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Enhanced Oil Recovery Using Micron-Size Polyacrylamide Elastic Microspheres (MPEMs): Underlying Mechanisms and Displacement Experiments Chuanjin Yao, Guanglun Lei, Jian Hou, Xiaohong Xu, Dan Wang, and Tammo S. Steenhuis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02717 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Industrial & Engineering Chemistry Research

Enhanced Oil Recovery Using Micron-Size Polyacrylamide Elastic Microspheres (MPEMs): Underlying Mechanisms and Displacement Experiments Chuanjin Yao,*,†,‡,§,|| Guanglun Lei,*,† Jian Hou,*,† Xiaohong Xu,† Dan Wang,† Tammo S. Steenhuis,*,§ †

College of Petroleum Engineering and ‡College of Pipeline and Civil Engineering, China

University of Petroleum, Qingdao, Shandong 266580, China §

Department of Biological and Environmental Engineering and ||KAUST-Cornell Center for

Energy and Sustainability, Cornell University, Ithaca, New York 14853, United States ABSTRACT: Micron-size polyacrylamide elastic microsphere (MPEM) is a newly developed profile control and oil displacement agent for enhanced oil recovery in heterogeneous reservoirs. In this study, laboratory experiments were performed to characterize the viscoelastic properties of MPEMs in brine water. A transparent sandpack micromodel was used to observe the microscopic flow and displacement mechanisms, and parallel-sandpack models were used to investigate the profile control and oil displacement performance using MPEMs in heterogeneous reservoirs. The results indicate that MPEMs almost do not increase the viscosity of injection water and can be conveniently injected using the original water injection pipelines. The microscopic profile

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control and oil displacement mechanisms of MPEMs in porous media mainly behave as selective-plugging in large pores, fluid diversion after MPEMs plugging, oil drainage caused by MPEMs breakthrough, and the mechanism of oil droplets converging into oil flow. MPEMs have a high plugging strength, which can tolerate a long-term water flushing. MPEMs can selectively enter and plug the large pores and pore-throats in high permeability sandpack, but almost do not damage the low permeability sandpack. MPEMs can effectively divert the water flow from the high permeability sandpack to the low permeability sandpack and improve the sweep efficiency of low permeability sandpack and low permeability area in the high permeability sandpack. The results also confirm the dynamic process of profile control and oil displacement using MPEMs in heterogeneous reservoirs.

1. INTRODUCTION With the rapid development of global economy, the demand for crude oil has grown constantly. However, the difficulty of oil exploration and development is ever increasing. Therefore, enhancing the oil recovery of developed oilfields becomes a serious challenge all over the world.1 In most oil reservoirs, there are many layers with different permeability. Even in the same layer, it can be heterogeneous or many preferential flow channels may exist.2,3 After the oil development reaches into the high water-cut stage, the long-term water erosion on the highpermeability layers leads to an increased heterogeneity of the oil reservoirs. In this case, the injected water will prefer to flow along the high permeability layers and the preferential flow channels, leading to prominent contradictions between layers which make it difficult to achieve a good profile control and oil displacement effect using conventional chemical flooding. The low permeability areas are still unswept and more than 70% of remaining oil is left in these unswept


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areas.4,5 Thus, improving the sweep efficiency of unswept areas is important to enhanced oil recovery in heterogeneous reservoirs. Many chemical agents have been applied to control profile and enhance oil recovery of heterogeneous reservoirs. There are many cases reported that bulk gel, weak gel, colloidal dispersion gel (CDG), and pre-crosslinked particle gel (PPG) can control profile and enhance oil recovery of heterogeneous reservoirs effectively and are widely used in oilfield development.5-15 In the early 21st century, a smart profile control and oil displacement agent with elasticity called “micron-size polyacrylamide microspheres (MPEMs)” was developed to control profile in oil reservoirs with high temperature and high salinity.16 The MPEMs are size- and strengthcontrolled, environmentally friendly, and not sensitive to reservoir minerals and formation water salinity. They can deform depending on their elasticity and pass through the pore-throats under a large pressure difference to achieve a dynamic process of profile control and oil displacement. MPEMs treatment has become a cost-effective method to modify flow heterogeneity and enhance oil recovery from heterogeneous reservoirs, as proven by field applications in Daqing, Shengli, Changqing, Jidong, Liaohe, Zhongyuan, Henan, Jiangsu, Xinjiang, and Bohai Oilfield in China.17-28 The design principle of MPEMs treatment is that according to the micron-scale characteristic of pores and pore-throats in reservoirs, the MPEMs whose particle size matches with the pores and pore-throats size are controllably prepared on ground. Then they are suspended in production water and pumped into the target layers through water injection wells. The MPEMs will constantly transport and plug in the high permeability layers and the preferential flow channels depending on their migration, plugging, elastic deformation, deformation recovery, remigration and re-plugging mechanisms in porous media, i.e., a moveable profile control and oil displacement process. Finally, they move into the deep area of oil formation, effectively plug the


