D.B. BENNION, F.B. THOMAS, A.F. BIETZ, D.W. BENNIONHycal Energy Research Laboratories Ltd.

AbstractThe entrapment of extraneous phases within porous media

pn occur in a number of different situations during drilling,~mpletion, work:over and production operations. The introduc-Ion of an additional immiscible phase, or an increase in an&isting phase saturation within porous media can cause delete-rious relative permeability effects which can substantiallyimpact the permeability and relative penneability to oil or gas.This phenomenon is commonly described as aqueOus phase trap-png or hydrocarbon phase trapping, depending on the situationunder consideration. This paper describes specific conditionsftquired for the establishment of aqueous and hydrocarbonphase traps and provides diagnostic equations to evaluate the~tential severity of an aqueous phase trap in a given reservoiratuadon. Specific procedures are recommended for the preven-t¥>n of aqueous phase traps during drilling, completion and pro-duction operations and, in a situation where phase traps arecktermined to exist in a reservoir, a variety of treatment tech-Diques are presented to attempt to remove or reduce the severityof the aqueous or hydrocarbon phase trapping phenomenon.

Bennion et al. (1994)(1) have provided a detailed discussion of thebasic mechanisms of aqueous and hydrocarbon phase trapping aIKIassociated reductions in permeability. Aqueous phase trapping CIDoccur in both oil and gas reservoirs and may be associated wid!reservoir situations where the reservoir exhibits a sub-irreducibleinitial water saturation. Specific additional documentation 00 sub-irreducible water saturation reservoirs is documented by Km etal. (1982)(2) and Masters et al. (1984)<3).

Hydrocarbon phase traps may be established in gas reservoirapplications where extraneous immiscible hydrocarbon phasellreintroduced into the reservoir, or in retrograde condensate reservoirapplications where the reservoir is produced at some pressurebelow the dewpoint resulting in the accumulation of liquid retr0-grade condensate within the pore space.

This paper discusses various types of phase trapping and po-vides criteria for the diagnosis of the potential severity of a 1'eSeI"-voir to be susceptible to aqueous phase traps. Additional discus-sion is then presented with respect to the prevention of aqueousand hydrocarbon phase traps in different reservoir operational sit-uations and potential remediation of oil and gas reservoirs whereaqueous or hydrocarbon phase traps have already beenestablished.

IntroductionMechanisms of Aqueous Phase Trapping

Aqueous phase trapping and hydrocarbon phase trapping repre-Ialt a significant mechanism of impaired productivity in many oiland gas reservoirs in various locations throughout the world.

CLEAN UP WITH GASFLUSH WITH AQUEOUS FLUIDINITIAL CONDITIONS

kab8= 0.1 mD. Sw = 0.60kg = 0.001 mD

kabs = 0.1 mO, SW = 0.80kg = 0.00 mO

kabs=0.1 mO. Swl =0.10kg = 0.05 mO

Water- G~

FIC;URE I: Pore scale mechanism of aqueous pha.w trapping in a gas reservoir,

20Decenm.r 1~. Volume 35. No. 10

Figure I provides a schematic illu.~tration of the establishmentof an aqueous phase trap within a low penneability gas reservoir

the initial water saturation and the ineducible water Iatun-tio'n, the greater the potential damage associated with apotential aqueous phase trap. This is schematically illustrat-ed in Figure 2 where it can be seen that the relative redIM:tionin gas or oil phase relative pemleability is significantlygreater if the ineducible saturation is located at point C at50% vs. point A at 20'11. The ineducible saturation is aener-ally a strong function of the capillary geometry of the pcwesystem. Generally, rocks with low permeability and C(XIe-spondingly small pore throats and pore diameters tend toexhibit higher ineducible water saturations. This phetlOllle-non is illustrated in Figure 3. It should be emphasized thatthe irreducible water saturations nonnaIIy predicted by C(XJ-ventionallaboratory drainage capillary pressure tests IXOvidegood approximations to the irreducible water saturationwhich may exist in porous media, but do DO( necessarily .-0-vide accurate estimates of the true initial water satuntion(which may be substantially lower).

8pplication. The initial basis for die establishment of an aqueous~ trap is a reservoir which exists at what is classified as a sub-ineducible saturation, where the initial water saturation in the~rvoir is less than what would be typically quantified as theirreducible water saturation which would exist in the porousmedia under the prevailing capillary conditions. Sub-irreducible.-orations are postulated to be established by a combination ofdehydration, desiccation, compaction and diagenetic effects which~ over the life of certain reservoirs.

