS RESERVOIRS

The significance of invasion of oil intoa gas reservoir can best be developed by re -

;PE 3504 N. E. METHVEN 5

ferring to the known relative permeabilityprinciples and the known gas reservoir phasediagrams. Subsequently, recently developedvaporization- equilibrium constants for com -plex hydrocarbons and their adaptation for oilfiltrates will be presented.

Relative Permeability to Gas

The relative permeability curves shownin Figure 3 represent the classical physicalprinciple that shows the permeability to gasin a porous media decreases as the liquid sat-uration increases. This principle partiallyaccounts for the damage mechanisms listedabove as D. This reduction in relative per-meability also holds for increases in oil satu-ration, Mechanism F.

For purposes of Sirlplifiication, we canuse Figure 3 to analyze the effect of oil inva-sion. This can be done by considering thatthe invading oil is additive to the existingliquid saturation. Hence, as oil invades, thetotal liquid saturation increases from the in-terstitial water content (38’70 on the abscissafor the example) and the permeability to gasdecreases from point A toward B in Figure 3.This reduction in permeability to gas can besignificant during certain production phasesof retrograde reservoirs and in dry gas res -ervoirs. These cases can best be describedthrough the use of phase diagrams. 12

Gas Reservoir Phase Dia~rams

Gas reservoirs can be categorized asthree general types through the use of a sche-matic phase diagram as shown in Figure 4.

Reservoir A initially occurs below thedew point, which means that there will be freeoil, or condensate, throughout the reservoir,even under static conditions. Also, when gasis produced, the pressure will be reduced (atleast around the wellbore) and the oil (conden-sate) saturation will be increased. Hence, in-vasion of a compatible oil filtrate would oftennot be an increase in immovable oil and wouldnot reduce the relative permeability to gas.

Reservoir B depicts a retrograde con-densat e reservoir. A retrograde reservoiroccurs above the dew point, which means therewill be only two phases, gas and water, in thereservoir under virgin conditions. When the

wellbore pressure is reduced below the dewpoint, then the liquid saturation will increasearound the wellbore and the reservoir per-forms in a manner similar to Resefvoir A,wherein invasion of a compatible oil is notdetrimental. However, if Reservoir B isproduced above or near the dew point, theeffect of oil filtrate invasion on relative per-meability to gas must be considered. WhenReservoir B is above the dew point, it issimilar to Reservoir C - a “dry” gas reser-voir and can be analyzed accordingly.

Reservoir C has a tem~erature that ex -ceeds the cricondentherm and will exhibitonly two phases - gas and water - when pro-duced isothermally. Reservoir B above thedew point will also exhibit only two phases.Therefore, we must consider the effects of~il invasiQn On the relative permeability togas. In this case, when oil filtrate is intro-duced into the formation, it would be a thirdand extraneous phase. Under these condi -tions, the relative permeability to gas wouldbe decreased as indicated by point B in Fig-ure 3. This reduction in relative permeabil-ity to gas would prevail until the oil was re-moved. Some of the invaded oil is mobileand will be removed by the flow process asindicated from point C to D on the Kro curvein Figure 5. If only the mobile oil was re-moved, the relative permeability to gas wouldbe substantiality reduced as indicated by pointE on Figure 5. However, if the immobile oilhas certain vaporization properties, it will beremoved by producing gas from the reservoir.

Vaporization of Oil Base FiltrateBy Gas Production

As previously mentioned, the oil fil-trate from an oil base fluid system may bevaporized by producing gas from a reservoir.Removal by vaporization can be analyzed bydetermining the reservoir parameters andthe vaporiz ation properties of the oil filtrate.Heretofore, the vaporization properties of oilfiltrate have been difficult to determine be-cause the oil filtrate is a complex hydrocar-bon. Fortunately, Cook, et al, of the U.S.Bureau of Mines have recently developedl 3a simplified method for determining the vapor-ization-equilibrium constants for complex hy-drocarbon.

