EXPERIMENTAL INVESTIGATION OF WETTABILITY EFFECT AND DRAINAGE RATE ON TERTIARY OIL RECOVERY FROM FRACTURED MEDIA

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Journal of Porous Media, 15 (12): 11111123 (2012)

EXPERIMENTAL INVESTIGATION OFWETTABILITYEFFECT AND DRAINAGE RATE ON TERTIARY OILRECOVERY FROM FRACTUREDMEDIA

P. Maroufi,1 H. Rahmanifard,1 H. K. Al-Hadrami,2 M. Escrochi,1

S. Ayatollahi,1, & A. Jahanmiri3

1EOR Research Center, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz713451719, Iran2Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Muscat Oman3School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 713451719, Iran

Address all correspondence to S. Ayatollahi E-mail: [email protected]

Original Manuscript Submitted: 5/3/2011; Final Draft Received: 3/15/2012

Vertical displacement of oil by gas is one of the most efficient methods for oil recovery from naturally fractured reservoirs.Unlike the homogeneous media, the ultimate oil recovery by gravity drainage in fractured media is more dependent on theproduction rate. Hence finding the optimum production rate for more oil recovery with respect to the properties of mediaseems to be essential. In this work, unconsolidated packed models of cylindrical geometry surrounded by fractures wereutilized to perform a series of flow visualization experiments during which the contribution of different parameters suchas the extent of matrix wettability and the withdrawal rate were studied. In addition, mutual effects of wettability andproduction rate on tertiary oil recovery efficiency through controlled and free fall gravity drainage processes were alsoinvestigated. Experimental results obtained from tertiary gravity drainage experiments demonstrated that just before gasbreakthrough, lower withdrawal rates facilitate the tertiary oil recovery under the film flow mechanism, which leads toa higher ultimate recovery factor. However, after gas breakthrough, monitoring oil recovery by gravity drainage showedthat higher production rates recovered more oil. Furthermore, under tertiary recovery processes in low-production cases,oil-wet systems achieved higher recovery factors, while at high withdrawal rates, more oil was recovered for 50% oil-wetmedia.

KEYWORDS: gravity drainage, production rate, wettability, capillary rise

1. INTRODUCTION

Naturally fractured reservoirs (NFRs) contribute a largeextent of oil and gas production to the ever-increasingmarket demand of fossil energy (Aguilera, 1995). Ac-cording to Papay (2003), more than 50% of the worldpetroleum production comes from fractured reservoirs.It has been proved that water injection into NFRs canconsiderably increase oil production (Babadagli, 2003).However, even in this case, due to the reservoir hetero-geneity, well placement, and capillary forces, a signif-

icant amount of oil is still trapped in matrixes. Hence,to recover the remaining oil, gas injection was proposedas one of the most efficient methods by several authors(Carlson, 1988; Fassihi and Gillham, 1993; Kantzas et al.,1988a). The idea of recovering the residual oil after wa-terflooding process by gas injection appeared first in Carl-sons paper (Carlson, 1988). In the same year, Kantzaset al. (1988b) reported the results of gravity drainageexperiments and showed the important role of gravitydrainage in the gas injection process, which was called thegas-assisted gravity drainage process (GAGD) (Chatzis

1091028X/12/$35.00 c 2012 by Begell House, Inc. 1111

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NOMENCLATURE

Hob height of oil bank Swc connate water saturationSoi initial oil saturation Sorf residual oil saturation in fractureSor residual oil saturation TGB gas breakthrough time

et al., 1995). Based on the types of production mecha-nism, the GAGD process could be divided into two types:forced/controlled gravity drainage (FGD/CGD) and freefall gravity drainage (FFGD).

There are several parameters, including matrix blockwettability, spreading coefficients [S = wg (wo +go)] of the involved fluids, injection or production rate,reservoir dip angles, three-phase relative permeabilities,and capillary pressures, which are of practical importanceto the performance of GAGD (Dullien et al., 1989; 1991;Oren and Pinczewski, 1992; Vizika, 1993; Zendehboudiand Chatzis, 2008; Chatzis and Ayatollahi, 1995).

