Simulation Study of Asphaltene Deposition and Solubility of CO 2 in the Brine during Cyclic CO 2 Injection Process in Unconventional Tight Reservoirs

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1 Smulaton Study of Asphaltene Deposton and Solublty of CO 2 n the Brne durng Cyclc CO 2 Injecton Process n Unconventonal Tght Reservors Rashd S. Mohammad, Shcheng Zhang, Sun Lu, Syed Jamal-Ud-Dn, Xnzhe Zhao Abstract A compostonal reservor smulaton model (CMG-GEM) was used for cyclc CO 2 njecton process n unconventonal tght reservor. Cyclc CO 2 njecton s an enhanced ol recovery process consstng of njecton, shut-n, and producton. The study of cyclc CO 2 njecton and hydrocarbon recovery n ultra-low permeablty reservors s manly a functon of rock, flud, and operatonal parameters. CMG-GEM was used to study several desgn parameters of cyclc CO 2 njecton process to dstngush the parameters wth maxmum effect on the ol recovery and to comprehend the behavor of cyclc CO 2 njecton n tght reservor. On the other hand, permeablty reducton nduced by asphaltene precptaton s one of the major ssues n the ol ndustry due to ts pluggng onto the porous meda whch reduces the ol productvty. In addton to asphaltene deposton, solublty of CO 2 n the aqufer s one of the safest and permanent trappng technques when consderng CO 2 storage mechansms n geologcal formatons. However, the effects of the above uncertan parameters on the process of CO 2 enhanced ol recovery have not been understood systematcally. Hence, t s absolutely necessary to study the most sgnfcant parameters whch domnate the process. The man objectve of ths study s to mprove technques for desgnng cyclc CO 2 njecton process whle consderng the effects of asphaltene deposton and solublty of CO 2 n the brne n order to prevent asphaltene precptaton, mnmze CO 2 emsson, optmze cyclc CO 2 njecton, and maxmze ol producton. Keywords Tght reservors, cyclc O 2 njecton, asphaltene, solublty, reservor smulaton. T I. INTRODUCTION HE consumpton of petroleum hydrocarbons worldwde has been gradually ncreasng. Ol producton from the unconventonal reservors s one of the key energy resources that can meet the growng demand of the world s energy. Exploraton and producton of unconventonal reservors has attracted attentons snce t s avalable n large quanttes worldwde. Tght reservors are resources that contan hydrocarbons n extremely low permeable type formatons that are neffcent to produce at economcal rates wth the applcaton of conventonal approaches [1]. Therefore, the advanced horzontal drllng and multple hydraulc fracturng Rashd. S. Mohammad s wth the Ol-Gas Feld Development Engneerng Department, College of Petroleum Engneerng, Chna Unversty of Petroleum-Bejng, Changpng, Bejng Chna (e-mal: @qq.com). Shcheng. Zhang, Sun Lu, Jamal-ud-Dn and Xnzhe Zhao are wth the Ol-Gas Feld Development Engneerng Department, College of Petroleum Engneerng, Chna Unversty of Petroleum-Bejng, Changpng, Bejng Chna (e-mal: zhangsc@cup.edu.cn). are commonly used to penetrate unconventonal resources such as tght ol, shale gas, and coalbed methane; those resources become very essental to provde enough hydrocarbon to balance the shortage of conventonal resources. The average daly ol producton n the U.S. (90% from the unconventonal Bakken) was 977,000 barrels from 10,457 producng wells [2]. Hence, each well approxmately produces around 95 bbl./day as shown n Fg. 1. The ol producton from unconventonal reservors drops very rapdly around 75% wthn the frst two years n the lfe of the well; due to the lack of connectons between pores, whch makes t very challengng to mprove t further. However, the prmary recovery of unconventonal reservors remans low to only 8% of the ntal ol n place, even though long horzontal wells have been drlled and massvely fractured [3]. Fg. 1 Prmary producton of unconventonal reservors n U. S. [4] Waterfloodng s the most commonly used secondary ol recovery technque n conventonal reservors, but t s not a feasble choce n ultra-low permeablty reservors, manly due to the extremely low njectvty, clay swellng, and poor sweep effcency ssues [5]. Recent studes have shown that gas njecton may be a good choce. The reasons for ntatng these studes nclude shortage of effcent and economc technques for mprovng ol recovery n tght reservors, neffcency of conventonal methodologes such as waterfloodng, and the unque nature of unconventonal reservors lke ultra-low porosty and permeablty [6]. On the other hand, CO 2 s usually njected for unlockng tght ol formatons. Cyclc CO 2 njecton process s found to have acheved a good recovery performance n unconventonal reservors and a promsng EOR technque that could overcome some major problems assocated wth contnuous CO 2 floodng, such as early CO 2 breakthrough, hgh 485