In Figure I it can be seen that, in die initial desiccated condi-timl, due to die low pre-existing initial water saturation existing indie porous media, die majority of die cross-sectional area is avail-8ble for gas flow resulting in high initial relative pemleability topl. Subsequent flushing of this zone with a water-based filtrate(i.e., drilling mud flltrate, completion fluid, kill fluid. etc.) resultsiD die establishment of a high water saturation in die zone imme-diately surrounding the wellbore or fracture face and die establisb-.-lit of a critical gas saturation. Subsequent drawdown of theICKrvoir results in die affected zone reverting to dJe irreduciblewater saturation dictated by die capillary mechanics of the system,r81ber than back to die potentially very low initial water saturationwhich was initially present. If die irreducible water saturation issipificantly greater than the initial water saturation, this causes aconsiderable restriction in the cross-sectional area available forfhIid flow, as observed in Figure I, and causes a correspondingreduction in relative permeability to gas. This mechanism is ilIus-Ir88r:d schematically in Figure 2, which shows the change in rela-tiw permeability characteristics as a function of the establishmentof an aqueous phase trap. At the initial water saturation of 10%, as~ in Figure 2, high initial relative permeability to gas is appar-.. As the water saturation in the flushed zone is increased to themaximum value (i.e., 80%), a critical gas saturation is established.On subsequent drawdown, to attempt to mobilize the water fromdie porous media, the water ph8se relative penneability goes tozero at the true irreducible saturation value (50% in this example).This results in a situation where the entrapped water saturationC8DOt be physically reduced to a value less than the irreduciblelevel of 50% at the capillary gradient which can be realisticallyipplied in the reservoir, and the resulting permeability to gas atdie irreducible water saturation can be seen to have been substan-tially reduced (by approximately 95% in this particular exaDlple).

2. 11Ie configuraJion of the oil or gas phase relative penRMbiJ.ity curves-strongly controls the apparent severity of dam-age associated with aqueous phase b'apping. This phelKRDe-non is schematically iUusttated as Figure 4. It can be ~that a very linear set of relative penneability curves (as indi-cated by the optimal set of relative penneability curves inFigure 4) provides a system where relatively small inClalelin water saturation (such as that at point A) result in rea.ive-ly insignificant reductions in the relative permeability char-

Factors Which Affect the Severity ofAqueous Phase Trapping

The severity of an aqueous phase trap is controlled by fourmajor parameters. these being:

J. ~~ ~ th,. iIIilial wal,.' .uJIMI'fIIiOII and tk i,-n-"'i~ wal~' sal.mt;on-the larger die diff~ between

The Journal ~ c-cI8n P8b*-n T ~~

w~CK

Fract..-nGURE 5:IDustration of effectof mvasloo depth8Dd dnwdowoandieDt 00 aqueouspII8Se tnpplna.

remove entrapped fluids. It can be seen, when invasion isrelatively shallow and reservoir pressure is high, high appar-ent gradients can be applied across the flushed and affectedzone which can generally result in relatively low irreduciblesaturations being ~. If die reservoir pressure is low CKthe invasion depth is significant, the obtained gradientbecomes much less steep resulting in non-effective mobi-lization of the entrapped fluid from the porous media and dieestablishment of a significant permanent phase trap.

5. Wettability-low initial water saturations in water-wet fw-mations (which most commonly exist in desiccated Iureservoir applications) exhibit significant problems withboth spontaneous imbibition and phase trapping. In general,most water-wet formations in oil reservoir applications tendto exhibit in situ initial water saturations with values clOle todie irreducible value (or exceeding the irreducible v~ ifthe formation produces free water). This means that thesetypes of formations are not nonnally susceptible to sevae,permanent aqueous phase trapping phenomena. Strongly oil-wet porous media often exhibit extremely low initial w8tel'saturations and can exhibit significant sensitivity to aqueousphase trapping. Bennion(l) provides some examples of thistype of phenomenon.

BctCristics to oil or gas. Convenely, it can be seen that a setof very concave relative permeability curves (as illustratedby the severe curve in Figure 4) results in a significantreduction in relative permeability to oil or gas with a rela-tively small increase in the trapped water saturation. This isdue to significant multi-phase interference effects betweenthe immiscible phases within the porous media. Someporous media may exhibit very flat relative permeabilitycurves to gas at low liquid saturations, indicating that thetrapped phase is primarily occupying ineffective microp-orosity and hence gas permeability is relatively insensitive tomoderate increases in trapped fluid saturation. This phenom-ena is very reservoir specific and is a strong fraction of theconfiguration of d1e gas phase relative penneability at lowliquid saturation which is influenced by the degree of micro-porosity and how this microporosity is distributed in thepore system.

Typically, aqueous phase trapping tends to be more problemat-K: . lower permeability ftX'lDations. In many situations, particu-18ty when permeability is extremely low, or in strongly oil-wets media. relative permeability curves exhibiting significantt\~ MVere concave curvature, as seen in Figure 4, are the norm.

3. ~ physical depth of invasion-strongiy controls the abilityof the available reservoir pressure to mobilize and removethe entrained aqueous phase trap. In general, the more sig-nificant the invasion depth, the more difficult it is to removetbe resulting entrained aqueous fluid and the larger theapparent permanent reduction in permeability due to aque-ous phase trapping.