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large pores and pore-throats there, and dramatically improve the sweep efficiency and enhance oil recovery of heterogeneous reservoirs.29,30 The flow and displacement of MPEMs in porous media of oil reservoirs is a complex issue. It not only relates to the transport, retention, plugging and deformation of MPEMs in porous media, but also refers to the interaction of MPEMs with the crude oil and reservoir rock. Although traditional laboratory column experiments can provide valuable insights into the profile control and oil displacement properties of MPEMs, they do not clearly distinguish the essential mechanisms of enhanced oil recovery using MPEMs in heterogeneous reservoirs.30,31 Etchedglass micromodels can be used to observe the flow and displacement phenomena and mechanisms of water flooding, foam flooding, ASP (alkali/surfactant/polymer) flooding, microemulsion flooding and microbial flooding.32-35 However, these models do not apply to MPEMs because of the difficulty in cleaning after MPEMs injection. In this work, laboratory experiments were performed to characterize the viscoelastic properties of MPEMs in brine water. A transparent micromodel packed with the translucent quartz sand was used to observe the microscopic flow and displacement mechanisms of MPEMs in porous media. The microscopic images of MPEMs flow and displacement were obtained using a bright field microscope. In addition, parallel-sandpack models were used to investigate the flow and displacement properties of MPEMs in heterogeneous reservoirs. Based on these images and the fractional flow and enhanced oil recovery results, the profile control and oil displacement performance of the MPEMs was analyzed in detailed. Thus, the aim of this research was to elucidate the mechanisms of sweep improvement and enhanced oil recovery using MPEMs in heterogeneous reservoirs. 2. METHODOLOGY AND MATERIALS


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2.1. Materials. The samples of micron-size polyacrylamide elastic microspheres (MPEMs) used in this study were prepared through the method and procedure reported by Yao et al.39 The MPEMs have a density of 1.01 g/cm3, and the pH value in deionized (DI) water is 7.0. The initial particle size of MPEMs is 16.5−63.6 µm, and the average value (dave) is 27.4 µm. The average particle size of MPEMs after swelling in 5000 mg/L NaCl solution at 60 °C for 10 days is 49.3 µm. Figure 1 shows the microscopic image of MPEMs after swelling in 5000 mg/L NaCl solution at 60 °C for 10 days. It can be seen that MPEMs are regular spherical particles with a narrow particle size distribution and can be well dispersed in brine water. MPEMs are nonflammable, nonexplosive, noncorrosive, and nontoxic to the environment.

Figure 1. Microscopic image of MPEMs after swelling in 5000 mg/L NaCl solution at 60 °C for 10 days. Translucent quartz sand provided by AGSCO (Hasbrouck Heights, NJ) was selected as the porous media. The sand is hydrophilic, the density is 2.65 g/cm3 and the pH value in DI water is 7.0. The major compositions of the sand are SiO2, Al2O3, Fe2O3 and K2O, where SiO2 contributes greater than 98% of the total mass. Prior to use, the sand was acid-washed and oven-dried to


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remove surface impurities and to minimize chemically attractive microsites on the sand.37 Then the sand was sieved to six size ranges of 74–106, 106–125, 125–177, 177–250, 250–420 and 420–500 µm. An oil sample with a viscosity of 16.0 mPa·s and a density of 942.7 kg/m3 at 60 °C was used for all the displacement experiments. The oil was prepared using degassed crude oil from a Shengli Oilfield reservoir and mixing it with a certain proportion of kerosene. The NaCl solution with a salinity of 5000 mg/L was used as the injected brine water. 2.2. Sandpack Models. 2.2.1. Transparent Sandpack Micromodel. In this study, a transparent micromodel with an interior chamber of 8.0 cm in length, 1.5 cm in width, and 0.3 cm in depth was used to investigate the microscopic flow and displacement process of MPEMs for enhanced oil recovery in reservoirs. The transparent micromodel was constructed from clear acrylic sheets and consisted of a base plate, a sealing gasket (yellow area) and a top plate, as shown in Figure 2. Translucent quartz sand was filled into the interior chamber through the inlet or outlet. While filling the quartz sand, the model was lightly rapped to ensure the sand filling dense and uniform. This model is a three-dimensional model, and can better reflect the distribution of pores and pore-throats in reservoirs. Besides, this model is convenient to clean and can be packed with sand repeatedly.

