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SPESPE 23952

Horizontal Injection and Production Wells forEOR or WaterfhodingJ*J, Taber and FLS. Seright, NM Petroleum Recovery Research Center

SPEMembers

Copyrlgh! 1992, Sc41y of Petroleum En@neers Ino

Tfrls paper was prepared for p#ewrrlation at the 1992 SPE Permian Baaln 011and Gas Recovery Conference held In Midland, Texas, March 18-20, 1992.

Thie paper was selected for presentation by an SPE Prcgram Commltfee follow!ngreview of informationcontained in an abstract submifted by the author(a).Contentsof the paper,aa proeeoted, heve not been revlawod by the Society of Petroloum Engirreereand are aub@ctto correctionby the author(a).The materiel, se presented, does not nemssarlly reflectany positionof the Societyof Pewo!eumEngineer$, Its officers, or members, Pepera presentedat SPE meetlngaare ?>5jecl to publicationreview by Edllorial Committeesof the Bocletyof PetroleumEn$ineere,Permleeion!0 co+IyIerestrictedto an abstractof notmorethen SDOwords.Ilfuetratbnsmay no:be qlad. The abstractshouldccmtelnconepkuousacknowWgmmtof where and by whom the paper is fxeeented, Write Llbrarlan Manager, SPE, P,O. Box 6S2S.?.?,Richardson, TX 750S2WSS. Talex, 720SS9 SPEDAL.

ABSTRACT INTRODUCTION

This paper describes the potential benefits of using The use of horizontal wells has been increasing verycombinationsof horizontalinjectionand productionwells for rapidly throughout the oil industry as advances in drillingEORprocessesor waWiooding, techniquescontinue. In manyreservoirs,horizontalwellscan

help solvea numberof oil productionproblems, However,inOur results show that a very favorable configuration spite of a tremendousincrease in literaturers&mces, little

occurs when two opposed horizontalwells are drilled from information is available on horizontal-wellapplicationsfixinjectionand productionwells so that the opposedlateralsare EOR methods. A survey of the extensive horizontal-wellparallel in the patterns, and extended until the horizontal literatureis beyond the scope of this paper, but reviews,l”Ssegments almost meet midway between UIc like wells. books,6-10SPEReprintSeries,ll}’2andpublishedarticles(overComparedto five-spotpatterns with vertied wells, opposed one thousand,even excludingnewspaperarticles) show thathorizontal wells can increase injectivity (injection rate per horizontalwellsare still usedprimarilyin problemmaervoirsapplied pressure drop) by as much as a factor of ten, or to solvespecificproductionproblems. Theseinclude !ow-dependingon well spacingand formationthickness. Areal permeabilityformations especiallyfracturedformationssuchsweep efficiencycan be increasedby 25% to 40%, The as the Austin Chalk,13-k0low-permeability as reservoirs,2*horizontal-welladvantagesarcgreatestfor thinformationswith &unusual as sourcessuchas coal-bedmethane ‘u or Devonianwide spacing, and decline signifiesrrtlyfor thick formations

‘5shale,24-6 as or waterconing,27’2sthin formations,29”31and

and/orclosespacing, Also, for a giveninjectiorrpressure,the viscousoil, 2’33prcssum gradient in the bulk of the reservoir CM averageseveraltimesgreaterwhenusingopposedhorizontalwellsthan Mostof the EORactivityhas bec;Iin the w of thermalwhen using verticatwells. This could significantlyimprove

d~ves~“sg where both horizontal injection wells andrecove , primarily for steam stimulation and steam

microscopicdisplacementefficienciesfor EORprocesses,suchas miceHsr/polymerflooding,that are sensitiveto interracial produ&ionwells have been tried. A potentialfor gas EORtensions, projectswith horizontalwellsis indkatd by recentsimulation

and model studies for inert gss’Oand C02 injection,” TheBecauseof the better sweepefficiencies,faster flooding interest in horizontal-wellwaterfloodingis very recent with

rates, and/or lower injectionpressuresthat are possiblewith mostreportsor publicationsappearingin 1991and 1992.424horizontalwells,SHBORmethodsshouldbenefitby theiruse,For example,polymerfloodscan be improvedby the higher Althoughthereis relativelylittlepublishedinformationoninjectivityand lower rates of shearat the injectionsandface, the use of horizontal_ wells, other than for thermalThe advantagesof horizontalwells for CO floodinginclude:

/(a) delayed C02 breakthroughbecauseo the &tter sweeprecovery,the needforpatternsof bothhorizontali@ction andproductionwells,or opposedverticalfmctures,to increaseth

efficiency,(b) the potentialfor maintainingthe MMP in more 8rate of flooding in EOR processeshas been mentionedt”-sof the reservoir with no increase in the ir@tion pressure, Thispaperdescribesthe improvementsin sweepefficiencyand(c) better injectivity at the same pressures, and (d) the floodingrstes that are ~ssible if horizontalwellsare usedforopportunity to convert more pumped producers to flowing waterfloodingor for any EOR methodwhichrequiresthe usewells, ThermatEORwasnot investigatedin thiswork,but the of both htjection and production wells. In addition, thecited references show that horizontal wells have been potentialfor increasedmicroscopicdisplwxmentefficiencyatsucesssfulin aeveratfieldapplicationsand moreprojectsare the faster rates (with no increase in well-headpressure) Isbeingplanned, examinedfor someof the EOR methods.

