Pym Soleimni , Seyed Rez Shdizdeh , Riyz Khrrt

a Department of Petroleum Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran

b Department of Petroleum Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran

c Department Petroleum Engineering, Montanuniversit¨at, Leoben, Austria

ABSTRACT Different methods of enhanced oil recovery have been used to produce trapped oil.One of these methods is carbonated water injection in which CO2 contained water is injected in reservoirs in order to decrease free CO2 injection mobility, increase water viscosity and store/remove produced greenhouse CO2 gas safely.Another enhanced oil recovery method is smart water injection at which the ions in brine are modified in order to make controlled reactions with distributed ions on the surface of rock to cause more hydrocarbon recovery.Therefore, combination of these two methods may also have a great effect on enhancing oil recovery or may result in recovery factor less than each method used alone.In this paper hybrid smart carbonated water injection method is investigated to study its applicability in oil recovery using core flooding setup.The experimental core flooding setup was designed to perform different types of EOR methods for the sake of recovery comparison with the new hybrid method.The effect of both brine content and volume of CO2 is determining in hybrid EOR assessment.The main findings of this work show that the hybrid smart carbonated water results in the highest recovery factor in comparison to the most well-known EOR methods for carbonate cores.

Keywords:Hybrid smart carbonated water Carbonated water Smart water Core flooding Carbonate cores

1.Introduction

Enhanced oil recovery (EOR) has been always an interest for petroleum engineers due to increasing demands to fossil fuels especially oil.In EOR, external forces are used for changing chemical and physical properties of oil and porous media containing it to produce more oil.Different methods have been introduced to the petroleum industry.All EOR methods are categorized in main groups including miscible flooding (such as injection of CO2, nitrogen, hydrocarbon or solvent), chemical flooding (such as injection of polymers, surfactants, alkaline, emulsion, foam and their combinations), thermal recovery (such as steam assisted gravity drainage (SAGD)) and microbial recovery.Injection of water or immiscible gases such as nitrogen and carbon dioxide only help to maintain the pressure of the reservoir.One of the most common EOR methods is carbonated water injection at which CO2is dissolved in the water to be transferred to the reservoir, then CO2is introduced to the oil and increases oil recovery.In this method the advantages of both water injection and miscible CO2injection are deployed, ignoring each methods problems when used alone [1].

Carbonated water injection prohibits high mobility CO2which results in CO2escape or water shielding (water layers are present between isolated oil ganglia) and high-water saturation in water injection.Also shortage in CO2volume need for CO2injection and CO2sequestering for removing greenhouse gas effects are all solved by carbonated water injection(CWI)method.Viscosity and density of water increases when CO2is dissolved in it.Also salinity of connate water plays an important role in final recovery [2].Visualization of interactions between CO2, water and oil by micromodels made a sense by several researchers and persuade them to investigate the interaction using corefloods.Micromodel and coreflood experiments have shown that recovery factor is increased when carbonated water is used both for secondary and tertiary mode of injection [3-10].Hebach et al.correlation and extrapolation of Garcia data are used for measuring density of carbonated distilled water [11] and density of carbonated brine [12], respectively.Dong et al.[13] presented a summary of experimental tests performed at laboratory to investigate carbonated water performance.Experimental investigation of carbonated water injection in micromodels and cores had some restrictions in manipulating laboratory conditions.Therefore, carbonated water investigation through the models was also interested[14-18]in order to reach to acceptable results for field pilot projects of CWI[19-24].One of the main concerns in carbonated water injection works has been the amount of carbon dioxide solubility in water.Different correlations have been used for evaluation of CO2solubility in water [25], oil[26,27] and aqueous solutions such as methanol and acetone whether it contains sodium chloride salt[28].Solubility behavior of methane, ethane, propane and n-butane in heavy crude oil have been under interest like CO2[29].Mosavat and Torabi [30,31]investigated secondary carbonated water injection in pressure ranges from 0.7 to 10.3 MPa and observed that recovery factor increases sharply to pressure 5.9 MPa and then increases slowly above that pressure due to amount of CO2solubility.Wettability alteration using contact angle measurements have been reported in some papers for finding out the effect of varying CO2concentration on wettability change and recovery factor [32-36].It has been observed that when concentration of carbonic acid increases in the system,wettability changes to more water wet or intermediate wet.When CO2is dissolved in brine,it diffuses into the oil then diffuses to rock surface replacing heavier components of hydrocarbon which results in wettability change.Ruidiaz et al.[37] and Riazi &Golkari [38] have measured IFT of carbonated brine/oil/CO2and brine/oil/CO2systems and find out by adding CO2to brines having optimum concentration,IFT decreases.Rock integrity[39],porosity and permeability changes [40,41] have been investigated when carbonated water is injected in the reservoirs in order to predict probable deformations in reservoir situation and final recovery factor changes.CO2liberation from carbonated water when pressure decreases results in blow down effect during at which oil is recovered after carbonated water injection terminated and depressurization continued[42-44].New phase generation due to carbonated water and crude oil contact has been investigated[45-47].It was concluded that CO2partitioning is stronger when live oil is used due to faster generation of new phase resulting in faster transfer of CO2to oil.Viscosity reduction could be inferred from Lederer correlation[48]which is used for predicting viscosity.This correlation could be used in black oil simulators for heavy viscous oil simulations.

