A New Approach of Pressure Profile and Oil Recovery during Dual and Single Coreflooding of Seawater and CO 2

Transcription

1 A New Approach of Pressure Profile and Oil Recovery during Dual and Single Coreflooding of Seawater and CO 2 Injection Process for a Carbonate Reservoir Xianmin Zhou, Fawaz M. AlOtaibi, Dr. Sunil L. Kokal and AlMohannad A. Al-Hashboul ABSTRACT Sweep efficiency during waterflooding and carbon dioxide (CO 2 ) miscible injection can be challenging because of channeling and bypassing of injected fluids. Some of the factors that contribute to this include reservoir heterogeneity, permeability contrast and gravity override. All can lead to reduced volumetric sweep efficiency in both sandstone and carbonate reservoirs. To study the effect of reservoir heterogeneity on pressure profile and oil recovery, and accurately describe the displacement mechanisms during seawater and CO 2 flooding, an effective experimental methodology including the laboratory setup and procedures is proposed in this article. Dual coreflooding experiments were conducted at reservoir conditions using live oil, seawater, supercritical CO 2 ( ) and two composite plugs with different permeabilities. The two composite core plugs a high permeable core plug (HPCP) and a low permeable core plug (LPCP) were placed in parallel in a dual coreflooding apparatus. No cross flow of fluids occurred between the high and low permeable plugs. The sequence of different injection schemes included seawater flooding, initial CO 2 injection, gel slug injection for conformance control and a second CO 2 injection. In addition, two experiments with seawater and continuous CO 2 injection were conducted under the same conditions in a single coreflooding apparatus for comparison with the dual coreflooding experiments. The results indicate that the dual coreflooding technique is an effective method to evaluate the performance of improved oil recovery (IOR) and enhanced oil recovery (EOR) processes, especially involving CO 2 or gas injection. The profiles of differential pressure across both the HPCP and LPCP show a demonstrable distinction. The differential pressure across the LPCP is higher than that of the HPCP for both seawater and injection before breakthrough, and it drops to levels similar to those of the HPCP after breakthrough. A substantial increase in differential pressure across the HPCP (up to 200 psi) was observed during base gel slug injection. This indicates that in situ CO 2 emulsion was generated and was able to block the high permeable zone, displacing fluids into the LPCP and thereby improving sweep efficiency in the LPCP. The oil recovery by seawater and injection and the pressure profile of both the dual coreflooding experiments and the single coreflooding continuous CO 2 process (horizontal and vertical injection) are compared in this article. INTRODUCTION Both injectivity and mobility play important roles in improved oil recovery (IOR) and enhanced oil recovery (EOR) processes, and both have to be taken into consideration in any laboratory or field application. The stabilized oil-water front depends on the injectivity and the mobility ratio of oil and water or the mixture of supercritical carbon dioxide ( ) and oil-water under miscible flooding. The injection and differential pressure profiles strongly affect the oil-water front, or the front formed by and oil-water, and can contribute significantly to the displacement efficiency of oil by water, chemical solution or flooding. Research on the pressure profile of low salinity waterflooding has been conducted 1, 2. In one study, two coreflooding experiments using long carbonate composite cores, cm and cm in length, were carried out at reservoir conditions with seawater diluted 2, 10, 20 and 100 times, changing the flow rate at the end of each slug injection. The results show that as the salinity of seawater decreases, the differential pressure across the carbonate composite core decreases. Yousef et al. (2010) 1 explains that because the oil left in the core after regular seawater flooding is residual oil, the pressure drop reflects capillary forces, and that the constant reduction of pressure drop across carbonate composite cores with the injection of different dilutions of seawater is another brine/ oil/rock alteration. In a second study, three coreflooding experiments using low salinity water and alternating CO 2 were performed at reservoir conditions by Teklu et al. (2014) 2. The test procedure here was very similar to that of Yousef et al. (2010) 1 except for the CO 2 miscible injection added at the end of the low salinity waterflooding. Obtained by diluting seawater 2, 4 and 50 times, three levels of low salinity water were used to run coreflooding experiments at a temperature of 195 F and pore pressure of 2,300 psi. The results show typical differential pressure curves obtained during regular seawater flooding in terms of increasing pore volume (PV)