87

INCREASEDINJECTION AND PRODUCTIONRATESFOR HORIZONTAL-WELLPATI’ERNS

A relationshipbetweenthe rateof floodingandtheamountof oil displacedby surfactantsolutions was observed manyyearsago.sl Ingeneral,fasterdispimment ratesincreasedtheoil recoveryfor almostaIl Moratory experimentswithsurfaceactive agents. Althoughlater work showed that there is aspecific minimum rate for the displacement of oil with

!8,52 Themfom diff~~t possibilitiesforswfactants, floodn at rates above this criticalvaluealwaysproducedmom oil.increasing the rate of flooding ior the average pressuregradient)for surfactantfloodsin the fieldwereexamined. Amajor obstacle to efficient oil recovery was the inefficientradial flowbetweeninjectionwellsand productionwells, Asearly as 1933, studies showed that the available pressuregradientf~rabout75% of a waterfloodpatternwasonly Moftheaveragepressuregradient.53 To avoidhighpressurelossesfrom radial flow near wells, other geometric arrangementswere considered. The two best possibilitiesappearedto beopposed lateral drainhole (now usually termed “horizontalwells”) and opposed vertical fracture& Because !akraldrainhole hadnot beendrilledmorethan MOfeetin the fifties(althoughEastman claimed that they could be extended togreater distances),54and the costs were much greater thanverticalwelIs, the paralleland opposedvertical fractures(tocreatea linear flood)weregiven moreattention,47

Before discussing the benefits of hoti?id injectionwells, it is instructiveto examinethe differemccbetweentheflcodingrates in a normalfive-spotpatternand the theoreticalrate thatcouldbe achievedby a fill linearflood(possiblywithverticalfractures)with the samespacing.

The steady-sta~~rate of water injection in a five-spotpattern is givenby:33

% =LS4.khAp

(M%.: -0.420w )

where:

% = rate of water injection,bblahiayk = permeability,darciesh m ggndthicknegg,ft

Ap = pressuredifferencebetweeniqjectionand producingwells, psi

P = viscosity,cp~ M dis~~ between~, ftrw = rsdhs of injectionwell, ft

The linear tate in the same ..+ts is givenby

In this equation,A is the rectangularcross-sectionareaofthe sandfats betweentwo like wells, In Fig. 1, if the entire

fkce could be opened to the water (for example if preciseverticalfi’acturesreplacedthe lateralholes),A willbe equaltohW, Onthe otherhand, if the formationthicknessis close tothe diameterof the lateralholesshownin Fig. 1, E+. 2 shouldbe a goodapproximationfor thealmost-linearflowbetweenthehorizontalinjectionand producingwells in the pattern.

In Fig. 1, L is %W,and again, A is equalto AWiftheentiresandfaceis open to flow. Sincethe totalrate in the five-spot includesflowin both directionsfmm the horizontalwells(or verticalfractures),the totallinearrate in tkrrnsof h and Wis

Therefore, the linear rate for the five-spotbecomes

(3)

4.52khAqL($ = ~ ‘. (4)

Notethat the rate dependsmlv on the thicknessof the sandfora given k, P, and Ap. Ms, the linear rate wiil becompletely independent of the well spacing in five-spotpatterns.

Eqs. 1and4 canbe combinedto expressthe linearrate interms of the five-spotrote, The result is

( :-0”4+q’($)= 2.93 q5 @ (5)

Eq. 5 was used to prepare Table 1, which shows theamountthat the linearmte exceedsthe five-spotmteat variousspacings, Theoretically,the mtesare equalwhenthe injectionwells are 1.9 ft apart. However,the five-spotmte dr6paoffso rapidly with increasingdistance betwmn wells, that thelinearmte is 8.4 timesas fastas the five-spotrateat a spacingof 10 acres and can be more than 10 times as fast at widerspacings.

The horizontal-weilrate differs only slightly from thelinear mte in very thin formations. To determinethe amountthat the nte differs from the Ml linear rote, M approximatesolution can be obtained by solving the flow problem in amanner similar to determiningthe rate of flow for differentpermeabilitiesin series.

In Fig. 2, flowis radialbetweenthewelland the mdius,r From G, to q , the flow will be linear, and fkom~a to/w; the flowagain)s radial. ‘Ihe samepermeabilityexists inail regions,soq canbe expmsaedin termsof thepressuredropacross each section. In addkion, the total pressure drop0, - PJ till O@ the sum of the Pressuredifferentialsin

I theothti three s&ions or

(Pt-P4) -63- Pa)+ @- P9)+(PS-P4). (6)

w

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SPE 23952 J.J. TABERANDR.S. SERIGHT 3

&“ = x kL .

4r@ +uLrw

(9)

s61utioI~usingour symbolsis

RearrangingEq, 10 to the same form gives

2r#wApqm m ~,

jcJ +21n325 rw

The flowin the entirematrix can be expressedas

wherek’ is tileaverageor effectivepermtillity if the flowisassumedto be linearwith the end sectionsshowinga reducedpermeabilitycausedby the radial flow patterns. qm is therate for the combinationof radialand lincaxflowbetweenthetwo lateralholes.

