Retrograde Condensate Dropout Phenomena in Rich Gas Reservoirs

D.B. BENNION, F.B. THOMAS, B. SCHULMEISTERHycal Energy Research Laboratories Ltd.AbstractCja!i conllcn!i:IIC rcscrvllirJI cxhihiling cla!i!iic ..c.lcw p)in\ llrrclrllgr-oidc cllnc.lcn!i:1\C drop I'" heh.lvilltlr cxi!il in nulny arc.l~ inI~ world. Tht.-sc ~n'llirJI :trc unilluc in that. :t!i lhe ~n'llir(M\.~!iUre i!i I.k.",-"n ~. :t ccnOlin voluntl: of the l1.::tvy ~oo froclilm11I'lhe ga.. i.'i pn.",-"ipil:tI\.'tI in liquid rum1 fmm ~llulil1n in 111.: ~a!i.n1i!i cllnd~n!iOlle liquid may b~ Icmpl)rolrily I)r pc:rn1ancnlly1r-"p!)I..'tI in Ihl: re.'il:rvrnr. c:l'I!iin~ .'iI:Vl:rc n.'tIuctitln!i in ~0I!i prlldOClilll1rol,-~ aoo the pc:nn.lncnl 10!i... ur :I l..rgc Ixlnit)n uf Ihc\'uIOllik: ;tOO v:lluablc: \...mc.!cn.'iale liquitb Iduc: 10 cOlpill;ary pre!i-"lIre-iIWlM:\.'tI 1r;lpping .:rrt.",-"ls in the plnlU.'i tnc:tlial.Thi!i p.lp.:r rl:vic:\v~ Ih.: h:l~ic Ih':l'ry l)r g.l!i cund.:n!iollcJrI)pllllI alw c.Ic~bc!i. in l.k.'1O1il. d.lm:lgc pnml':l11!i Ih:ll nUlY 11.:;1!i~M:i:tIt.'tI with pnxlOClil1l1 of rc.'il:rvl)jrJI I" Ihi!i Iyp.:. Tc:chniquc!ifllr Inilig:tling coOOc:n!i:l\c droptlul prubl.:m!i I)n :I pnxlUClil)nhol!ii!i. a.. \vcll a~ !ililnulalilln 1t.",-"hni'IU\.~ ...uch a.. rcpre!i...uriwlilln.I.:an alw rich gol!i inj,:clil)n. !iurr:lctanl and ~)I\'.:nl inj.:ction. in!iilU l:'tlnhU!ilil)n :tnd w:tlcr/ga.. injt.",-"lilln. .Ire re\"ic\vt.'tI. :too 11)t::KI\"anI3g':!i 31xJ di ll.lv3nl:t~'C!i l" 111.: 1t.'ChniqlJl:!i di~'t!i."'-'tI.IntroductionRich gas or retrograde condensate gu reservoirs are commonon a worldwide basis.Fi~ 1 JXOvides a pressure-composition diagram for a typicalhydrocarbon system at a fixed temperature level. The shaded p>rtionof this figure represents an area of two-phase equilibrium atthe specified composition and pressure condition. This is generallya region where an immiscible hydrocarbon liquid and gu phaseco-exist in thermodynamic equilibrium. Outside this area is a singlephase region, where only one continuous and homogeneousuniform phase exists.Potential Problems AssociatedGas Condensate Systemswith RichThe~ are two main categories of problems commonly associat-\.-d with rich gas reservoir systems:I. Fonnation damage effects associated with the condensatedropout2. Pennanent loss of valuable light condensate liquids due totrapping effects in the reservoir..~is paper addresses both of these issues and various techniquesthat can be used to rMuce their impact on productivity andrCCllVcry.Formation Damage Issues in GasCondensate ReservoirsThe literature is replete with detailed discussions on fonnationdamage

ing situations, as many gas condensate reservoirs at discoveryconditions are at or very near the dewpoint pressure of the gas.Even modest drawdowns applied in order to achieve economicproduction rates result in sub dewpoint pressure productionaround the producing weDs.Drawdown can be reduced in many cases by increasing effectivebottomhole flow area through the use of high density perforating,open hole completions, horizontal wells and, in somecases, even by small fracture treatments (if fracturing wiD notresult in water/oil production) in order to increase bottomholeflow area which can reduce drawdown and damage effects.Mitigation TechniquesIn most situations, a certain amount of damage due to condensatetrapping is unavoidable. This is even the case in a gas cyclingoperation as, in general, although the bulk of the reservoir ismaintained at some pressure above the dew point value, the areaimmediately adjacent to the wellbore is often subjected to highdrawdowns which still result in localized condensate dropout Ingood quality reservoirs where the value of the critical mobile sablrationis low, this dropout may not appreciably affect flow rates(at least while reservoir pressure is high), and conventiODal primaryproduction may be the economic and technical option ofchoice. In many cases though, periodic treatments to attempt to"remove" a portion of the trapped condensate liquids from aroundthe wellbore are often used to try to increase production rates.There are a number of potential techniques that have been suggestedor used recently for this purpose with varying degrees ofsuccess. They include:1. Static repressurization and imbibition2. Lean gas injection3. Rich gas injection4. Solvent injection5. Mutual solvent injection6. In situ combustion7. W ater injection/displ~tStatic