Rock wettability is a major factor controlling the lo-cation, flow, and distribution of fluids in a reservoir. De-pending on specific interactions of rock, oil, and brine,system wettability can range from strongly water wet tostrongly oil wet. So far, several works have been pub-lished that evaluated the effect of wettability on gravitydrainage performance. Kovscek et al. (1993) studied thedisplacement of thin wetting films and the effective forcesin a collection of different capillary tubes to describemechanisms of oil production in the mixed-wettabilitystate in reservoir rock. In his work, greater prospecting oilproduction potential of heterogeneous wetting media wasconfirmed. Zhou and Blunt (1998) claimed that residualoil saturation after secondary gas injection was increasedas the portion of oil wet sands increases in fractional wetsand mixture. Rezaveisi et al. (2010) used a combina-tion of clean water-wet glass beads and silane-treated oil-wet ones to assess the effect of wettability alteration to-ward more oil wetness on the recovery efficiency duringFFGD process in a synthetic fractured medium. Parsaeiand Chatzis (2011) showed that having favorable wetta-bility conditions in homogeneous porous media resultedin slightly lower reduced residual oil saturation after theGAIGI process compared to heterogeneous media withthe same condition of withdrawal rate.

Another important parameter in the implementation offorced/controlled gravity drainage is to find the optimumproduction/injection rate with regard to economical andtechnical concerns. Terwilliger et al. (1951) showed that

ultimate recovery was independent of the production rate,and the maximum theoretical gravity drainage rate wasnot significant. In contrast, the same or even better ulti-mate oil recovery at higher production rates in controlledgravity drainage (CGD) was reported by Chatzis and Ay-atollahi (1993). In their experiment, homogeneous, water-wet unconsolidated media and positive oil-spreading co-efficients were utilized. In addition, Zendehboudi et al.(2009) did a sensitive analysis of CGD in a fracturedporous media in which the effects of fracture aperture,matrix height and permeability, well spacing, and fluidproperties on the magnitude of critical pumping rate andmaximum possible withdrawal rate were investigated. Inanother work, Zendehboudi and Chatzis (2011) showedthat the characteristic rate depends only on the dimen-sions of the fracture and properties of the test fluid andnot on the properties of the matrix.

So far, many efforts have been made to critically ad-dress the recovery of residual oil in fractured media bygravity drainage processes (Da Sle and Guo, 1990; Deanand Lo, 1988; Paul and Zoback, 2007; Quintard andWhitaker, 1996; Salimi and Bruining, 2008). However,there are still challenging areas related to the applicationof GAGD process in fractured media, which lead to con-tradictory results such as wettability, production rate, andmatrix properties. Consequently, an experimental designin which only one parameter is changed while all otherparameters remain constant seems to be necessary. There-fore the methodological plan of this study is to perform asensitivity analysis to determine the effects of wettabilityand production rate on tertiary oil recovery through CGDmechanism. Apart from that, for comparing the perfor-mance of CGD and FFGD mechanisms based on differentwettabilities, some experiments are also conducted.

2. DESIGN OF EXPERIMENTS (DOE)Design of experiments (DOE) is widely used in engi-neering and the natural and social sciences (Montgomery,2008). The steps involved are explained in this section.

Journal of Porous Media

Tertiary Oil Recovery from Fractured Media 1113

2.1 Research ObjectivesThe objectives of this study were chosen as follows:

Investigation of various withdrawal rates in CGDprocess in fractured media;

Comparison of CGD and FFGD performance on thetertiary oil recovery process;

Obtaining the tertiary recovery performance of themodels with various wettabilities under CGD andFFGD mechanisms;

Simultaneous consideration of the effects of wetta-bility and production rates on tertiary recovery effi-ciency

2.2 Process Variables

The effect of wettability on CGD and FFGD processeshas not been investigated in fractured reservoirs; there-fore, drawing on our understanding of CGD and FFGDprocesses, the following two parameters influencing theprocess were specified as variables: wettability and pro-duction rate.

2.3 Experimental Design

According to the number of factors evaluated in this paper(two parameters) and the nature of the process, it appearsthat the most appropriate experimental design method isas follows.