For example, Figure 6 presents the dis -

tribution percentage of the oil fraction num-bers of an oil base filtrate. This distributionpercentage was determined from the micro-distiiiation anaiysis of the fiit~at.~.ml---+-41LllcllJ.=trate had been accumulated at the end of a pre-viously gas -saturated core under temperature,pressure, and dynamic-static mudding condi -tions described in the Laboratory Results sec-tion of this paper.

The vaporization- equilibrium constantsfor the oil fractions can then be obtained fromFigure 7. Then6e, we can use the computerprogram of Reference 13, or we can calculatethe rat e of vaporiz at ion of the invaded filtrateas a function of the gas produced. For our ex-ample, the results are presented in Figure 8.We can then relate the rate of vaporizationfrom Figure 8 to the effect on the relative per-meability to gas - from point D toward pointA - in Figure 5.

The effect on well productivity as a func-tion of time can be approximated by enteringFigure 9 with the data from Figures 5 and 8.The actual well performance for two cases ofthe example has been calculated and shown inFigure 10. It can be seen from Figure 10 thatthe small amount of invasion normally experi -enced from the oil base mud system would beessentially removed, and the well productivitywould be in excess of 95% of its virgin capa-bility, within one hour.

Should there be an abnormal invasion ofa “compatible oil”, then, as an example, the30” invasion curve of Figure 10 shows thatover 907’oof the well productivity would be re-covered within 15 hours. These return per-meability values will continue to improve withcontinued production. For example, for thenominal invasion case, the well productivityor completion efficiency will exceed 997’owith-in 5 hours. These productivity values will beeven more favorable (i. e. , less time for va-porization of the oiI) for higher reservoir tem-peratures, pressures, and production rates.

LABORATORY RESULTS

Formation damage and well impairmenttests have been conducted in the Oil Base, Inc.laboratories on core samples from numerousreservoirs. These tests have been conductedunder reservoir pressure and temperatureconditions to dynamic mudding conditions and

to liquids that would be expected to be pro-duced from the reservoir (i. e. , oil or gas).

Tb.iS i~tt~~ pOi!lt* testing with oil or gas,is an exceedingly important condition if theoperator’s objective is to drill for and producegas (and/or oil). Unfortunately, many labora-tory formation damage tests reported in theliterature have used various water or brinesolutions to determine the permeability beforeand after subjecting the core to mud in the lab-oratory. While brine is sometimes less dam-aging than, say, fresh water, it is sometimesmore damaging. In summation, use of suchaqueous fluids for damage studies will giveerroneous results as they will cause damagein themselves and will mask the damage thathydrocarbons will “see”. Perhaps this re -states the basic physical truth that aqueousfluids are cliff erent than hydrocarbons.

It is difficult, if not usually impossible,to define the amount of productivity impair-ment that is attributable to the particular dam-age Mechanisms A through F. However, fora particular reservoir rock, the effects of A,B, D, and E can be isolated from C and F inthe laboratory by saturating the cores with oil,subjecting the cores to various mud systems,and then determining the return permeabilityto gas. Also, Mechanism C can be evaluatedfrom sorption isotherms. 9

Oil or Condensate Flow

We have conducted formation damagetests by flowing oil through undamaged or “re-stored” cores to determine the initial perme-ability. Subsequently, mud is circulated pastthe face of the core at differential and confin-ing pressure and temperature conditions simi -iar to what ‘w-OiiiC.ibe CApCI LC1lVCU .1. - Ur. . ..r.g----- .----.-A :- “ A +lli

well. These tests have included the use ofwater base, invert emulsion, and oil basemuds. An example of the results of such testsare shown in Figure 11 and aiso in ‘Tabie LFor various formations, the results would bet-nrr~.qnnndin~lv different,--. . -- r-----.-= -U

In the Table I example, the return per-meabilityfor the water base is in the order of30 to 4@70, while the invert emulsion is in theorder of 50 to 60Y0, and the oil base returnpermeability is on the order of 90 to 9470.These date give an indication of the effects offormation damage Mechanisms A, B, D, and E.