In the smart water injection, the salinity of injected brine is varied to change the wettability of rock and therefore increasing recovery factor of reservoir.Salinity variation causes wettability change from water wet to mixed wet in which small pores are water wet while large pores are oil wet[49].Many researches also have been performed to investigate low salinity water injection in the reservoirs.Fine migration occurs in porous media if the ionic strength of the injected water is less than a critical flocculation concentration [50].When low salinity water is injected the electrical double layer in aqueous phase would expand and strip the oil coats, turning the mixed wet system into water wet.So the recovery factor would increase [51].By injecting low salinity water,hydrogen ions are released from water to rock surface and the sodium ions are adsorbed from the rock surface resulting in increment of pH.When the pH is increased, the organic materials are desorbed from the surfaces and though the Zeta potential of the surface would change.When the pH is increased, organic acids in crude oil react with each other and produce surfactants(soaps),so the IFT between the crude oil and water would decrease [52].Divalent ions at the rock surfaces have strong bonds with polar compounds in oil such as resins and asphaltenes forming organometallic compounds.When low salinity water is introduced to the system, the multicomponent ion exchange occurs and the organometallic compounds and oil wet surfaces are removed, causing higher oil recovery [53].The combination of Van der Waals attraction and the electrostatic repulsion due to counter ions is called double layer effect.When low salinity water is injected through the porous media, the charged surface-charged surface attraction force is reduced due to electric double layer effect [54].When the salinity of injected water is decreased, more organic materials (such as oil droplets) would dissolve in water (salt-in effect)and recovery factor would increase[55].Oil droplets act as a semi-permeable membrane.So there would be an osmotic pressure gradient which relocates oil by expanding an inaccessible aqueous phase in a porous medium.The low salinity water would relocate oil and open new water pathways through the oil called diffusion[56].Low salinity water causes the contact angle decrease.So, the wettability would change into more water wet resulting in higher recovery factor[57].

The combination of carbonated water injection with other EOR methods have been examined in the literature.Fathollahi and Rostami [58] performed coreflood experiments of pressure range 2000-3500 psi on water wet outcrop core having 18%porosity and 57 mD.They observed that by adding silica nanoparticles to carbonated water recovery factor increases because CO2concentration increases in the carbonated water due to cracking of initial large CO2bubbles into smaller bubbles.They concluded that there is an optimum concentration of Nano silica which results in higher recovery factor.In higher concentration,Nano silica deposits on the surface of rock.They observed the same result when used brine based carbonated water.Bakhshi et al.[59] concluded that carbonated water injection results in more recovery factor in carbonate rocks rather than sandstone,in lighter oil rather than heavy oil and in low salinity brines rather than saline brines.Honarvar et al.[60]concluded that lower IFT is measured for carbonated sea water (35079 ppm) instead of carbonated formation water(97645 ppm).