2 of seawater injection leading to a decrease in the differential pressure. With reduced monovalent ion concentrations in the seawater diluted 2 and 4 times, the differential pressure was decreased, indicating that the oil remaining after regular seawater flooding was mobilized in the core and recovered. No more oil was produced during the waterflooding that used seawater diluted 50 times, and the differential pressure dropped down only slightly in the experiments. Miscible CO 2 injection at the end of the waterflooding with 50 times diluted seawater was conducted at a temperature of 195 F, at a rate of 0.3 cc/min for about 14 PV, and at a pore pressure of 2,500 psi, which is the minimum miscible pressure (MMP). A level of approximately 14.2% incremental oil recovery was obtained. Differential pressure and injectivity are also critical parameters during secondary and tertiary polymer and alkaline-surfactant (A/S) flooding. Researchers have investigated this issue 3-6, reporting on the injection pressure profile vs. injection time in sandpack samples for three alkaline surfactant flooding experiments 3. The resulting injection pressure profile of the waterflooding was a normal differential pressure curve, and oil recovery increased with an increase in the differential pressure during A/S slug injection following waterflooding. Wang and Dong (2010) 3 explain that the mechanism is caused by the ultra-low interfacial tension and oil-water emulsions, and that the entrapment mechanism accounts for the increase in the differential pressure leading to an improvement in oil recovery. Han et al. (2012) 4 reports on the injectivity of sulfonated polymer solutions after measuring the differential pressure across carbonate core samples. The results show that the differential pressure across the core sample is proportionate to the increase in injection rate. Vermolen et al. (2014) 6 is a study of the systematic effect of the viscosity of polymer solutions and injection rate on differential pressure and on the reduction in residual oil saturation for two types of crude oils, a low viscous crude and high viscous crude. Several coreflooding experiments were performed in the vertical Bentheimer outcrop cores: (1) injection of three polymer solutions in series with a high viscous crude; (2) injection of the same three polymer solutions in series with viscosity; (3) injection of the low viscoelastic polymer in a process of increasing the viscosity and/or injection rate with a low viscous crude, and (4) injection of the high viscoelastic polymer in secondary mode with a low viscous crude. The results showed that even though increasing the viscosity of the polymer solutions and their injection rates led to increasing differential pressure, the residual oil saturation was not reduced (experiment 1). No extra oil was recovered and the differential pressure was changed only slightly by increasing the viscosity of the viscoelasticity of the polymer solutions at a constant injection flow rate (experiment 2). No additional oil was recovered despite the increase of the differential pressure caused by increasing the viscosity of the low viscoelastic polymer solutions up to 300 cp, but a little oil was produced with an increased injection flow rate (experiment 3). When high viscoelastic polymer solutions were injected at a constant rate and a constant viscosity to displace a low viscous crude, the residual oil saturation was reduced by an increasing differential pressure (experiment 4). The objectives of this present study are to experimentally investigate and determine the pressure profile across dual core plugs using different permeability core plugs in a dual coreflooding apparatus and understand the mechanism of displacing oil during seawater and miscible injection into dual cores. In addition, a comparison of the differential pressures obtained from the single and dual coreflooding experiments is discussed. EXPERIMENTAL WORKS Preparation of Fluids Brines: Two types of brines were used in this study: field connate water and seawater. The field connate water was used to saturate the core plugs to achieve an initial water saturation (S wi ), and seawater was used for the waterflooding. The components of both brines are listed in Table 1. The total dissolved solids of the field connate water and seawater were 213,734 ppm and 57,670 ppm, respectively. The densities and viscosities of these brines at ambient and reservoir conditions are listed in Table 2. Dead and Live Crude Oils: A dead crude oil from a carbonate reservoir was used in this study to set up the S wi in the core plugs. Separator crude oil and gas were collected from the same reservoir and recombined to create the live crude oil sample, which was then used as an oil phase for the waterflooding and the miscible flooding experiments. The viscosity and density of the dead and live crude oils at reservoir temperature are also listed in Table 2. The molecular weight of the recombined live crude oil in this study was 121. Sc-CO 2 : was used as a displacing agent for tertiary oil recovery at a pressure of 3,200 psi and temperature of 102 C to create the miscible condition of live crude oil in Component Field Connate Water (g/l) Seawater (g/l) NaCl CaCl 2.2HO MgCl 2.6H 2 O Na 2 SO NaHCO Table 1. Recipes of field connate water and seawater

