Conformance control by a microgel in a multi-layered heterogeneous reservoir dur

Zheyu Liu,Jian Zhang,Xianjie Li,Chunming Xu,Xin Chen,Bo Zhang,Guang Zhao,Han Zhang,Yiqiang Li,*

1 State Key Laboratory of Heavy Oil Processing,China University of Petroleum (Beijing),Beijing 102249,China 2 College of Petroleum Engineering,China University of Petroleum (Beijing),Beijing 102249,China 3 China National Offshore Oil Corporation Research Institute,CNOOC,Beijing 100028,China 4 School of Petroleum Engineering,China University of Petroleum (East China),Qingdao,Shandong 266580,China

Keywords:Carbon dioxide Microgels Enhanced oil recovery Conformance control Porous media

ABSTRACT Injecting CO2 into the underground for oil displacement and shortage is an important technique for carbon capture,utilization and storage (CCUS).One of the main problems during the CO2 injection is the channeling plugging.Finding an effective method for the gas channeling plugging is a critical issue in the CO2 EOR process.In this work,an acid-resistance microgel named dispersed particle gel (DPG) was characterized and its stability was tested in the CO2 environment.The microgel size selection strategies for the homogeneous and heterogeneous reservoirs were respectively investigated using the single core flooding and three parallel core flooding experiments.Moreover,the comparison of microgel alternate CO2(MAC)injection and water alternate CO2(WAC)injection in the dual core flooding experiments were presented for the investigation of the role of microgel on the conformance control in CO2 flooding process.The results have shown that the microgel featured with -NH and C-N groups can keep its morphology after aging 7 days in the CO2 environment.Where,the small microgel with unobstructed migration and large microgel with good plugging efficiency for the high permeability zone were respectively featured with the higher recovery factor in homogeneous and heterogeneous conditions,which indicate they are preferred used for the oil displacement and conformance control.Compared to WAC injection,MAC injection had a higher incremental recovery factor of 12.4%.It suggests the acid-resistance microgel would be a good candidate for the conformance control during CO2 flooding process.

1.Introduction

The carbon dioxide (CO2) emission caused by human activities is more than 25Gt every year,which is a major reason for the global temperature rise [1].Carbon capture,utilization and storage(CCUS) is an effective method to control the CO2concentration in the atmosphere[2,3].The captured CO2can be used in the chemical synthesis industry[4],refrigeration[5]and oil production process [6].Among them,injecting CO2into the underground reservoirs for oil production has presented a large potential.The excellent characteristics of mass transfer between CO2and crude oil make CO2significantly enhance oil recovery via oil swelling,viscosity reduction,interfacial intension(IFT)reduction or miscibility,and solution gas drive [7–9].CO2enhanced oil recovery (EOR)technology has been successfully applied in worldwide oil companies[10].The Weyburn–Midale oilfield in southeastern Saskatchewan had restored 27 million tons of anthropogenic CO2in the ground from 2000 to 2016,which made oil production increase from 8000 bbl.d-1（1 bbl=0.159 m3）in 2000 to approximately 26,000 bbl.d-1by 2014[11].The pilot of CO2flooding in H-59 block of Jilin Oilfield yielded 14.5 × 104tons of crude oil after 16 × 104tons of CO2injection,which achieved an incremental oil recovery factor of 10% original oil in place (OOIP) [12].

CO2is feathered with the common characteristics of other gas.Its viscosity and density are both less than the liquid,which will inevitably cause viscous fingering and gravity override during the CO2flooding [13,14].Al-Wahaibi et al.(2010) [15] conducted CO2displacement experiments in a visible beadpack model and found the gas channeling reduces mass transfer between the oil and CO2,which affect the swept volume.Zhao et al.(2011) [16] also found CO2channeling reduce the sweep efficiency in the immiscible flooding using MRI technique.Ren et al.(2015) [17] believed the gas channeling leads the miscibility instability and nearmiscible flooding is preferred for the CO2-EOR.How to control the gas channeling is a critical issue for the successful CO2-EOR projects application.