4. Available reservoir pressure to mobilize the entrappedreservoir fluids-generally, the higher the available reser-voir pressure, the higher the capillary gradient which can beapplied and the lower the resulting irreducible liquid satura-tion which can ultimately be obtained. Reservoir pressure istightly coupled with the previous mechanism of invasiondepth. As the reservoir pressure is usually distributed overthe length of the flushed zone, it is not so much d1e absolutevalue of the reservoir pressure but rather the availableapplied gradient in kPa per metre or psi per foot whichallows the entrained fluids to be physically mobilized andproduced from the porous media by allowing viscous forcesto overcome the retentive capillary force. This phenomenonis achem8K:ally iliustraAed in Fiaure 5 showina the effects of~ sl.l1ow - deep invasion . bc8 hip - low .--voir pressure conditions on the ability to mobilize and

Correlations to Predict the PotentialSeverity of Aqueous Phase Trapping forBoth Oil and Gas Reservoir UniformMatrix Situations

In many situations. an evaluation of the potential severity ofaqueous phase trapping associated for a given reservoir situ8tionis desired. The following set of e(Juations provides a set of euy-to-u~e diagnostic tools to evaluate the potential sen~itivity andseverity of a reservoir to the establishment of an aqueous pbuetrap. These equations are based upon regression analysis of a I~number of aqueous phase trap tests which have been conductedover a wide range of penneabilities and lithologies for gas ~-voir applications and sub-irreducibly saturated oil reservoir appli-cations (generally of an oil-wet nature).

The initial formulation uses a new methodology known as the~.!f phue tnPIJina iJldex (APT;) to di.,nose the potentialseverity of i'f~.1i ..1OCi~ with ~ phase tr8AJing. The

December 1996 . VokIme 35 . No . 1 0 31

basic fonnulation of this equation is based solely upon perme8bi1-ity aIKi initial water saturation values (which generally Ire the twomost strongly controlling characteristics f(W a given !K)tnOaeneousporous media) for either gas or oil reservoir applications.Although only a correlation, the APT; formulation generally pro-vides a conservative estimate of the potential for an aqueous phasetrap in the vast majority of situations. The formulation is:

0,§,

r:0~~E

l

~'2

~

8~~

APT;;(t)

1where:APT;t.Swi

aqueous phase trap indexuncorrected average formation air permeability (mD)initial (not ilTeducible) water saturation (fraction)

0.1

Ringe of validity:k. =0-5QOOmDS. =0-1.00WI

0.010.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Initial Water Saturation (fraction)~ No ..- d8lloae -""' - - -- --

Interpretation:APT; ~ 1.00

(NOTE: APT - ~ "" -- por--r« _mn - .,->FIGURE 6: illustration of apt correlation for preliminarydiagnosis of aqueous phase trap problems.

0.. ~ APT; .~predictive equation for IP a is given by:

APT; < 0.80

formation unlikely to exhibit significantpennanent sensitivity to aqueous phasetrappingformation may exhibit sensitivity toaqueous phase trappingfonnation will likely exhibit significantsensitivity to aqueous phase trapping

0.08 [loglo (Id + 0..4)]IP,A more rigorous evaluation of APT; can also take into consid-ention the following factors which may have a contributory effectm d1e severity of aqueous phase trap concerns, these being:

I. Relative penneability adjustment factor, RPa2. Invasion profile adjustment factor, IPa3. Reservoir pressure adjustment factor, PRa

.(4)

~ = radial (wellbore) or linear (frac face) interstitial invasiondepth (cm) ,

(0 ~ ~ ~ S~ Cm, IP a varies from -0.032 to +{J.216)

Reservoir Pressure Adjustment Factor, PR.The greater the reservoir pressure, the higher the gradient

which can be mobilized to overcome capillary retentive effectswhich aggravate aqueous phase trapping concerns. Therefore. PR.is given by:

Relative Permeability Adjustment Factor,RPa

In general, the oil or gas phase relative penneability curve canbe approximated by an equation of the form:

o.75PR. 0.15 [loglo (PItr- (Sw ~r. (S. .(S)

e:k.a,o (S,

where PR = current reservoir pressure (MPa), (0.1 0 ~ Pr ~ 50MPa, PR. varies from -0.325 to +{}.080)Therefore, the complete fonn of the APTj equation with allidjust-ment factorils:

t n

(Swmox-Sw)

APT;In

+.2.2 is RP, IP, + PR.(6)w,

(SW~_SWmiD)

X = relative permeability shape factor (x ~I.O)The greater the value of the relative permeability shape factor,

the more concave the shape of the resulting oil or gas phase rela-tive permeability curve and the more significant the effect of aniBCrease in trapped water saturation on oil or gas phase permeabil-ity. Therefore, the relationship for RP. is:

RP 0.26 [log/a 0.5)1x

.&S x .s 8. RPa varies from -0.078 to +0.228

The final formulation of the APT; equation, as illustrated byEquation (6), includes allowance for the effects of formation per-meability, initial water saturation, severity of relative perme8bilitycharacteristics, allowance for fluid invasion depth and availablereservoir pressure to facilitate clean-up. In general, these C(II1'eC-tionsare subtractive which increase the severity of the damageassociated with aqueous phase trapping but, in some situations,the corrections are additive (such as the case of extremely bighreservoir pressure, very low invasion depth or favourable relativepermeability) which tend to reduce the overall significance ofproblems associated with aqueous phase trapping in a given reser-voir situation.