3 Reservoir Condition Ambient Temperature (25 C) (102 C and 3,200 psi) Fluids Density (g/cc) Viscosity (cp) Density (g/cc) Viscosity (cp) Field Connate Water Seawater Dead Crude Oil Live Oil X X Sc-CO 2 X X Table 2. Fluid properties the reservoir. The viscosity and density of the is also listed in Table 2. The MMP between the live oil and was 2,600 psi. Materials Coreflooding Apparatus: Both single and dual coreflooding apparatus with carbonate core plugs were used in this series of experiments, which involved displacing oil via seawater and miscible flooding and evaluating the injectivity and differential pressure profiles at reservoir conditions. The single coreflooding apparatus used in this study has been previously described 7. Two coreflooding experiments were completed with the different orientation of horizontal and vertical flooding using long composite core plugs, up to 25 cm in length 8. A dual coreflooding apparatus was custom designed to perform tests on two stacked or composite core plug samples to determine the impact of reservoir heterogeneities, such as permeability contrast and gravity override, on oil recovery performance. A schematic of the flow chart, Fig. 1, and a detailed description of the dual coreflooding apparatus is given in Zhou et al. (2015) 9. T = 102⁰C MV#31 MV#29 MV#30 P = 3200 psi MV0 MV#19 High Permeability Core Plug MV#25 MV#35 MV#43 AV0 MV#18 MV#20 MV#27 MV#26 P = 4500 psi MV#36 AV3 MV#17 MV#40 MV#42 MV#39 MV#41 MV#47 AV4 AV5 AV6 MV#45 MV#14 MV#15 MV#16 MV#11 MV#12 MV#13 MV#22 MV#24 P = 3200 psi MV#46 Live Oil Foam CO 2 Connate Water Sea Water Dead Oil MV#21 MV#32 Low Permeability Core Plug MV#28 MV#34 MV#37 MV#23 MV#38 MV#33 MV#44 N 2 MV#48 MV#8 MV#9 MV#10 MV#5 MV#6 MV#7 MV#4 MV#3 Pump 2 (Confining Pump) MV#2 MV#1 Pump 1 Pump 3 MV#49 Fig. 1. Flow chart for dual coreflooding experiment setup at reservoir conditions.

4 Properties of the Core Plugs: The core plugs were selected from a carbonate reservoir and scanned to ensure consistency, i.e., no fractures or permeability barriers. Nuclear magnetic resonance (NMR) analysis was also conducted to ensure that all core plugs were of a similar rock type. Routine core analysis was first conducted to measure the dimensions, air permeability, porosity and helium PV of the core plugs. Based on the routine core analysis, the NMR analysis and the computed tomography scan results, two composite cores each composite core consisted of five core plugs were chosen for the single coreflooding experiments. For the dual coreflooding experiments, two core plugs were selected for each of the high permeable core plug (HPCP) and low permeable core plug (LPCP) composites. The core plugs were then saturated with field connate brine, and the PV was calculated by the material balance method. Table 3 lists the routine data of the core plugs used in this study. S wi and Original Oil Saturation (S oi ): The individual dry core plugs were vacuumed for 24 hours and then saturated with field connate water. Brine volume and porosity were determined from the change in weight. The saturated core plugs were left immersed in field connate water for about 10 days, to establish ionic equilibrium between the rock constituents and the field connate water. The original connate water was then displaced with about 10 PVs of fresh connate water during the course of measuring the individual core plug brine permeability. For the single coreflooding, core plugs for composite-1 and composite-4 (single) were desaturated using a centrifuge method to set up the S wi. For the dual coreflooding, core plugs #35 and #36 were stacked together to form the HPCP composite-1 (dual), and core plugs #285 and #286 were stacked together to form the LPCP composite-2 (dual). The composite core plugs were then assembled into a stack using Teflon tape, aluminum foil and a Teflon shrink tube. The aluminum foil functioned as a diffusion barrier between the core plug and the overburden sleeve. The field connate water of each composite core plug was then displaced by dead crude oil at a variable injection flow rate of 0.1, 0.2, 0.4, 0.8, 1.0 and 2.0 cc/min at ambient conditions. At each flow rate during this dead crude oil flooding, the amount of connate water produced and the differential pressure across the composite were recorded, continuing until no more water was produced. During oil flooding, the direction of oil flow was reversed to alleviate possible end effects. At this stage, the S wi and S oi were calculated by material balance, and the effective oil permeability (K eo ) was calculated at S wi, Table 4. Type of Apparatus Single Coreflooding Dual Coreflooding Composite ID Sample ID Length (cm) Diameter (cm) PV (cc) Porosity (%) Air Permeability (md) Composite-1 (single) Horizontal Total Composite-4 (single) Vertical Total Composite-1 (dual) Total Composite-2 (dual) Total Table 3. Routine data of core plugs for single and dual coreflooding experiments