Figure 2. Transparent sandpack micromodel (unit: mm).


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2.2.2. Parallel-Sandpack Models. In this study, parallel-sandpack models were also used as the heterogeneous porous media for conducting flow and displacement experiments. Translucent quartz sand of different particle sizes was packed into acrylic glass holders to make sandpacks. The sandpacks were 17.50 cm in length and 1.95 cm in diameter. The desired permeability of a sandpack was obtained by packing translucent quartz sand with suitable particle size (between 74 and 500 µm). 2.3. Viscoelastic Measurements. The apparent viscosity of MPEMs dispersed in brine water with a salinity of 5000 mg/L NaCl was measured using LVDV-II rotational viscometer (Brookfield Corporation, USA), at a shear rate of 7.32 s-1 and 60 °C. M5600 rheometer (Grace) was used to measure the dynamic modulus, i.e., the storage modulus (G’) and loss modulus (G”). The stress sweep was first carried out with a temperature range of 20–180 °C, and at a constant frequency of 1.0 Hz. Then the frequency sweep was carried out with a frequency range of 0.1–10.0 Hz, and at a fixed temperature of 60 °C. 2.4. Flow and Displacement Experiments 2.4.1. Micromodel Studies. Figure 3 shows a schematic of the micromodel flow test. The experimental setup consisted of a micro pump (CoMetro6000LDS, USA), three piston containers, a plastic capillary, a stainless steel ruler, a transparent sandpack micromodel, a jacket heater, a circulating water bath, a sample collector, a vacuum pump, a bubble tower, a computer, a bright field microscope (KH-7700 Hirox-USA, River Edge, NJ), and imaging software in the computer. The micro pump was used to inject fluids at a desired flow rate. The jacket heater and circulating water bath were used to fix the micromodel at a desired temperature. All micromodel tests were conducted at 60 °C.


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Figure 3. Schematic of micromodel tests. Fresh water-wet translucent quartz sand was first packed into the interior chamber of the micromodel to a porosity of 0.38 cm3/cm3 and a permeability of 10.2 µm2. Before the tests, the micromodel was vacuumized and saturated with 5000 mg/L NaCl brine water, and then oil was injected into the micromodel to set the initial oil saturation. After the micromodel was saturated with the oil and heated in an oven at 60 °C, for 2 h, the fluids were added in three phases. In phase 1, the brine water with a salinity of 5000 mg/L NaCl was pumped into the chamber with a flow rate of 0.10 mL/min until the water cut in the effluent was up to 98%. For phase 2, 1.5 PV (pore volume) of 2000 mg/L MPEMs in 5000 mg/L NaCl solution was supplied to the chamber with the same flow rate of 0.10 mL/min. In phase 3, the brine water with a salinity of 5000 mg/L NaCl was injected again with a flow rate of 0.10 mL/min until the water cut in the effluent was up to 98%. The microscopic images of MPEMs flow and displacement, as well as the global and local images of remaining oil distribution in the micromodel were collected with the bright field microscope throughout the displacement process.38 The injection pressure was measured with a small piezometer filled with water at the micromodel inlet. 2.4.2. Parallel-Sandpack Tests. Figure 4 gives a schematic of the parallel-sandpack tests. The principal components of the experimental apparatus included a constant-flux pump, three piston containers, a thermostat, two sandpacks, two measuring cylinders and a pressure transducer that


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was connected to a computer for continuous recording of the injection pressure. The constantflux pump was used to inject fluids at a desired flow rate. The thermostat was used to fix the sandpacks at a desired temperature. All parallel-sandpack tests were conducted at 60 °C.