In Fig. 2, A is actuallyW(2r,)(neglectingwellradius),so~, 7 is rewrittenas

fi” Wr,(pl - P4)9HW= (8)

pL “

The flow for each of the other sections is written insimilar units and each equation is solved for the pressuredifferential. The resultsalongwith Eq. 8 are substitutedinto~. 6 and solvedfor k“. The result is

Substitutionof this valuecf k’ into Eq. 8 and multiplyingby 2 to give the total flowin both directionsyields

Eq. 10is an approximationof the flowcapacityof a five-spotpatternwhereinthedistancebetweenlikewellsis spannedby lateral holes, After obtaining the above equation, J,E.Warrennotedthat theproblemis comparableto Muskat’sline-drive flood56if thedistancebetweeninjectionwellsis replacedby the formation thickness iii Fig, 2, Muskat’sline-drive

(11)

(12)

Eqs. 11and 12are similar, as long as L is muchgreaterthan the formationtldcknew To determinehow closelythe

flow betweenthe lateral holesapproachesa linear flood,qr~~ be eXpltSXdill tCM’lSOf q’ (* substituting2f, tid%h for W and rI in I@ 12). ~e result, after changingtologlo, is

‘m=q’(o[%L+4;:*(?)} ’13)

The approximaterelationshipin Eq. 13 shows thatq~and qL@am almost equalfor very wide spacingand/or thinformations. Thus, if five-spotpatternscan be drilledwith theopposedand parallelhorizontalwellsas shownin Fig. 1, thelinear rates in Table 1 may be taken as the theoreticalmaximumrates for very thin formations. For thickpay zmesand/or close spacing, the horizontal-well rate departsappreciablyfrom the linear-flowmaximumas more of theinjectedfluid followsthe radial-flowregime.

Fig, 3 was prepared to provide a direct comparisonbetweenthe flowratesof horizontalwellsandverticalwellsinfive-spotpatterns, Horizontal-wellrateswerecalculatedwithEq. 11 (from Muskat) and compared to the five-spotrates(Eq. 1) for differentspacingsand as a functionof formationthickness. The flow-roteadvantageof horizontalwellsfor thinformationsand widespacingis clear in Fii. 3. For example,for ten-ft formations,the horizontal-wellflowrates sre eightto ten times the mtes for the usual five-spot, vertical-wellpatterns. The advantage of horizontal wells is reia!ivedyinsensitiveto increasingformationthicknessuntilthepayzonesexceed100feet. Evenwith KN10-footformations,horisontal-WW flow ra&a are double those of the usual five-spotwaterfloodingpatterns unless the spacing is ckxwr than 20acres ‘fliehorizontalnet mte couldbe even Wter than thatindicatedby Fig. 3 if the water entersthe formationt%omtheverticalportionof the well along with the water enteringthelateralhole. For the simplegeometryin Figs. 1 and 2, it isassumedthe vertical portion of the well is cased and waterenters the lateral hole only, The potential for improvedoilrecoveryandbetteroperatingconditionswiththesefastermtes(atno increasesin injectionpressure)willbe examinedin latersections.

SWEEP EFFICIENCY WITH OPKMID HORIZONTALINJECTION ANDPRODUCTIONVb~.LLS

In thissection,wecomparesweepefficienciesexpectedinopposedhorizontalwells withthose in verticalwells, Craig57has extensivelyreviewedareal‘sweepefficienciesfor variouspatterns of unfractured vextical wells. In a developed,homogeneous five-spot pattern with a unit-mobilitydisplacement,the areal sweep et%cieneyat breakthroughisabout70%, accordhg to mostauthors. For example,Muskatpredcted a sweepefflci~cy of 71.5% for verticalwalls in ahomogeneousk-spot pattern.%

I Muskat also developed an quation to predict sweepefficiency for a unit-moMlitvdisplacementh a direct-linedrive. I; Eq. 14, we have- Muskat’sdirect-linedriveequation to estimate weep efficiency (Zf~ for opposed

80

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HORIZONTALINJECTIONAND PRODUCTIONWELLSFOR EOR OR WATERPLOODING SPE 23952..-. ——-. .-. .— -..-——. . . .

horizontalwells as a fimctionof formationtldcknesa(h) and Wick~Oi~ withbSrriCSSto VCltiCd flOW. WIS will be anpatternsize (L). importantarea for futureresearch.

E*= I-* (14)USE OF HORIZONTAL WELIJi 70 IMPROVEDISPLACEMENTEFFICIENCY OF EOR PROCESSES

Eq. 14 was used to generate Fig. 4, which comparesanticipated sweep eff’cicncies (at breakthrough for unit- AllEORor waterfloodingprojectsshouldbenefitfromthe

mobility displacement in homogeneous formations) for faster injection-productionrates and better sweepefficiencies

opposedhorizontalwellswith thoseforverticalwellsin a five- thatare possiblewithhorizontalwellpatternssuchas shownin

spotpattern. In this idealizedmmparison, sweepefficiencies Fig. 1. However, for those processeswhich exhMt higher

for the vertical weils are independentof well spacing and recoveries because of improved microscopicdisplacement

formation thickness. In contrast, for the horizontal wells, efficiencies at higher rates, the horizontal injection wells

sweepefficiencydecreaseswith increasedformationthickness shouldprovidea significantadditionalbenefit.

and with decreasedwell spacing. Using dpposedhorizontalwells, sweep efficiencies can exceed 90% for formation In Fig. 5, the EOR methodsare arrangedin order of the

thicknessesup to 100 fl. However, as formationthicknessAPI gravitk of the oils to be dispkwcd.w The figurecanbe

increasesabove300 ft., sweepefficienciesquicklyfalIbelow usedas a verygeneralscreeningguideto matchtheoil gravity

thosefor theverticalwells,especiallyfor tighterwellspacings.(or correspondingoil viscosity) to the moat effectiveEORmethod. The rangeof APIgravitiesfor mostof theEOR field

Much has been published on the effects of vertical projectslistedin recentsumeys’sare givenby the limitsof the

fractureson areal sweepefficiency.S8-71In ~n~pt, PSdkl solid lines whichenclosethe nameof each method. A rough

verticalfracturesin both injectionand productionwellscouldindication of the importance of the method (in terms of

provide areal sweep efficienciesapproaching 100% if theincrementaloil production)is shownby therelativesi= of the

fractureshave the proper orientation, length, and height, so letters.

that flow is linear between opposed vertical fractures.Unfortunately,fractureorientation,len th, andheightareoften

#Pig. 5 indicatesthatadditionalbenefitsshouldbe observed

difficultto controlduring fracturing. With recentadvances foreachof the threegeneraltypesof EORmethodx miscible,

in the precise placementof horizontalwells,m’” the use of water-basedchemical, and thermal. l’he masons for the

opposed horizontal wells may now offer advantagesover improvementsthat accompanythe faster rates and/or lower

vertical fractures, with respect to improved mwd sweep pressuresthat are possiblewith horizontalwellsare examined

eftlciency.in the next sectionfor someof the EORpmcewea.