Figure 6 provides an illustrative example of a graphical repre-sentation on semi-log coordinates of the basic APT; correlationshowina lines of constant APTj quality. The region between APT;of I and APT i of 0.80 shouJd be considered to be a re,ion ofpotential sensitivity to aqueous pIwe trappin,. Permeability and

Invasion Profile Adjustment Factor, IP a11Ie Ife*r the apparent invasion depth, the more significant

the severity and difficulty of aqueous phase trap clean-up. The

The ~ of c.-.n Petrolewn T ec:- .-1i*'G'/.

relative permeability to oil or gas at a givenvariable water saturationmaximum endpoint relative permeability tooil or gas at the irreducible water saturationnormalized saturation function:

water S81Ur8bOO resions falling below an APT; of 0.80 stk)uId beconsidered to be at a risk for significant potential damage associ-ated with aqueous ~ b'apping.

Fcxmations exhibitiDB w~ S8tUr8tioo and penneability chlr-ICteristics with APT, values in excess of I generally are not at sig-Dificant risk for long-term permanent damage associated with~ phase trapping. In some situations. ttansitory reductionsin permeability may be observed if significant amounts of aque-ous invasion into the porous media occur which may result in alipificant time for downloading of the fonnation and productionof the water. This particular phenomenon is commonly coinedaqueous phase loading. which represents a transient and non-per-~t variant of aqueous phase trapping. Aqueous phase loading(API...) can cause substantial short-to medium-term reductions inmI or gas production rates and. hence, may also have economic~t in a given ~servoir application.

Accurate Determination of InitialWater Saturation

The accurate detemlination of initial water saturation is a keyf8Ct0r in diagnosing the potential severity of problems associatedwith aqueous phase trapping. Recent techniques to supplement~ventiona1log based water saturarion detennination evaluationsiDCllKie the use of speciality coring systems using hydrocarbon-baed drilling muds to provide non-altered initial water saturationsvia direct core analysis. Other methods include the use of lowmvuion coring systems with water-based muds with radioactive(lricium or deuterium) or chemical tracers and subsequent evalua-- and analysis of the extracted water from the obtained corematerial to provide an accurate evaluation of initial wateruturation.

gas, flue gas or LPG, has been attempted iD some situations toavoid the Cootact of water- or hydrocarbon-based fluids with theformatiOD. ID some situatioDs. uDderbalaDCed drillinl bas alsobeen utilized as a techDique to avoid deleterious effects associatedwith aqueous phase trapping. Completion techniques with mutualsolvents (i.e., methanol) have also been successful in some appli-cations. Compatibility testing should be undertaken before usingmutual solvents in an oil reservoir application to ensure adequatemiscibility and physical compatibility with the in situ crude oil.

Underbalanced drilling should be utilized with caution if dieprimary mechanism of damage is postulated to be aqueous J8aIetrapping. as strong countercumnt spontaneous imbibition effectscan still occur due to very adverse capillary pressure cbark:tens-tics which exist in sub-irreducibly saturated reservoirs. S~case studies and discussion of the mechanism of spontaneouscountercurrent imbibition during underbalanced drilling opera-tions is provided in detail in Bennion et aI' (1994)(4).

If technical, operational or economic constraints indicate thatwater-based fluids must be used in a situation where aqueouspI1ase trapping is problematic. care should be taken to minimiapotential invasion depth so that a high gradient can be applied tomobilize any entrapped water which does invade into fOnnab~.For drilling fluids, this would generally involve the use of verylow API fluid loss systems complete with artificial bridJiDIagents to establish a very low permeability sealing filter cakerapidly on the face of the fonnation. A similar approach should beutilized for completion or kill fluids to avoid the infiltrati<MI ofwater based fluids into the formation.

For fracture stimulation treatments, if water based fluids ale tobe considered. ultra low fluid loss slK>uld also be considered 81 aprime objective of the fracturing program and consideratioDshould be given to the usage of high viscosity cross-linked fluidswith appropriate time or temperature activated breakers to mini-mize potentiallwoff to the formation at high fracture ovcrt.l-ancc pressures. Although conventional wisdom indicates that frK-ture face damage can reach very high values (over 98%) duriDIfracture treatments and the ultimate productivity of the well i.18illcontrolled by fracture conductivity rather than fracture face per-meability, the mechanism of aqueous phase trapping in low per-meability gas reservoir applications has been documented to causecomplete reductions in penneability over large apparent secti<MIsof the exposed fracture face. This is due to the fact that. once ftuidis invaded into the very tight matrix, if invasion depth is signifi-cant at the drawdown pressures available. mobilization ~cannot be exceeded. resulting in a large portion of the expoeedfracture face having zero or exceptionally low conductivity. ThiIresults in a significant reduction in the apparent inflow charac istics of the water based frac. The use of gas charged (CO2 <W N~

fracture fluids is also advantageous in these situations as it .-0-vides both superior shrinkage characteristics for any invadiDgfluid as well as provides a zone of high energy for subsequentblowdown of the invaded zone upon return production.