Figure 4. Schematic of parallel-sandpack tests. Fresh water-wet translucent quartz sand was used for each parallel-sandpack test. Table 1 gives some key parameters of the parallel-sandpack models used in this study. In Table 1, the porosity was measured using the weight method and the permeability was determined using Darcy’s Law of single water flow. One parallel-sandpack flow test (1#) was conducted for analyzing the profile control of MPEMs in heterogeneous reservoirs. Firstly, 0.5 PV of brine water with a salinity of 5000 mg/L NaCl was pumped into the parallel sandpacks, followed by 0.5 PV of 2000 mg/L MPEMs in 5000 mg/L NaCl solution, using the constant-flux pump, at a flow rate of 1.0 mL/min. Then the brine water with the same salinity of 5000 mg/L NaCl was injected again at a flow rate of 1.0 mL/min to investigate the resistance of MPEMs to long-term water flushing. The injection pressures and effluent water volumes of the two sandpacks were measured. In the parallel-sandpack displacement test (2#), the sandpacks were first saturated with 5000 mg/L NaCl solution, and then displaced with oil until the water cut in the effluent was less than 1%. After the oil saturated, the brine water with a salinity of 5000 mg/L NaCl was first injected until the water cut in the effluent was up to 98%. Then 0.5 PV of 2000 mg/L MPEMs in 5000 mg/L


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NaCl solution was injected into the sandpacks, followed by extended displacement of brine water with a salinity of 5000 mg/L NaCl until the water cut in the effluent was up to 98%. The effluent fluids were collected and the volumes of oil and water in the samples were recorded, respectively. The injection rate in all the phases was fixed at 1.0 mL/min. Table 1. Kew parameters of parallel-sandpack models. Parallel-sandpack model Test 1# Test 2#

Permeability/µm2 High 8.5 Low 1.7 High 8.8 Low 1.9

Permeability ratio 5.0 4.6

Porosity/% 37.56 38.47 37.15 38.32

Initial oil saturation – – 0.76 0.63

3. ANALYSIS OF THE EXPERIMENTAL RESULTS

3.1. Viscoelastic Properties of MPEMs. Figure 5 plots the apparent viscosity of MPEMs in brine water with a salinity of 5000 mg/L NaCl at 60 °C. For the MPEMs suspension, the apparent viscosity increases smoothly with increasing concentration of MPEMs. High concentration causes the volume fraction of MPEMs in water to increase and the entanglements of branched chains also increase, and thus the apparent viscosity increases. In addition, the viscosity of 500−5000 mg/L MPEMs suspension is less than 1.4 mPa·s, which is much lower than that of conventional particles for profile control and oil displacement. MPEMs almost do not increase the viscosity of injection water, and as a result, can be conveniently injected using the original water injection pipelines. The viscoelasticity of MPEMs was also measured in this study. Due to the stress-strain relaxation behavior of MPEMs, the variation of the measured strain is always lagging behind that of the applied stress. The ratio of the stress to strain is called “dynamic modulus”. The dynamic


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modulus includes two parts, i.e., the storage modulus (G’) and loss modulus (G”). The storage modulus represents the energy stored during the deformation process of MPEMs and can be recovered after the stress is eliminated. The storage modulus comes from the transient change of network chains conformation and thus is used to characterize “elasticity”. The loss modulus represents the energy which has been used for the deformation of MPEMs or internal friction energy. The loss modulus mainly results from the change of network chains conformation occurred by absorbing external energy and thus it can be used to characterize “viscosity”.39 Figure 6 shows the variation of dynamic modulus (G’, G”) of MPEMs versus frequency at 60 °C. With a frequency range of 0.1–8.0 Hz, both of the storage modulus and loss modulus increase linearly with the frequency increase. The storage modulus and loss modulus are 4.1 Pa and 21.5 Pa, respectively, indicating MPEMs have good dynamic viscoelasticity. Additionally, it can be clearly observed from the variation of dynamic modulus (G’, G”) of MPEMs versus temperature at 1.0 Hz (Figure 7) that the storage modulus and loss modulus of MPEMs all decrease with the increase of temperature. The reduction rate of the loss modulus is obviously greater than that of the storage modulus with the increasing temperature, indicating that the viscosity of MPEMs is more sensitive to temperature. Eventually, the storage modulus and loss modulus all stay at a high level when the temperature is up to 180 °C, indicating the good viscoelasticity of MPEMs at high temperature. The viscoelastic properties of MPEMs reflect the elastic nature of the MPEMs, which can deform to change their shape because of the drag fore caused by water flow. The deformed MPEMs can recover to their original shape and size after leaving the small pore-throats to large pores. It is expected that the dynamic profile control and oil displacement process of MPEMs can effectively improve the sweep efficiency and oil recovery in heterogeneous reservoirs.