Previous analysesof the impact of fractureson sweepefficiencycan be of valuein assessingthe meritsof horizontal The use of }oriamtsl injection wells can improveversus vertical wells. When comparing fractured versusunfractured vertical wells, some importam published injectivityand reduce polymer degradationduring polymer

conclusionsare:5s*61 floods and other chemicai floods. Becauseof the viscousnatureofpolymersolutions,irijectivhyinunfiactumdwellscan

1. Fractureorientationcan stronglyaffixt sweepefficiency. be substantiallyleas during a polymer flood than during awaterflood.’c ‘l%ehigher injectivitiesallowedby horizontal

2. The impactof fractureson sweepefficiencyincreasesas injectionwellscan help to alleviatethis problem,

the mobility ratio increases. In particular, for a givenadverse mobility ratio, the presence of an opposed For a given injectionpressure, the fluid velocityat the

vertical-fracturewell pattern would improve the areal wellboresandfacecan be significantlyless in a horizontalwell

sweepefficiencycomparedto that for unfracturedvertical thanin a verticalwell. Pig. 6 demonstratesthat tMsdifference

wells. is most pronouncedin thin formationsand with large wellspacings.The reducedsandfacevelocityallowelby horizontal

3, The greatest impact on areal sweep efficiencyoccurs injectorscould significantlyreducemechanicaldegradationof

whenbothproducersand injectorsare fractured,whileth~ polymersolutions.

smallest effect occurs when only the producers arefractured. The higherinjectionratesassociatedwithhorizontalwells

couldalso help to mitigatethe effectsof chemicaland thermsd

4. The ratio o! fracture len&#hto interwelldistance, x~L, degradationof polymersand other chemicalsduring floodsin

mustbe greaterthan0.1 (andsometimesgraster than0.4) high-temperatureresmoirs, Sincehigherinjectionrates lead

in walerto significantlyaffectsweepefficiency. to lower maidencetimesfor injectedfluids, the requirementsfor long-termstabilityfor the chemicalscan be relaxed.

By analogy, we anticipate that similar conclusionscan bedemonstratedfor horizontalwells.

Im -

In stratifiedreservoirs, the performanceof a horizontal In this section, we discuss the potential for increased

well will be affectedby its verticalplacement,especiallyin micro=pic displacementefficiencywhen the faster rates of

w

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SPR 23M2 1.1. TAFIER AND R.S. SEIUGHT s-. - ----- ---- . ..-—. ---

horizontalwellsare used for those ~ that produceoilby virtue of low oil-watexinterfacialtensions(IIT).

Theaforementionedrelationshipbetweenthe floodhg rateand oil recoveryby surfactantsolutionsis often correlatedbya dimcnsionleasgrouptermedthecapillarynumtm, designatedNCA,Nvc, or Nc. The numberis usefid for expressingthemagnitudeof the ratio betweenviscous and capillary forcesduring the displacemat of one phase by another in porousmedia, such as water, polymer, or surfactantfloods. Fig, 7showsexamplesof experimentalandcalculated(byStegemeier)capillarydesaturationcurves.48,52,77,78 The oil Sahmjtions~.normalizedso that the displacementsthat started at high oilsaturations (continuous oil) can be compared to thedisplacementsof residual oil. Many capillary desaturationcurves for water or surfactantsolutionshave appearedin thelitmture (for examples,see Refs. 48, 52, 77-83), All of thecurves for the displacementof residualoil, by increasingthecapillary number, have characteristics similar to theexperimentalresults shownin Fig. 7. For adstones similarto Bsrea, residualoil displacementstarts at a “critical”NCAvalue of 1V5-10A,and about 50% of the oil is usuallyrecoveredby an order of magnitudeincreasein the capillarynumber. At least95% of the oil is normallyrecoveredif theNCAvaluereaches103 to 10-2.The highNCAvalue? requiredfor tMs improvedoil recoveryare achievedin the field byadding surfactantsto reduce the oil-brineinterfaciaitension,sincefieldfloodhg rates (andpressuregradients)are normallysignificantly below that required to be in the criticaldesaturation region in Fig. 7. Maintaining low IFI’sthroughout the resmoir is difficu$tbecause of adsorptionandlordilutionof componentsin the surfactantslug. Tlds &tpartly explainswhy oil recoveryhas beamvery disappointingfor many micellar/polymer field projects. However, ifhorizontalwells could be used to increasethe floodingrate,and thus the capillarynumber, some increasein oil recoverywould be expected. For example, a tenfold increase infloodingrate should double the oil recovery for a dynamicsystemthat has an NCAvalueof 3 x 1~ beforethe increaseinrate (see Fig. 7).

Laboratorydata for the displacementof residualoil from15sandstoneswithpermeabilitiesrangingfrom40 to 2,190 mdare presented in Table 2.*1 Although the dimensionless“critical”capillarynumbers(theminimumNCAvalueat whichresidualoil is displaced)are similarfor the differentracks, the“critical” pressure gradient per unit of interracial tension(AP/La) was much higher for the low-pernmkdlityrocks.Sincehorizontalwellscamraise the avemgepressuregradientsin the reservoir, horizontal wells could extend surfactantfloodingto tighterSandstones.