Aqueous Phase Trapping inFractured Systems

In general, in systems which contain large open fractures, thephysical diameter of the fracmres is sufficient that very low capil-lary pressures are obtained and, in many situations, permanent~s phase trapping is no( a significant concern. In some cases~ massive fluid loss has occuned, it may take a considerableck)wn1oading time to remove the water transferred to the fracture.ystem which may result in an aqueous phase load and a longperiod of clean-up and reduced flow potential to oil or gas.Oenera11y, if fracture apertures are greater than approximately 100~. significant problems with pennanent aqueous phase trap-~ are DOt a concern. In small fractures (10 - 100 microns) somepermanent retention of hydrocarbon- or water-based fluids mayoccur resulting in moderate reductions in permeability. In micro-fncIIa'ed systems (fracture apenures of less than 10 microns), sig-8ificant capillary retentive effects can still occur resulting in the~on of relatively high aqueous phase saturations within themicro-fracture system and corresponding performance relativelysimilar to that observed in matrix quality systems.

Treatment of Existing Aqueous orHydrocarbon Phase Traps

In many situations, the productivity of an oil or gas prod8M:iDgwell has been significantly impaired by the establishment of aphase trap. This phase trap may have been established iniballyduring drilling or completion or subsequently induced inlMlver-leDtly through the use of water-based kill fluids, completion fluidsor workover fluids. When a phase trap has been established. theprime objective is lo restore productivity by physical removal orreduction of the entrapped water saturation. Once this water or oilsaturation is physically removed, the apparent relative penncabili-ty lo oil or gas should technically increase. There are a variety ofmeth<x1s postulated lo remove previously exi~ting aqueous ~traps. Generally. the mechanism to remove an existing aq~sphase trap is based on one of four major techniques, these being:

I. Ia:IaIe c-.,ila.y .-a- .. ..-ce die radial was' or oilsaturatiCMI.

Prevention of Aqueous Phase TrappingIf aqueous phase trapping is considered to be a significant

mecbanjsm of potential formation damage in a given reservoirapplication based upon the results of the previously discusseddi8&DQstic tests, consideration should be given to the design of the*illing and completion JXUgraJn to minimize the impact of COII-t8I::t of water-based nuids on the formation. This can be done in avlriety of ways, such as the use of compatible hydrocarbon-basedfluids in oil reservoir applications and also in some gas reservoirs1,1plications [see Bennion et al. (1994)11) for further discussionand case studies on the use of hydrocarbon based fluids in lowpermeability gas relervoir apptic8tions]. The use of gu duringdrilling end completion ~ ~b 81 air. nitroten. n8Iural

~,oi)8f 1988. V~ 35. No. 10 33

2. Reduce tile Ipparent interfacial tension between tile waterand oil or water and gas phases to reduce the resulting capil-lary ~ssure lad hence generate a lower irreducible watersan..tion at a given available reservoir IXeIIUre ckawdown.

3. Change the physical geometry of the pore system to increasethe radius of curvature of the phases present in the porousmedia and thereby reduce the capillary pressure and allowproduction of tile trapped phases.

4. Physically remove the trapped water saturation throughevaporation or heat treatment techniques.

Each of these mechanisms will now be discussed in greaterdetail.

Physically Increasing Drawdown PressureIt can be seen from examination of capillary pressure curve

sxesented as a portion of Figure 5 that, in general, the higher theapplied capillary pressure, the lower the resulting residual watersaturation which is obtained. In general, capillary pressure curvestaMi to go rapidly vertically asymptotic near the irreducible satu-ration, which results in very large iJK:reases in applied capillary.-essure being required to achieve relatively small reductions inreaidual water or oil saturation. Although, technically speaking,any porous medium can be reduced to a near zero residual satura-- by the application of sufficiently high capillary gradients, in(XKticality, this is not feasible. It can be seen from an examina-tion of the data of Figure 5 that, if high reservoir pressures areavailable and invasion depth of the aqueous phase fluid is relative-ly sballow, relatively high instantaneous capillary JXeSSure gradi-.. could technically be applied in the reservoir which may resultin a significant reduction in residual water saturation. However, ina.-t situations, the combination of reservoir pressure and inva-IiC8 depth result in a gradient which is generally insufficient toDJVe a sufficiently far distance back along the capillary pressurecurve to result in a significant reduction in the residual water satu-ration. Thus, in many cases, although increased drawdown ratesmay be efficient in reducing the water saturation to the normallytylXfied irreducible value, they often are not a significant mecha-niam for attempting to reduce saturation to the original sub-irre-~ble value. Increased drawdown is generally a g~ method forralM)ving an aqueous phase loading problem as reductions to theineducible water saturation can be reasonably achieved.

prooUCOVIty. UIe mrroaucuon 01 ~<1lnlo me tOmlaUon may hIr-ther compound the problem associated with aqueous phase tnp-ping rather than providing stimulation. Therefore, acid stimul8bonin carbolJate formations which ale susceptible to aqueous pbuetrapping shouJd oll1y be undertaken after extensive evalu~ orthe potential severity of aqueous phase trapping which migiK beassociated with the physical use of acid. Acidization in non-reK-tive sandstone formations is not ~nded as a method fw ...removal of an ;MJueous phase trap and will likely fulther e~-bate water retention problems as additional aqueous phase isbeing displaced into the formation.