5 Type of Apparatus Single Coreflooding Dual Coreflooding Composite ID Composite-1 (single) Composite-4 (single) Composite-1 (dual) Composite-2 (dual) Orientation of Flooding Length (cm) PV (cc) S wi (%PV) S oi (%PV) K eo at S wi (md) Horizontal Vertical Horizontal Horizontal Table 4. Routine and dynamical data of core plugs for dual and single coreflooding experiments Aged Composite Core Plugs with Live Oil at Reservoir Conditions: After S wi and S oi were determined, live oil flooding was conducted for all composite core plugs at a reservoir condition having a pore pressure of 3,200 psi, a confining pressure of 4,500 psi and a temperature of 102 C. For three weeks, one PV of live oil was injected into each composite core plug per day at a flow rate of 1.0 cc/min to check the stabilization and effective oil permeability of the core plugs. After the core plugs were aged with the dead and live oil used in this study, these carbonate plugs were expected to be weakly oil-wet or mixed-wet Experimental Procedure of Seawater and Sc-CO 2 Flooding Secondary Oil Recovery by Waterflooding Using Single and Dual Coreflooding Apparatus: After the four composite cores were aged with live oil at reservoir conditions, the composite cores with initial water and original oil were flooded by seawater, using both the single and dual coreflooding apparatus. Different injection rates were used during the seawater flooding process. For single coreflooding, the flow rate was first set at 1.0 cc/min, followed by 2.0 cc/min and ultimately 4.0 cc/min. For the dual coreflooding experiment, the flow rate was 0.5 cc/min at the beginning, followed by 1.00 cc/ min and then 2.00 cc/min before the end of the experiment. Seawater flooding was generally stopped when the water cut reached ~99%. For the single coreflooding, the produced oil was collected in a separator placed inside the oven that generated the reservoir conditions. During dual coreflooding, the seawater was injected simultaneously into both composite core plugs, HPCP and LPCP. The water and oil production was measured at ambient conditions using centrifugal tubes. Two back pressure regulators were used to hold the pore pressure of composite-1 and composite-2 (dual). The upstream and downstream pressure, the differential pressures across the composite core and the injection rate were also recorded automatically during the single and dual coreflooding experiments. Tertiary Oil Recovery by Miscible Flooding Using Single and Dual Coreflooding Apparatus: Following the initial seawater flood, continuous was injected into both single composite cores and dual composite cores to displace the remaining oil. In the single coreflooding experiments, continuous was injected at a constant flow rate of 0.5 cc/min until no more oil is produced. In the dual coreflooding experiments, was injected simultaneously into both dual composite core plugs, HPCP and LPCP, at a rate of 0.2 cc/min. The recovered oil was collected separately from the two composite core plugs until no more oil is produced, and the differential pressures were recorded across both. Diverting System and Second Injection: After completing the first injection for the dual composite, a diverting system was injected into the HPCP for conformance control. The LPCP was isolated during this diverting system injection. Following that, a 0.4 PV diverting system was injected into the HPCP, and a second or post injection was conducted, following the same procedure as the first injection. Both HPCP and LPCP were open for the second injection. A more detailed description is given in Zhou et al. (2015) 9 and AlOtaibi et al. (2015) 13. RESULTS AND DISCUSSION To study the differential pressure across the core, plus the performance of oil recovery by seawater and continuous, three coreflooding experiments were conducted using both single and dual coreflooding apparatus. Two experiments on the composite core plugs identified as composite-1 (single) and composite-4 (single) used a single coreflooding apparatus, described in AlOtaibi et al. (2015) 8. One experiment on composite core plugs identified as composite-1 (dual) and composite-2 (dual) used a dual coreflooding apparatus. All experiments were carried out using live oil, seawater and continuous at reservoir conditions with a pore pressure of 3,200 psi, temperature of 102 C and confining pressure of 4,500 psi. Typical results from each of the three sets of displacement experiments are presented and discussed separately.