The greaterareal sweepand fmter floodingnatesthat arepossible with horizontal wells can provide a number ofeconomic and operational improvementsfor two sodled“miscible” EOR method~ i.e., C% flooding andhydrocarbon-misciblemethods. The hydrocarbonpmcesseainclude high-pressure(vaporizing)gas driva and enriched(condensing)gas drives. The horizontal-wellbemefitaapplyprimafilyto thoseprocesea whichgeneratemiscibilityin the

.—

i-.

resemir by one of he multiple-contactpromwes. CQfloodingis the fasteatgrowingEOR methodtmla~7s*U andmuch of the following discussion of the horizontal-weilimprovementsappliesprimarilyto COpiscible flooding.Thehorizontal-well advantages and operational improvemcatsinclude:

- Betterareal sweepand delayedbreakthroughof C%.- Increasedinjectionratea(withno increasein pressure).- Mhdmummiscibilitypressure(MMP)maintainedinwider

area of reservoir.- More productionwellsconvertedto flowingwells,- Better oil recovery in near-miscibleams (just below

MMP)becauseof higherNCAvaluesfromfasterinjectionrates.

Importantaspects of these five projectedimprovementswith horizontal wells for C02-miscible projects will bediscussedbriefly.

. .and ~ .

Althoughmuch of the flush oil productionin C% floodingcomesafter C02 breakthrough,the early gas breakthroughatproductionwells adds to the projectcwts and eauseavariousoperational problems. Increaxcl areal sweep efficiency,illustratedin FVg.4, Smu!ddelay the C~ breakkmgh fornew C% projects that use horizontalinjectionwells. If theoperatormwtdecidebetweenhorimntd h@etoraorproducecs,horizontalinjectionwellsshouldprovidethemostimprovementof are4 sweep. TIds suggestionMOWSfrom the work ofBargas and Yanosik?s who inveatigetedthe u of verticalfractures to improve sweep efficiencyduring dispkemenbwith unfavorablemobilityratioe.

.Fig. 3 shows that injection rates for C% floods can beincmsed several fold with no increasein wellheadpreaaurewhenhorizontalwells are used insteadof verthd-well, five-spotpatterns. Injectivityis oftena seriousproblemin tertiaryC% floods. Althoughoperatorshaveexperiencedinjectivitiesthat are both higherand lowerthan predkted from waterandC02 viscosities(or mobilities),w-ma loss of injectivityforborhC~ and water, especiallyafter switchingWAGcycles,is common. The potentialfor faster injectionrata, with noincrease in pressure, could mitigate various injectivityproblems,especiallyfor thoseC02 floodswhichare operatedwitha pressureconstraint,i.e., at pressuresnearthe formationpartingpressure.

.~. operatorsshouldbe able to maintainmoreuniformpressuredistributionsin C% floods that utilize horizontal Wells. Because lesspressureis lost nearthewellbore,hi@erpreasutea(ator aboveMMP) can be achieved in a much broader area of thereservoir,perhapswithevena loweriqjecdonpressure, MostC~-miscible floods are operatedai i~ection pmasum wellabovethe MMP to try to includees muchof the redemir aspossiblein the multi-contact-miscibleflowregime, Horizontalinjectionand productionwells shouldreducesignificantlytheamountof over-prasure required, Again,if it ia necemry todecidebetweeniqjectorsw producers,the switchto horizontal

81

HORIZONTALINJECTIONAND PRODU~

injectionwells shouldb ernoreusefulfimmaintain”g MMPuniformlyat lower injectionpmaures.

possibilityfor much hater floodingrates, with no incr&e inwdk!ad pmsaure, should pCOVideqaators with moresh@&~~~&m@ producers to flowing wells for

.

of ~A~. Shyeh-Yunghas shownthatthenear-miscible

pressureregionsmaybe very importantin C% displacementsof oil.m In laboratory studies from outcrop samples, sheshowedthattheC02 mobilityis lessat pressuresjust belowtheMMP, and that oil recoveryis quite good, i,e., only a deless than the high recoveriesabovethe MMP, She attributessomeof these high oil recoveriesto the very low IFT valuesand attendanthigh NCAVSIUCS at pressures~ the MMP.Therefore, one would expect even higher recoverb at thehigher rates possible with horizontalwells, if the IFh andC02 displacementrates are already in the sensitivepart of acapillarydesaturationcuwe (asshownin Fig. 7). Shyeh-YunghascomparedC02 displacementwithswfactant-fkoddisphce-mcnt (see Fig. 4 of Ref. 90) and finds that more oil isdisplacedby C% thanby surfactantsat theequivalentc@larynumbers shown. However, it appearsthat she did not vwyNCAvalues by floodhg at differentrates, but obtahwdbertiues by calculatingthe NCAVSIuefor the IFI’s ~td ti

different floocthgpressuresnear the MMP. To makea directcomparison,experimentsat different rates at each pressurewouldbe hdpfil!.