2 operaton costs, demand for CO 2 source, etc. Thus, t s essentally mportant to evaluate performance of cyclc CO 2 njecton process, whch can dmnsh early CO 2 breakthrough wth the produced flud. The earler studes showed the necessty for optmzng cyclc CO 2 njecton process. Gamad et al. [7] performed an expermental work for shale ol reservor and found that cyclc CO 2 njecton process s capable of recoverng ol from unconventonal shale ol reservors. Lkewse, Chen et al. [8] evaluated the effect of reservor heterogenety on the CO 2 huff-n-puff recovery n a shale matrx usng numercal smulaton and the ablty of CO 2 to penetrate near-fractured regons. Smlarly, Song and Yang [9] conducted expermental work as well as numercal smulaton to assess the performance of mmscble and mscble cyclc CO 2 njecton process n shale formatons. However, t s mportant to study the most sgnfcant parameters domnatng the CO 2 njecton process. The applcaton of CO 2 to enhance ol recovery can nduce asphaltene precptaton whch counts as one of the major ssue n the ol ndustry due to ts pluggng onto the porous meda, whch reduces the ol productvty. To avod asphaltene precptaton, reservor bottom-hole pressure must be greater than the precptaton onset pressure. However, cyclc CO 2 njecton process has provded enough support for pressure mantenance, whch helps n avodng asphaltene precptaton [10]. Hamouda et al. [11] reported that, at hgh CO 2 concentraton, asphaltene dssolved n ol begns to flocculate below the onset pressure because the njected CO 2 dssolves n ol, leadng to ol swellng and vscosty reducton, thus solatng the lght hydrocarbon fractons from the ol and promptng flocculaton. Furthermore, Al-Qasm [10] studed asphaltene precptaton, flocculaton, and deposton durng CO 2 floodng for conventonal reservors. Leontarts and Mansoor [12] stated that one man ssue durng CO 2 njecton s asphaltene uncertanty, whch nduces precptaton and may cause pore-throat-pluggng or wettablty alteraton. Okwen [13] and Srvastava et al. [14] concluded that the presence of water can reduce the asphaltene precptaton. Therefore, t s necessary to have a demonstratve reservor smulaton model that can antcpate the phase behavor of asphaltene deposton precsely. Global warmng and clmate change s caused by CO 2 emsson, manly from the combuston of fossl fuels. Thus, solublty of CO 2 n the aqueous phase s one of the safest trappng technques whle consderng the CO 2 storage mechansms n the geologcal formatons. However, dssoluton of CO 2 depends on pressure of the system, pressure buldup durng njecton process could possbly produce or ntate fractures, provdng mgraton pathways for CO 2 that requres safety of storage [15], [16]. Most of researchers are motvated by rsng attenton n CO 2 sequestraton n deep brne aqufers and CO 2 enhanced ol recovery (EOR). Trappng CO 2 n brne aqufers s observed as one of the best applcable technque to reduce CO 2 emsson although t s manly amed at enhancng ol recovery and controls depleton mechansms of ol reservors. Furthermore, t s observed as an economcally feasble technque for underground CO 2 storage. Brne aqufer manly conssts of NaCl whch s consdered as the major component of many formaton brnes. Therefore, many studes have been conducted on the solublty of CO 2 n NaCl brnes, whch are bascally a bnary mxture of dstlled water and NaCl. Drummond [17] measured more than 500 data ponts of CO 2 solublty n NaCl brnes of dfferent salntes. However, CO 2 solublty s an mportant concern for CO 2 EOR snce CO 2 s much more soluble n the ol than other gas components [18]. Therefore, CO 2 must be taken nto account n desgn and smulaton of a CO 2 EOR process. Ths study asssts n better understandng the behavor of cyclc CO 2 njecton process n tght reservors whle consderng the effects of asphaltene deposton and CO 2 solublty. A numercal reservor smulaton model has been generated to evaluate some parameters of the cyclc CO 2 njecton scheme, n order to mprove technques for desgnng cyclc njecton treatments [19]. II. RESERVOIR MODELING APPROACH Reservor smulaton s an expensve and usually deal method to assess the concern of such complex formatons due to low permeablty of tght reservor. The complex nature of pre-exstng natural fractures and ther network wth hydraulc fractures are combned wth horzontal well completon. Numercal smulaton performances are commonly recognzed n the petroleum ndustry [1]. Therefore, such tools are consdered beng fast and smple, whch are supportve n makng decson for unconventonal wells. The reservor smulaton work for the applcaton of cyclc CO 2 njecton process was studed usng compostonal smulator n Computer Modelng Group CMG-GEM. The dmensons of the reservor were 2800, 1700, and 20 ft n the x, y and z-drectons, respectvely. The tght reservor s stmulated; n ths smulaton work, only a sngle half-fractured regon was smulated on the bass of flow symmetry and havng the dmensons of 300, 800, and 20 ft n the x, y and z-drectons, respectvely as shown n Fg. 2. However, t can save lots of grdblocks, computaton complextes, and tme wthout sacrfcng the computng accuracy [20]. A. Flud Propertes Phase behavor smulator s dependng on the compostonal data of the flud samples, njected flud and on reservor pressure and temperature. Reservor flud propertes provde the man nput for any smulator used to predct thermodynamc propertes of the flud based on some relable data provded by the operatng company. Therefore, accurate PVT propertes are requred to get approprate and representatve smulaton results. CMG-WnProp was used for generatng flud model. The flud sample was taken at an 8500-ft depth. Reservor ntal pressure, temperature, and saturaton pressure (bubble Pont) at reservor temperature were 5820 ps, 238 o F and 2652 ps, respectvely. Phase behavor smulaton showed that frst-contact mscblty between the reservor flud and CO 2 s frst establshed at 3375 ps, whle the mult-contact mscblty occurs at 3125 ps. CO 2 dffuson s a crtcal factor n CO 2 EOR and ts dffuson coeffcent between component s calculated 486