Based on system wettability, all tests were classifiedinto five groups (water wet, 30% oil-wet, 50% oil-wet,70% oil-wet, and oil-wet). Thereafter, for each group, theexperiments were conducted for five different productionrates (0.1 cc/min, 1 cc/min, 10 cc/min, 29 cc/min, andfree fall). Hence 25 tests were performed. Of course, toexamine repeatability, and verify accuracy, some experi-ments were repeated several times (about 30 trials), andat the end of each experiment, average results were used.Therefore, on the whole, in the current study, to investi-gate the effects of independent variables (wettability andwithdrawal rate), 55 trials were needed.

3. EXPERIMENTAL WORK

In this section, details of the experimental setup, test flu-ids, and experimental procedures are presented.

3.1 Experimental Setup

Figure 1 shows schematics of the experimental setup usedin this study. In this novel model, the inner core holderplaced within the outer core holder in which a bed of well-screened glass beads was carefully packed with a slimsize distribution by a vibrating table. The annular spacebetween two holders is simulating vertical fracture. Tojoin matrix and fracture, using laser technology, the in-ner core holder was perforated. The number of holes was187 per each 10 cm of the core holder height, and their

FIG. 1: Schematic of the experimental setup apparatusfor controlled gravity drainage

Volume 15, Number 12, 2012

1114 Maroufi et al.

diameters were 0.1 mm, which was significantly smallerthan glass beads diameters (0.50.8 mm). The propertiesof the fractured model are given in Table 1. It is essen-tial to mention that the matrix porosity was determinedby using a porosity meter apparatus, and it was concludedthat all systems had approximately the same porosity. Ad-ditionally, steady state permeability measurement tech-nique was used to measure the permeability of differentemployed models.

3.2 Test Fluids

Gravity drainage tests were performed using dyed n-heptane with Sudan-red, air, and distillated water to sim-ulate oil phase, gas phase, and water phase, respectively.The physical properties of test fluids are shown in Table 1.

3.3 Wettability Alteration Procedure

Since clean glass beads are known to be naturally wa-ter wet, to check the wettability effects; systems withdifferent wettabilities were prepared using glass beadswith altered wettability through the silanization process,which is described as follows (Grattoni and Dawe, 2002;Kovscek et al., 1993; Zhou and Blunt, 1998; Rezaveisi etal., 2010):

1. To remove any contamination during the process andtransportation, glass beads were first cleaned usingHCL solution (%20 vol.).

2. They were rinsed with distilled water and dried for 2hours at 100.

3. They were then immersed in a diluted solution oftrichloromethyl silane (2% vol.) and dehydratedtoluene for 15 min, when a thin film of the silanecoats the grains.

4. The beads were rinsed with methanol and then driedat 100. This heating favors the cross-linking reac-tion and formation of monolayer silane coating.

Wettability alteration through this procedure is confirmedby contact angle measurements on beads presented inFig. 2. As it is obvious, the contact angle of distilled waterdrop in an n-heptane container surrounding the beads wasconsiderably changed from water wet toward oil wet.

3.4 Drainage Experimental ProcedureA set of 25 CGD and FFGD tests were carried out forfive different wettability states and five various productionrates. To verify the accuracy of tests, some of them wererepeated several times, and average results were used bythe end. The procedure involved the following steps:

1. The glass beads were packed in the inner core holder(matrix), using a vibrating table, which was thentransferred into the outer core holder.

2. Prior to saturating the model with any liquid, airpackets in the matrix part of the model were flushedout using several pore volumes of CO2.

TABLE 1: Physical properties of the experimental setup and test fluids

Experimental setup

Length of inner holder 59 cmTotal length of outer holder 60 cm

Inner diameter of outer holder 4 cmOuter diameter of inner holder 3.9 cmInner diameter of inner holder 3 cm

Fracture aperture 0.5 mmAbsolute permeability of matrix 710 Darcy

Porosity 38%Size distribution of glass beads 0.80.5 mm

Test fluids

Density (g/cm3) Surface tension (mN/m) Viscosity (Pa.s)water 1 72 .8 8.9e-4

n-heptane 0.684 20.14 3.86e-4air 1.25e-3 1.8e-5



FIG. 2: Apparent contact angle on (a) fresh glass beads and (b) treated glass beads

3. To saturate the system with water, at least four porevolumes of distilled water were injected from thebottom at very low rates to ensure complete satura-tion of the matrix block. In this step, CO2 moleculeswithin the pores were dissolved in water, and conse-quently, the model was fully saturated with water.