Although C% displacementshave been performed atvarious rates above the MMP, we arc not aware of C%experimentswhere the displacementrates havebeenvariedatpressuresbelow, and at, the MMP. However,oil-recovery-versus-rateexperimentshave been conductedwith all-liquidmiscible displacements(alcoholdisplacingoil with connatewater resent),and oii recoveryimprovedmarkedlyat higher

7rates.9 The higheroil recoverieswereattributedto reducedtrappingand/or increased~ displacementof the oilphaseas miscibilitywas approachedwith the oil-waterIFT atvery low valu~, tmdthereforeat even higherNCAwIues, ssthe displacementrates were increased

Wepostulatethat similarincreasedoil recoverywouldbeexperiencedwith faster mtes for C~miscible displacementsnear the MMP. Stem92did observea smalldecreasein SO(an increasein oil recovery)when going from a rate of O%ft/day to 1.0 R/day, but this was followedby an increaseinS- whenthe rate was raisedto 5 ft/day. Watkins93showedan evengreateradverseeffectof hi@er mteson oil recovery.However,both Stem and WatkinsperformedtheirC02 floodsat pressures much higher than the MMP, where thecontribution from high-capillary-number, immiscible-displacementmechanismswould be reladvely less, and thecontributionfkomextractionwould be greater.m Thus, thecompetingefftxXs-improvedrecovery from high NCA,anddecreasedrecoveryffom poorer extractionat high mtes-canexplain the optimumrmvery obsemd by Stern at 1 R/day(based on only three different rates) even though his

..

NWELLSFOR EOROR WATERPUXIDINQ SPB239S2

experimentswerewcuabove MMP, bcuuaetherate waaatillslowenoughfor efhctive extracb,

Therefore,untilmoreco2displacementsareccmductedatmanydiffbremtrates (qeeially at pressuresnear the MMP),adefinitive conclusioncannotbereachsd ontheeffectofmteon the microscopicdisplacemente!llciencyof C% floods. Inany event, if the optimumconditionsof pressureand floodhgrate are ever determined more precisely fm C% floods,horizontal wells would be very useful to establish thoseoptimumconditionsin the field,

In a recentarticle in the SPE TechnologyToday series,Joshi pointed out thatmost of the applicationsof horizontalwells for EOR to datehavekm for steamprojects.M I&eatreviews and other publicationsexplaknhat horizontalwellsshouldprovidethe sametypesof befits for thermalrecovmyas for waterfloodingand other EOR methods, i.e. bettersweepefficiencyand faster injection/productionrate3.~3”39Inaddition,horizontal-wellsystemscan be devisedto permit theinjectionof the requiredheat into moreparts of the reservoir,and in a shorter time than with the usual vertical wells.However, reservoirs with viscous oils present differentchallengesand opportunitiesfor horizontalwell systems. Forexample,thermalprojectshaveusedmorehorizontalproducingwells than injectors, whereas we anticipate that horissmtalinjectors should be nwm beneficialthan produem @r mostotherEORprocases. Oneproblemwithlonghorizontalsteaminjectorsis the condensationof steam in the oold part of thedlwti~a~ti titiatimmlyatimofthe injector’sfull length. Preheatingthe longwellboreshouldreduceor eliminatethis problem.x

Eecause of the need for early heat to reduce the oilviscositybefore it can flow at a ussfbl rate, many differentpatternsand arrangementsof hozixmtalwells havebees trialor suggested,eqccially configumtionsdesignedto place thehorizontal injectors and producers close together. Foradditionalinformation,the reader is referred to some of theliteratureon the subject,32-39~WM

HORIZONTALWELU3 lWR WATERFLOODING

Althoughmost of this papsr deals with the potentialofhorizontal wells for EOR methods, the benefits of usinghorizontalwells for waterfloodingshouldnot be overlooked.Manyimprovementsin waterfloochgopemtionsshouldcomefmm the fasterinjectionmtesandbettersweepefficicociesthatarepossiblewithhorizontalwellsas illustratedh Figs. 3 and4.

The faster mtes, shown in Fig. 3 for horizontal-wellpatterns, should provide significantoperatingand economicadvantagesfor waterflood, especiallyif the horizontalwellscan be drilled in thinner formations at wide spacing fixreasonablecosts. (A recent survey showed that horizontalwells cost only 17% more per foot even thoughthe averagetotalmeasured‘depth’ of the horbntal wellwasalmosttwicethat for vertical wells,97) We emphasii that the increasedfloodingmtes shownin Fig. 3 are achievedwith no increase

U2

orxa WZae ● .a. B m- ruvu W.U, u—v.. a

in pesaure, i.e. the rates are comjxucdto standwdfive-spot horizontalwells fromanewinfill welltoapprmch thepattemrates atthesame pressureandspacing. typeshownby Fig.1. Inthiscsse, thelateral segmentswotdd

notneed toextend as fartowsrds theexisting h@tionwellsSinccrhe flowrate isdh’ectly propordodb presaum torecavermost of thepattern-bypassedoil, Carefideconomic

differencesbetweenthe bottom-holepresums of the iqjection studies are needal @ detedne if any of these retrofitandpKXhKtiOnWCh, Fi. 3 couldSdSObe USed to estimate the possibilitiescould recoveroil profitablyfrom existingfloods.lower pressures that would result for a horixmtal-well At present, it appears that wme type of horizontal-wellwaterfloodcompared to an equivalmt five-spotflood at the application should compare very favorably with the infiUasme rate. In general, the pressure requiredfor horizontal- drillingof verticalwells, eitherin newor maturewatcsfloods.well floodiig at the same rate as an equivalentvertical five-spotpatternshouldbe the reciprocalof the ratentio shownonthe ordinateof Fig. 3. For example,in a 10-ftformationat

CONCLUSIONS

80-acre spacing, the prmure required to maintainthe sameinjectionrate with horizontalwellswould be about one-tenth

1. The fiwterfloodingratesand improvedsweepefficicmcies

the presswerequiredfor normalfive-spotfloods. Maintainingthat are possiblewithcombinationsof horizontalinjection

equivalent mtes at lower pressures would have many atld production wells should be very beneficial for

advantages for waterflood operators. For floods wherewaterfloodngand the followingEORpmcessew

injection rates are limited by !he parting pressure of therecovery, C02 flooding,hydrocarbon-misciblefloodhg,

formation,much higher rates couldIMachievedat acceptablemicdkidpolymer (low IFT) floodhg, and polymer

pressures. In practice, some combinationof lowerpressuresflooding. An espeddy favorable configuration ofparallel and oppscd horizontalinjectionand production

and faster rates would probablybe selectedas the optimumcondhionto maximizeoil recoveryand profits.

wells is ahownin Fig. 1.