Water alternated gas(WAG),CO2foam and hydrogel treatment are commonly used methods to control profile in CO2flooding.The existence of the free gas phase and various gas and water saturation in the reservoir during WAG injection will cause the fluid redistribution and increase the swept volume [18,19].Christensen et al.[20] reviewed the WAG field experience in about 60 projects and concluded a 5 to 10% incremental oil recovery could be achieved with WAG injection.Ghahfarokhi et al.[21] reported compared to the less-heterogeneous zone of SACROC,WAG injection was more favorable to enlarge swept volume in moreheterogeneous zones.Song et al.[22] investigated the effect of reservoir properties and operation parameters on WAG performance using the numerical simulation and found WAG injection has a higher incremental recovery factor of 17.64% as compared to water flooding.Shakiba et al.[23]and Yu et al.[24]respectively tested active carbonated water alternating gas (ACWAG) injection in the cores with permeability of 10 mD and 0.1 mD (1 mD=10-3μm2).They found more than 30% original oil in place(OOIP) were recovered for both tight cores and low permeability cores using ACWAG injection.These reports indicate WAG seems to be an effective technique in CO2EOR process.However,the loss of injectivity,wax deposits and corrosion caused by the dissolved CO2in water are common issues in WAG projects,which may restrict the application of WAG injection.Using CO2soluble surfactants to generate CO2foam for conformance control is also proposed and investigated by a series of study.Ren et al.[25] found CO2foam generated by adding two kinds of 2-ethyl-1-hexanol into aqueous phase will increase recovery factor from CO2flooding of 24% to CO2foam flooding of 71% and 92%.The surfactant partitioned more to the CO2phase leads a higher recovery factor.Zhang et al.[26]used two nonionic surfactants mixing to increase the solubility of single surfactant in CO2,which can significantly improve CO2foam performance for its blockage.Norris et al.[27] reported the application of CO2foam in the Salt Creek Field can successfully decrease the gas–liquid ratio and expand swept volume during CO2flooding process.Using foam as the conformance control agent in the pilot also faced huge challenges.Unlike the foam generation by co-injection of surfactant and CO2in the laboratory,gas and surfactant are usually separately injected in the pilots in the form of multi-slug to avoid too high injection pressures.The complicated operation process makes the application of CO2foam more difficult.Meanwhile,gas and surfactant are unable to meet in the severely heterogeneous reservoir,which may fail to generate foam in this condition [28].

Hydrogel treatment has overcome the above problems in WAG injection and CO2foam flooding,which make it become a promising,cost-efficient method for conformance control in the heterogeneous reservoirs[29,30].Hydrogel treatment can be classified into the in-situ gelation and preformed particle gel (PPG).In-situ gel treatment refers injecting monomer,crosslink agents and some additives into the reservoir to block preferential flow channels after these components reaction and gelation [31].Unfortunately,the chromatographic separation of the components and shear degradation during its injection make the in-situ gels only work near the wellbore [32].PPG is an already crosslinked gel which is made in the dry powder form.The range of PPG sizes are usually from several microns to several millimeters.Moreover,it can swell up to dozens of times in the brine.The good performance in shear resistance and simple injection operation makes PPG has widely used in the water treatment in the high water cut oilfield[33,34].To expand this idea,microsphere with smaller sizes are proposed to implement conformance control in the low permeability reservoir.