4. The dyed oil was injected from the top of column us-ing a syringe pump, while water was displaced fromthe bottom of column. The injection process contin-ued until water saturation reached to the irreduciblestate. The transparency of the model and dyed oilallowed us to monitor the oil-water interface duringoil flooding. Initial oil in place and the connate watersaturation at this step were measured by volumetricbalance.

5. To establish waterflood residual conditions, waterwas injected at the constant rate of 11.66 cc/minfrom the bottom of the model. The volume of oilproduced from the top was measured volumetrically.The water injection was continued until no more oilwas produced. The amount of residual oil was calcu-lated by subtracting the volume of oil produced bywater flooding process from the volume of initial oilin place (determined in step 4).

6. Finally, by opening the top valve of the column toatmospheric air and draining liquid from 6. the bot-tom valve at a constant rate with a syringe pump,CGD process was started. Oil bank formation in thecolumn and oil recovery up to gas breakthrough atdifferent production rates were our main goals dur-ing CGD process. Beside CGD experiments, FFGDtests were also performed to compare the outcomes.In performing FFGD experiments, all steps prior tostep 6 were the same as CGD tests. However, in the

final step, top and bottom valves of the model wereopened suddenly to atmosphere.

4. EXPERIMENTAL RESULTS AND DISCUSSION

Table 2 is a summary of main results of 25 trials fromwhich many of conclusions have been directly derived. Itis wise mentioning that porosity, permeability, and waterinjection rate in water flooding (step 5) are the same forall tests.

4.1 Production Characteristics at GasBreakthrough and Thereafter

4.1.1 At Gas Breakthrough

After water flooding (step 5), due to negligible capillarypressure and high permeability of fracture when drainagewas performed at a high pumping rate, the fracture liq-uid was drained much faster than at a slow pumping rate.So the RF value just before the gas breakthrough wouldbe lower than in the slow drainage case, where the liq-uid had enough time to communicate fully between thematrix and the fracture (Chatzis and Ayatollahi, 1995).Figure 3 shows tertiary oil recovery at different oil pro-duction rates at gas breakthrough for various wettabilityratios. As it is expected, increasing the production rate re-duced oil recovery at gas breakthrough for all differenttests.

4.1.2 After Gas BreakthroughThe pump was turned off; the bottom valve was com-pletely opened, and oil production continued until nomore oil was produced. As was mentioned already, thehigher withdrawal rate caused a larger amount of oil to


1116 Maroufi et al.

TABLE 2: Results of controlled and free fall gravity drainage experiments for 25 testsEx

p.no.

Wet

tabi

lity Initial Water Results of gravity drainage process

Ulti

mat

eR

F(%

)

conditions flooding

Swc

(%)

Soi

(%)

RF

(%)

Sor

(%)

Prod

uctio

nra

te(cc

/min)

RF

atG

B(%

)

Hob

(cm)i

nfra

ctur

e

TGB

Sor

@gb

(%)

RF

afte

rGB

(%)

Tert

iary

RF

(%)