Repressurization and ImbibitionThe tenant of Ibis technique is based on the assumption of !hermodynamicreversibility. Examination of Figure 1 indicates that,theoretically, if production from the well is baIted (shut in) and ifthe bulk reservoir pressure is still high enough that over time thepressure in the depleted region around the weUbore is increased toabove the original dew point pressure, the condensate liquidswhich are trapped in the pore system should be "re-vapourized"into the gas phase.Unfortunately, this technique enjoys limited actual success inporous media due to the limited interfacial area, the revapouriza-lion process is extremely mass transfer limited (diffusion motivated)and hence very slow. Also, since such a limited volume of gasis in contact with a given volume of condensate in d)e pore space(the condensate which is ttapped having been precipitated out of ahuge volume of gas w~h has flowed through d)e pore space incomparison to the relatively small volume present in contact withit under static exposure), this once again severely limits theamount of revapourization that can occur.In a similar fashion, static imbibition (capillary pessure effects"wicking" the ttapped condensate away from d)e wellbore deeperinto the formation) are only effective if the reservoir exhibitsstrongly oil wet natural wetting tendencies. If d)e reservoir is neutralor water wet, there will be no spontaneous imbibition affinitypresent to move the trapped saturation away from the near wellboreregion. Since many gas reservoirs do not contain a liquidhydrocarbon saturation initially, by default, in many cases, d)eyare water-wet in nature, and hence this negates imbibition as ameans of drawing retrograded condensate liquids away from thewellbore.Examination of Figure I also indicates for some reservoir situationsthat if the pressure is dropped low enough, condensatebegins to revapourize once again. In some instances, it is possible.at very low pressures and high temperatures, to pass completelyout of d)e two-phase region (through d)e "bottom" dew point lineas illustrated in Figure I). Practically, once again, this med1od islimited in application. In most cases, the pressure at which substantiverevapourization begins to occur is well below the ec0-nomic abandonment pressure of the reservoir. Even if this is notd)e case, the re-vapourization process tends to be extremely slowand mass transfer-dominated when occurring in a static situationin porous media (in comparison to an agitated visual cell systemin which the PVT measurements are normally conducted).Lean Gas InjectionThis technique enjoys more success, but generally requires relativelyhigh bottomhole pressure in order to be successful. Leangas injection is generally conducted using either dry methane ornitrogen gas and uses high pressure vapourizing miscibility toextract the trapped condensate from the injection region ~-ing the wellbore. In this case, because the injection gas is lean innatum and contains 00 heavy end fraction, it has the capability ofextracting a considerable amount of heavy ends from the system.6 Journal of Canadian Petroleum T~-:.ok)gy

Since injection is occurring and new fluid is constantly movingthrough the pore system, convective and constant mass transfer isalso possible in comparison to the previously discussed static shutin methods. Hence, more rapid mass transfer and vapourizationeffects are present.The pressure required is highly dependent on the compositionof the trapped condensate aOO specific temperature and gu pr0perties.Generally higher pressures, usually in the 40 - 50 MParegion, are required for vapourlnng miscibility with pure nibogengu. Methane tends to have lower vapouri7iDg miscibility pressuJewith most condensates, often in the 30 40 MPa region.This technique is commonly used u a periodic stimulationmethod in many gas cycling operations as a readily availablesource of lean gas and compression facilities are already presenton-site. In many gu cycling operations, production wells are periodicallystimulated for a few days by dry gas injection to removeaccumulated condensate and increase production rates and reducedrawdown pressures.Rich Gas InjectionIn many reservoir situations. insufficient presswe is present togenerate vapourizing miscibility with lean gas or nitrogen injection.In these cases. similar miscible extraction/displacement ofthe condensate can be effected by the use of