Acid-resistance and the particle size selection are two key issues for conformance control by microgel in the reservoir of multi-layered heterogeneous CO2flooding.Conventional polymer in the gel is easy to degrade under the acidic conditions which created by CO2dissolved in the formation water.But this problem has been solved by the constant efforts.Abbasy et al.(2008)[35]tested the long-term stability and plugging efficiency of polyacrylamide synthetic polymer,chloroprene rubber and nitrile rubber under the H2S and CO2environments and concluded all three materials can bear H2S and CO2.Albidrez et al.[36] reported a crystallized superabsorbent copolymer with the sizes range from 1 to 14 mm featured the ability of CO2-resistance and bacteria-resistance,which was used in the Permian Basin for the treatment of highpermeability channeling during CO2injections.Zhou et al.[37]added dimethyldiallylammonium chloride as the acid resistance monomer and synthesized an acid-resistant preformed particle gel.It exhibited the good plugging performance in the supercritical CO2flooding.Since the plugging efficiency of these granular microgels is determined by the relationship between microgel sizes and pore throats sizes,how to choose the microgel sizes based on pore throat sizes in high-permeability channeling has been attracting lots of attentions.Yao et al.[38] found it can balance the microgel plugging and migration ability when the ratio of particle size to pore throat diameter is 1.35–1.55 using sand pack model.Chen et al.[39]believed microgel had the best plugging efficiency when matching factor ranged from 1 to 1.3 via the resistance factor and plugging rate test.However,Dai et al.[40]optimized the matching factor from 0.21 to 0.29 based on the injection pressure and the plugging rate by considering the in-depth migration and plugging ability of microgel.Yang et al.[41]proposed the plugging efficiency was affected by the elasticity of microgel and our previous work also presented the microgel had four migration and plugging patterns in the porous medium,which were separately corresponded to different matching factors [39,42].These works clarified the microgel size selection method for conformance control to a certain extent,but there are several flowing channelings in the multi-layered heterogeneous reservoir.It is still unclear how to use microgel to simultaneously realize plugging in the large size channeling and displacing in the porous medium in this condition.

The objective of this work is to investigate microgel size selection strategy for conformance control in the multilayer heterogeneous reservoir and test plugging efficiency by an acid-resistance microgel during the CO2flooding process.Single core and three cores in parallel flooding experiments are performed to optimize the ratios of microgel sizes to pore throat sizes via the ultimate oil displacement efficiency.The plugging efficiency of an acidresistance microgel was tested by the comparison of the injection pressure of water alternate supercritical CO2(WAC) injection and microgel alternate supercritical CO2(MAC) injection in the cylindrical core.The proposed size selection strategy is also verified by the oil displacement efficiency of dual core flooding experiment.This work will provide an insight for the microgel size selection for conformance control in CO2utilization for EOR.

2.Experimental

2.1.Materials

The brine used in this work was a simulated formation water with the salinity of 9374 mg.L-1.The brine composition was listed in Table 1.The mineral oil dyed with Sudan red had a viscosity of 45 mPa.s and a density of 0.835 g.cm-3at 30 °C.Two microgels were used in this work are both organic polymer gel.One wassynthesized with various sizes in our previous work [42] and the second named dispersed particle gel (DPG) was provided by Professor Dai’s group at China University of Petroleum (East China)[43].CO2with a purity of 99% were purchased from an agent in Beijing.To highlight differences of each scheme,a hydrolyzed polyacrylamide polymer commercialized used in Dagang oilfield with an average molecular weight of 20 million Da and a degree of hydrolysis of 25% was added into the microgel solution as the mobility control agent in core flooding experiments for microgel sizes selection.The compound system had a viscosity of 32 mPa.s which was measured using a Brookfield DV II viscometer at the shear rate of 6 s-1with the polymer concentration of 1500 mg.L-1.Synthesized resin cores with a size of 4.5 cm×4.5 cm×30 cm and cylindrical outcrop cores with a diameter of 2.5 cm and length of 30 cm were respectively used in the microgel size selection experiments and CO2channeling plugging experiments.Three synthesized cores with the gas permeability of 2800 mD,780 mD and 360 mD were in parallel to mimic a heterogeneous reservoir.Since the high permeability zone and middle permeability zone were the treatment targets for conformance control,the pore sizes distribution of the synthesized cores with the permeability of 2800mD and 780mD were tested by nuclear magnetic resonance (NMR) as shown in Fig.1.The outcrop with a permeability of 15 mD was served for microgel plugging efficiency test and then paralleled with another outcrop with a permeability of 5 mD as the dual core model for microgel conformance control study in the CO2flooding process with the optimized microgel size.