In this work, a combination of carbonated water injection and smart water injection method was studied to see the effect of both brine content and volume of CO2on oil recovery.Applicability of the introduced method is investigated using coreflood tests on carbonate cores and the results are compared with other EOR methods.

2.Materials and methods

2.1.Cores and fluids properties

Carbonate cores from one of Iranian reservoir outcrops were used to perform the experiments Table 1 presents the properties of these cores.As measured, the initial properties of carbonate cores from the same lithology are similar because the cores are selected from the same location of outcrop with distance of less than 0.5 cm from each other(Fig.1).As is shown in this figure all the plugs used for the experiments are taken from the same point of the reservoir in order to reduce the heterogeneity effect.In other words, when the plugs are provided in this way, heterogeneity becomes as minimum as possible for providing the most similar core plugs.Because the intrinsic properties would be similar in this way.All the cores have length of 6.7 cm and diameter of 3.8 cm.

Helium porosimeter and gas permeameter were used for measurements of rock porosities and permeabilities respectively.Aging process is performed for cores in order to provide initial reservoir rock properties.As it is known oil migration to reservoirs results oilto fill the pore space and change wettability.The properties of reservoir crude oil used are shown in Table 2.

Table 1 Experimental cores physical properties.

Two sources of water are used for preparing carbonated water in field scale.Persian Gulf water is used as sea water source and one of Iranian reservoirs formation water as reservoir water.Ion concentration for different water sources are presented in Table 3.The third line of Table 3 shows other brine source used (0.01% volume formation water (FW) that has been diluted with distilled water).Sea water is used for providing brine water having concentration in range of 40000 ppm.Reservoir formation water is diluted 100 times to reach in range of 2000 ppm salinity concentration.Also,this 100 times diluted formation brine is used for preparing smart carbonated water.

Table 2 Physical oil properties.

Table 3 Ion concentration of different source water.

2.2.Core flooding setup

In this work main apparatuses used are constant-temperature oven, a high pressure core holder, a vacuum pump, two high pressure high performance liquid chromatography (HPLC) pumps,high pressure fluid transfer vessels,a back pressure regulator(BPR),high-pressure N2cylinder to provide the supporting pressure for BPR a visual separator,a gas meter,an overburden pressure pump,a differential pressure transducer (DPT), a CO2gas cylinder, a high pressure double pump, a graduated cylinder for collecting produced oil,a gas flow meter for measuring the amount of gas coming out, a vessel for transferring separated gas into it, a soxhlet apparatus for washing cores, and an electronic scale for weight measurements.

A schematic diagram of core flooding setup is shown in Fig.2.Each of the cores is vacuumed initially.Then formation water is injected to the vertically situated core holder from the bottom by rate of 0.1 cc/min to saturate all parts completely.Then oil is injected in the same manner (vertically and rate of 0.1 cc/min) to establish initial water saturation.Then the cores are aged in the oven(at temperature of 140 ℉)for 4 weeks.All tests are performed at temperature of 140°F and overburden pressure of 2500 psi.Rate of injection for carbonate cores is 0.05 cc/min.

2.3.Core flooding methods

For investigating hybrid smart carbonated water injection applicability for oil recovery,it is intended to compare this methodwith other most well-known EOR methods.Five EOR methods used in this work are; smart water injection, brine water injection,continuous CO2injection, water alternating gas (WAG) injection and 100 times diluted formation water(0.01 FW)based carbonated brine representing hybrid smart carbonated water injection.

Smart Water Injection Method:smart water injection is the way in which the salt concentration is changed throughout the injected fluid.In this method,2000 ppm(100 times diluted formation water or 0.01 FW) concentration representing as low salinity brine is injected through the cores.

High Salinity Brine Injection Method: in this method 40000 ppm (Persian Gulf water) concentrations representing as high salinity brine is injected through the cores.

Continuous CO2Injection Method: continuous CO2injection is performed by filling the pressurized cell shown in Fig.2 as carbonated water by CO2.A gas flow meter also is used for evaluation of CO2volume injected in the core holder.When CO2is injected continuously, overburden pressure and backpressure cell should be set in order to avoid early gas breakthrough from the core.

