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\ llr

rclrllgr-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 prll-dOClilll1 rol,-~ 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.

Formation Damage Issues in GasCondensate Reservoirs

The literature is replete with detailed discussions on fonnationdamage<I-I). This paper <kIes not address different types of damagewhich can occur during conventional drilling, complerion and pr0-duction operations, but concentrates solely on issues associatedwith condensate liquid dropout

The primary damage mechanism ~ulting in a reduction in gasproduction rate in a rich gas-condensate reservoir is generallyassociated with capillary pressure-motivated phase trappingeffects. These effects result in permanent reduction in the effec-tive pemJeability to gas in the region affected by the condensatedropout surrounding the wellbore. This phenomenon is iIIusttatedpictorially in Figure 2 and using a set of gas-condensate relativepermeability curves in Figure 3.

As the condensate drops from solution in the gas while still inthe porous media, capillary pressure effects are ~nt [due to thegeneration of a second immiscible phase which has a finite inter-facial tertsion (IFIj between it and the gas phase]. This capillarypressure traps the discrete drops of condensate in the central por-tion of the pore system and does not allow them to move until thesaturarion increases to the point where the individual droplets ofcondensed hydrocarbon liquid can accrete together and form acontinuous condensate "film" in the porous media. Once thisoccurs, the condensate phase acquires finite relative permeabilityand can then flow as a separate and distinct phase in the rock. Thevalue of the condensate saturation, which must build up bef~mobility occurs, is commonly referred to as the "critical" or"mobile" condensate saturation. It can have values ranging fromless than I % (at low IFf conditions and in high permeabilityrocks which have very low capillary pressure), to values in excessof 40% in poorer quality porous media.

IntroductionRich gas or retrograde condensate gu reservoirs are common

on a worldwide basis.Fi~ 1 JXOvides a pressure-composition diagram for a typical

hydrocarbon system at a fixed temperature level. The shaded p>r-tion of this figure represents an area of two-phase equilibrium atthe specified composition and pressure condition. This is general-ly a region where an immiscible hydrocarbon liquid and gu phaseco-exist in thermodynamic equilibrium. Outside this area is a sin-gle phase region, where only one continuous and homogeneousuniform phase exists.

Potential Problems Associated with RichGas Condensate Systems

The~ are two main categories of problems commonly associat-\.-d with rich gas reservoir systems:

I. Fonnation damage effects associated with the condensate

dropout2. Pennanent loss of valuable light condensate liquids due to

trapping effects in the reservoir..~is paper addresses both of these issues and various tech-

niques that can be used to rMuce their impact on productivity andrCCllVcry.

This paper is being published as a technl

5Dec:&.RJe 2001, VokJrne 40, No. 12

Techniques to Avold/Reduce CondensateDropout Effects

Condensate dropout is a difficult problem to combat, simplybecause it is a natural thermodynamic property of the reservoirgas under consideration. As long as the reservoir temperatUIe andgas composition remain constant, producing at a bottomhole pres-sure below the dewpoint will result in retrograde dropout

One approach is pressure maintenance combined widl reduceddrawdowns and production rates such that the flowing bottomholepressure around the production wells remains at all times abovethe dewpoint value. This is generally impractical in most operat-

ical note and has not been peer reviewed.

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 effec-tive bottomhole flow area through the use of high density perfo-rating, 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.

lion process is extremely mass transfer limited (diffusion motivat-ed) 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 neu-tral or water wet, there will be no spontaneous imbibition affinitypresent to move the trapped saturation away from the near well-bore region. 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 situa-tions that 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 sub-stantive revapourization 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).

Mitigation TechniquesIn most situations, a certain amount of damage due to conden-

sate trapping is unavoidable. This is even the case in a gas cycling

operation as, in general, although the bulk of the reservoir is

maintained at some pressure above the dew point value, the area

immediately adjacent to the wellbore is often subjected to high

drawdowns which still result in localized condensate dropout In

good quality reservoirs where the value of the critical mobile sabl-

ration is low, this dropout may not appreciably affect flow rates

(at least while reservoir pressure is high), and conventiODal prima-

ry production may be the economic and technical option of

choice. In many cases though, periodic treatments to attempt to

"remove" a portion of the trapped condensate liquids from around

the wellbore are often used to try to increase production rates.

There are a number of potential techniques that have been sug-

gested or used recently for this purpose with varying degrees of

success. They include:

1. Static repressurization and imbibition

2. Lean gas injection

3. Rich gas injection

4. Solvent injection

5. Mutual solvent injection

6. In situ combustion

7. W ater injection/displ~t

Lean Gas InjectionThis technique enjoys more success, but generally requires rel-

atively high bottomhole pressure in order to be successful. Lean

gas injection is generally conducted using either dry methane or

nitrogen gas and uses high pressure vapourizing miscibility to

extract the trapped condensate from the injection region ~-

ing the wellbore. In this case, because the injection gas is lean in

natum and contains 00 heavy end fraction, it has the capability of

extracting a considerable amount of heavy ends from the system.

Static Repressurization and ImbibitionThe tenant of Ibis technique is based on the assumption of !her-

modynamic reversibility. 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-

6 Journal of Canadian Petroleum T~-:.ok)gy

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 conden-sate and 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 objec-tive is to reduce the gas-condensate IFf which makes it easier torecover the ttapped condensate (e.g., lowers the value of the criti-cal condensate saturation). Many alcohols have sludge and emul-sion problems 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 effective-ness of any agent of this type for condensate removal.