a higher molecularweight injection gases such as ethane, propane or carbon dioxidewhich have much lower miscibility pressures with the trappedcondensate. Although effective. these medKIds are fairly expensiveto apply and concerns exist in some situations with respectopotential deasphalting of the condensate liquids by rich gas contact.Compatibility tests between the proposed injection gas andthe reservoir condensate should be conducted to evaluate thisissue prior to executing a treatment of this type.Solvent InjectionThis process consists of the injection of a liquid phase hydrocarbonsolvent (generally toluene, xylene or distillate) into d)e formation.Although this method is often effective for removingwax/paraffin deposits that may be associated with ~~te pr0-duction in some wells, as I method for removal of "trapped" c0ndensate,the method is normally ineffective as one simply substi-lutes one miscible hydrocarbon phase for another. In many cases,the IFf between the reservoir gas and an organic solvent, such axylene. is actually higher than that between the trapped condensateand the reservoir gas, which may actually result in an increasein trapped hydrocarbon phase saturation.Mutual Solvent/Surfactant InjectionThis includes the injection of high molecular weight alcohols(i.e., butanol), ocher mutual solvents and surfactants. The objectiveis to reduce the gas-condensate IFf which makes it easier torecover the ttapped condensate (e.g., lowers the value of the criticalcondensate saturation). Many alcohols have sludge and emulsionproblems with condensates and careful compatibility testingshould be conducted. Often with many agents of this type, theactual reduction in gas-oil IFf is relatively slight and the overallstimulation effect may be marginal. Careful 1FT and lab screeningshould be conducted prior to execution to determine the effectivenessof any agent of this type for condensate removal.In Situ CombustionThis is a fairly novel technique which attempts to use air injectionto ignite and "bum" the trapped condensate sablration out ofthe region in the near wellbore &rea. Most condensates are volatilein nature and will spontaneously ignite at reservoir temperaturesover about 120' C.Concerns with this method include high bouomhole temperaturesaDd cement degradation, effective propagation of the flood,ccxrosion CUICemS and well flashback. (explosion) coocems if allof the injected oxygen is not consumed by combustion andLTO/HTO oxidation reactions with the in situ crude oil.Considerable resean:h wort is cunently underway evaluatingthe use of this method. Accelerated rate calorimetric studies

sate is removed from dte near wellbore &rea, continued productionbelow the dewpoint pressure results in the recurrence of accumulationproblems. The economics of the treatment will thus beProvenance-Original Petroleum Society manuscript,Retrograde Condensate Dropout Phenomena in Rich Gashighly dependent on the magnitude and length of the "flush" pr0-Reservoi~Impact on Recoverable Reserves, Permeability,duction period that is created after the stimulation treatment isDiagnosis, and Stimulation Techniques (TN2001-078), firstpresented at the Canadian International Petroleum Conferencecompleted.June 12-14, 2001, in Calgary, Alberta. Abstract submitted forreview December 12, 2000; editorial comments sent to theauthor(s) August 20, 2001; revised manuscript receivedConclusionsSeptember 19,2001; paper approved for pre-press October II,The mechanism of condensate dropout from rich gases has 2001, final approval December 3, 2001.&been discussed as one of the potential mechanisms reducing theproduction rate aJKI ~verable reserves of condensate liquids. InAuthors Biographiesgeneral, the value of the critical condensate saturation in porousmedia varies from I - 40% and is affected by IX>le geometry, wettability,interfacial tension and capillary pressure and drawdownBrant Bennion is Hycal's president and isI a project engineer with over 20 years ofeffects. In most situations, porous media with in situ peImeabilitiesgreater than 1,000 mD exhibit low (less than S%) critical con-domestic and intematiOllaJ technical expertisein the area of formation damage anddensate saturation values. Fonnaboo damage aJKI trapping issuesfluid flow in porous media. Brant hasare generally more severe in lower quality (sub-lOO mD) formationswhich exhibit more adverse capillary pressUIe characteris-I authored or