Water Alternating CO2(WACO2) Injection Method: 40000 ppm sea water (Persian Gulf water) is used as injected water slugs in water alternating CO2(WACO2) method.When water alternating CO2experiments are to be run,one slug volume of CO2is injected,then one slug volume of brine is injected and this cycle is repeated until desired pore volumes of fluids injected.Slug volume ratio during these tests is one.

Hybrid Smart Carbonated Water Injection Method: 100 times diluted formation water (0.01 FW) based carbonated brine is used for injection.In order to prepare hybrid smart carbonated brine,the volume of CO2needed is calculated using Chang’s CO2solubility correlation[25]and verified using Duan’s CO2solubility model[61].100 times diluted formation water is injected in the pressurized cell.Then the pump is connected to it and set at the pressure(pressure of 2500 psi and temperature of 80 ℉)intended to prepare carbonated brine.The calculated volume of CO2is passed through a line having gas flow meter and then injected through a gas booster to increase the pressure of it and decrease its volume.Pressure is increased till not pass the carbon dioxide liquefaction pressure.If the CO2injected in the brine cell be separated and not be dissolved in water, due to high pressure of cell, CO2liquefaction occurs and the experiments would fail.Hybrid smart carbonated water cell is ready now.It is positioned vertical and connected to the lines.

High Salinity Carbonated Water Injection Method: The same procedure of hybrid method(low salinity carbonated water)is used for preparation, except that Persian Gulf water is used instead of 100 times diluted formation water for preparation of carbonated water.

3.Results and discussions

In the following, results of tests for different most well-known EOR injection methods in carbonate cores are compared with the hybrid smart carbonated method results.

Fig.2.Schematic diagram of experimental setup used during the tests.

3.1.Smart water injection method versus hybrid smart carbonated water injection method

Fig.3 compares smart water and hybrid smart carbonated water injection methods results.

As is observed Part a)of Fig.3,ultimate oil recovery(%OOIP)for smart water injection is nearly about 55% and for hybrid smart carbonated water is about 70%.Similar brine(2000 ppm)is used for both smart and hybrid brine preparation but oil recovery differs about 15%.The reason is couple effects of both smart and carbonated water injection in hybrid method.In lower pore volumes injected (less than 0.3 PV) oil recovery for smart water injection method is more than hybrid brine injection due to activation of smart water injection mechanisms such as wettability alteration, IFT reduction, ion exchange and pH variation as explained in literature.But smart brine mechanisms activation during injection of hybrid brine is delayed due to space occupation of CO2.

Fig.3.a) Oil recovery factor and b) differential pressure; for Smart brine (2000 ppm) and Hybrid smart carbonated water injections.

As explained earlier the cores become oil wet during aging process in order to provide similar conditions to reservoir.When the carbonate core is aged, its surface is coated with oil droplets resulting in negative surface.When smart water is injected through the porous media, the wettability changes.The reason is that negative ions present in the injected brine(such as SO42-)substitute the negative oil droplets on the surface.Then positive ions present in the injected brine (such as Ca2+, Na+or Mg2+) are absorbed to SO42-ions resulting in ion exchange.This ion exchange causes wettability alteration from oil wet to water wet.

When smart carbonated water (SCW) is exposed to the porous media, additional phenomenon occurs.During two sets of equations, cations (H+and Ca2+) are produced more.

When more cations are provided, they will be absorbed to surface rock substituted anions resulting in higher wettability alteration.

Another phenomenon during injection of smart carbonated water is transfer of CO2from brine to the oil and negatively charged oil distribution on rock surface due to higher solubility of CO2in oil with respect to brine.Dissolved CO2swells the negatively charge oil distribution on the rock surface cause it to be lighter and therefore the oil is separated easier from the surface.

Therefore, higher wettability alteration from oil wet to water wet for carbonate cores occurs when smart carbonated water is injected compared to smart water injection.