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Figure 5. Apparent viscosity of MPEMs in brine water.

Figure 6. Variation of dynamic modulus (G’, G”) of MPEMs versus frequency (60 °C).

Figure 7. Variation of dynamic modulus (G’, G”) of MPEMs versus temperature (1.0 Hz).


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3.2. Transparent Micromodel Test. 3.2.1. Displacement Characteristics of MPEMs. Figure 8 gives the injection pressure (water column height) change curve throughout the displacement process. The global and local images of remaining oil distribution are shown in Figure 9 and Figure 10. In Figure 10, images Ia-Ie give the local images of remaining oil distribution after initial waterflood, and images IIa-IIe present the local images of remaining oil distribution after extended waterflood. During the injection of MPEMs in phase 2, the pressure at the inlet increased gradually and presented a characteristic of fluctuant variation, as shown in Figure 8, indicating that the conductivity of the micromodel decreased due to the blocking of pore-throats and the MPEMs constantly migrate and plug in the porous media. After changing to MPEMs-free background brine water in phase 3, the pressure at the inlet decreased gradually, but stayed at an elevated niveau, as shown in Figure 8, indicating that some pore-throats remained blocked, and MPEMs have good resistance to long-term water flushing. The injected MPEMs prefer to enter the preferential flow channels caused by initial waterflood, and then effectively plug the large pore-throats, producing an additional flow resistance in the preferential flow channels. The additional flow resistance forces the followed water to change its flow direction and to displace the remaining oil in the unswept low permeability areas of the micromodel. With the continuous injection of MPEMs, the remaining oil in the unswept low permeability areas of the micromodel was constantly displaced by water, as shown in Figure 9(a)-(c). In general, a greater number of MPEMs injected into the micromodel causes more swept volume and more additional resistance to water flow, and thus more remaining oil in the unswept low permeability areas of the micromodel will be displaced. In phase 3, due to the effective plugging and fluid diversion effect of many MPEMs, the remaining oil in small pore-throats of


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the micromodel was also displaced by the MPEMs-free background brine water, as shown in Figure 9(d).

Figure 8. Injection pressure (water column height) change curve throughout the displacement process.

Figure 9. Global images of remaining oil distribution throughout the displacement process.


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Figure 10. Local microscopic images of remaining oil distribution before and after MPEMs injection. Actually, the profile control and oil displacement of MPEMs is a dynamic process. At the beginning of phase 2, because of the low injection pressure, MPEMs first plug large pore-throats near the inlet and cause the followed water to flow into the unswept low permeability areas near


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the inlet, and to displace the remaining oil there, as shown in Figure 10(a). However, the plugging of MPEMs near the inlet is temporary. When the pressure difference is more than the critical plugging strength of MPEMs, they will remigrate into the deep micromodel depending on their elastic deformation and re-plug large pore-throats there, further forcing the followed water to flow into the surrounding unswept low permeability areas and to displace the remaining oil, as shown in Figure 10(b)-(d). The fluctuant variation of the pressure at the inlet also indicates the dynamic process of profile control and oil displacement using MPEMs. These results indicate that MPEMs have good capacity of in-depth fluid diversion and can effectively improve the sweep efficiency and enhance oil recovery. In the next sections, we will consider the mechanisms of transport, plugging, fluid diversion and oil displacement of MPEMs in porous media of the micromodel. 3.2.2. Selective-Plugging of MPEMs in Large Pores. The migration of MPEMs is always along the directions with low flow resistance, which are usually the preferential flow channels caused by long-term waterflood. At a certain injection pressure, the injected MPEMs first transport along these directions, but almost do not enter the unswept oil-bearing areas. Additionally, the followed water cannot enter the pore-throats blocked by the MPEMs. In other words, MPEMs have a selective-plugging effect of blocking water without blocking oil. Figure 11 shows the microscopic images of MPEMs selective-plugging in large pores. It can be clearly observed that during the injection of MPEMs in phase 2, the migration of MPEMs in large pores is very quickly (Figure 11(a)-(b)). When the MPEMs enter the pore-throats between large pores, they plug these pore-throats through three ways of capture-, superposition-, and bridge-plugging, as shown in Figure 12(a)-(d). These plugging processes will cause the injection pressure to increase and thus restart the remaining oil in the low permeability areas. With the continuous injection of MPEMs, some MPEMs will continue to migrate in large pores under the


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carrying of water flow. When entering the pore-throats between these large pores, these MPEMs will be retained again and continue to change the flow directions of the followed water, as shown in Figure 11(c)-(d). Thus the sweep efficiency and oil recovery will be improved further.