3 by Sgmund, 1976 [21]: D j 0 0 ρldl = ( ρlr (1) ρ l ρ 2 lr ρ 3 lr ) where ρ 0 0 s the product of densty and dffusvty at zero l D l pressure, ρ l s the densty of the l phase and ρ lr s the reduced densty. Fg. 2 Stmulated reservor wth 10-hydrualc fractures havng 200-ft fracture spacng and 250-ft half-fracture length The modfed Peng-Robnson (1978) equaton of state was used, and the crtcal propertes of the heavest component together wth the bnary nteracton parameter were tuned usng regresson settngs to ft the flud propertes at ntal reservor condtons [22]. Used Peng-Robnson EOS s: p = RT = v b a ( v + b )( v + b + 2c ) + ( b + c )( v b ). The energy parameter of component s estmated by a a c (2) 2 2 R Tc = Ωa a( Tr ) (3) P ( T ) [ + a ( 1 T ) + a + a ( 1 T )( 0. T )] 2 c = (4) 1 o r 1 2 r 7 r For non-polar (hydrocarbon) components, α 1, α 2 are equal to zero, for the PR EOS (1978), α 0 s gvng by: 2 ( ω ω ), a = ω (5) 0 2 a 0 = ( ω ω (6) ω ), ω b = Ω b 2 R T P c 2 c c where c= the volume-shft parameter of component, for the Peng-Robnson: Ω a = , Ω b = The dmensonless volume shft parameter s s defned as c s = b + c c = RT Ωb P The volume shft parameters are determned by matchng the c c (7) (8) 487

4 expermental densty data at T r = 0.7 for a lot of components, the s values are stored n the smulaton. If the values are not avalable, the parameters for lght components are calculated by Peng-Robnson EOS: s w (9) = For a heavy component, the volume shft parameters are determned by matchng ts specfc gravty (SG) at standard condtons. Applcaton of the EOS to flud mxtures requres a mxng rule n order to descrbe the mxture from the propertes of ts pure consttuents. For hydrocarbon systems, the Van-Dar-Waal s mxng rules are commonly used, here t s used wth only temperature ndependent d j ( k ) jzz j aa j 1 (10) j a = b (11) = zb c (12) = zc The d j are usually referred as Bnary Interacton Parameters (BIP) and are usually calculated by parameterzng the EOS wth expermental K-values. The BIP values n the smulaton and the alternatve method for evaluatng BIPs were proposed by Mehra (1981) and L (1983) [23]: d j 2 v = vc 1 3 c v 1 3 cj + v 1 3 cj n (13) where n=1 (constant) and v c = The crtcal molar volume of the component. The crtcal propertes of the heavest component together wth the bnary nteracton parameter were used as regresson varables to ft provded thermodynamc propertes of the flud and pressure saturaton data. Table I shows reservor flud components and Table II shows predcted PVT propertes. On the other hand, asphaltene precptaton s modelled usng a multphase flash calculaton n whch the flud phases are descrbed wth an equaton of state and the fugactes of components n the sold phase are predcted usng the sold model. The approach for modelng asphaltene precptaton s descrbed [24], [25]. The precptated phase s represented as an deal mxture of sold components [23]. The fugacty of a precptatng component n the sold phase s: ln f = ln f s H R tp * s vs + R 1 1 T T * p p T C R p tp p * T ln T p T * * tp T tp 1 1 T T * (14) where f s s the fugacty at pressure p and temperature T, f s * s the fugacty at pressure p * and temperature T *, H tp s the heat of fuson at the trple pont, C p s the sold-lqud heat capacty dfference, p tp and T tp are the trple pont pressure and temperature, and R s the unversal gas constant. For sothermal predctons, ths equaton can be smplfed to gve: * * p p ln f = f + v s ln (15) s s RT TABLE I RESERVOIR FLUID COMPONENTS Comp Pc (ps) Tc (F) Mol Wt. (lb/lb-mol) Mol. Frac (%) N CO CH C 2-C C 4-C C 7-C C 13-C C TABLE II PREDICTED PVT PROPERTIES PVT Propertes Sample Model AARE (%) Sat. Press. (ps) Vscosty (cp) Ol Densty (lb/ft 3 ) AARE (%) The crucal step n modelng asphaltene precptaton s the characterzaton of the solds formng components, both n soluton and n the sold phase. Furthermore, the BIP between the lght components and the asphaltene component have sgnfcant mpact on the predcton of the asphaltene precptaton. A greater value for BIP between lght components and asphaltene wll cause a superor amount of asphaltene precptaton [10]. In ths study, the amount of asphaltene precptaton was 0.26 wt.% at the onset pressure of 2702 ps and 238 F. Fg. 3 shows asphaltene precptaton curve after correctng BIP between the lghter components (C 1 - nc 5 ) and asphaltene component to get the precse shape of the precptaton curve. Reservor flud components are shown n Table III and calculated mole fracton of the asphaltene component s shown n Table IV, whereas Fg. 4 shows phase envelope for the asphaltene. However, t was found that by splttng the heavest component nto two components, as a non-precptatng and a precptatng fracton, these two components have the same crtcal propertes and acentrc factor. Therefore, the mole fracton of heavest component (non-precptatng) and asphaltene component (precptatng) can be calculated by: MW Mole ) MW Ol %( asph) = weght %( asph (16) Ol Asph MW = YMW (17) Addtonally, solublty of CO 2 and hydrocarbon components n the aqueous phase s computed by Henry s law. Reference 488