Carbonated water mechanisms such as wettability alteration,IFT reduction,new phase generation,ion exchange capacity,double layer effect,oil viscosity reduction and oil swelling are also delayed until gradual CO2transfer from brine to oil starts and completes.In higher pore volumes injected, carbonated and smart water injection mechanisms are activated and result in higher oil recovery factor for hybrid method.

As is observed in Part b) of Fig.3, differential pressure for both methods is nearly similar before injection of 0.3 pore volume fluid but increases more in smart water method for higher pore volumes.CO2transferred from water to oil in hybrid method, causes oil swelling and viscosity reduction.Therefore,the trapped oil in small pores and throats, is extracted out from these critical points and injected water replaces it.Therefore, less pressure is needed for injection of fluid in hybrid method.Another reason is that, CO2dissolution in brine causes decrease in pH and acidic environment which results in minerals dissolution and higher permeability.Small traces will be appearing on throat walls due to mineral dissolution.Therefore, less viscous swelled trapped oil (due to better CO2transfer)will be swept through new small paths.Mineral dissolution occurs very slowly, so in low pore volumes injected differential pressure is the same.

3.2.High salinity brine injection method versus hybrid smart carbonated water injection method

Fig.4 compares high salinity brine and hybrid smart carbonated water injection methods results.

Part a) of Fig.4 shows that ultimate oil recovery factor(%OOIP)for high salinity (40000 ppm) brine and hybrid smart carbonated water injection methods are 49% and 70%, respectively.By comparing Part a) of Fig.3 and Part a) of Fig.4 it is observed that increasing salinity of injected fluid from 2000 ppm to 40000 ppm increases ultimate oil recovery factor difference between hybrid brine and high salinity brine injection method to 21%.

Behavior of wettability alteration during injection of high salinity brine injection in porous media is exactly the same as the one discussed for smart water injected except that there is a limit for ion reactions.When the salinity increases more than an optimum limit,the effect of brine ions changes.When brine containing ions is injected to the medium, positive and negative ions react with each other which causes SO42-absorption to the rock surface resulting in negatively distributed oil separation from rock surface.The substitution will continue till an extremum value resulting in wettability alteration.When SO42-concentration exceeds this extremum value, more reactions occur which results in ion residence and precipitation on the rock surface.This causes blockage in pores and throats of porous media.But when smart and smart carbonated water are injected,SO42-concentration does not exceed extremum value.In addition,for the case of smart carbonated water injected, carbonate mineral dissolution occurs due to acidic environment provided, therefore because more space is provided for possible precipitations,pores and throats are not blocked.Another difference between saline brine injection and smart carbonated water injection is that when the salinity increases, due to more absorption of anion SO42-and following blockages,less percentage of cations Ca2+and Mg2+could be transferred from the medium.

In addition, oil recovery factor of high salinity brine increases faster than the smart water(2000 ppm)injection method at lower pore volumes of injected fluid.In other words, the difference between high salinity brine injection method and hybrid smart carbonated water injection method oil recovery factor is more noticeable at low pore volumes of injected fluid.The reason is that at low pore volumes of injected fluid, due to higher concentration of 40000 ppm high salinity brine,the injected fluid is more packed,causing more oil displacement.But at higher pore volumes injected, smart and carbonated water injection mechanisms are activated during injection of hybrid method.Higher differential pressure for high salinity brine injected compared to hybrid water injected shown in Part b)of Fig.4 also verifies packed movement of oil.At low pore volumes of injected fluid, due to higher concentration of 40000 ppm high salinity brine,more differential pressure is needed to displace oil.When breakthrough occurs, due to concentrated injected fluid,higher percentage of injected fluid will be produced and brine pushes oil away from its path and causes high oil trapment.The difference between differential pressure of high salinity brine injection and hybrid smart carbonated water injection after breakthrough at different injected pore volumes(Part b)of Fig.4)is more than the difference of differential pressure for smart water injection and hybrid smart carbonated water injection (Part b) of Fig.3).

Fig.4.a) Oil recovery factor and b) differential pressure; for High salinity (40000 ppm) brine and Hybrid smart carbonated water injections.