6 Pressure Profile and Oil Recovery during Seawater Flooding Two single coreflooding experiments (horizontal and vertical displacement), using long cores up to 25 cm in length, and one dual coreflooding experiment, using short cores up to 10 cm in length, were conducted at reservoir conditions. The purpose of the seawater flooding tests using both the single and dual coreflooding apparatus was to determine the oil recovery factor and evaluate the profile of differential pressure across core plugs and remaining oil saturation (S or ) before the injection. Pressure Profile of Seawater Flooding: Table 4 also provides the initial water and oil saturations for all composite core plugs at the beginning of seawater injection. Figures 2 and 3 show the injection flow rates for the seawater flooding during the single and dual coreflooding tests, respectively. For the single coreflooding experiments, the differential pressure increases with increasing injection flow rate, as expected. Differential pressure increases with an increase in the PV of seawater injected before breakthrough and decreases with the increase in the PV of seawater injected after breakthrough until reaching S or or maximum water saturation. In a comparison of the two curves, composite-1 (single, horizontal) with lower permeability shows higher differential pressure than composite-4 (single, vertical) with higher permeability for all flow rates. For results of the dual coreflooding experiment, Fig. 3 shows the differential pressure curves across the high and low permeable core plugs vs. the total PV of seawater injection for both cores. It reveals a substantially different differential pressure curve for the LPCP compared with that observed in the single coreflooding experiment. For the LPCP, the differential pressure across the core reaches a maximum value of 11 psi and then drops down to a value of about 0.2 psi, or the same value as the HPCP, at an injection rate of 0.5 cc/min. For the HPCP, the differential pressure curve is similar to that in the single Fig. 2. Profile of differential pressure vs. PV of seawater injected for single coreflooding (horizontal and vertical). Fig. 3. Profile of differential pressure vs. the sum of PV injected during simultaneous seawater injection for LPCP and HPCP composites using dual coreflooding apparatus. coreflooding experiment. Until breakthrough, the differential pressure across the core reaches a maximum value of 0.8 psi, and then drops to a value of about 0.2 psi at an injection rate of 0.5 cc/min. Oil Recovery by Seawater Flooding: Table 5 shows oil recovery for all the coreflooding experiments. Figure 4 shows oil recovery by seawater flooding using the single Oil Recovery Type of Apparatus Composite ID PV S wi Waterflooding Injection (cc) (%) (%) (%) Single Coreflooding Composite-1 Single Composite-4 Single Dual Coreflooding Composite-1 Dual (HPCP) Composite-2 Dual (LPCP) Table 5. Oil recovery by waterflooding and injection

7 Fig. 4. Oil recovery by seawater flooding using single coreflooding apparatus (horizontal and vertical). Fig. 6. The profile of pressure drop during horizontal and vertical injection by single coreflooding. Fig. 5. Oil recovery by simultaneous seawater injection for both HPCP and LPCP composites at reservoir conditions using dual coreflooding apparatus. coreflooding apparatus. Figure 5 shows the performance of seawater flooding in terms of oil recovery from the HPCP and LPCP composites using the dual coreflooding apparatus. As shown, no more oil or only a little oil was produced for the LPCP after breakthrough occurred in the HPCP until the injection flow rate reached 2 cc/min. This is a result of seawater bypassing through the HPCP. About 8% additional oil from the original oil in core (OOIC) was produced from the LPCP when the rate was changed to 2 cc/min. The poor displacement efficiency observed in the LPCP is due to the breakthrough of seawater in the HPCP. The S or after seawater flooding was about 49% OOIC for the HPCP and 59% OOIC for the LPCP. The results show that oil recovery by seawater flooding depends on rock permeability, injection rates and bypassing flow. The results also indicate that more oil is produced from the higher permeable cores, as expected. Pressure Profile and Oil Recovery during Sc-CO 2 Injection The experiments of continuous injection after waterflooding were conducted using both the single and dual coreflooding apparatus. Figure 6 shows the differential pressure profiles during horizontal and vertical injection in the single coreflooding experiment. The breakthrough occurs at about 0.2 PV of injected. In comparison, the pressure profiles in the dual coreflooding experiments are impacted Fig. 7. Comparison of differential pressure vs. the sum of PV injected during simultaneous injection