Reductions in Interfacial TensionCapillary pressure is a direct linear function of the interfacial

teuion between the immiscible phases present in the porousmedia (either oil or water or water and gas). If some means can befound to significantly reduce the apparent interfacial tensionwhich exists between the two fluids. the capillary pressure may be~cally reduced, thereby allowing the physical mobilizationof. significant portion of the entrapped water-based phase.

A variety of materials have been utilized in both oil and gasreservoir applications as interfacial tension reducing agents tocidICr remove existing aqueous phase traps or prevent the originalbmation of aqueous phase traps when water-based fluids are ini-tially introduced into the formation. ror gas reservoirs, commoninterfacial tension reducing agents which have been utilized_tude a variety of alcohols (commonIy methanol), carbon diox-* gas and chemical surfactants.

O1emical surfactants have had limited utility, due to a disparityia ~ molecular structure between water and gas, in obtaining theI-. reductions in interfacial tension which are required to signif-~y reduce the residual saturation. Alcohols and carbon diox-ide have a moderate reducing effect on the interfacial tension buthave additional potential beneficial properties. C~ is relativelyhi,hly soluble in lhe enlrapped water phase so in addition lO~ocing ga.~-water interfacial lension, il also has lhe effect ofadding some residual swelling and charge energy lO the water inthe entrapped area. This allows instantaneously high pressure gra-dien~, much higher than could normally be achieved during nor-Ina1 rela'Voir drawOOwD ~ during IUbIequeAt bIoWWWDafter . ~ R8tn1ent has been conducted. .

Direct Mechanical Removal of EntrappedWater by Evaporation or Heating

Physical long-term production of reservoir gas generally ck)esnot result in a significant dehydrating effect in the formaUCMI Uthe gas is generally saturated with water vapour at reservoir .-ea-sure and temperature conditions. However, it has been doc~-cd that injection of dry pipeline spec gas into gas storage ~-voirs bas caused significant long-term increases in injectivity -the physical desiccation of the water saturation in the near well-bore region. This phenomenon can also be utilized in gas reservoirapplications where aqueous phase trapping has occurred. Shun-term injection of pipeline spec gas, geneBJly for a period of ~to two weeks, results in the establishment of both channels of hipgas saturation and permeability in the region directly around diewellbore and a physical desiccation of a portion of the entrappedwater by evaporation effects.

C~ .-w be t8keD in the UIe of dry pi injection .. . medIodfcw removiftI -.uecMII ~ Inpa M, due to die hiih mobility of

34 The .bAm8I of C8n8ci8n PetroIeI..n T.s lk)gy

AIcoiK)Is ~ ~ interf&"ial tension and are mutually mis-cible with the entrained water phase which tends to elevate Ippar-- vapour pressure and increase volatility of the CDtnpped water,For oil reservoir applications. alcohols have also been utilized but,in general. heavier alcohols are required (such as isoproponal orbutanol) to avoid potential sludging problems with the crude oiland to obtain a reasonable degree of miscibility between bodt diecrude oil and the entrained water phase,

A variety of chemical surfactants have been utilized which C8Dobtain near zero interfacial tensions between water and oils, 11IeIetypes of surf&"tants are relatively efficient at mobilizing en~water but are prey to high adsorptive losses in clay rich clastic b-mations which may result in large volumes of surfactant beingrequired and difficulty in effective contacting of the entire aque-ous trapped zone.

Carbon dioxide has also been utilized in oil reservoir applica-tions as it has extremely high solubility in both hydrocarblm -aqueous solution and, once again, tends to have a reducing effecton interfacial tension and the benefit of an increased zone ofcharged energy in the aqueous phase trap region. Care should betaken in oil reservoir applications. with the use of both alcoiM)Isand carbon dioxide, to ensure that compatibility exists betweenthe formation fluids and the introduced alcohols and CO2, and thataspbaitic sludges or emulsions are not ~ipitated or generated bythe contact of the oil by these fluids.

Changes in Physical Radius of Curvature ofthe System

In carbonate reservoir applications where acidization can beeffectively utilized as a stimulation mechanism, some benefit maybe realized through the use of acid as a method for physicallyincreasing the pore size of the system and thereby reduciD& dieapparent sensitivity to aqueous phase trapping. Acidization ispotentially a two-edged sword with respect to aqueous phase tnp-ping. In the &"id contacted area. the acid may enhance apl8aJtpem:leability and porosity characteristics and reduce the pro~-ty for aqueous phase trapping, However. as the acid reacts withthe carbonate-based formation, the acid spends resulting in theintroduction of additional aqueous phase into the reservoir, If diezone of stimulation is not sufficiently large enough to enbalK:e. .". .. . ' '" . , . ~, ~

TMnperaturenGURE 7: Condensate trapping and vapourizatlon mechanisms.

through die area). Also, the pocess of condensate revaporizMionin porous media is extremely mass transfer limited and requires along period of ti~, even if the reSel'Voir is P'e55~ to a silDifi-cant pressure above d1e original ciewpoint pressure.