Figure 11. Microscopic images of MPEMs selective-plugging in large pores.

Figure 12. Microscopic images of the three ways of MPEMs plugging in pore-throats. The selective-plugging strength of MPEMs in large pores is clearly affected by the pore-throat size, MPEMs particle size, MPEMs elasticity, MPEMs quantity for superposition, MPEMs


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quantity in bridges, and the compaction degree of MPEMs in bridges. The selective-plugging mechanism of MPEMs in large pores is important for MPEMs to fully play their capacity of fluid diversion in heterogeneous reservoirs. 3.2.3. Fluid Diversion of MPEMs Plugging. During the displacement process in phase 2 and 3, the real microcosmic displacement mechanism is mainly oil-water displacement mechanism. The chance for MPEMs to directly contact the remaining oil in unswept areas is negligible. Under the long-term erosion of waterflood, most of the oil in large pores and pore-throats has been swept by water and the remaining oil is mainly distributed in small pores and pore-throats with high flow resistance. MPEMs can almost not enter these small pores and pore-throats. Nevertheless, most of the MPEMs will flow into large pores and pore-throats to plug them effectively and force the injected water flow through the small pores and pore-throats that still contain oil. From this perspective, the main role of MPEMs in the profile control and oil displacement process is fluid diversion, i.e., the water flow is diverted into the unswept low permeability areas to displace the remaining oil. Figure 13 and Figure 14 show the microscopic images of fluid diversion and oil displacement caused by the plugging of MPEMs at two different locations. It can be seen that under the fluid diversion effect of MPEMs, the followed water is continuously diverted into the small pores and pore-throats and displaces the remaining oil. Thus, the sweep efficiency of the micromodel is improved obviously. Additionally, fluid diversion of MPEMs can not only occur near the injection end of the micromodel (Figure 13), but also exist in the deep location of the micromodel (Figure 14), which is different from traditional hard particles of only causing fluid diversion near the injection end. The reason is that with the increase of pressure at the inlet, MPEMs captured by pore-throats can deform depending on their elasticity and pass through the pore-throats, as shown in Figure 15(a)-(c). Afterward the deformed particles recover to their


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original shape and size quickly (Figure 15(d)), and then migrate into the deep micromodel to produce in-depth plugging and fluid diversion. Thus, with the ability to plug pore-throats of different locations, effectively and movably, MPEMs can improve the sweep efficiency in different areas of the micromodel. The in-depth fluid diversion of MPEMs fully reflects the dynamic profile control and oil displacement process.

Figure 13. Microscopic images of fluid diversion and oil displacement caused by the plugging of MPEMs (1.0 cm from the inlet).

Figure 14. Microscopic images of fluid diversion and oil displacement caused by the plugging of MPEMs (4.0 cm from the inlet).


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Figure 15. Microscopic images of MPEMs remigration in pore-throats (3.7 cm from the inlet). 3.2.4. Oil Drainage Caused by MPEMs Breakthrough. When MPEMs plug pore-throats, there is still a possibility for them to pass through the pore-throats, which depends on the elasticity and fluid pressure difference. In Figure 16(a), ten MPEMs are captured by a porethroat. When the pressure difference between both ends of the pore-throat, the MPEMs begins to deform and loose, as shown in Figure 16(b). Then the breakthrough of MPEMs will cause an instantaneous negative pressure between the pore-throat and pores that connect with the porethroat. As a result, the followed fluid flow overcomes the flow resistance of small pores and pore-throats, carries out the remaining oil, and then flows through the pore-throat quickly, as shown in Figure 16(c)-(d). The oil drainage mechanism caused by the breakthrough of MPEMs is important for MPEMs to restart the remaining in small pores and pore-throats and to improve the sweep efficiency of waterflood.

Figure 16. Microscopic images of oil drainage caused by the breakthrough of MPEMs (2.5 cm from the inlet).