5 Henry s constant, molar volume at nfnte dluton, and reference pressure are the propertes used n calculatng the solublty of CO 2 n the aqueous phase. Henry s constants are calculated from: ln H = ln H 0 v + 0 ( p p ) RT (18) where the superscrpt o refers to the reference condton. If the expermental solublty data are to be matched usng regresson, ths method for defnng the solublty parameters must be used. However, the Ol-Gas-Water (OGW) calculaton nvolves a three-phase calculaton where the vapor and lqud phases are modeled wth an EOS, whle the aqueous phase s modeled wth Henry's law. L and Nghem [26] recommended the use of Henry's law constants for component solublty n the aqueous phase. The fugacty coeffcent of component n the aqueous phase φ w s gven by H lnhϕ w = ln (19) P TABLE III RESERVOIR FLUID COMPONENTS Comp Mol. Fract. (%) MW YMW N CO CH C 2H-nC C 6-C C 13-C C 20-C C 26-C C 31-C C 36-C C Asph Moreover, water modelng n GEM does not allow vaporzaton of the water component. water densty s calculated from a lnear model n terms of compressblty where water vscosty s constant [23]. Though, Rowe-Chou aqueous densty correlaton and Kestn aqueous vscosty correlaton are used to allow the water densty and water vscosty n GEM to be calculated respectvely as a functon of pressure, temperature, and salnty. CMG-WnProp usually Fg. 3 Asphaltene precptaton curve estmates solubltes for all components up to C 8. However, n ths study, t s desred to model only the solublty of CO 2 n the aqueous phase. Brnes are modeled by assumng that the total salnty s due only to Na + and Cl- ons and the total salnty of the brne s 100,000 ppm. Therefore, Henry s constant calculated for both CO 2 and H 2 O whle consderng rest of the components as nsoluble. Table V shows reservor flud components and calculated solublty propertes. TABLE IV MOLE FRACTION OF THE ASPHALTENE COMPONENT Weght % of Asph Asph MW Ave. MW Ol (Mol.%) Σ YMW C 20+ & Asph (Mol.%) C 20+ (Mol.%) Asph. (Mol.%) B. Rock Propertes A dual-permeablty model was bult n computatonal doman consstng of an deal-fracture wth a wdth of 1 ft and a permeablty of 40-mD, nstead of a real fracture wth a permeablty of 40,000-mD and ft wdth, n order to 489