Table 1 Composition of the simulated formation water

2.2.Characteristic of microgel that used for conformance control in CO2 flooding

The synthesis method and characteristic of microgel used in the size optimization experiments were introduced in our previous work[42].Herein,the characteristic of an acid-resistance microgel named DPG which was provide by China University of Petroleum(East China)[43]was investigated with Fourier transform infrared spectroscopy(FTIR),element analysis and scanning electron microscope (SEM).The characteristic functional groups of DPG in the 400–4000 cm-1range was analyzed by a TENSORIIFTIR spectrometer (Bruker,Germany).Vario EL Cube elemental analyzer (Elementar,Germany) and Rapid OXY Cube elemental analyzer(Elementar,Germany) were respectively used for C,N,H and O quantitative detection.To obtain the DPG morphology,the samples were frozen at-80°C for 6 h,tableted and sputtered with gold and then observed with a SEM measurement (Hitachi S-4800,Hitachi,Japan).

Fig.1.Pore diameter distribution of two permeability cores with NMR test.

2.3.Microgel acid-resistance test

The DPG morphology and size distribution before and after aging in the CO2environment were compared to show its acidresistance ability.The CO2was added into a sealed accumulator which contained DPG solution and then increased its internal pressure to the reservoir pressure of 22 MPa with a booster pump.The microgel was taken out for the morphology observation with ESEM and size distribution test with dynamic light scattering(DLS)apparatus (Malvern Instruments Ltd.,UK) after aging 7 days.

2.4.One-dimensional core flooding and parallel core flooding experiments for microgel size optimization

One-dimensional cores flooding experiments were used to investigate the displacement efficiency of compound systems with different sizes microgel in the homogeneous reservoir.The two sizes of microgel with the median diameter of 18.7 μm and 42.8 μm mixed with the polymer were respectively injected into the cores with the permeability of 2800 mD after water flooding,which corresponded the ratio of microgel diameter to pore throat diameter of 0.45 and 1.03.The polymer flooding was conducted after water flooding as a comparison.The detailed experimental procedure is as follow:vacuum the core and saturated with brine for the porosity measurement,then used oil displacing aqueous phase to create the irreducible water saturation.The prepared core samples were flooded with water until water cut reached 90%,then separately conduct three kinds of tertiary oil recovery methods,including 0.8 PV(pore volume,PV,unit is cm3) of two compound systems of polymer microgels and 0.8 PV of polymer solution,followed by post water flooding until the water cut reached 98%.An accumulator with stir was used for the microgel injection.The pressure,oil production,and water production were continuously recorded.

The permeability of three cores were 2800 mD,780 mD and 360 mD in the parallel core flooding experiments.The ratios of two microgel diameters to pore throat diameter of 780 mD core are 0.97 and 2.22.Unlike the scheme in one-dimensional cores flooding experiments,0.3 PV of microgel of two sizes without mixing with polymer were separately injected after water flood to the water cut reached 90%,then followed by 0.4 PV of polymer flooding.The experiments were shut off until the water cut reached 98%during the post water flooding.The concentration of polymer and microgel were both 1500 mg.L-1in both one-dimensional core flooding and parallel core flooding experiments.The schematic diagram of two experiments were shown in Fig.2.

Fig.2.Schematic diagram of single core and parallel core flooding experiments for microgel size optimization.

2.5.Dual core flooding experiments for conformance control by DPG in CO2 flooding

The acid-resistance DPG with a concentration of 1500 mg.L-1used for conformance control was characterized in Section 2.2.The dual cores with permeability of 15 mD and 5 mD were saturated oil by following the procedure in Section 2.4.The back pressure of dual core model was set at 22 MPa and CO2was immiscible with crude oil at this condition.CO2was continuously injecting till CO2breakthrough.Then,three rounds of WAC and MAC were preformed and compared.Each alternate round contains 0.5 PV of aqueous phase (water or microgel solution) and 1 PV of CO2.The oil production and injection pressure of WAC and MAC were recorded in the whole displacing process.The schematic diagram of dual core flooding experiments was shown in Fig.3.