PE 3504 N. E. it

Using Figure 1, these return permeabilitiesindicate completion efficiencies as follows:

Mud CompletionType Efficiency-7”

Water Base <50Invert Emulsion 80Oil Base >95

Gas Flow

Laboratory tests have also been con-ducted by flowing gas through a core in a man-ner similar to the preceding section. Fig-ure 12 indicates the return permeabilitiesthat have been experienced after mudding offwith water base and oil base muds. Figure12a presents data after mudding off with oilbase mud. These data indicate return perme-abilities to gas after subjection to water mudsof Oto 6070. The oil base return permeabilitieswere 40 to 90’70 (and still increasing) at com-parable differential pressures. These datagive an indication of the effects of formationdamage Mechanisms A, B, D, E, and F.

FIELD RESULTS

Field data have been obtained from wellsdrilled with various mud systems subsequentto conducting the laboratory formation damagetests discussed above. The field data includecore analysis, production performance, pres -sure buildup analyses, and back - pressuretests. Data have been collected from oil wellswhich are likened to gas-condensate reser-voirs and from gas wells for comparative pur-poses.

n+l n- P.. ._Pn An”c!l+a IXTS1lQWA.u. Uau Lu.nu-a.”-ti.- . . “...-!

Production and pressure data from waterbase-drilled, invert emulsion- drilled, and oilbase-drilled wells corroborate the laboratoryformation damage tests. An example is fromthe Cotton Valley formation as reported inTable II which is an extension of Table I.

Based on the laboratory formation dam-age studies, completion efficiencies of 50’7’0were predicted after use of water base mud,approximately 8070after use of an invert emul-sion mud, and over 95% was predicted afteruse of the oil base mud. The actual field dataindicate that the water base-drilled wells indi -

7TH VF!N 7

cated normalized completion efficiencies of44- 69Y0, while the invert-drilled and -com-pleted well gave 7470 efficiency, and the oilbase- completed well gave approximately 1009’ocompletion efficiency.

The mechanisms of well impairment ex-perienced in this field are attributed primarilyto the effects of water filtrate. In the case ofthe water base mud, the filtrate is surely wa-ter, and an analysis of the formation indicatesthe presence of swelling and dispersible claysin situ. It also appears that there is somepermeability loss due to water blocking.

In the case of the invert-drilled well,the invert emulsion had water in its filtrate astabulated in Table III, wherein the propertiesof the mud were obtained from a field sample.Note that the filtrate had 5070 water. This isperhaps contrary to the advertised claims ofthe invert emulsion muds; however, it hasbeen noted many times in the past that field in-vert emulsion muds often have water in theirfiltrate. This is particularly true when theinvert emulsions are operated at the higherreservoir temperatures. Laboratory~reparedsamples of inverts may not have water in theirfiltrate; however, when this mud is hot rolled-- which is a method for simulating the tem-peratures to which the mud would be subjecteddownhole -- and then test ed, the invert muds,as shown in Figure 2, will have water in theirfiltrate and may have high fluid loss charac-teristics.

Gas Wells

Numerous articles in the literature pre-sent data on gas reservoirs that have beefiAmillaA in+m with watcsr hats mild. ~~qiu~p-~~y,u. ...=- . ...” v. . . . . . . -.-. --u- . -----

the production performance and pressurebuildup surveys indicate a substantial amountof formation damage. Examples are given inReferences 14 and 15.

Figures 13, 14, and 15 present compara-tive well performance data on wells that weredrilled with water base mud and oil base mud.These examples are in the Frio formation inTexas. The four-point production tests indi-cate that the oil base completions have greaterdeliverabilities and absolute open-flow poten-tials, even in those cases where less sand wasencountered.

EFFECTS OF DRILLING MUD~

Detailed data have also been obtained onWilcox formation wells drilled with an invertemulsion and with oil base mud. These dataare summarized in Table IV. Here again, theinvert emulsion-drilled well had a normalizedcompletion efficiency of only 88%, while theoil base drilled well had a normalized comple-tion efficiency of 10wo. As noted in this table,the invert-drilled well is in a zone that had ahigher capacity in millidarcy-feet as indicatedby the core analysis and the pressure buildupsurveys than did the oil base-drilled well. Yetthe oil base- drilled and -completed well had asubstantially greater deliverability initiallyand after three years of performance as shownin Figure 16.