Other techniques which have been more successful in theremoval of condensate phase traps include the injection of drypipeline specification gas. Dry gas injection tends to be ~effective than simple repressurization due to the fact that the IC8Dgas which is injected contains no significant heavy end conteDtand can thereby vapourize and contain in solution a considerablyhigher amount of the entrained retrograde condensate than theexisting gas in die reservoir. Also, if die formation permeability isrelatively low, high rate injection can result in the establishmentof a zone of high pressure around the injection well whichincreases the propensity for volatilization. Since an injectionprocess is occurring, there is generally some degree of dynamicflow in the near wellbore region in the zone associated with diecondensate phase trap. This decreases dynamic mass transfereffects and once again increases d1e volatilization characteril8icsof the trapped fluid. In other situations, carbon dioxide or LPOgas has been utilized to miscibly or supercritically extnctentrained condensate at lower pressures and has had succea incertain reservoir applications.

1M. effective contact of the aqueous phase trapped zone is some-ti~ problematic and difficult. Also, in some situations, if theiJlv.Ied aqueous fluid is highly saline or bas a high total dissolv~IOlidI content (such as saline drilling mud filtrates, kin fluids,workover fluids, or spent reacted acid), physical desiccation ofdIeIe fluids can result in the physical precipitation of a large quan-tity of these solids within the interstitial pore space as only wateris NlDoved by the desiccation and any dissolved solids are leftbcIIiIKI in a crystalline fonn. In some cases, the physical precipita-tiOD of the solids may largely negate the potential stimulativeeffect of the removal of the entrapped water saturation.

Near weUbore heat tJeating bas also been recently postulated as. ~ntial mechanism of removing near weUbore penneabilityimpainnent. lamaluddin et al. (1995)<5) details the use of specialOOwDhole heating tools for extreme high temperature (i.e., ~ -aoo-c) injection of nitrogen gas to supereriticaUy desiccate reser-voir mnes and thermaUy decompose expanded clays in gas reser-voir ~licatiODS.

Hydrocarbon Phase TrappingHydrocarbon phase traps can be established in a variety of sitU-

8Iioos where hydrocarbon fluids ~ introduced into gas ~servoirscoataining no previously existing hydrocarbon saturation or a situ-IIQl wbe~ a retrograde condensate reservoir is depleted belowthe dewpoint and liquid hydrocarbon saturation retrogrades and*X:umulates in the zone directly adjacent to the wellbo~ or frac-ture face. This pheoomenon is schematically illustrated as Figure7 8Id a set of ~lative permeability curves is provided showing d1eelt8blishment of a critical condensate saturation and the ~sulting~on in productivity to gas due to adverse ~lative permeabil-ity dl8racteristics. The establishment of a critical condensate satU-ration, mechanistically speaking, is very similar to the establish-.-at of an aqueous phase trap with respect to the fact that the..-~ extraneous phase has an overall reducing effect on theIpp8Ient ~lative permeability to gas.

A variety of techniques have been postulated over the years to.-apt to remove condensate traps from porous media in retro-~ ~servoir situations. Thermodynamically speaking, physicalrqxasurization of d1e reservoir by shutting in the affected wellaDd .uowing d1e bottomhole pressure to increase back. up abovethe dewpoint p~ssure (from P2 back to PI in Figure 7) indicatesdI8I the majority of the entrapped condensate should be able to be~ly ~vapourized by ~versible thennodynamic ~action. InactUality, this process is generally ineffective due to the fact thatthe limited volume of gas in contact with the ~trograded hydro-carbon is already relatively highly satUrated with heavy ends andcan absorb only a limited equilibrium amount of additional heavyCC8188* (wlMch h8I been ~ted in the reIerVoir by die pre-vious retrocrade behaviour of a hule volume of gas moving

ConclusionsAqueous phase trapping can be a significant mechanism fw die

impaired productivity of oil and gas in many reservoir situalicms.Situations which compound the severity of aqueous phase trap-ping include:

I. Low initial water saturation at some level significantly &ellthan what would typified as the irreducible water satur8tiOllfor a given reservoir.

2. Low permeability porous media exhibiting adverse reI8ivepenneability characteristics to the gas or oil phase at low 181-uration levels.

3. High invasion depths of die aqueous or hydrocarbon J*8Ieoccurring during drilling, completion or stimulationoperations.

4. Low available reservoir pressure for drawoown resultin& inboth more significant invasion due to high overbalanceeffects during initial drilling and completion as well as limit-ed available capillary gradient available for subsequentcleanup of the invaded fluid.

5. Specific correlations have been presented to calculate theaqueous phase trapping index for a variety of matrix qualitycarbonate and sandstone formations for both oil and ga!ireservoir applications to diagnose d1e potential sensitivity ofIbesc f~ 10 aq~ Iia8Ie ar.., ,. An APT i ilMJexof greater than one senera1ly indicates a reaervoir apptica-

D8c6.T~ ,~. Vok.- ~. No. 10 ~

Authors' BiographiesBrant Bennion is president of HycalEnergy Research Laboratories Ltd. and isresponsible for research and developmentin multiphase flow in porous media andformation damage. He has worked forHycal since 1979. He graduated with dis-tinction from the University of Calgary in1984 with a B.S. in chemical engineering.He has authored more than 60 technical

I papers and has lectured in North and South

America, Asia, Europe. Africa and Australia. Brant is a memberof APEGGA and serves as a director in the Calgary Section ofThe Petroleum Society.