3.Results and Discussion

3.1.Characteristic of microgel (DPG) used in CO2 flooding

FTIR data of microgel was acquired via the 32 scans accumulation and presented in Fig.4(a).The broad peak at about 3322 cm-1indicated the presence of-NH groups in secondary or tertiary carbon amine.The peak around 1477 cm-1revealed the presence of C-N in the microgel.The peak around 1642 cm-1is ascribed to the presence of C=O in bond.The peaks at 3009 cm-1and 2913 cm-1were related to the stretching vibration peak of unsaturated and saturated C-H bond.The peaks at 1609 cm-1and 1212 cm-1were caused by the stretching vibration of C=C bond and C-C bond in DPG sample.Fig.4(b) showed C and O were the most abundant element in the microgel,followed by N and H.These elements were identical to the interpreted elements of peaks in FTIR spectra.

The microscopic morphology of DPG in the original status was observed with SEM and its size distribution was counted via the image identification using the IMAGE J package.The DPGs were tightly packed together and presented regular spherical in Fig.4(c).The range of the DPG sizes were from several hundred nanometers to several micrometers,and its median diameter was around 1 μm.Since DPGs had shaded each other,the statistics of DPG sizes in Fig.4(d) can only be used for reference.

3.2.Morphology and size distribution of DPG in the acid environment

The microgel morphologies obtained from SEM in Fig.5(a) and(b) presented the effect of acid environment on its stability.With the aging in the CO2environment,the uniform and dispersed particles began coalescence because of their large surface energy.Dissolved metallic ion in brine will also neutralize the negative charge of DPG surface,which lead to the coalescence [44].The median diameter of DPG increased from the initial size of 1 μm to 4 μm after 7 days aging was found from DLS measurement in the Fig.5(c).The size increasing was caused by the particle coalescence rather than the degradation.The aggregated DPG would contribute to the plugging in gas channel,which resulted in an effective conformance control.The intensive particle size distribution and regular microscopic morphologies of DPGs demonstrated they were feathered with the good acid-resistance ability.

Fig.3.Schematic diagram of dual core flooding experiments for the comparison of WAC and MAC during CO2 flooding process.

Fig.4.(a) FTIR spectra,(b) element analysis and (c) SEM of an acid-resistance microgel (DPG) (d) Size distribution analysis of DPG via image identification.

Fig.5.Effect of acid environment on DPG morphology and size distribution.(a)DPG morphology before aging in the acid environment;(b)DPG morphology after aging in the acid environment;(c) Size distributions of DPG before and after aging in the acid environment using DLS measurement.

Fig.6.Oil recovery curves and incremental oil recovery factor of polymer with and without microgel.

Fig.7.Injection pressure of polymer with and without microgel during the whole flooding process.

3.3.Oil displacement efficiency of two microgel sizes in the onedimensional core flooding experiments

Oil displacement efficiency of polymer with and without microgel were compared to optimize microgel size selection strategy in the relative homogeneous reservoir.The ratio of microgel size to pore throats sizes of 0.45 and 1.03 were respectively corresponded the unobstructed migration and slight blockage.The oil recovery curves of polymer flooding and polymer with two size microgel combination flooding in Fig.6 presented the system of polymer with small size microgel featured with the best performance in the tertiary oil recovery stage.Oil recovery factors of three experiments during water flooding stage were similar,which indicated the good repeatability of these experiments.With the chemical solution injection,the oil recovery factors rapidly increased and gradually became smoothly.The incremental recovery factor of polymer with small microgel was 30.5%,followed by the polymer with large microgel of 27.2% and polymer of 25.5%.

The injection pressure curves of three systems in Fig.7 may help to explain the reason.All of them firstly increased to the peak and then decreased to the flat because of the reduction of flow resistance of oil and water during the water flooding stage.The difference appeared as the chemical solution injection.For the polymer flooding,the injection pressure sharply increased to the plateau due to the high viscosity of displacing phase and then gently decreased because the oil bank was pushed out.For the solution combined by polymer and microgel injection,their pressures did not reduce after reaching the peak .The pressure of the small microgel nearly kept constant during the injection process,which indicated the small microgel could migrate and remedy the pressure dropping caused by the oil saturation reduction.However,the pressure of the large microgel continuously increased during its injection.And the pressure descent rate of large microgel was much less than those of other two experiments during the post-water flooding.Both meant the sever blockage was occurred.Compare to the small microgel with the size ratio of 0.45,the large microgel with the size ratio of 1.03 may be unable to enter into small pores of the core,which failed to achieve a more incremental recovery factor.