for LPCP and HPCP. Fig. 8. Oil recovery by injection using the single coreflooding apparatus. by each other, as is shown in Fig. 7 (HPCP pressure profile in blue and the LPCP pressure profile in red). The pressure profile of injection for the LPCP composite during dual coreflooding is quite different from that for the composite during single coreflooding. Multiple breakthroughs of are observed for the LPCP during injection, compared with the single core injection. The maximum differential pressure at first breakthrough is 10 psi at 0.16 PV of the injection. The second breakthrough occurs at 5.5 psi of differential pressure and 0.33 PV of injection. After the second breakthrough of, the differential pressure curves for both core plugs in the dual coreflooding experiment are similar until the end of the

8 test. This phenomenon is due to the generation of multiple oil banks during miscible injection. Figure 8 shows ultimate oil recoveries by the continuous injection process for composite-1 (single) and composite-4 (single) core plugs using single coreflooding. Comparing the results for composite-1 (single, horizontal displacement) and composite-4 (single, vertical displacement) shows that higher oil recovery was obtained by vertical displacement. This high recovery in the vertical orientation indicates that as long as the CO 2 is able to contact the remaining oil, it has the potential to recover it due to a gravity-stable injection front. Figure 9 presents oil recovery factors for both the LPCP and the HPCP during injection using the dual coreflooding apparatus. About 47% and 22% of OOICs were recovered from the HPCP and LPCP, respectively. Less than 2% of residual oil was left in the HPCP after injection. The breakthrough point occurs at about 0.2 of total PV of injected. For the LPCP, the amount of oil recovered was substantially lower compared to the amount for the HPCP composite. This indicates that as long as there is a more permeable path for the injected fluids to travel through, the oil recovery in the less permeable parts, where the injected fluids have bypassed, will be much lower. The results after injection indicate that most of the oil was produced from the HPCP composites and a significant amount was left behind in the LPCP. To recover this remaining oil and improve displacement efficiency from the LPCP, a slug about 0.4 PV of a diverting system was injected into the HPCP at an injection rate of 0.5 cc/min. During the process of diverting system injection, the LPCP was isolated, and the differential pressure across the HPCP was monitored. The maximum injection pressure recorded for the diverting system was more than 200 psi at reservoir conditions. After plugging the HPCP with the diverting system, was injected again into both the composites to determine the oil recovery performance from the LPCP. Figure 10 shows the comparison of differential pressure curves across the LPCP for injection before and after diverting system injection. The differential pressure curve Fig. 10. Comparison of differential pressure across LPCP for initial and second injection before and after diverting system injection. of the LPCP shows a type of differential pressure curve obtained in the second injection, unlike the curve before diverting system injection. About 19% of the OOIC was recovered at the end of the second after an injection of about 1 PV 9. The results indicate that the leftover or bypassed oil can be recovered if the injected fluids are made to travel through the bypassed zones. These results have great implications for understanding sweep efficiencies in gas flooding, and more importantly, in designing the injection strategy in such floods. CONCLUSIONS Based on experimental results and observations from tests of seawater and flooding using both single and dual coreflooding apparatus at reservoir conditions, the following conclusions can be drawn: The differential pressure profile is similar in single and dual coreflooding for seawater or injection before breakthrough, but is quite different after breakthrough. For the dual coreflooding, the differential pressure across the LPCP is higher than that across the HPCP for both seawater and injection before breakthrough and drops to levels similar to those for the HPCP after breakthrough. This has important implications for reservoir simulation during IOR and EOR processes, especially for gas injection processes. The amount of oil recovered for the LPCP was substantially lower than that for the HPCP composite. It indicates that as long as there is a more permeable path for the injected fluids to travel through, the oil recovery in the less permeable parts, where the injected fluids have bypassed, will be much lower. Fig. 9. Oil recovery by injection for HPCP and LPCP using the dual coreflooding apparatus. Poor sweep efficiency in the lower permeability zone caused by the bypassing of fluids through the higher permeable zone is experimentally evidenced during the seawater and coreflooding tests. Permeability contrast therefore has a significant impact on oil recov-