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3.2.5. Mechanism of Oil Droplets Converging into Oil Flow. It can also be observed from micromodel test that the remaining oil carried by the diverted fluid flow out from small pores and pore-throats can easily enter large pores and then converge into a large flow of oil. Afterward the oil flow continues to move in large pores and pore-throats toward the outlet of the micromodel. The mechanism of oil droplets converging into oil flow is beneficial to the rapid outflow of the remaining oil in heterogeneous reservoirs. 3.3. Parallel-Sandpack Flow and Displacement Test In order to further validate the above mechanisms, parallel-sandpack models were used to simulate the heterogeneity of reservoirs. In these tests, the effluent fluids of the two sandpacks were collected and the volumes of water and oil in the samples were respectively recorded to investigate the fractional flow properties and enhanced oil recovery in the two sandpacks with different permeability. 3.3.1. Fractional Flow Analysis. The fractional flow and injection pressure curves in parallel sandpacks throughout three phases of the initial waterflood, MPEMs injection and extended waterflood are shown in Figure 17. It can be seen that the fractional flow properties are as follows: (1) During the initial waterflood in phase 1, the fractional flows of the high and low permeability sandpacks are 100% and 0%, respectively, which reflects the water injection profiles of heterogeneous reservoirs by waterflood. (2) In phase 2, the fractional flow of the high permeability sandpack decreases, but the fractional flow of the high permeability sandpack increases with the injection of MPEMs. The injection pressure also increases with the injection of MPEMs. After injecting 0.5 PV MPEMs, the fractional flows of the two sandpacks all tend to a same value of 50%, and the


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injection pressure increases from 2.9 kPa to 12.5 kPa. The results indicate that MPEMs can effectively improve the water injection profiles of the two sandpacks. (3) After changing to 1.0 PV of MPEMs-free background brine water in phase 3, the injection pressure still stay at a high level, indicating some pore-throats remained blocked. In addition, the fractional flows of the two sandpacks remain at about 50%, indicating that the MPEMs have a high strength of in-depth plugging, which can tolerate a long-term water flushing.

Figure 17. Fractional flow and injection pressure curves in parallel sandpacks throughout the injection process. Another important result can be made from the change in permeabilities of the two sandpacks before and after the injection of MPEMs. In Test 1#, the initial permeability ratio of the two sandpacks was 8.5:1.7 µm2/µm2. After a 0.5 PV MPEMs slug was injected, the permeability ratio changed to 2.0:1.5 µm2/µm2. This result indicates that the MPEMs can selectively enter and plug the large pore-throats in high permeability sandpack, but almost do not damage the low permeability sandpack. 3.3.2. Enhanced Oil Recovery Analysis. Figure 18 shows the cumulative oil recovery in parallel sandpacks throughout three phases of the initial waterflood, MPEMs injection and


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extended waterflood. In Figure 18, there are three curves representing the high permeability sandpack, the low permeability sandpack, and the total oil recovery, respectively.

Figure 18. Cumulative oil recovery in parallel sandpacks throughout the displacement process. During the initial waterflood in phase 1, the oil recovery of high permeability sandpack is higher than that of low permeability sandpack. When the water cut in the effluent is up to 98%, the oil recovery of the high permeability sandpack is 48.18%, compared with 27.37% of the low permeability sandpack, as shown in Table 2. This means that there is still a lot of remaining oil in the two sandpacks, and much more in the low permeability sandpack. Actually, the remaining oil cannot be recovered by conventional water flooding. In phase 2, 0.5 PV MPEMs slug was injected to modify the water injection profile and to improve the sweep efficiency and oil recovery of the heterogeneous parallel sandpacks. It can be seen from Figure 18 that during the injection of MPEMs slug, the oil recovery of the two sandpacks rises, and more recovery comes from the low permeability sandpack. For Test 2#, after the extended waterflood, the cumulative oil recovery of the low permeability sandpack was 58.55%, which was only 0.51% lower than that of the high permeability sandpack. Correspondingly, the enhanced oil recovery of the low permeability sandpack was up to 31.18%, which was much higher than 10.88% of the high permeability sandpack. The results indicate that


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the injected MPEMs can effectively divert the water flow from the high permeability sandpack to the low permeability sandpack and improve the sweep status and oil displacement effect of low permeability sandpack and low permeability area in the high permeability sandpack. Table 2. Potential of enhanced oil recovery in heterogeneous parallel sandpacks. ParallelPermeability Permeability MPEMs sandpack /µm2 ratio slug/PV model High 8.8 Test 2# 4.6 0.5 Low 1.9