Other field examples include well pro-ductivity before and after killing wells forworkover operations. Oftentimes, well pro -ductivity is reduced after use of damagingfluid systems. Use of an oil base system forworking over has proven non-damaging as re-ported in References 15 and 16.

SUMMARY OF RESULTS

The field data corroborate the laboratoryformation damage test data. These data indi-cate that the water base and invert emulsion—..2- : .....11 --n A,,~+<..;+..1....,-.-e -w *nmamuus lRkp=ir WCLL p UUULLIV LL.y Uy UIL= u. . ..uA =

of the formation damage mechanisms.

The data also support the theoretical andcomputational data on formation damage and,in particular, oil invasion into a gas reservoir.The theoretical and laboratory data indicatethat the oil base mud system will be non-dam-aging.

The non- damaging concept of the oil basemud system is also supported by comparativewell performance data. It is recognized thatthe well performance data maybe used only asa hypothesis as there are no other known prac-tical methods to preclude formation damagewhere liquid mud systems are used. Nonethe-less, the preponderance of the data indicatethat the oil base-drilled and -completed wellwill have the maximum natural well produc -tivity.

NOMENCLATURE

qa = productivity with damageqb . nrodllc+ i w“tv w~~h~[u~ d~.m~ge=. W-..-- . . . ..J

3N GAS WELL PRODUCTIVITY SPE 350

k = permeability - mdka = permeability after damage - mdkb = permeability virgin or before damage-md

krg = permeability to gask=k~~l = permeability to oil

h = thickness of pay - ft.

Sw = water saturation

CONCLUSIONS

Data preserlt,ed irl this reqmrt ~lmie~,d~e

computed laboratory and field results of drill-ing and completing in oil and gas reservoirs.These data compare the effects of water base,invert emulsion, and oil base mud systems.Conclusions that can be drawn from these re -s ults include the following:

1.

2.

3.

4.

Water base and invert emulsionfluids may damage gas - or oil-bearing formations through oneor more of six damage mecha-nisms. Such formation damageis indicated by theoretical anal-ysis, laboratory tests, and fieldstudies.

Theoretical, laboratory, andfield data indicate that an oilbase fluid system can eliminateGA-6+ 1 . . .

=. Least .T. i.n.HT.l Z ~ fnwmn+innAU. . . . . . . . . .

damage in gas zones as well asgas - condensate or oil zones.

Prevention of formation damageis achieved by precluding waterfiltrate, minimizing particle in-vasion, and (in gas reservoirs)by use of oil that does not effecta permanent reduction in rela -+iva nsrmeahilitv ~0 g~se. ..- y- . ..--W. -...J

The potential reduction in rela-tive permeability to gas, ascaused by oil invasion, can benegated by selection of compati -ble oils when formulating an oilbase mud.

a. A compatible oil filtrate hasvaporization-equilibrium con-stants in a range dependent

.PE 3504 N. E. h

b.

5. A

uponthe reservoir pressuresand temperatures.

Most dry gas reservoirs willoccur at higher temperaturesand pressures and, in thesecases, oil filtrates that havedistillation temperatures of600”F or below will be readilyvaporized by the produced gasand thereby will not signifi -cantly reduce the relativepermeability to gas.

method is herein presented foranalyzing oils to determine theirvaporiz at ion ratios and the effect ofthese oils on the performance of agas well. Included are the princi-ples of the relative permeability togas, the effects of radial flow, theeffects of the amount of the inva-sion, and the effects of gas produc-tion from the reservoir on the va-porization of any invading oils.

REFERENCES

1. “Principles of Drilling Fluid Control”,Petroleum Extension Service, 1969.

~, P.ogers, W. J, , Composition and Proper-ties of Oil Well Drilling Fluids, Gulf Pub-lishing Company, Houston, Texas, 1963.