6 reduce computatonal tme and numercal complexty [20]. The horzontal well was drlled and perforated along ts complete length, and the entre producton horzon was fxed for fve years, bottom-hole producton pressure kept constant at 2000 ps and bottom-hole njecton pressure at maxmum 7000 ps were used n smulaton. As shown n Table VI and Fg. 5, doman man propertes and relatve permeablty curves were used n ths study, respectvely. The deal-fracture permeablty, k deal s calculated by [27]: k fw f k = (20) deal wdeal where w f s the real-fracture wdth, k f s the real-fracture permeablty, and w deal s the deal-fracture wdth used n the smulator. Fg. 6 shows the comparson between real-fracture and deal-fracture. Thus, ths procedure can reduce the smulaton runnng cost and tme III. RESULTS A. Effects of Dfferent Parameters on Ol Recovery The study of cyclc CO 2 njecton and hydrocarbon recovery n ultra-low permeablty reservors s manly a functon of several parameters: frstly, the structural parameters such as fracture conductvty, fracture half-length, fracture spacng, and matrx porosty [28]; secondly, the operatonal parameters such as bottom-hole pressure, prmary depleton tme, CO 2 njecton tme, and number of cycles. However, the most mportant parameter n the ol ndustry s bottom-hole pressure of an ol well at any exstng operatng condton and ts relaton wthn the formaton pressure, n order to determne the most effcent methods of recovery and lftng procedure. Thus, t ncreases ol producton, elmnates sand producton by controllng the drawdown, adjusts njecton rates, optmzes operatonal costs, and determnes the natural drve mechansms n the reservor. Fg. 7 (a) shows the effects of mnmum bottom-hole producton pressure on the ol recovery. As the ol-well kept on producng, the reservor pressure started declnes smoothly wth ol producton. However, as the reservor pressure drops below the bubble pressure 2652 ps, hgher ol recovery was observed about 11.53% of orgnal ol n place at pressure 2000 ps; ths due to the soluton gas whch was dssolved n the ol, becomes free gas and drves the crude ol to the wellbore. However, further declne n the reservor pressure up to 500 ps showed reducton n the ol recovery due to allevaton of gas drve mechansm. Therefore, n ths work, the baselne was created at 2000 ps as mnmum bottom-hole producton pressure to be used for further assessment of other parameters effectng cyclc CO 2 njecton n tght ol reservors. TABLE V RESERVOIR FLUID COMPONENTS FOR SOLUBILITY OF CO 2 IN THE BRINE Comp Mol. Frac Ref. Hnry Vol. nfo Pres. ref (%) (atm) (l/mol) (atm) N Insoluble CO CH Insoluble C 2H-C 3H Insoluble C 4-C Insoluble C 7-C Insoluble C 13-C Insoluble C Insoluble H 2O Fg. 4 Asphaltene phase envelope 490

7 the number of cycles. However, the ol recovery from prmary producton was about 11.53%, whereas the ncremental ol recovery of the frst two cycles was approxmately 2.8%. Moreover, the ncremental ol recovery from the rest of the cycles (thrd to sxth) produced only 3.8%. Thus, the ol recovery declned n the later cycles mght be due to the reducton n CO 2 effcences or dmnsh of the ol n the reservor. On the other hand, exstence of fssures or nduced hydraulc fractures provdes a large contact area for the CO 2 to dffuse through and penetrate nto the low-permeablty formaton. Fg. 5 Relatve permeablty curves In order to evaluate the performance of cyclc CO 2 njecton process based on the prmary depleton perod, the dfferent tme steps are chosen for prmary producton. As noted from Fg. 7 (b), the early CO 2 njecton has less mpact (postve) on ol recovery about 14.5% of OOIP, whereas delayng CO 2 njecton has no further effect on the ncremental ol recovery; therefore, the best tme for CO 2 njecton as observed s to be after 18 months wth hghest ol recovery around 16% of OOIP. Thus, the results perfectly matched the prmary recovery of unconventonal reservors whch remans low at only 5-8% of OOIP, even though long horzontal wells have been drlled and massvely fractured as reported by Department of Mneral Resources, North Dakota, USA [4]. Number of cycles s also an mportant parameter of the cyclc CO 2 njecton process. As shown n Fg. 7 (c), the ol producton boosted wth ncrease n TABLE VI DOMAIN MAIN PROPERTIES Propertes Fg. 6 Comparson between real fracture and deal-fracture Value Unt Matrx Porosty 0.05 % Fracture Porosty % Matrx Permeablty 0.01 md Fracture Permeablty md Hydraulc Fracture Permeablty md Ideal-Fracture Permeablty 40 md Hydraulc Fracture wdth ft Ideal-Fracture wdth 1.00 ft Hydraulc Fracture spacng 200 ft Intal Reservor Pressure 5820 ps Mnmum Producton Pressure 1000 ps Maxmum Injecton Pressure 7000 ps Reservor Temperature 238 o F Reservor Depth 8500 ft Intal Waster Saturaton 0.16 % Generally, natural fractures n the matrx are not connected to each other. Therefore, they are consdered as non-productve unless they have been lnked to hydraulc fractures. In order to evaluate the effect of hydraulc fracture spacng on the ol recovery, hydraulc-fracture half-length was kept constant at 200 ft, whle hydraulc-fracture spacng vared n order to nfluence the ol recovery. As can be seen from Fg. 7 (d), the nearer spacng between the fractures leads to a hgher ol recovery than those havng wder spacng durng cyclc CO 2 njecton process. Furthermore, the hydraulc fracture half-length plays an mportant role n recoverng more ol from ultra-low permeablty reservors. The longer fracture half-length connects wth a larger porton of the reservor volume allowng more CO 2 to dffuse and penetrate nto the 491