Fig.8.Images of three effluent during tertiary oil recovery process with different displacing solutions.(a)Polymer with large size microgel flooding;(b)Polymer with small size microgel flooding;(c) Polymer flooding.

The effluents in Fig.8 vividly show the oil production processes of three experiments.Compare to the polymer flooding,the compound system of polymer with microgel can rapidly reduce the water cut.The minimum water cut of microgel with size ratio of 0.45 was lower than others and featured a longer duration.It could be concluded the migration of microgel was more important than its blockage in the homogeneous condition.The small microgel would be recommended for the oil displacement.

3.4.Microgel size selection strategy through three parallel core flooding experiments

Three different permeability cores were paralleled for core flooding experiments to investigate microgel size selection strategy in the multi-layered heterogeneous reservoir.From the Section 3.2,it can be known the large microgel will plug the high permeability core and the small microgel can easily migrate in it.The two microgels with the size ratio of 0.97 and 2.22 to the middle core respectively indicated the slight plugging and serious blockage for their transportation.Herein,two microgel size selection strategies which respectively realized the plugging of the high permeability flow channel or the migration in middle permeability flow channel were compared and discussed in this section.

Two parallel cores flooding experiment results shown in Table 2 and Fig.9 indicated the large microgel had a higher ultimate oil recovery,which was favored for the conformance control in the multi-layered heterogeneous reservoir.Table 2 demonstrated the small microgel could effectively enhance oil recovery of high permeability core,while the large microgel benefited the oil recovery in the middle and low permeability cores.The greatest contribution to the incremental oil recovery was the microgel injection stage in the experiment using the large microgel,which was the polymer flooding stage in the experiment using the small microgel.Fig.9 shows the oil recovery of the core flooding experiment using small microgel was firstly higher than that using large microgel.The oil recovery incremental rate of small microgel obviously increased with the microgel injection,which lead a higher recovery factor at the later stage of microgel injection.With the polymer injection,the oil recovery factor of two experiments firstly became similar and then the large microgel featured a higher recovery factor at the later stage of polymer flooding and the whole post-water flooding stage.

The injection pressure of the large microgel in Fig.10 was obviously higher than that of the small microgel during the microgel and polymer injection stage,which was similar with the core flooding experiments in the homogeneous condition.They both decreased in the post water flooding stage and the injection pressure of the small microgel presented a higher descending rate.The fractional flow curves of two microgels in Fig.11 vividly showed the liquid produced proportion of each layer during the whole flood process,which may help to understand the reason for the better performance of large microgel in the three parallel core flooding experiments.Since the permeability ratio of high permeability core to low permeability core was nearly 8,most injected fluid went along the high permeability core,which caused the low swept efficiency in other two cores.The fractional flow of middle and low permeability core increased with the microgel injection.This increment was more pronounced for the large microgel,especially in the low permeability core.The reason was compared to the small microgel,the larger resistance caused by the blockage of large microgel in the high permeability core forced the fluid flowing into the other cores.The inversion point appeared as the polymer flooding initiation.The fractional flow of each layer kept constant in the experiment of small microgel injection.But for the experiment of the large microgel injection,the fractional flow of high permeability core and low permeability core in the polymer flooding stage went against that of microgel injection stage,which meant flow resistance of each core was redistributed.The occurrence of the fractional flow inversion was a little later in the middle permeability core,which was the reason for the largest ultimate incremental recovery factor achieved by the large microgel in the middle permeability core.

Fig.9.Oil recovery curves and incremental oil recovery factor of two sizes microgels followed by polymer flooding.

Fig.10.Injection pressure curves of two sizes microgels followed by polymer flooding.