3. Black Magic Handbook, Published by oilBase, Inc. , Houston, Texas, 196 .

4. The Word is Oil Base, Published by OilBase, Inc. , Houston, Texas, 1967.

5. Gray, D. H. and Rex, R. W., “FormationDamage in Sandstones Caused by ClayDispersion and Migration”, FourteenthNational Conference on Clays and ClayMinerals.

;THVEN

6.

7.

ii.

9.

10.

11.

12.

13.

14.

15.

16.

Darley, H. C.H. , “Physical and ChemicalFactors Affecting Borehole Stability”,SPE Paper 2400.Chenevert, M. “Shale Control With Bal-anced - Activity Oil - Continuous Muds”.SPE Paper 2559, Presented at the 44thAnnual Fall Meeting of SPE, Denver,Colorado, September 28 -october 1, 1969.Roliins, M. B., “Seal@ Ssnds with Water-borne Bentonite”, Proceedings of theASCE, December, 1967.Chenevert, Martin E. , “Shale HydrationMechanics:!, SPE Paper 2401, 1969.Krueger, R. F., “Evaluation of DrillingFluid Filter Loss Additives Under Dvan-.mic Conditions”, Journal of PetroleumTechnology, January, 1963.Farris, S. W., Black, E. P., and Clelland,J. B., “Aromatic Structures in AsphaltFractions”, ASC. 1966.Kate, D. L., -et al, Handbook of NaturalGas Engineering, McGraw-Hill Publish-ing Company, New York. 1959.Cook, Alton B. , Walker, C. J. , SpencerG. B., “Realistic K Values of C7 +Hydro -carbons for Calculating Oil VaporizationDuring Gas Cycling at High Pressures”,Journal of Petroleum Technology, July,1969.Graham, J. R., and Boyd, W. E. , “AnAnalysis of Changing Backpressure TestCurves From Some Gulf Coast Area Gas..-. ., 11 1 ..r m.~- 1---- I=anhmnlnmx,w eils” “s j~ii~~~l ~1 ~~LL”oLCULU L GL.AL.U+5J,

December, 1967.Agadzhanow, R. S., “Effect of Clay-BasedDrilling Fluid and Well Completion onProductivity of Gas Wells”, Nauch. -Tekh.S.B. Ser. , Gazovoe Delo, USSR.Watkins, T. E., Tucker, J. W., and White,E. H., “Effective Completion Fluid Pro -grams - Texas Gulf Coast and Mid- Con-tinent Regions”, Petroleum Engineer,April, 1967.

TABLE I

COTTON VALLEY FORMATIONJONES COUNTY, MISSISSIPPI

FORMATION DAMAGE TESTS ON CORES

MUD TYPEWater Invert Oi~Base Emulsion Base+

Sample Depth - Feet 14, 020 14,060 14,060

Permeability to Oil - md 2.1 12.6 1.1

Return ‘Permeability - ~, 30-40 50-60 90-94

Completion Eff. Calc. - YO <50 80 >95

*Black ‘MagicR

TABLE II

COTTON VALLEY FORMATIONJOINES COUNTY, MISSISSIPPI

WELL PERFORMANCE REVIEW——

MUD TYPE=ter Invert OiBase Emulsion Bas

Permeability to Oil - mdReturn Permeabi].ity - %Completion Eff.

Well NumberKh - md ft.Kavg. - mdSkin EffectCompletion Eff.

Completion Eff.

* Black Magic13

caic. - %

- ‘%

Normalized ~.

2.130-.40<50

3-1 6-1282 5379.7 12.8

-1.6 3.5119 75

69 44

12.650-60

80

4-1853.2

-1.6127

74

1.90-

>9

7-52

-3.1

1

TABLE IV

INVERT EMULSION MUD PROPERTIES

WELL PERFORMANCEAND

PRESSURE BUILDUP ANALYSIS

Density - ppgRetort - ~.

WaterOilSolids

11.6

216316

WILCOX FORMATION, HARR [S COUNTY, TEXAS

Hot Rol] Before After——

Farm Properties @ 151t1°FVa - cpsVp - CpsYP - lb/100 ft2Gels - lo’~-lol - 16/100 ftz

26 2623 24

6 43-5 2-3

WELL 1-1 3-1

DRILLING FLUI:D Invert C)il Base*

kh - Cores-red ft.h - Cores - ft.

h - Perfs - ft.