8 formaton matrx, resultng n hgher ol producton. As observed from Fg. 7 (e), ths ncremental ol recovery s further enhanced as the fracture half-length s ncreased. The unconventonal recourses have complcated characterstcs such as mcro-fractures, natural fracture and ultra-low permeabltes were varyng from mcro to nano-darces. In such reservor, the fluds flow from matrx to the natural fracture, from natural fracture to the hydraulc fracture and from hydraulc fracture to the wellbore. Fg. 7 (f) shows the effects of natural fracture permeablty on the ol recovery. Its noted that, f reservor natural fracture permeablty s mproved from md to 0.2 md, the ncremental ol recovery wll boost sharply due to hgh conductvty path for the ol to flow easly from formaton to the wellbore. Fg. 7 Effects of dfferent structural and operatonal parameters: (a) effects of BHP (b) effects of prmary depleton effect (c) effects of number of cycles (d) effects of HF spacng (e) effects of HF half-length and (f) effects of NF permeablty on the ol recovery B. Cyclc CO2 Injecton Process Durng cyclc CO 2 njecton process, CO 2 s njected nto the reservor at a certan pressure. The njecton wells are shut-n to allow CO 2 and crude ol to soak for a perod of tme pror to beng swtched back on for the producton as shown n Fg. 8. Ths sngle cycle may be repeated for a couple of tmes untl an economcal producton level s acheved. CO 2 behaves as a supercrtcal flud under most reservor condtons (gas-lke 492

9 vscosty and lqud-lke densty) allowng superor volume of CO 2 to be stored n the reservor, causng crude ol volume to swell, ts vscosty s decreased, nterfacal tenson s reduced, crude ol s drven by soluton ol gas, and lght-components are extracted to the njected CO 2 phase, and thereby mproves ol dsplacement effcency. Moreover, the CO 2 dffuson mechansm durng soakng perod s consdered as the fracture s saturated wth the njected gas (CO 2 ), and the low-permeablty matrx s saturated wth the reservor flud (ol). It s assumed that there were no vscous forces, gravty, and capllary pressure between two phases n the fracture and matrx. However, there s only dfference of gradent mass or concentraton of CO 2 and components n the ol and gas phases [29]. Fg. 9 shows CO 2 dffuson mechansm durng soakng perod. As the CO 2 mass fracton was greater than 0.3, sgnfcant mpact on the saturaton pressure has become greater due to the nteracton between the hydrocarbon molecules whch were affected n such a way that a heavy phase solates the lght hydrocarbon fracton n the form of vapor C. Senstvty Study of Cyclc CO 2 Injecton Process The uncertanty n tght reservor s due to the several parameters ncludng reservor permeablty, number of hydraulc fractures, fracture half-length, and fracture conductvty. Addtonally, the parameters assocated to cyclc CO 2 njecton process are also uncertan, ncludng CO 2 njecton perod, soakng perod, number of cycles, and CO 2 dffusvty. Hence, seven uncertan parameters were nvestgated. It s concluded that the ol recovery factor ncreases wth ncrease n the CO 2 njecton perod, followed by number of cycle, CO 2 dffuson, CO 2 soakng tme, Fg. 8 Cyclc CO 2 njecton process phase as the pressure s lowered; the system was dvded nto three phase fractons as shown n Fg. 10: the lght phase (gas), the ntermedate phase (CO 2 rch-lqud), and heavy ol phase (ol rch-lqud), leadng to ol swellng and vscosty reducton, as t was observed from our prevous lab work [30], [31]. The determnaton of the above-mentoned desgn constrants to optmze the njecton process s a dffcult job. In order to evaluate the effects of the ndvdual perod (njecton, shut-n or producton) on the ol recovery, the other two perods were kept constant. Table VII shows three dfferent cases for each perod, and the analyss s summarzed n Fg. 11 where the ol recovery factor boosted from 18% as planned study (red lne) to 22.5% as optmzed study (green lne) wth overall 4.5% ncrement n ol recovery. One can conclude that more CO 2 njecton wll lead to more ol recovery, and the producton perod mostly depends on the njecton perod, whereas the soakng perod has specfc perod and further extendng the soakng perod would not affect any addtonal ncrement on the ol recovery. permeablty, and fracture half-length, whle t decreases wth ncreasng fracture conductvty [32]. The effects of uncertan parameters on ol recovery factor are shown n Tornado plot n Fg. 12. It can be observed that the most essental parameter s CO 2 njecton tme, followed by number of cycle and CO 2 dffusvty. 493