3.3.High salinity brine based carbonated water injection method versus hybrid smart carbonated water injection method

Hypothesis of this work was that a combination of two methods of smart water and carbonated water may have a great effect on enhancing oil recovery or may result in recovery factor less than each method used alone.The low salinity based carbonated water(low salinity CW) injection is compared with high salinity brine carbonated water (high salinity CW) injection method here.In order to perform this comparison, low salinity water of 2000 ppm(100 times diluted formation brine)and high salinity of 40000 ppm(Persian Gulf water) are used for preparation of carbonated brine.Core flooding results are presented in Fig.5.

The ultimate RF for 100 times diluted formation carbonated brine (0.01FW) which is considered as low salinity based carbonated water is about 70%; when Persian Gulf water is used for preparation of high salinity based carbonated water, ultimate recovery factor becomes 62%.This shows that when the salinity of carbonated water increases, less volume of CO2is dissolved in the brine due to competency between ions and CO2(salting-out phenomenon), therefore recovery factor decreases.In this case other carbonated related mechanisms are activated [62,63] which summary are presented in Table 4.

Table 4 Low salinity CW and high salinity CW parameters comparison.

pH increases for low salinity CW,but remains constant for high salinity CW.The injected low salinity brine contains more volume of CO2rather than high salinity brine resulting in more acidic environment.Therefore, dissolvable minerals such as Ca2+and Mg2+, are dissolved causing the brine pH increase which is about 21.42% for low salinity CW case.Due to this mineral dissolution permeability and porosity also change.Because dissolution occurs in traces and small pores,amount of permeability increase is more sensed than large scale porosity variations.Higher amount of mineral dissolution occurs in low salinity CW with respect to high salinity CW,therefore higher permeability and porosity increase is observed in this case.Low salinity CW injected has caused the wettability change from oil wet to water wet and high salinity CW has caused change from oil wet to neutral wet.When surface ions are substituted and exchanged as discussed earlier, wettability alteration occurs which is higher in low salinity CW case.In low salinity CW case, in addition to Ca2+, Na+or Mg2+absorption to surface substituted SO42-, due to decrease in brine salinity, higher CO2is dissolved, therefore H+and Ca2+and surface oil lightning plays more important role.

This is also shown in Zeta potential variation parameter which increases 206% for low salinity CW with respect to 179% of high salinity CW.In other words, negatively oil droplets distributed on the surface are separated and the surface becomes more positive for low salinity case due to Ca2+,Na+or Mg2+absorption to surface substituted SO42-.Viscosity variation is not very noticeable although a higher relative decrease is observed in case of low salinity CW with respect to high salinity CW due to oil swelling effect.Different parameters are measured before running the tests and after completing the tests.

Fig.5.a) Oil recovery factor and b) differential pressure; for high salinity carbonated water and low salinity carbonated water injections.

Fig.6.a) Oil recovery factor and b) differential pressure; for Continuous CO2 Injection and Hybrid smart carbonated water injections.

3.4.Continuous CO2 injection method versus hybrid smart carbonated water injection method

As is shown in Part a) of Fig.6, oil recovery factor increases sharply and steadily at lower and higher injected pore volumes of CO2, respectively.Breakthrough of injected fluid occurs later for continuous CO2injection (0.7 PV CO2injected for continuous CO2injection and 0.4 PV smart carbonated water injected for hybrid smart carbonated water injection) and at breakthrough point, oil recovery for CO2injection is higher than hybrid brine injection about 2%.But as injection continues, hybrid brine recovery factor gradually increases and at final point ultimate oil recovery of hybrid brine method (70%) becomes 5% more than the one for CO2injection method(65%).In CO2injection more fluid should be injected to cause CO2diffuse in pores and throats and finally dissolution in oil.After breakthrough in CO2injection, only dissolved CO2causes oil production, so less percentage of ultimate oil recovery will be obtained.But for hybrid brine injection,more mechanisms related to smart and carbonated water injection become activated after breakthrough, therefore gradual increase in oil recovery factor of hybrid water continues more after breakthrough.

When CO2is dissolved in brine and then injected to the medium,the following equations resulting in mineral dissolution occur.