1 WW 23.36 76.64 89.43 10.57 0.1 4.42 6 2088 6.14 0.24 4.66 94.092 WW 26.04 73.96 88.53 11.47 1 3.33 7 157 9.53 2.23 5.56 94.093 WW 26.4 73.6 88.42 11.58 10 2.2 4 18.27 9.39 3.15 5.35 93.774 WW 23.4 76.6 87.02 12.98 29 2.03 3.5 7.31 10.94 3.18 5.21 92.235 WW 25.97 74.03 86.55 13.45 FF 1.96 0 0.25 10.81 5.57 7.53 94.0821 30% OW 12.86 87.14 78.64 21.35 0.1 11.65 8 1770 9.69 0 11.65 90.2922 30% OW 10.36 89.64 79.41 20.59 1 10.3 7 159 10.24 4.74 15.04 94.4523 30% OW 13.13 86.87 78.4 21.6 10 5.16 3 19.3 16.43 10.09 15.25 93.6524 30% OW 14.07 85.93 76.27 23.73 29 2.54 3 5.3 21.86 12.99 15.53 91.825 30% OW 14.14 85.86 79.54 20.46 FF 2.27 0 0.3 18.18 13.07 15.34 94.8816 50% OW 13.75 86.25 76.35 23.65 0.1 17.1 10.5 1644 6.55 0 17.1 93.4517 50% OW 10.44 89.56 77.77 22.22 1 14.4 10 162 7.77 2.65 17.05 94.8218 50% OW 11.42 88.58 77.41 22.59 10 6.45 3.5 20.15 16.12 10.13 16.58 94.0919 50% OW 13.11 86.89 78.69 21.30 29 2.67 3 5.14 18.63 13.6 16.27 94.9720 50% OW 14.82 85.18 78.58 21.42 FF 2.34 0 0.33 19.43 14.07 16.41 94.9911 70% OW 16.17 83.83 76.96 23.04 0.1 16.01 10 1539 7.06 0 16.01 92.9712 70% OW 16 84 76.19 23.81 1 10.1 7 156 13.72 3.17 13.27 89.4613 70% OW 15.38 84.62 76.36 23.64 10 6.36 4 19.23 17.27 7.5 13.86 90.2214 70% OW 16.46 83.54 76.81 23.18 29 2.89 3 5.25 20.28 11.45 14.34 91.1515 70% OW 14.63 85.37 78.85 21.15 FF 2.28 0 0.83 18.85 12.86 15.14 93.996 OW 11.97 88.03 74.7 25.3 0.1 17.35 8.5 1223 7.94 0 17.35 92.057 OW 11.05 88.95 75.7 24.3 1 8.48 8 125 16.79 7.9 16.38 92.088 OW 9.05 90.95 76.75 23.25 10 4.51 3.5 17.05 18.73 9.71 14.22 90.979 OW 8.58 91.42 76.04 23.96 29 2.08 3 5.5 21.87 11.72 13.8 89.8410 OW 9.35 90.65 77.83 22.17 FF 1.54 0 0.26 20.61 12.15 13.69 91.52

remain in the matrix, which led to the dominancy of grav-ity forces compared to the capillarity. Therefore, after gasbreakthrough, models with higher production rates (up togas breakthrough) produced more oil (see Table 2).

4.2 The Effect of Various Production Rates

During early stages of production at a constant with-drawal rate, there was no matrix gas invasion. However,because of low resistance of fracture to flow, it was in-vaded immediately. On the other hand, pumping liquid

from fracture imposed a pressure difference (P ) be-tween the top and bottom of the model (Zendehboudi etal., 2009). After a while, the liquid head (gas liquid in-terface) inside the fracture dropped, and at a particulartime, it became feasible for gas to begin to invade thematrix, while it was continuing to flow in the fracture.At this instant, the gas invasion driving force (i.e., pres-sure difference) through both matrix and fracture wouldbe equal, and liquid drainage from the matrix began andjoined the flux from fracture. Thus, to have a better inves-tigation of the influence of withdrawal rate on oil recovery



FIG. 3: The profiles of oil recovery for the media of dif-ferent wettabilities versus production rate at gas break-through

at gas breakthrough and thereafter, for each wettability ra-tio (ww, ow, 70% ow, 50% ow, and 30% ow), five differ-ent production rates (0.1, 1, 10, and 29 cc/min and FFGD)were considered.

4.2.1 100% Water Wet

Figure 4 displays ultimate tertiary oil recovery versustime (exp. 15) for water-wet systems. In fact, in a testwith the production rate of 0.1 cc/min, because of posi-tive spreading coefficient and the fact that this test tooklonger than the others, the injected gas had enough timeto reconnect oil blobs and made a larger oil bank, whichresulted in the highest recovery up to gas breakthrough forthis system (see Fig. 4). However, after gas breakthrough,as remaining oil was very close to the capillary end, smallamounts of oil could be producible. Hence the lowest oilrecovery after gas breakthrough and under the tertiary re-covery process was achieved for this test. On the otherhand, in FFGD test (exp. 5), due to the highest withdrawalrate (critical rate1), early gas breakthrough was unavoid-able, which caused a large remaining amount of oil in themodel. Nevertheless, after gas breakthrough because ofgravity forces, more oil was recovered, which resultedin the highest tertiary oil recovery among other tests. To

1The maximum vertical oil production rate allowable in a givenreservoir to achieve a stable flood front is called the critical rate.

FIG. 4: Comparison of tertiary oil recovery of variousproduction rates versus time in water-wet media

some extent, for other production rates, a combination ofpreviously mentioned mechanisms led to approximatelythe same recovery efficiencies.