9 ery by seawater and injection. Multiple breakthroughs of were created during the initial injection. They contributed to removing the remaining oil, building an oil bank and recovering oil. Dual coreflooding experiment techniques provide some new insights into the phenomenon of a two-phase flow oil-water and oil- during seawater and injection, including pressure profiles of higher and lower permeable zones, and the performance of oil recovery processes. A substantial increase of differential pressure in the HPCP (up to 200 psi) was observed during base gel slug injection, which indicates that in situ CO 2 emulsion was generated and was able to block the high permeable zone. This resulted in displacing fluids into the LPCP, thereby improving sweep efficiency in the LPCP. The results indicate that the leftover or bypassed oil can be recovered if the injected fluids are made to travel through the bypassed zones. These results have great implications for understanding sweep efficiencies in gas flooding, and more importantly, in designing the injection strategy in such floods. ACKNOWLEDGMENTS The authors would like to thank the management of Saudi Aramco and EXPEC ARC for their support and permission to publish this article. Special thanks to Amin M. Alabdulwahab, Faris Alghamdi and Mohammed Al-Dokhi for the preparation of the coreflooding experiments. This article was presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Crown Perth, Perth, Australia, October 25-27, REFERENCES 1. Yousef, A.A., Al-Saleh, S., Al-Kaabi, A.U. and Al-Jawfi, M.S.: Laboratory Investigation of Novel Oil Recovery Method for Carbonate Reservoirs, SPE paper , presented at the Canadian Unconventional Resources and International Petroleum Conference, Calgary, Alberta, Canada, October 19-21, Teklu, T.W., Alameri, W., Graves, R.M., Kazemi, H. and Al-Sumaiti, A.M.: Low Salinity Water Alternating CO 2 Flooding Enhanced Oil Recovery: Theory and Experiments, SPE paper , presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, November 10-13, Wang, J. and Dong, M.: Simulation of O/W Emulsion Flow in Alkaline/Surfactant Flood for Heavy Oil Recovery, Journal of Canadian Petroleum Technology, Vol. 49, Issue 6, June 2010, pp Han, M., Zhou, X., Fuseni, A.B., Al-Zahrani, B.H. and AlSofi, A.M.: Laboratory Investigation of the Injectivity of Sulfonated Polyacrylamide Solutions into Carbonate Reservoir Rocks, SPE paper , presented at the SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, April 16-18, Cottin, C., Bourgeois, M., Bursaux, R., Jimenez, J. and Lassalle, S.: Secondary and Tertiary Polymer Flooding on Highly Permeable Reservoir Cores: Experimental Results, SPE paper , presented at the SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, March 31-April 2, Vermolen, E.C.M., van Haasterecht, M.J.T. and Masalmeh, S.K.: A Systematic Study of the Polymer Viscoelastic Effect on Residual Oil Saturation by Coreflooding, SPE paper , presented at the SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, March 31-April 2, Zhou, X., AlOtaibi, F.M. and Kokal, S.L.: Laboratory Evaluation of Performance of WAG Process for Carbonate Rocks at Reservoir Condition, SPE paper , presented at the SPE Kuwait Oil and Gas Show and Conference, Kuwait City, Kuwait, October 7-10, AlOtaibi, F.M., Zhou, X. and Kokal, S.L.: Laboratory Evaluation of Different Mode of Supercritical CO 2 Miscible Flooding for Carbonate Rocks, SPE paper , presented at the SPE Saudi Arabia Section Annual Technical Symposium and Exhibition, al-khobar, Saudi Arabia, April 21-23, Zhou, X., AlOtaibi, F.M., Kokal, S.L., Alhashboul, A.A., Balasubramanian, S. and Alghamdi, F.A.: Novel Insights into IOR/EOR by Seawater and Supercritical CO 2 Miscible Flooding Using Dual Carbonate Cores at Reservoir Conditions, Saudi Aramco Journal of Technology, Summer 2015, online content. 10. Kasmaei, A.K. and Rao, D.N.: Is Wettability Alteration the Main Cause for Enhanced Recovery in Low Salinity Waterflooding? SPE paper , presented at the SPE Improved Oil Recovery Symposium, Tulsa, Oklahoma, April 12-16, Okasha, T.M., Funk, J.J. and Al-Enezi, S.M.: Wettability and Relative Permeability of Lower Cretaceous Carbonate Rock Reservoir, Saudi Arabia, SPE paper 81484, presented at the SPE Middle East Oil Show and Conference, Bahrain, April 5-8, Okasha, T.M., Funk, J.J. and Al-Rashidi, H.N.: Fifty Years of Wettability Measurements in the Arab-D Carbonate Reservoir, SPE paper , presented at the SPE Middle East Oil and Gas Show and Conference, Bahrain, March 11-14, 2007.