883631

47.24145

HP-HT Fluid Loss@ 300”F, 500 psi - 30”

Oil - cc corr.Water - cc corr.Total - cc corr.

2.0 2.8T 2.8

2. o+ 5.6kh - Buildup - md ft.Completion Eff. -70Completion Eff. Normalized - YO

22.37688——

20.888100

MMCF/ D - InitialPwf -psi

2.0077175

3.9153762

MMCF/ D - RecentTubing Pressure - psi

2.31425

3.651425

Comparative Quality of Completion:By F]1ow El!f.By Present Production

Adjusted for Buildup kh

116701267017070

*Black MagicR

>0 100

90

80

70

60

50

40

3C

2C

10

0

IIIII

I 1 1 1 I 1 I I I I 1

0 4 8 12 lb 20 24 28DEPTH OF DAMAGE,

Fig. 1 - Effect of wellbore damage on

100 %

5070

30%

I

-L32 36 40 44 48 52

INCHES

productivity.

~L!trzIte !.Yorn Invert Oil Mud

Wilcox Formation, HarrM County, Texas

After Static Aging@ 350”F-4ooPSI-N2Imposed 16Hrs.

Mud WeightTemperature

Pressure

Time Run

- ,.—-..,,.,,,—”P ,Lcrzlte““L.’ IL.=

F,ltrate Water ContentFiltrate Emulsion Content

Filtrate Water Percent

15. 8#/gal.3500F

500 psiNo Control

after 16 mm~~.o cc

1.5 cc0.8 CC

4.87,

Fig. 2

\ L

b’k’g :

1-

W

tl

i

[ /’vIRGIN GAS \A

> — —— ——— .— glx 50 PERMEABILITYw ~\ ;c1.

: 40J\;

:a w

I~ 30

I \ \

I

L

II

20 –I

I10 – I

IB

o - I I I I I I

o 1“ 20 30 4“ 5“ 6“ 7“ 8“ 90 I

TOTAL LIQUID SATURATION, “lo VO

Fig. : _ Relati,,eper~eability gas and oil xitk.interstitial water.

)0

RESERVOIRqzINITIAL

A B

/4

‘<; PRESSURES

I i

I!

II~ASSUMED

IsOTHERMAL

I EXPANSION

I

II

—

TEMPERATURE~

Fig. b - Phase diagram showing liquid-gas volume distribution.

n

\

mIn

uuoECL

._ ‘f’ nr ~

PERMEABILITY L——-AF T:R ViP8R;Z AT 10–N- j~A z,

5

i\

J GSWJ \:’ z>

I

v

LIJCYII 6I

‘PERMEABILITY AFTER I

‘%(I

‘\ ,’—. -----l -_ D

FLOW ~ROC; SS 1-

k

Iro~

c//

“o 10 20 30 40 50 60 70 80 90

TOTAL LIQUID SATURATION, ‘/o Vo

Fig,:- P,ela:i:epermeability gas and oil wit?.interstitial water.

J/FILTRATE

. f

(\>OB

SOLVENT

—

)0

o 20 40 60 80 100DISTRIBUTION PERCENTAGE

.Fig. +,

-.. .- uIl Irac Lion distribution Bbc’k Magic filtrate .Hu UU .V4. =L1..

.. . nm .-1 . ..-+

‘o-k—————l

+x

II LFRACTION 15u

m- 10-5

$<N

1

1

I /I /-

,.-3

0-4

0-5

I 18

I

I I I I 1

19

1000 3000 5000 3000 5000

PRESSURE, psio

F

1 //’/’.//.

/

/’/’/ /-,

~//,

$’,’ ,;

~

/b$.’ /%’

7. /’*●

/20

3000 5000

Fig. 7 - (Kx C<+) values for hept~~esplus fractions/ 15 through 20. ‘u

175° F-2000 psi

CUMULATIVE GAS PRODUCTION

PER BARREL OF FILTRATE MCF/STB

Fig. 8 - Vaporization of Black Magic oil filtrate as function ~fgas production.