Where M represents a divalent metal ion(Ca2+,Mg2+,…),H2CO*3is the sum of dissolved molecular CO2(aq)and H2CO3in the aqueous system,and k1, k2and k3are rate coefficients.

Therefore, in addition to the equations resulted in wettability alteration discussed in earlier sections, mineral dissolution will occur to higher extent when smart carbonated water is injected to the medium compared to only CO2injection.

Also, because of low permeability and special pore size distribution in carbonate cores, early breakthrough of carbon dioxide is prohibited.So, carbon dioxide will invade more pores and throats and passes longer distances.In addition,due to CO2compressibility,injected CO2is dissolved in oil and then additional CO2is compressed because of its lower density with respect to oil.The compression lasts till CO2could move oil.

As is observed in Part b)of Fig.6,differential pressure for hybrid brine injection will increase and reach to breakthrough peak faster than continuous CO2injection due to earlier fluid breakthrough.At breakthrough,higher differential pressure is needed for continuous CO2injection to force CO2diffusion in pores and throats or cause dissolution in oil due to high compressibility of CO2.In higher pore volumes injected, acidic environment provided by hybrid brine injection, will result in mineral dissolution and permeability increase.

3.5.Water alternating CO2(WACO2)injection method versus hybrid smart carbonated water injection method

Fig.7.a) Oil recovery factor and b) differential pressure; for Water alternating CO2 (WACO2) injection and Hybrid smart carbonated water injections.

As is shown in Part a) of Fig.7, ultimate oil recovery factor for water alternating CO2(WACO2) injection is about 62% and for hybrid smart carbonated brine is 70%.Injecting water and CO2alternatively would combine effects of both water injection and CO2injection.Alternative changes in slope of recovery factor curve versus pore volume of injected fluid after breakthrough is observed in WACO2injection method.In this method, 0.5 PV of water is injected then 0.5 PV CO2is injected and continues till total 4 PV fluid is injected.After breakthrough if water is injected, the recovery factor increases more sharply due to more effective push of water, but if CO2is injected, increases steadier.Because of smart water injection mechanisms activation which could not be observed in WACO2injection due to 40000 ppm based brine used,hybrid method ultimate recovery factor is more at lower injected pore volumes.When water is injected in WACO2, the difference between recovery factor curves of two methods decreases because highly concentrated brine pushes oil better and also CO2is forced to diffuse through the pores and throats at which oil is trapped,while in hybrid method gradual increase in recovery factor is observed due to mechanisms related to CO2dissolution.

When CO2and water are present in the medium, reactions resulting in mineral precipitation occurs.Because Calcium or Magnesium are present in the carbonate medium the following reactions occur:

Therefore,Ca(HCO3)2andMg(HCO3)2will precipitate.For smart carbonated water, mineral dissolution also occurs due to acidic environment,as discussed earlier.Mineral dissolution in carbonate media is noticeable during injection of smart carbonated water injection.In smart carbonated water injection both mineral dissolution and precipitation occurs but for WACO2injection only mineral precipitation occurs.

Differential pressure for WACO2is observed in Part b)of Fig.7.In WACO2method,although injection of water needs higher pressure change, injection of periodic CO2will decrease this need (in contrary to continuous CO2injection).At high pore volumes of injected fluid injection of WACO2will need higher differential pressure with respect to hybrid method.CO2would diffuse from water to oil due to higher solubility of CO2in oil droplets (this manner of diffusion could not be observed in WACO2method)that causes oil swelling,oil viscosity reduction and oil mobility increase resulting in oil to come out from small pores.

In addition, when pressure decreases in differential pressure oscillations of WACO2injection, Ca2+(or Mg2+or such divalent ions) carbonate will precipitate.

In addition,mineral dissolution causes fluid movement by lower differential pressure.It is interesting that method of injection is effective on changing pore size distribution and resulting permeability.

3.6.Overview of all methods

The overall results and ultimate oil recovery factors are compared in Fig.8 and Table 5.

Table 5 Oil recovery factor for different EOR methods conducted on carbonate cores.