10 13. AlOtaibi, F.M., Zhou, X., Kokal, S.L., Senthilmurugan, B., Alhashboul, A.A. and AlAbdulwahab, A.M.: A Novel Technique for Enhanced Oil Recovery: In-Situ CO 2 Emulsion Generation, SPE paper , presented at the SPE Asia Pacific Enhanced Oil Recovery Conference, Kuala Lumpur, Malaysia, August 11-13, BIOGRAPHIES Xianmin Zhou is a Petroleum Engineer with 40 years of experience working in Saudi Aramco s Exploration and Petroleum Engineering Center Advanced Research Center (EXPEC ARC). His focus areas are at present in paleo oil and heavy oil recovery studies. Prior to joining Saudi Aramco in 2010, Xianmin worked as a Senior Petroleum Engineer/Senior Special Core Analyst for four major oil companies: Daqing Petroleum Research Center, China; Core Lab Inc., U.S.; Omni Labs Inc., U.S.; and Intertek/ Westport Technology Center, U.S. His areas of expertise include special core analysis; CO 2 and chemical enhanced oil recovery studies; reservoir characterization that includes developing methods for measuring two-phase and three-phase relative permeability; single and dual coreflooding apparatuses at reservoir conditions; and wettability studies. Xianmin has authored or coauthored 30 papers on the above subjects in Chinese and Canadian journals, and in several Society of Petroleum Engineers (SPE) journals. He has published four patents. In 1976, Xianmin received his B.S. degree in Petroleum Engineering from Daqing Petroleum Institute, Heilongjiang, China, and in 1996, he received his M.S. degree in Chemical and Petroleum Engineering from the University of Wyoming, Laramie, WY. Fawaz M. Al-Otaibi is a Petroleum Engineer at Saudi Aramco s Reservoir Characterization Department. Prior to that, he worked as a Supervisor of the Petrophysics Unit in the Exploration and Petroleum Engineering Center Advanced Research Center (EXPEC ARC). Fawaz has worked in many technical positions and a variety of disciplines, including production engineering and reservoir management, within Saudi Aramco. He has led research projects on both enhanced oil recovery using carbon dioxide (CO 2 EOR) and reservoir fluids. Fawaz has evaluated different CO 2 EOR methods, such as water-alternating-gas (WAG) and tapered WAG during CO 2 EOR flooding. He has also taught courses on CO 2 EOR and coreflooding theories and applications. Currently, Fawaz is leading a group of scientists and technicians to conduct studies to investigate several techniques in overcoming the gravity override during CO 2 EOR. He is an active member of the Society of Petroleum Engineers (SPE) and has published numerous SPE papers and technical journals. Fawaz also has five filed patents. He is a Certified Petroleum Engineer and has received several awards and other recognitions from SPE. In December 1997, Fawaz received his B.S. degree in Chemical Engineering from King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia.

11 Dr. Sunil L. Kokal is a Principal Professional and a Focus Area Champion of enhanced oil recovery (EOR) in the Reservoir Engineering Technology team of Saudi Aramco s Exploration and Petroleum Engineering Center Advanced Research Center (EXPEC ARC). Since joining Saudi Aramco in 1993, he has been involved in applied research projects on EOR/improved oil recovery, reservoir fluids, hydrocarbon phase behavior, crude oil emulsions, and production related challenges. Currently Sunil is leading a group of scientists, engineers and technicians to develop a program for carbon dioxide EOR and to conduct appropriate studies and field demonstration projects. Prior to joining Saudi Aramco, he worked at the Petroleum Recovery Institute, Calgary, Canada. Sunil is a member of the Society of Petroleum Engineers (SPE), and is a Registered Professional Engineer and a member of the Association of Professional Engineers, Geologists and Geophysicists of Alberta (Canada). He has written over 100 technical papers. Sunil has served as the associate editor for the Journal of Petroleum Science and Engineering, and SPE s Reservoir Evaluation and Engineering Journal, and earlier served on the Editorial Review Board of the Journal of Canadian Petroleum Technology. He is the recipient of the prestigious 2016 SPE Honorary Member Award, the 2012 SPE DeGolyer Distinguished Service Medal, the 2011 SPE Distinguished Service Award, the 2010 SPE Regional Technical Award for Reservoir Description & Dynamics, and the 2008 SPE Distinguished Member Award for his services to the society. Sunil also served as a SPE Distinguished Lecturer during Currently he is the Chair of the SPE Distinguished Lecturer Committee. In 1982, Sunil received his B.S. degree in Chemical Engineering from the Indian Institute of Technology, New Delhi, India, and in 1987, he received his Ph.D. degree in Chemical Engineering from the University of Calgary, Calgary, Alberta, Canada. Almohannad A. Al-Hashboul is a Petroleum Engineer with Saudi Aramco s Reservoir Engineering Technology Team at the Exploration and Petroleum Engineering Center Advanced Research Center (EXPEC ARC). Since joining Saudi Aramco in July 2014, he has been involved in enhanced oil recovery (EOR) research projects, specifically carbon dioxide (CO 2 ) EOR. Almohannad is a key member of the multidisciplinary CO 2 EOR Demonstration project team and was part of the commissioning team of the first EOR project in Saudi Arabia. He is currently on assignment with the Southern Area Production Engineering Department and earlier completed a year-long assignment with the Reservoir Management Department in Almohannad has published several technical papers, mainly related to CO 2 EOR, and has a patent application that was submitted in He has been an active member of the Society of Petroleum Engineers (SPE), where he served on several teams that supported young professionals activities and conferences in 2015 and Currently, Almohannad is the Marketing and Communications Director of the SPE-KSA Section s executive board for the term. In 2014, he received his B.S. degree in Petroleum Engineering along with a Minor in both Mechanical Engineering and Mathematics from Texas Tech University, Lubbock, TX.