4.2.2 100% Oil Wet

For oil-wet systems, oil occupies smaller pores and sticksto the surface of larger ones, while water lies at the cen-ter of larger pores. Therefore, during the water flood-ing process (step 5), water mostly displaced oil in largerpores, while bypassing the smaller ones. Consequently,a large quantity of oil remained in the system (see Ta-ble 2). Up to gas breakthrough, for lower withdrawalrates, since viscous and gravity forces were more dom-inant than capillary forces, gas as the nonwetting phaseoccupied larger pores and pushed water into the smallerpores, which led to the production of more amounts ofoil and higher recovery factors. However, increasing thewithdrawal rate (the same as water-wet systems) causedan early gas breakthrough, as it is shown in Fig. 5, leav-ing a large amount of oil in the system and lesser recov-ery efficiencies (see Table 2). After gas breakthrough, forhigher production rates due to the dominancy of gravityforces as compared to capillarity, more oil was recovered.Generally, since in the case with the rate of 0.1 cc/min,there was a more piston-like displacement (without by-passing pore volumes due to the higher withdrawal rates),it had got the highest recovery efficiency during the ter-tiary recovery process.


1118 Maroufi et al.

FIG. 5: Influence of production rates on the overall oilrecovery for an oil-wet system versus time

4.2.3 70% Oil Wet

Recovery efficiency versus time (exp. 1115) for the 70%oil-wet model is depicted in Fig. 6. Up to gas break-through, the same trends as compared to the oil-wetmodel were observed. After gas breakthrough, becauseless oil remained in the system and for those water-wetportions of the system, the capillary threshold was higher

FIG. 6: The variations of recovery efficiency versus timefor different withdrawal rates in 70% oil-wet model

than oil-wet parts (Erle and Waqi, 2008), and oil recov-ery efficiencies decreased as the production rate changedfrom free fall to lower rates (see Fig. 6).

4.2.4 50% Oil Wet

In 50% oil-wet systems (exp. 1620), in the same way asbefore, the highest oil recovery prior to gas breakthroughbelonged to the test with the rate of 0.1 cc/min, while aftergas breakthrough, the FFGD test exhibited the highest oilrecovery, as indicated in Fig. 7. Furthermore, the ultimatetertiary recovery was approximately the same for all rates,which showed that for 50% oil-wet media due to similarwater-wet and oil-wet portions, similar production fromlarge and small pores before or after gas breakthrough wasobtained.

4.2.5 30% Oil Wet

Increasing the water-wet ratio caused the system to be-have like a water-wet system. As discussed in Sec-tion 4.2.1, the lowest overall recovery was found for thetest with a production rate of 0.1 cc/min. Moreover, in-creasing the production rate led to more oil recovery aftergas breakthrough and higher tertiary recovery efficiency(see Fig. 8). It is wise to mention that in smaller pores ofthose oil-wet portions, due to lower capillary forces, moreoil was also produced, which resulted in higher tertiary re-covery efficiency as compared to water-wet systems (seeTable 2).

FIG. 7: Recovery factor variations versus time for differ-ent production rates in 50% oil-wet porous media



FIG. 8: The variations of recovery efficiency versus timefor different withdrawal rates in the system of 30% oilwetness

4.3 The Effect of Various Wettability Ratios

4.3.1 Production with the Rate of 0.1 cc/minIn the water-wet system, since residual oil saturation af-ter water flooding was extremely low as compared toother systems (e.g., oil wet) and oil was not the con-tinuous phase and existed in blob form in the center oflarger pores, during the test with a low production rate,lesser bulk films of oil formed. Thus gravity forces be-came less efficient than capillarity, and consequently; thelowest amount of oil was recovered. However, the storyfor oil-wet media is completely opposite. In this system,after water flooding due to significant reduction of oil sat-uration, water became the continuous phase and gas, thenonwetting phase, expelled water out of larger pores andpushed some of it into the smaller ones, which causedmore oil production (Erle and Waqi, 2008; Anderson,1986). Thereby, for the oil-wet system, the highest oil re-covery under tertiary recovery process was achieved. Inthe system with 50% oil wetness, in water-wet parts, airmostly swept oil in larger pores, while in oil-wet partsdue to lower capillary forces, smaller pores in addition tolarger ones were also depleted. Hence the formation of alarger oil bank led to more oil production and approxi-mately the same recovery factor with the oil-wet system.Finally, for systems with 70% or 30% oil wetness, de-pending on being more oil wet or water wet, the systembehavior was similar to oil-wet or water-wet media. It