Brent Thomas holds a doctorate in chemi-cal engineering. He has worked onenhanced oil recovery applications fCK dielast ten years, including gas injection,chemical flooding, solids precipitation andthennal applications. He is presently vicepresident of Hycal Energy ResearchLaboratories Ltd.

tion with no significant potential concUns for long-term per-manent aqueous phase trapping, although short- to medium-term transient effects due to aqueous loading with waterbased fluids may be a~t. At APT; values less than 0.8,consideration should be given to the use of preventativetechniques to avoid problems associated with aqueous phasetrapping.

6. These preventative techniques would include either thephysical elimination of the use of water- based fluids(through the use of liquid hydrocarbon, mutual solvent orgaseous fluids) or aqueous-based fluids with very low fluidloss to minimize invasion depth into the formation.Underbalanced drilling may be susceptible to countercurrentspontaneous imbibition effects in desiccated reservoir appli-cations using water-based fluids. Therefore, underbalanceddrilling should not be considered as a mechanism to totallyeliminate potential problems associated with phase trappingin dehydrated reservoir applications, although it may havewidespread specific application as a means of mitigatingmany other types of invasive formation damage. Fracturedsystems are generally not susceptible to permanent aqueousphase trapping. The exception to this would be micro-frac-tured systems with fracture apertures of less than approxi-mately 10 microns where significant aqueous phase retentiveeffects can occur in some situations.

7. Treatment of aqueous phase trapping depends on the specificmechanics of the system and whether a gas or oil reservoir isunder consideration. Common treatment methods includeutilization of higher capillary pressures through increaseddrawdown, physical reduction in effective capillary pressureby the use of interfacial tension reducing agents such asmutual solvents, surfactants or miscible gases such as carbondioxide or LPG or the use of direct mechanical treatmenttechniques such as dry gas injection or near wellbore heatstimulation techniques.

Ron Bietz is manager of research engineer-ing at Hycal Energy Research LaboratoriesLtd. He completed his B.Sc. in petrolewnengineering from the University ofWyoming in 1987 and has been wOltingpredominantly in the area of special coreanalysis since that time. Much of his expe-rience has been in coreflood simulationswhich are designed to investigate a vmetyof formation damage phenomena and dis-

placement characteristics in porous media with specific empbasison horizontal well applications and secondary recovery processes.

Douglas Bennion received his Bachelor'sdegree from the University of Oklahoma.and his Masters and Ph.D. fromPennsylvania State University, all in petr0-leum engineering. He worked for Mobil Oilin Canada for six years between degreesand taught at the University of Calgary inthe Department of Chemical and Petroiewn…

I Engineering for 21 years. In 1986, beretired from the University of Calgary and

became C.E.O. for Hycal Energy Research Laboratories Ltd. Hehas guided Hycal to become an international company cunendyworking in Europe, Asia. Africa. Australia and North and SouthAmerica. with branch offices in the U.S.A. and South America.

Acknowledgement

Provenance-Original Petroleum Society manuscript, Water andHydrocarbon Phase Trapping in Porous Media - Diagnosis,Prevention and Treatment, (95-69), first presented at the 46thAnnual Technical Meeting, May 14-17, 1995, in Banff, Alberta.Abstract submitted for review October 31, 1994; editorial com-~ts sent to the autbor(s) February 15, 1996; revised manuscriptreceived April 1, 1996; paper approved for pre-press April 5.1996; final approval July 16, 1996.1

The Joum81 of Canadian PetroIe\Mn T ~ .- ,;q,:M

The audlors wish to express appreciation to die management ofHycal Energy Research Laboratories Ltd. for pennission to pub-IiIh this paper.

REFERENCESI. BENNION, D.B., BIETZ, R.F., THOMAS. F.B. and CIMOLAI,

M.P., Reductions in the Productivity of Oil and Low PernleabilityGas Reservoirs Due to Aqueous Phase Trapping; Journal ofCanadian Petroleum Technology. November 1994, p. 45.

2. KATZ, D.L. and LUNDY, C.L., Absence of Connate Water inMichigan Reef Gas Reservoirs. An Analysis; AAPG Bulletin. Vol.66. No.1. January 1982. p. 91.

3. MASTERS, I.A., ELMWORTH, Case Study of a Deep Basin GasField; AAPG Memoir 38. 1984.

4. BENNION, D.B.. THOMAS, F.B., Underbalanced Drilling ofHorizontal Wells, Does It Really Eliminate Formation Damage?;SPE 27352. presented at the 1994 International Symposium onFonnation Damage Control. February 1994. Lafayette. LA.

.5. IAMALUDDIN, A.K.M., VANDAMME, L.M., MANN, B.K. andBENNION, D.B., Formation Heat Treaunent (FHT): A State of theArt Technology for Near Wellbore Formation Damage Treatment;paper CIM 95-67 presented at The Petroleum Society 45th ATM.May 14 - /7. 1995. Banff, Alberta.