The priority of different methods of injection for carbonate core are hybrid smart carbonated water(70%),continuous CO2injection(65%), High salinity CW (62%), water alternative CO2injection(62%), 2000 ppm brine injection (55%) and 40000 ppm brine injection (49%).The slope of recovery factor curve versus PV in continuous CO2injection method, has a sharper increase with respect to other methods in low pore volumes of injected fluid due to high mobility of CO2with respect to water.In other words, recovery factor increases sharply by CO2injection till breakthrough occurs.After breakthrough lower increase in recovery factor occurs due to high mobility and escape of CO2.In addition, the breakthrough occurs with more delay in continuous CO2injection and happens earlier for brine injections due to higher power of water to find channels through oil.

By comparing the RF versus PV curves for 40000 ppm brine and 2000 ppm brine injection it could be inferred that higher salinity results in lower recovery factor for carbonate cores.One main reason for decrease in recovery factor is ion exchange capacity and changing wettability of core.The other main reason of decrease in recovery factor by increasing salinity to 40000 ppm is that salt dissolved in water comes out due to pressure drop in throats and pores.When pressure drops suddenly after breakthrough,salt will reside on pore and throat bodies.Although it was mentioned earlier that lower permeability will result in higher pressure distribution in pores and overcoming threshold pressures but the sudden pressure drop may not be compensated by increase in pressure distribution.

Fig.8.a) Oil recovery factor and b) differential pressure; for all injection methods.

Final recovery factor of high salinity CW injection nearly equals with WACO2representing that when the salinity of carbonated brine increases, pattern of injection in carbonated mode is not as effective as low salinity carbonated water injection.The problems occurred during brine injection and CO2injection are not involved in hybrid smart carbonated brine injection.One important point in smart carbonated flooding is to keep the pressure constant at the surrounding of core using overburden pressure and at the end of core using back pressure to avoid pressure drop which causes CO2come out of brine solution.Therefore, carbonated water will be injected throughout the core without CO2coming out of solution.Hybrid smart carbonated water activates more mechanisms than wettability as compared with smart and high salinity water injection methods, provides acidic environment resulting in mineral dissolution which increases permeability compared to WACO2method, and activates mechanisms of pH variation, permeability increase, wettability alteration compared to high salinity carbonated water injection method.Due to the reasons mentioned in different brine injections, 2000 ppm brine was selected for preparing carbonated brine injection.Due to lower brine concentration, higher volume of CO2is dissolved in brine, higher swelling factor occurs and higher oil recovery will be obtained.

4.Conclusions

The following conclusions are obtained by comparing hybrid method with most well-known frequently used EOR/reservoir pressure maintenance methods:

(1) Hybrid smart carbonated water injection results in highest recovery factor among different EOR methods for the studied carbonate cores.This shows that both CO2volume and brine content (compared with low salinity water injection, high salinity carbonated water injection and CO2injection methods) and manner of CO2and brine combination(compared with WACO2method and high salinity carbonated water injection) are important to have higher oil recovery.

(2) When salinity of injected brine is increased from 2000 to 40000 ppm, the recovery factor is decreased for carbonate core in both cases of water injection and carbonated water injection.Therefore, optimum concentration of salt is used for preparation of carbonated water called smart carbonated water injection method.This smart brine resulted in 70%recovery for carbonate core which is the highest recovery obtained using different methods.

(3) When CO2is used in injecting fluid(smart carbonated water,continuous CO2and WACO2and high salinity carbonated water injection) the recovery factor is more than the time when CO2is not used in injection.This is due to more distance and more pores CO2will pass before breakthrough.Also, CO2is dissolved in oil resulting in oil swelling that brings trapped oil droplets out of the pores.

(4) Differential pressure curves for different EOR methods show that hybrid method results in lowest values of differential pressure.This shows that less pressure change is needed in hybrid method for oil displacement across the carbonate cores.

(5) Manner of injecting CO2with brine for EOR results in different recoveries.Hybrid smart carbonated water injection results in ultimate recovery factor of 70% while high salinity carbonated water and WACO2results in recovery factor of 62%.

Declaration of competing interests

The authors declare that they have no conflict of interests.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.petlm.2020.03.006.