is essential to mention that in the 70% oil-wet system,since the oil phase was not the continuous phase any-more, the recovery factor for this system was less than forthe oil-wet medium. While for the 30% oil-wet systemdue to higher residual oil saturation after water flooding,higher recovery efficiency as compared to water-wet me-dia was achieved (see Fig. 9). Another point is the timethat oil started to be produced, which was the longest forwater-wet systems. The reason for this long delay was thehigher capillary height and, consequently, stronger cap-illary forces for this system, which acted against gravityforces.

4.3.2 Production with the Rate of 1 cc/minFigure 10 shows the effect of different wettability ratioson tertiary recovery efficiency for a withdrawal rate of1 cc/min. As is evident, increasing the production rateby tenfold caused shorter gas breakthrough time for sys-tems with various wettabilities. Again, due to lesser resid-ual oil saturation after water flooding and higher capil-lary forces, the water-wet system has got the lowest oilrecovery factor. However, according to oil residual sat-uration at gas breakthrough of the oil-wet medium (seeTable 2), it is obvious that increasing the withdrawal ratecaused air to choose ways with lower resistance (lessercapillary forces) through the matrix. In other words, airbypassed some small pores in the matrix part (gas chan-neling), which decreased the recovery factor. In the 50%

FIG. 9: Influence of different wettability ratios on oil re-covery versus time for production rate of 0.1 cc/min


1120 Maroufi et al.

FIG. 10: Oil recovery factor variations versus time fordifferent wettability ratios with the withdrawal rate of1 cc/min

oil-wet system, as similar portions of oil-wet and water-wet glass beads were thoroughly mixed; a medium withheterogeneous wettabilities was prepared in which thedistribution of water-wet beads among oil-wet parts pre-vented gas channeling or caused an approximate piston-like displacement of the gas front. Hence, for the 50% oil-wet system, the highest tertiary recovery efficiency wasachieved. For the 70% oil-wet system, because the oilphase was not the continuous phase anymore, and also inoil-wet parts, smaller pores, because of their higher cap-illary forces, were bypassed due to the higher withdrawalrate (gas channeling), and a lower recovery factor as com-pared to oil-wet media was expected. Finally, in the 30%oil-wet system, higher capillary forces among water-wetparts hindered gas channeling, while, due to higher resid-ual oil saturation after water flooding and the productionof oil from smaller pores in addition to larger ones in oil-wet parts, more amounts of oil in comparison with 70%oil-wet and water-wet media were produced.

4.3.3 Production with the Rate of 10 cc/minAs depicted in Fig. 11, again for water-wet medium dueto lesser residual oil saturation after water flooding andhigher capillary forces, the lowest tertiary recovery fac-tor was achieved. In oil-wet medium, another increase inthe production rate by ten folds intensified gas channel-ing phenomenon and therefore decreased tertiary recov-

FIG. 11: Comparison of tertiary oil recovery of vari-ous wettability ratios versus time for production rate of10 cc/min

ery factor significantly. However, in 50% oil-wet system,because of the piston-like movement of gas front and oilproduction from large and small pores in oil-wet parts,the highest recovery factor was obtained. For 70% oil-wet medium, since oil was not the continuous phase andincreasing the production rate worsened gas channelingphenomenon, again lower recovery factor as compared tooil-wet system was achieved. It is wise mentioning thateven in this system, water-wet portions in limited extenthindered gas channeling phenomenon. Finally, in 30%oil-wet medium, due to the same reasons (high residualoil saturation after water flooding, prevention of gas chan-neling, and production from large and small pores in oil-wet parts) higher amount of oil as compared to water-wet,oil-wet, and 70% oil-wet systems was recovered.