2. Injection/productionratescanbe increasedby as muchasten times (with no increase in pressure) by usingcombinationsof horiimntslWction and wduction wells

Figures 3, 4, and 6 show that the Wan- ofin tldn formationsand at wi& spacing. *advantage of

combinationsof horizontalinjectionand productionwells forfaster mte3 with hori20nadwells (comparedto vertical”

Waterfloodingare greatest for patterns with wide spacing.wellpattems) deGreUa fixthUerfbrmWna andkr

Therefore, equivalent (or better) water injection and oilcloser well spacing.

pfOdUCtiOi mk can b achievedwith ti h horizontalwellsthanwithverticalwells. Althoughindividualhorizontal

3. Compared to vertical wells, fewer horizontaliqjectiord

wellscostmore, the totaldrillingcostscouldbe muchless thanproductionwellsare neededto maintainthe fast flooding

for vertical-wellmtterns becausefewerwellsare drilledat therates possiblein patternswith wide spacing.

wider spacing. From Table 1 and Figures3 and4, it appearstialymbbti~ltid~~inp~oftit10 verticalwells to achievethe sameoverall floodingrate inthin formationsat wide spacing. However, a ratio of onehorizontalwellforeachthreeto eightverticalwellsis probablyabetter estimateto achievetheequivalentproductionratesandbetter sweerIefficienciesin thickezformations. Oneopemtor,

4. Significantincrease-sinarealsweepefkiency arepossiblewithhorizontalinjectionandproductionwells. l%esweepadvantageis greatest (up to 99% areal sweep)* tldnformationsand wide spacing, For very tidck formationsand closer spacing, it appears that there will be noadvantageover vertical-wellpatterns,

who has &ady drilled at least one horizontalinj=-tor and 5, The fMer rated possible with’horizontal wells shouldproducer in SoutheastTexas, estimates that “two horizontalwellscouldtakethe placeof six vemicaiwells,Wowin higher

hincrease the microscopicdisplacementefficienciesforEOR methods such as micelhdpolymer (surfhctant)

injection/productionrates and boostingoil recovery.’ flooding, that show increased recoveries at higher

‘lTie sweep-efficiency advantage of horizontal-well,floodingpatternswouldbe observedbest for newwaterfloodor an expandeddevelopmentof existing floods. ‘l’hehighersweep efficiencies shown in Fig. 4 (especially for thinformations)should delay water breakthroughfor new floodsand providemuchhighernet oil recoveryin a shortertime,

Forexistingwaterflood, infilldrillingis showingpromiseas a goodmethodfor recoveringthe oil that is bypassedby thepoor areal sweep of normal, vertical-well waterhodingpattems.9*”’~ Horizontal-well systems should comparefavorably with new infill wells, even in mature floods,especiallyif thecostaof horizontalwellscontinueto fhllwhilethe techniquesbecome more versatile. Mathods are now

availablefor drillinga horizontalwell froman existingwell sothat a pattern geometry similar to Fig, 1 cwld be achievedevenin olderwaterflood. Analternativewouldbe to drill two

6.

7,

8,

capill~” numbers.

Polymer floods should be improved by the higherinjectivity and lower rates of shear at the injectionsandface (see Fig. 6) that are possible with horizontalwells.

Advantagesof horizontalweM for C% floodinginclude:(a) delayedC02 breakthroughbecauseof thebettersweepefficiency,(b) the potentialfor maintainingthe MMP inmore of the reservoir with no increase in the injectionpressure, (c) better i@ctivity at the samepressures,and(d) the opportunityto convertmorepumpedproducerstoflowingwells.

Further research,development,and economicsmdiesareneeded to determine the most beneficial ways for theapplicationof horizontal-welltduwlogy to all i@ntionmethodsand EORpmcesaes.

●

●

8 HORIZONTALINJECTIONAND PRODUCTINOMENCLATURE

A.

Eh =

k=

k“ =

h=

L=

NCA sc

P=

q =

%r =

qL =

qL($ =

(?s =

r, =

rW =

s- =

w ~~

Xf =

v=

a=

CrOSS-SdOld a Of the sandface,ft

areal sweepefficiency

pernwibility,darcies

averagepermeability,darcies

formationthickness,ft

dMancebetweenopposedhorizontalwells infive-spotpattern(Fig. 1), ft

dimensionlesscapillarynumber(alsoNc orhkmAp

Nvc), definedas — or ~ (severalotherULU a

forms listedin Ref.“49)

pressure,psi

flOW rate, bbhkiay

flowrate betweenopposedparallelhorizontalwells, bbhdday

linear flowrate betweenparallelfacesof a reservoirblock, bbhhy

linear flowrate in five-spotpattern, bblsklay

water injectionrate for five-spotpattern,bbls/day

radius to externalboundary,ft

wellfkceradius, ft

misciblefloodresidualoil saturation

distancebetweenlike wells, tl

il; cture length, ft

visawy, cp

interracialtension(F17 betweenoil and wateror betweenoil and’C02-richphase,dynekrn

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,—--U7

● ✌✌

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Sarem, A,M, S,, Fuller, S.M., and Gould, T.L.:“ScreeningWaterflood for Infill Drilling opemtions,’paper SPE 22333 to be presentedat the 1992 PermianBasinOil and Gas RecoveryConference,Midland,TX,March 18-20.