In Situ CombustionThis is a fairly novel technique which attempts to use air injec-

tion to 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 tempera-tures aDd 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<IO) area common screening med1od. coupled with combustion flow tests,to determine the suitability of this method for reservoirapplication.

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 pr0p-erties. 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 peri-odically stimulated 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 to

generate vapourizing miscibility with lean gas or nitrogen injec-tion. 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 expen-sive to apply and concerns exist in some situations with respect topotential deasphalting of the condensate liquids by rich gas con-tact. Compatibility tests between the proposed injection gas andthe reservoir condensate should be conducted to evaluate thisissue prior to executing a treatment of this type.

Water InjectionThis has long been proposed u a technique to recover trapped

condensate liquids from a depleted gas condensate reservoir. Themotivation is in many reservoir cases, the irreducible condensatesaturation to water displacement is lower than the ineducible con-densate saturation to gu displacement It is thought that injectedwater may mobilize a portion of the trapped condensate saturationin the bulk reservoir mattix. allowing recovery of a portion of this"trapped" IeSOUJ1:e.

Practical experience indicates that the value of the trapped con-densate saturation must be very high for this to be practical. In allcases where the author bas evaluated this method u an EaRmethod, very limited recovery wu observed on a bulk reservoirbuis which suggests in most applications, the potential for suc-cess is marginal.

Water injection has been used u a stimulation method in so~high permeability (10,000 mD plus) gu condensates to displacethe zone of high mobile condensate saturation away from the bot-tomhole region in a situation where pressure has dropped to theextent that the wells are loading and self killing with condensate.This is then followed by gu injection to displ~ die water and re-establish high gu saturation and permeability with die "undam-aged" portion of the reservoir. In lower permeability rocks, thismethod is not advised, u trapping and capillary IXes5UJ'e effectsusociated with the introduction of a water phase in die near well-bore region may create additional damage effects.

Solvent InjectionThis process consists of the injection of a liquid phase hydro-

carbon solvent (generally toluene, xylene or distillate) into d)e for-mation. 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" c0n-densate, the method is normally ineffective as one simply substi-

Duration of StimulationIt should be noted that all of the afo~mcntioncd stimulation

techniques do not solve d1e root cause of d1e damage. this beingthe retrograde nature of the gas. In many cases. these treatmentsmust be repeated on a regular basis as. even though the coDden-

DecerYt)ef 2001, V~ 40, No. 12 7

sate is removed from dte near wellbore &rea, continued productionbelow the dewpoint pressure results in the recurrence of accumu-lation problems. The economics of the treatment will thus behighly dependent on the magnitude and length of the "flush" pr0-duction period that is created after the stimulation treatment iscompleted.

Provenance-Original Petroleum Society manuscript,Retrograde Condensate Dropout Phenomena in Rich GasReservoi~Impact on Recoverable Reserves, Permeability,Diagnosis, and Stimulation Techniques (TN2001-078), firstpresented at the Canadian International Petroleum ConferenceJune 12-14, 2001, in Calgary, Alberta. Abstract submitted forreview December 12, 2000; editorial comments sent to theauthor(s) August 20, 2001; revised manuscript receivedSeptember 19,2001; paper approved for pre-press October II,2001, final approval December 3, 2001.&

ConclusionsThe mechanism of condensate dropout from rich gases has

been discussed as one of the potential mechanisms reducing theproduction rate aJKI ~verable reserves of condensate liquids. Ingeneral, the value of the critical condensate saturation in porousmedia varies from I - 40% and is affected by IX>le geometry, wet-

tability, interfacial tension and capillary pressure and drawdowneffects. In most situations, porous media with in situ peImeabili-ties greater than 1,000 mD exhibit low (less than S%) critical con-densate saturation values. Fonnaboo damage aJKI trapping issuesare generally more severe in lower quality (sub-lOO mD) forma-tions which exhibit more adverse capillary pressUIe characteris-tics. Means of reducing dropout problems, as well as remediationtechniques such as represurization, lean and rich gas injection,solvent injection and in situ combustion have been presented andthe specific advantages and disadvantages of each methodreviewed.

Authors Biographies

Brant Bennion is Hycal's president and isI a project engineer with over 20 years of

domestic and intematiOllaJ technical exper-tise in the area of formation damage andfluid flow in porous media. Brant has

I authored or co-autbored over 170 technicalpapers on a variety of subjecu, includingmulti-phase flow in porous media, forma-tion damage, underbalanced drilling, fluidphase behaviour and enhanced oil recovery.Brant received his B.Sc. in chemica] and

petroleum engineering from the University of Calgary with dis-tinction in 1984. BraDt received Best Technical Paper of the Yearawards from the Petroleum Society in 1993 and 1995. Brant is adirector of the Petroleum Society and is also a ~ber of APEG-GA and SPE (registered P .Eng). Brant wu also chosen to be a"Distinguished Lecturer" for the SPE for 200 1.

Brent Thomas is Hycal's senior vice-president and is a project engineer workingin the area of numerical simulation and gas

I injection. He received his Ph.D. fromWashington University in chemical engi-neering. Brent has over 20 years of domes-tic and international experience in die area

I of numerical simulation, gas injection,phase behaviour, solids precipitation, and- - chemical and thermal application. He 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 techni-cal background in the areas of petrophysicsand fonnation damage assessment and con-trol. Experienced in managing and conduct-ing applied studies in all ~ of advanced-- - core analysis. Currently responsible for

overseeing 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.

AcknowledgementsThe authors express appreciation to Hycal Energy Research

Laboratories for permission to present this material and to VivianWhiting for her assistance in the ~on of the manuscript.

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8 Jownal of Canadian Petroleum Technology