4.3.4 Production with the Rate of 29 cc/minIncreasing the production rate to 29 cc/min caused shortergas breakthrough time and almost same recovery factorsat gas breakthrough for all experiments (see Table 2).Similarly to before, the lowest tertiary recovery factor isfor water-wet systems. For oil-wet systems, increasingthe withdrawal rate caused that air bypassed more porevolumes in which it did not enter even after gas break-through, and therefore, lower recovery factors after gasbreakthrough and on the whole for tertiary recovery pro-cess were achieved. The 50% oil-wet medium, due to the



same reasons (gas front piston-like displacement and pro-duction from small and large pores), had got the high-est oil recovery factor (see Fig. 12). In the 70% oil-wetmedium, those water-wet portions somehow preventedgas from channeling, which caused the higher tertiary re-covery factor as compared to the oil-wet system. And the30% oil-wet medium had better performance rather thanthe water-wet, oil-wet, and 70% oil-wet systems.

4.3.5 Production with Free Fall Gravity Drainage

As in the previous section, prior to gas breakthrough,almost the same recovery factors were achieved, whichshowed the important role of after-gas breakthrough pro-duction on tertiary recovery efficiency for the FFGDprocess. Water-wet media had the lowest tertiary recov-ery factor, while, in oil-wet systems prior to gas break-through, the same as the case with a withdrawal rate of 29cc/min, air did not expel oil from some small pores, andafter gas breakthrough, because of gas channeling occur-rence, a lower amount of oil was recovered (see Fig. 13).Based on the similar reasons, the 50% oil-wet system hadthe highest tertiary recovery factor. In the 70% oil-wetmedium, due to the partial prevention of gas channelingphenomenon by those water-wet portions of the system, ahigher tertiary recovery factor as compared to the oil-wetsystem was achieved. And finally as depicted in Fig. 13,the 30% oil-wet medium had better performance com-pared to water-wet, oil-wet, and 70% oil-wet media. It

FIG. 12: Tertiary oil recovery for systems with differentwettabilities versus time at production rate of 29 cc/min

FIG. 13: Recovery efficiency changes in systems withdifferent wettabilities versus time for free fall drainage(FFGD)

is wise to mention that in the 30% oil-wet system, dueto the high flow rate in the FFGD process, the gas chan-neling phenomenon also occurred (see TGB column inTable 2), which caused approximately the same recoveryfactor with the 70% oil-wet system.

5. CONCLUSIONSIn this study, experimental results were obtained in a longvisual laboratory model for a single matrix block sur-rounded by a vertical fracture. Experiments have beenconducted using a wide range of physical and operationalconditions, where the wettability of the porous medium aswell as the production rate were changed to acquire opti-mum oil recovery criteria through CGD and FFGD mech-anisms. According to the experimental studies and phe-nomenological analyses, the following conclusions arededuced:

Monitoring oil recovery rate by gravity drainage af-ter gas breakthrough shows that tertiary oil recoveryis strongly dependent on residual oil saturation, wet-tability of the matrix, and production rate.

At high production rates up to gas breakthroughtime, the recovery is approximately independent ofwettability.

Prior to gas breakthrough, increasing the productionrate leads to low tertiary oil recovery. However, after


1122 Maroufi et al.

gas breakthrough, cases with higher withdrawal rateshave recovered more amounts of oil.

For water-wet media, an incremental trend of theproduction rate caused higher tertiary recovery,while in oil-wet systems, this results in reduction ofthe recovery factor under tertiary recovery mecha-nisms.

Water-wet models exhibit the lowest tertiary recov-ery efficiency compared to the other tests.

Production rate variations do not have any consider-able influence on the tertiary recovery factor in 50%oil-wet systems.

At lower rates, an oil-wet system produces higheroverall recovery, while for higher withdrawal rates(more than 1 cc/min), a 50% oil-wet medium hasbetter performance.

Finally, we should point out that dimensionality and ho-mogeneity of the system, porous media that consist ofglass beads instead of reservoir rock, experimental con-ditions (atmospheric pressure and temperature), and uti-lized fluids (C7 and air as oil and gas phases) limit thegeneralizability of the results presented here. In addition,since this study is a single matrix block (a matrix blocksurrounded by a fracture), block to block effects (capil-lary continuity and reinfiltration) that belong to a stack ofmatrix blocks are not considered.

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EXPERIMENTAL INVESTIGATION OF WETTABILITY EFFECT AND DRAINAGE RATE ON TERTIARY OIL RECOVERY FROM FRACTURED MEDIA