10 TABLE VII HUFF AND PUFF PERIODS Huff (days) Soakng (days) Puff (days) Fg. 10 Three phase system durng mscblty process [30], [31] Fg. 9 CO 2 dffuson durng soakng (shut-n) perod Fg. 11 Optmzed cyclc CO 2 njecton process 494

11 D. Asphaltene Deposton Fg. 12 Effect of uncertan parameters on ol recovery Fg. 13 Ol recovery factor and pressure drop wth and wthout asphaltene precptaton Generally, the presence of asphaltene contents n the reservor flud s not an ssue, but ther precptaton and deposton mght occur due to the mscblty process takng place between CO 2 wth the reservor flud, causng an rreversble damage specally n tght ol reservor rocks wth smaller pore throats subjected to more severe formaton flow mparment. In compostonal modelng, t s mportant to select approprate bnary coeffcent parameters for the sake of better predcton of asphaltene precptaton curve, and the volume shft s used to mnmze the errors related to estmaton of sold molar volume. However, when the bottom-hole pressure drops below the asphaltene precptaton pressure, where the average reservor pressure wll be allowed to decrease below the bubble pressure, allowng more gas to be lberated, whch n turn, reduces the solublty of asphaltene and nduces precptaton. In order to prevent asphaltene precptaton, the reservor bottom-hole pressure must be greater than asphaltene AOP 2702 ps. The effects of the njected flud (CO 2 ) concentraton play a sgnfcant role n preventng asphaltene deposton, the hgher CO 2 concentraton leads to hgher precptaton. However, the presence of brne could also prevent asphaltene deposton, enhance the ol recovery, and reduce CO 2 emsson. Fg. 13 shows the ol recovery factor and pressure drop wth and wthout asphaltene, whereas Fg. 14 shows the daly ol producton wth and wthout asphaltene. As observed from 495

12 Fgs. 13 and 14, the ol producton rate obtaned wth asphaltene precptaton s lower than that of wthout the asphaltene precptaton. Ths s due to the consequences of asphaltene precptaton on permeablty reducton whch s manly dependng on some factors such as reservor permeablty, pore sze dstrbuton, and the amount of asphaltene deposton. In most of the stuatons, precptaton occurs close to the producton vcnty, probably because of the pressure drop assocated wth the producton vcnty. However, these factors may lead towards not only near wellbore formaton damage, but also t mght cause some formaton flow mparment deep n the reservor. E. CO 2 Solublty n the Brne CO 2 njectons are used n ol ndustry for several optons, njectng CO 2 nto deep salne aqufers for sequestratng CO 2 n geologcal formatons, whle njectng CO 2 nto mature or depleted ol and gas reservors for the purpose of effectve sequestraton and enhancng producton. Generally, deep salne aqufers have no economc value. In order to reduce CO 2 emsson n the atmosphere, CO 2 dssoluton needs to be measured under reservor condtons. However, the solublty of CO 2 n the aqueous phase s manly functon of temperature, pressure, and salnty. The volume of CO 2 that can be dssolved n aqueous phase can be estmated by phase behavor module (WnProp). A seres of solublty curves were generated for CO 2 dssolutons at dfferent pressure and temperature as shown n Fg. 15. Concernng the pressure, t can be noted that ncrease n pressure ncreases the dssoluton of CO 2 n the aqueous phase. However, the pressure dependency of the solublty ncreases as the pressure decreases. The effect of temperature on the solublty s contrary; a reducton n the temperature substantally ncreases the solublty, regardless of the salnty of the brne. However, at hgher temperature above 251 F, the temperature curve trend changed after 3500 ps, and CO 2 dssoluton ncreased wth ncreasng temperature. Ths s because the reservor ntally contans three-phase system (aqueous, lqud and vapor), and as the pressure and temperature ncreased, the lqud phase started evaporatng and phase behavor of the fluds changed nto two-phase system (aqueous and vapor) above 3500 ps, therefore the CO 2 dssoluton ncreases wth ncreasng temperature as shown n Fg. 15. Furthermore, the brne salnty (NaCl) concentraton also has an mpact on the CO 2 dssoluton. Fg. 16 shows CO 2 solublty curves at dfferent brne salnty (NaCl) concentratons. It can be concluded that the salnty also has a contrastng mpact on the solublty as ncrease n the brne salnty leads to a reducton n CO 2 solublty, regardless of ts pressure and temperature. Fg. 17 shows that the salnty of the aqueous phase s ndependent of pressure and temperature, usng percent solublty as a functon of salnty. Fnally, the result obtaned from CMG-GEM shows that CO 2 can be utlzed as EOR technque as well as stored n deep aqufer CO 2 storage and sequestraton processes. From Fg. 18, after njectng CO 2 n the aqueous phase, gas recovery factor (CO 2 ) revealed negatve behavor, reflectng dssoluton of CO 2 n deep brne aqufer whle enhancng the ol recovery up to some extent. Fg. 14 Daly ol producton wth and wthout asphaltene precptaton 496