—

100sog

; 70 –IMPROVEMENT WITH VAPORIZATION

=: 60 – Is~ 50 – IZ I

g 40 – I

G 3o1- 1 I

100 “/9

50%

30 “1.

10Y.

>Gg 20

t!

I

n

; 10 6“

iII

30”

1 Io I 1 i I I I 1 I 1 I I I I J

O 4 ~ J2 16 20 24 28 32 36 40 44 48 52

DEPTH OF DAMAGE, INCHES

Fig. 9 . Effect of wellbore damage on productivity.

/ 6“ OIL INVASION

OIL INVASION

q = 1,000 MC F/DAY

($ = 300/0

h = 20’ PAY

S.w = 38.8 O/.

rw = 3“

T = 175° F

pf = 2600 psi

o ~1 I Io 5 10 15 20 25

GAS FLOW PERIOD- HOURS

Fig. 10 - Well productivity vs gas fiow for Klac’ki,iagicoil filtrate.

50 psi I 100 psi

CORE B -14,020’

KOil 12.6 md100 psi

INVERT EMULSION MUD

Ic

7CORE C -14,060

KOil 2.1 md

CLAY WATER MUD

I 25 psi I/I I I I I I I I I

o 10 20 30 40 50 60 70 80 90 100

TIME, HGURS

Fig. 11 - Return permeability result: of Cotton Valley cores after exposure to various muds for24 hours at 240 F and 900-1400 psi differential pressure.

v’50

-8.5 Ppo BLACK MAGIC

‘gas = 20.7 md

L8,8 pw WATER BASE

)y8

k~a~ = 4.4 md

5

3

50

Wwlo 100 200 300 400 500

TIME-MINUTES

UN LABLED NUMBERS = DIFF PRESS. -psi

~~g. u . ~et,wz uermeakilityto ~as \:ilcoxformation,Texas.

100

80

Ap= 7psi —

o 10 20 30 40 30

TIME -HO’JRS

@* 9.2 PP9 BLACK MAGIC

~ic. 12.A- Rci,urr.~ermeabilit,yto gas after subjectionto oil base mud*,}Iat,nlein, v!. Germany.

100

16’

z

FEET OF PERFS.

No. 3-2 WATER BASE 3’

10 —_ No, 2-2 BLACK MAGIC

o

L

.

m-:

N~.

1.0

‘ -/

COMPARATIVE AOFP’S

2.1 3.5 BLACK MAGIC

I I i I I 11 Io.1 I I I I I I II I I I I I

0.1 1.0 10 100

GAS PRODUCTION -MMCF/D

Fig. 13 - Wellproductivityy comparisons,Frio sand,MatagordaCounty,Tex.

loo~ I

FEET OF PERFS.

6’ 9’

NO. 3-2 WATER

NO. 2-2 BLACK MAGIC

COMPARATIVE AOF P’s

4.2 12.0 BLACK MAGIC

1 I I 1 I ]11 I I I I I I II IT-II(10.1 1.0 10 100

.

GAS PRODUCTION -MMCF/D

Fig. 14 . Well productivitycomparisons,Frio sand,MatagordaCounty,Tex.

—

—

FEET OF PERFS

NO. 1 WATER BASE >

NO. 3-2 wATER BASE

NO. 2-2 BLACK MAGIC

/

1- /COMPARATIVE AOFP’S

B.9 11.o 16.3BLACK MAGIC

01. I I I I I Ill I I I I I I I I I I I I I-.0.1 1,0 10 I 00

GAS PRODUCTION -MMCFID

?ig,. IJ . :;ell productivitycmpris~ns, Frio sand, Matagorda COuntY, Tex.

BLACK MAGIC——

10 —

ag“

w-.-

1.

~.

I .0

Fttto,~01 1.0 10

wELL PRODUCTIVITY -M MCF/D

10

Fig. 16 - Well productivitycomparisons,Wilcox formation,Harris County, Tex,