Initial oil saturation

Recovery of waterflood/%

Final Enhanced oil recovery/% recovery/%

0.76

48.18

59.06

10.88

0.63

27.37

58.55

31.18

As discussed with previous microscopic mechanisms in micromodel and fractional flow curves in parallel-sandpack model, the injected MPEMs selectively flow first into the swept high permeability areas with large pores and pore-throats, but almost do not enter the unswept low permeability areas. The MPEMs effectively plug large pore-throats of the high permeability areas through three ways of capture-, superposition-, and bridge-plugging, and then produce additional flow resistance to divert water flow into the unswept low permeability areas with small pores and pore-throats near the inlet. Depending on the elasticity, MPEMs captured by pore-throats can pass through the pore-throats and flow into the deep area to realize in-depth plugging and fluid diversion. The breakthrough of MPEMs can cause an instantaneous negative pressure to restart the remaining in small pores and pore-throats. The oil droplets displaced form small pores and pore-throats converge into oil flow and quickly move in large pores and pore-throats toward the outlet. With the ability to plug pore-throats of different locations, effectively and movably, MPEMs can improve the sweep efficiency of heterogeneous systems. The parallel-sandpack flow and displacement results further confirm the dynamic profile control and oil displacement properties of MPEMs, which effectively enhance the oil recovery of heterogeneous reservoirs.


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4. CONCLUSIONS In this work, laboratory experiments were performed to characterize the viscoelastic properties of MPEMs in brine water. Transparent sandpack micromodel tests were conducted to observe the microscopic flow and displacement mechanisms, and parallel-sandpack models were used to investigate the performance of profile control and enhanced oil recovery by injection of MPEMs in heterogeneous reservoirs. The major conclusions indicated from this research are as follows: (1) MPEMs almost do not increase the viscosity of injection water and can be conveniently injected using the original water injection pipelines. Besides, MPEMs are capable of elastic deformation and thus can flow into the deep area of heterogeneous reservoirs to realize indepth plugging and in-depth fluid diversion. (2) The transparent sandpack micromodel tests show that the profile control and oil displacement mechanisms of MPEMs in porous media mainly behave as selectiveplugging in large pores, fluid diversion after MPEMs plugging, oil drainage due to MPEMs breakthrough, and the mechanism of oil droplets converging into oil flow. (3) The parallel-sandpack flow and displacement tests show that MPEMs have a high plugging strength, which can tolerate a long-term water flushing. MPEMs can selectively enter and plug the large pore-throats in high permeability sandpack, but almost do not damage the low permeability sandpack. MPEMs can effectively divert the water flow from the high permeability sandpack to the low permeability sandpack and improve the sweep efficiency of low permeability sandpack and low permeability area in the high permeability sandpack. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (C.Y.); [email protected] (G.L.); [email protected] (J.H.); [email protected] (T.S.S.). Phone: +86 151 6528 3060 (C.Y.); +86 138 5463 7389 (G.L.); +86 139 5467 0741 (J.H.); +1 607 255 2489 (T.S.S.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors greatly appreciate the ﬁnancial support from the Project Funded by China Postdoctoral Science Foundation (Grant No. 2015M570622), the Project Supported by National Natural Science Foundation of China (Grant No. 51574269), the Fundamental Research Funds for the Central Universities (Grant No. 15CX08004A) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1294). REFERENCES (1) Chen, Y. M. Modern Reservoir Management; China University of Petroleum Press: Dongying, Shandong, PRC, 2007. (2) Sun, H. Q.; Wang, T.; Xiao, J. H.; Chen, H. Novel technique of in-depth profile control step by step by polymer microspheres. Petrol. Geol. Recovery Eff. 2006, 13(4), 77-79. (3) Xiong, C. M.; Tang, X. F. Technologies of water shut-off and profile control: an overview. Pet. Explor. Dev. 2007, 34(1), 83-88. (4) Liu, Y. Z.; Bai, B. J.; Wang, Y. F. Applied technologies and prospects of conformance control treatments in China. Oil Gas Sci. Technol. 2010, 65, 1. (5) Sang, Q.; Li, Y. J.; Yu, L.; Li, Z. Q.; Dong, M. Z. Enhanced oil recovery by branchedpreformed particle gel injection in parallel-sandpack models. Fuel 2014, 136, 295-306.


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