NWELLSFOR EOR OR WATERPIXX3DINC3 SPE 2395

Table 1, The htiO of the LinearRate to the Vertica&WelJFive-SpotRate●t VariousSpacings

Spacing(Acres) w (Feet) 9&l%

1,9 1,0

0,625 16s,0 6,71,000 209,0 7,02s00 330,0 ‘?,55NO0 467,0 8,0;O,ooo 660,0 8,42o1100 935,0 8,940,000 1320,0 9*3

80,000 1859,0 9,7160.000 2640.0 10,2320,000 3734,0 10.6

q&($- 2’93q’[10g:-0’42W = distam betweenlike wells

9s = five-spotmte for verticalwells

rw - well radius = 0.33 t%

Table2, The Displacementof ResidualCMfromSandstoneand AiundumCores

Initial Percentof Maximum critical

core core Porosity Permora:;;ity Residual ResidualOil A?/Lovalue of critical

Number Typcx (%) oil Recoveredat (psi/ft) A~ko Valueof(millidarcies) Saturation Maximum (#l) Nti(dydcm) (dyndCni)(%) A P/Lu

2-269 ..... 11,1 40,8 41.7 17,8 52,3 13*MI 1,23 X 1042-205 ..... 13,1 52,1 42,4 9*3 52,8 20.10 2,33 X 10s

D-8-9 ..... 17,3 9sJf 5S,6 4,8 82.6 23,80 S*O5x lo~J-13 Jclm 17,2 189,0 37,1 6!.3 16,30 6483X 104S-12 ..... 17,6 242,0 40,4 ‘1::; 61,9 11.40 6,13 X 10sON-2 ..... 18,3 320,0 33*O 28*2 86.7 4,03 2,87 X 105

TA Tensleep 18,6 381,0 36,6 22,4 56,2 S*I2 4634x 10sBer-1 Mm 19,1 346,() 40,5 2s. 1 46,2 6,13 4,72 x lo~D.1”8 ...-. 19*7 503,0 a 49,5 17.I 26,3 1,71 1,91x 1(Y$SP-6 St, Peters 22*1 533,0 45*3 24,8 IS*2 2,25 x 10sJK-i Alundum 28,2 585.0 34,1 11,7 20.4 ;:: 4*96x 10sSP-5 St, Peters 23,1 613,0 48,6 27,2 30,9 1,12 1,s3 x lo~DOT ..... 29,9 635,0 32,1 29,8 21,s 2*O1 2,84 x 1(Y521X Berea 21,6 752,0 4s,3 26.4 2,78 X @

80 ...... 14,4 1450,0 49*7 34,1 E:; ;:: 2,35 X 104Mci-6 ..... 16,6 2190,0 49,3 48.2 12,2 0!31 1,51x lo~

,.. , . . .‘All coresexceptAlundum(A120) were sandstone, Many were mm otl.pfoauclngreservoirsand the oil companhsaaskedthat the namesnot be released. ~ata fromRef. 81,

R-Jl====f-T—’—~(or 2r@)

Fig. 1. Five-spot patterri vvith horizontalinjection and production wells.

1 4 ?3 P

~ C*—..

‘,,,A— J~’ /+

\.=. ” “’””’:’‘~ol~..””f ,’ -r@. . w

,, ‘,.,, ”.. .’ .,.$,.” . . . . . . . . . . . . . . . . ...”

,..,.

L[Al--’—.. ——

Fig, 2. Segment of parallel and opposedhorizontal injection and production wells.

,- sy&l320— ——__

80I

10 : -%“Q*..**....* ... ,,.O--=JO ““..,, ‘\ I

8“ --% “,5

‘, \‘+ *.

\ \\\ “.,

\ ‘o\ ‘. \

6 “ \ ‘b\ “,

\\

*0,\ ‘.

4 - \ ‘*\=’0 ●*

2

0 I10

=Constant Injectionpressure

20 30 60 100 20’0300 Sw 1,00Formation thicicne$% ft

Fig. 3, Comparison of injection rates forhorizontalweils reiative to verticai weils

70 - Verticai weiist

\i,

8 \,\

,\l \t

I I I I I \80 ‘

10 20 30 60 100 200300 500 1,0(

Formation thickness, ft

Fig. 4$ Cornp8riaonof sweep efficienciesforverticai weiis versushorizontalweiiai(paralleiand opposed in a five-spot pattern, unhmobiiity t’tNiO)

-—. -L.——.-

14 HORIZONTALINJECTIONANDPRODIJCT1OiUwl?l.1.~ I?OR mn m WATURm nnnmufi @nn 9%1

Olln GRAVITY ‘APIBENEFITS POSSIBLE WITH

PIORIZONTAL WELLS

s, Ps,s, P

WATER

[<<

ALKAL13JE

POLYMER FLOODS

S, P, R?

S, R

s, R

!3, P, R?

s, P

s, P

Fig. 5. Possible benefits from ~ontal we IJsfor EOR methods:- Better areal sweep (S),- Improved recovery from faster flooding rates (R),- Lower injection pressures (P).

1

0.6

0.3

0,2

0! 1

0.06

0.03

0,021’/’ Five-spot pattern

Unit mobility ratioHomogeneousformationConstant injection pressure

0!01~ I I I I I I I 1

10 20 30 50 100 200300 500 1,000

Formation thickness, ft

Fig, 6, Comparison of fluid velocities at thewellbore for horizontal versus vertical wells

Fig, 7, Potential for improved oil recovery fromfaster floc+ding rates with horizontal injectionand production wells, Original figure withcalculated capillary number vaiues (anddefinitions of symbols) from Stegemeier;78data from Refs, 48, 52, and 77,

..-