From the above results,it could be known the blockage of high permeability zone was the first step to realize conformance control in the heterogeneous reservoir.The large microgel was preferred though it may cause the serious plugging in the middle and low permeability zones.In this work,the unexpected fractional flow changing during the polymer flooding process after large microgel injection may be caused by the too high flow resistance of microgel in the middle and low permeability zones.However,it was controllable via manipulating the injection amount of microgel.

3.5.Comparison of conformance control by water alternate CO2(WAC)injection and microgel alternate CO2 (MAC) injection in dual core flooding experiments

The acid-resistance microgel with the median size of 3.5 μm was used for the conformance control in the dual core flooding experiments.The high permeability core of 15 mD featured with the median pore size of 4.2 μm,which indicated it can be blocked by the selected microgel based on the above study.Water alternate CO2(WAC) injection and microgel alternate CO2(MAC) injection were respectively conducted and compared after CO2flooding.Fig.12 presented the oil recovery curves and incremental oil recovery of the each cycle injection.It showed MAC injection had the ultimate oil recovery of 76.7%,which was higher than that of WAC of 64.3%.Since the oil recovery after CO2flooding was similar in two experiments,the difference of the ultimate oil recovery factors between WAC and MAC could reflect the role of microgel in conformance control.The incremental oil production was both from the first round of aqueous and CO2alternated injection for WAC and MAC.The higher recovery factor of MAC was also attributed to the first round cycle injection because the microgel could occupy the gas channel and forced the aqueous phase into the unwept volume.There was not much remaining oil in the areas which aqueous and gas could reach for the followed cycle injection.

The injection pressure of MAC and WAC in Fig.13 reflected the plugging efficiency of two methods.Due to the impact of gas compressibility,the pressure curves increased during the aqueous phase injection stage and decreased once CO2was injected.Compare to the increment of flow resistance during WAC injection caused by the Jamin effect and three phases flowing effect,the flow resistance increased by the microgel blockage was much larger during MAC injection.The increased pressure of MAC injection was 1.5–3 times higher than that of WAC injection,which was beneficial for the swept volume enlargement.The high injection pressure also verified the good performance of acid-resistance microgel for conformance control.

Fig.11.Fractional flow curves of three cores during the whole displacement process (a) small microgel;(b) large microgel.

Fig.12.Oil recovery curves and incremental oil recovery factor of WAC and MAC in each injection stage.

Fig.13.Comparison of injection pressure curves of MAC and WAC with the back pressure of 22 MPa.

4.Conclusions

An acid-resistance microgel used for conformance control in CO2EOR stage was characterized and evaluated via a serious of static tests.The microgel size selection strategy for the efficient blockage in the multi-layered heterogeneous reservoir was investigated using the single core and three parallel core flooding experiments.The following conclusions were drawn:

(1) A microgel named DPG featured with—NH and C—N groups with the average size of 3.5 μm can keep intensive particle size distribution and regular microscopic morphology after aging 7 days in the CO2environment,which indicated it had a good acid-resistance ability.

(2) A small microgel with the ratio of microgel diameter to pore throat diameter of 0.45 presented a higher oil recovery factor in the single core flooding experiment,which indicated the unobstructed migration was important for the application of microgel toward the effective displacement in the homogeneous condition.

(3) The large microgel injection featured an incremental recovery factor of 30.4%,which was 6.6%higher than that of small microgel injection in the three parallel core flooding experiments.It indicated the blockage of high permeability channel was preferential for the microgel selection in the multi-layered heterogeneous reservoir.

(4) The pressure of microgel alternate CO2(MAC) injection was 1.5–3 times higher than that of water alternate CO2(WAC)injection and the incremental oil recovery of MAC was 12.4% higher than that of WAC.The oil production in MAC and WAC were both from the first cycle of aqueous and gas injection.These results indicated the acid-resistance microgel would be a good candidate for the conformance control during CO2flooding process.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(52004305),the Postdoctoral Research Foundation of China (2021M693497) and the Science Foundation of China University of Petroleum,Beijing (2462020XKBH006).We thank Prof.Caili Dai from the China University of Petroleum(East China)for providing the DPG sample