co-autbored over 170 technicalpapers on a variety of subjecu, includingtics. Means of reducing dropout problems, as well as remediationmulti-phase flow in porous media, formationdamage, underbalanced drilling, fluidtechniques such as represurization, lean and rich gas injection,solvent injection and in situ combustion have been presented andphase behaviour and enhanced oil recovery.the specific advantages and disadvantages of each methodBrant received his B.Sc. in chemica] andreviewed.petroleum engineering from the University of Calgary with distinctionin 1984. BraDt received Best Technical Paper of the Yearawards from the Petroleum Society in 1993 and 1995. Brant is aAcknowledgementsdirector of the Petroleum Society and is also a ~ber of APEG-The authors express appreciation to Hycal Energy ResearchGA and SPE (registered P .Eng). Brant wu also chosen to be aLaboratories for permission to presenthis material and to Vivian"Distinguished Lecturer" for the SPE for 2001.Whiting for her assistance in the ~on of the manuscript.REFERENCESI. ASADI, M., et aI., Effect of the Perfontion Damage on WellProductivity; pap.r SPE 27384 pr,s.nt.d at th, InternationalS)'mpo$iIIIII 011 FomllJdOll Damag' Control, lA/aY't", U, FebnlDry7 -10, 1994.2. BENNION, D.B., et aI., Remediation of Water and HydrocarbonPbue Tnppinl Problems in Low Permeability Gas Reservoirs;P.trol,um Soci", paper No. ~80, pru,nt,d at tM 4~ ATM ofth,P~- Society, Calgary, AS. 1_10-12, 1996.3. ECONOMIDES, M.J., DEHGHANI, K., OGBE, D.O., andOSTBRMANN, RD., HysteIais Effects fw Gas CondensaIe WellsUlMierIoing Bwldup Tau Below the Dew Point PressuM; pre.rmudat die 62..1 ATC of tile SPE, DaJJos. TX. Sepi.mber 27 - 30, 1987.4. HWANG, M.K. and ODEH, A.S., Estimation of CondensateDropout Effects On Well Productivity as Skin Changes WithMultiplicative Interactions Among Skin Components; ~r SPE29894 pres,nt,d at th, SPE Middk East Oil Show, Bahrain, March11- 14, 1995.5. HAMBERUN, C.W., at aI., Combination of Selected Solvents andMutual Solvents Successful in Removing Hydrocarbon BasedFormation Damale; paper SPE 21572 pr,s,nt,dat th, JointPetroleum SoderysPE Technical M,eting. Calgary, AS, IUM 10 -13,1990.6. THOMAS, F.B., et al., Towards Optimizing Gas CondensateReservoirs; PIIroI Society paper No. 95-09, ~ at die 461'ATM ofdl, Pltroill/m Soci"" BaJ!ff; AB, lun.1995.7. THOMAS, P.B., et aI., Opcimizin, Productioa Prom a Rich GuCondensate Reservoir; paper SPEIDOE 35455, Tulsa, OK, April2/- 24,1996.8. YEAGER, V.J., Use of Downhole Diagnostics EnhancesDetermination of Damage Mechanisms; Pr,s,nt,d at th, SPE1ntmlatiOftal SJIIIposium 011 FomlOtion Damag' COIIIrol. 1A/aY'II"U, SPE 39466, F.bruary 18 -19, 1998-9. BENNION, D.B., PomI8tiUI Damaae During UncJerl)a1aIICed andOverbalanced Drillinl Operations; Ph.D. Th~sis, Univ~rsity 0/Calgary,200J.10. Y ANNlMARA, D. V. and -nPPIN, D.L, ScreeIIing of Oil fw In SituCombustion at Reservoir Conditions Using Accelerating RateCalorimetry; ~r SPE 27791, pru./lt.d and tM 98' SPE EaRSymposium. Tul.ta. OX, April 17 - 20, 1994.Brent Thomas is Hycal's senior vicepresidentand is a project engineer workingin the area of numerical simulation and gasI injection. He received his Ph.D. fromWashington University in chemical engineering.Brent has over 20 years of domesticand international experience in die areaI of numerical simulation, gas injection,- - phase chemical behaviour, and thermal solids application. precipitation, He and has...uthored or co-authored over 130 technical papers and receivedthe 1992 Best Technical Paper of the Year award from thePetroleum Society (Experimental and Theoretical Studies ofSolids Precipitation from Reservoir Fluids). He was selected as a"Distinguished Author" for die Petroleum Society in 1995.-- -Bernie Schulmelster is the engineeringmanager at Hycal. He brings with him 20years of industry experience, IS of whichare directly focused on laboratory researchand services. He maintains a strong technicalbackground in the areas of petrophysicsand fonnation damage assessment and control.Experienced in managing and conductingapplied analysis. studies Currently in all ~ responsible of advanced core foroverseeing the engineering function of our integrated projectswhich includes special core analysis and phase behaviour. Bernieis a graduate of the Southern Alberta Institute of Technology witha diploma in petroleum engineering technology.8 Jownal of Canadian Petroleum Technology