13 Fg. 15 CO 2 solublty curves at dfferent pressure and temperature Fg. 16 CO 2 solublty curves at dfferent brne salnty (NaCl) concentraton 497

14 Fg. 17 CO 2 solublty n brne Fg. 18 Ol recovery factor and CO 2 recovery factor durng CO 2 solublty process IV. CONCLUSIONS The reservor smulaton results show that cyclc CO 2 njecton process s an effectve EOR method and has ablty to produce an economcal amount of ol from unconventonal tght reservors. Ths study acknowledged perfect procedures of optmzng the performance of cyclc CO 2 njecton treatments and maxmzng the ol recovery factor. The followng conclusons can be drawn from ths work: 1. Prmary depleton perod plays an essental role n mprovng ol recovery. Therefore, startng CO 2 njecton too early or too late would adversely mpact the effcency of the process and dmnsh the projects net present value. So, sutable tme for CO 2 njecton n ths work was observed to be after 18 months of prmary depleton wth hghest ol recovery of about 15.94% of OOIP. 2. Hgher ol recovery s expected from longer njecton tme 498

15 n cyclc CO 2 njecton process, even though the producton perod depends on njecton perod, whle the soakng perod has specfc tme, and further extendng soakng perod would not affect an ncrement n ol recovery. 3. CO 2 dffuson durng soakng perod also plays an mportant role n penetratng the unconventonal tght ol reservors, as t s mscble wth ol, thus produces more lght ol n the early lfe of the well. 4. Optmzaton of cyclc CO 2 njecton process boosted ol recovery from 18.15% to 22.56% wth overall 4.41% ncremental n ol recovery. 5. The asphaltene precptaton ncreases as the njected flud concentraton ncreases. 6. Overall, CO 2 njecton causes more asphaltene precptated, and permeablty reducton. 7. Durng cyclc CO 2 njecton process, as CO 2 s soluble n the brne, CO 2 concentraton s reduced and asphaltene precptaton s mnmzed. 8. Hgh brne salnty leads to a reducton of CO 2 solublty. 9. The dssoluton of CO 2 n the brne aqufer ncreases wth ncreasng pressure and decreasng temperature. 10. 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16 [31] Yu, W., Lashgar, H., & Sepehrnoor, K. (2014, Aprl 17). Smulaton Study of CO 2 Huff-n-Puff Process n Bakken Tght Ol Reservors. Socety of Petroleum Engneers. [32] Wanfen Pu, Bng We, Fayang Jn, Ybo L, Hu Ja, Penggang Lu, Zhjuan Tang, Expermental nvestgaton of CO 2 huff-n-puff process for enhancng ol recovery n tght reservors, Chemcal Engneerng Research and Desgn, Volume 111, July 2016, Pages , ISSN , Rashd S. Mohammad s a PhD canddate n Ol-Gas Feld Development Engneerng, College of Petroleum, Chna Unversty of Petroleum-Bejng, Chna. He worked as lecturer n BUITEMS as well as research assstant n UAE Unversty. He holds a MS degree n petroleum engneerng from UAE Unversty, Al-an, Unted Arab Emrates. Hs current major feld of study s n unconventonal reservors development usng CO 2 njecton. Zhang Schcheng s a Professor n the college of Petroleum Engneerng, Chna Unversty of Petroleum-Bejng. Zhang holds a BS degree n producton from East Chna Petroleum Insttute. MS and PhD degree n ol and Gas Feld Development Engneerng from Chna Unversty of Petroleum-Bejng, Chna. He has served on the edtoral commttees of several journals and has authored or coauthored more than 80 techncal papers. Zhang currently workng as vce rector of Chna Unversty of petroleum-bejng, Chna. Sun Lu s a PhD canddate n Ol-Gas Feld Development Engneerng, College of Petroleum, Chna Unversty of Petroleum-Bejng, Chna. She holds a MS degree n petroleum engneerng from Chna Unversty of Petroleum-Bejng, Chna. Her major feld of study s n unconventonal reservors and smulaton studes. Syed Jamal-ud-Dn Shah s a Master n Petroleum Engneerng, Chna Unversty of Petroleum-Bejng. He worked as lecturer as well as holds BS degree n petroleum and gas engneerng from BUITEMS, Quetta, Pakstan. Hs research s manly focused on unconventonal reservors and smulaton studes. Xnzhe Zhao s a Master n Petroleum Engneerng, Chna Unversty of Petroleum-Bejng. She holds BS degree n petroleum engneerng from Yangtze Unversty, Hube, Chna. Her research s manly focused on unconventonal reservors and stmulaton process. 500