Energy Fuels 2010, 24, 6346–6352 Published on Web 11/16/2010
: DOI:10.1021/ef100849u
Research on Mechanisms of Alkaline Flooding for Heavy Oil Baodong Ding, Guicai Zhang,* Jijiang Ge, and Xiaoling Liu Petroleum Engineering College, China University of Petroleum, Qingdao 266555, China Received July 4, 2010. Revised Manuscript Received September 29, 2010
To investigate mechanisms of alkaline flooding for heavy oil, we designed three flooding systems, including NaOH, diethylamine, and compound alkali (a mixture of NaBO2 and Na2CO3; NaBO2/Na2CO3 = 2:1 by weight) and evaluated their performances for Binnan heavy oil by sandpack flooding tests. Results indicated that the displacement efficiency of alkaline flooding depended upon the concentration and type of alkali. Displacement efficiency increased with the increase of the alkaline concentration and basicity. It was relative to the type of emulsion generated by alkaline solutions to a certain degree but independent of the interfacial tension between alkaline solution/heavy oil. Although water-in-oil (W/O) emulsion may appear when heavy oil was mixed with some alkaline solutions in the phase test, not all of them could exhibit well performance in the sandpack flooding test. To further elucidate the mechanisms of alkaline flooding for heavy oil, it was compared to the water flooding process in a glass-etched microscopic model, and a conclusion was drawn that the penetration of alkaline solutions in crude oil was the main mechanism to improve sweep efficiency during alkaline flooding and W/O emulsion was just the derived result of alkaline solutions penetrating and not the basic reason for enhanced oil recovery (EOR) by alkaline flooding.
mechanism in improving oil recovery.2 Dong et al.3 constructed a displacing system with stronger emulsifying power and lower IFT, but the increment of tertiary oil recovery in the sandpack test was low. However, when the alkaline concentration was increased to 0.6 wt % (the corresponding oil-water IFT is no longer optimum), a good oil recovery was obtained. Conceptually, the alkaline flooding for enhanced heavy-oil recovery consists of the following processes: the injected alkaline reagents react with the surface-active materials originated in the oil, resulting in the in situ formation of surfactants. The adsorption of these in situ generated surfactants at oil/water interfaces can drastically reduce the oil/water IFT. The low IFT causes the emulsification of the heavy oil.4,5 Using lowfield nuclear magnetic resonance (NMR), Bryan et al. investigated the nature of the emulsions formed during chemical flooding and obtained the following results: when the chemical slug was prepared with distilled water, although oilin-water (O/W) emulsion appeared in the effluent, waterin-oil (W/O) emulsions occurred in the inlet of the core, judging from the NMR relaxing time; when the chemical slug was prepared by brine (2 wt % NaCl contained), the whole core had a NMR relaxing display of W/O emulsion.6 Dong et al. investigated the displacement mechanisms of alkaline flooding in a micromodel and observed that two mechanisms governed the enhanced oil recovery (EOR) process.7 One was the appearance of W/O emulsion and partial wettability
1. Introduction According to the statistics of the China Petroleum and Chemical Corporation (SINOPEC) at the end of 2007, about 30.1% of heavy-oil resources are recovered by waterflooding in China. Because of the adverse mobility ratio, the oil recovery is at least 10% lower than conventional oil. Therefore, it is urgent to improve the production of such reservoirs. By now, two methods have been suggested. One is thermal recovery, and the other is chemical flooding. The thermal method is inapplicable for deep or thin heavy-oil reservoirs, but chemical flooding is promising for these reservoirs. Since Subkow1 patented the injection of aqueous emulsifying agents for recovering heavy oil or bitumenthen in 1942, studies on heavy-oil chemical flooding had been carrying on until the end of the 1980s. Within this period, more attention was paid to the displacing system with a low interfacial tension (IFT) and the concentration of alkali selected in the tests was generally kept at a relatively low level. The results indicated that some succeeded and some failed. After entering the 21st century, a lot of studies on chemical flooding have been performed at Regina University and Calgary University, owning to the requirement of developing thin-layer heavy-oil reservoirs. The principle to design chemical flooding is completely different from the tradition. More attention is paid to the capacity of the chemical agent to improve sweep efficiency rather than lower IFT or reduce the viscosity of heavy oil. It was proposed by Bryan and Kantzas that the emulsification-entrapment mechanism is more effective than the emulsification-entrainment
(4) Ma, S.; Dong, M.; Li, Z.; Shirif, E. J. Pet. Sci. Eng. 2007, 55, 294– 300. (5) Liu, Q.; Dong, M.; Ma, S. Proceedings of the 2006 Society of Petroleum Engineers (SPE)/Department of Energy (DOE) Symposium on Improved Oil Recovery; Tulsa, OK, April 22-26, 2006; SPE Paper 99791. (6) Bryan, J.; Mai, A.; Kantzas, A. Proceedings of the 2008 Society of Petroleum Engineers (SPE)/Department of Energy (DOE) Improved Oil Recovery Symposium; Tulsa, OK, April 19-23, 2008; SPE Paper 113993. (7) Dong, M.; Liu, Q.; Li, A. Proceedings of the International Symposium of the Society of Core Analysts; Calgary, Alberta, Canada, Sept 10-12, 2007; SCA2007-47.
*To whom correspondence should be addressed. Telephone: þ8653286981178. E-mail:
[email protected]. (1) Subkow, P. U.S. Patent 2,288,857, July 7, 1942. (2) Bryan, J.; Kantzas, A. Proceedings of the 2007 Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition; Anaheim, CA, Nov 11-14, 2007; SPE Paper 110738. (3) Dong, M.; Ma, S.; Liu, Q. Fuel 2009, 88, 1049–1056. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 6346–6352
: DOI:10.1021/ef100849u
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Table 1. Basic Properties of the Oil Samples density viscosity normal heptane at 55 °C at 55 °C acid numbera resin asphaltene oil sample (g/cm3) (mPa s) (mg of KOH/g) (wt %) (wt %) Zhuangxi 0.9302 Binnan 0.9472 a
238 2000
0.80 2.69
19.7 19.5
0.835 2.033
Measured according to the GB/T 18609-2001 Standard.
alteration. According to this mechanism, the high permeability zones were blocked by W/O emulsion and the pore walls were altered to partially oil wet; therefore, an increase in the pressure drop and a high tertiary oil recovery were obtained.8 The other mechanism was the formation of O/W emulsion. According to this mechanism, heavy oil was emulsified in O/W emulsion, then entrained in the water phase, and produced out of the model. In comparison to surfactants and polymers, alkali is relatively inexpensive. If chemical flooding containing alkali can be successfully applied in a heavy-oil reservoir, the oil production cost will be decreased greatly. However, there is no definite viewpoint on whether oil/water IFT, phase behavior, or other more important factors can be adopted as criteria to design the displacement system for heavy oil. To elucidate this problem, three alkaline systems were used to recover heavy oil in sandpack flood tests, including NaOH, diethylamine, and compound alkali. This paper discussed the effects of IFT and emulsion types on oil recovery efficiency and also investigated the mechanisms of alkaline flooding for heavy oil by micropore model displacement experiments.
Figure 1. Instrument used to measure Rmin.
Figure 2. Variation of oil and water phases during Rmin measurement for the 0.2 wt % NaOH/Binnan oil system.
2. Experimental Section
2.2.2. Stirring Rod. To avoid agitating the interface directly, the stirring rod is specially designed to stir the alkaline solution at a certain depth under the interface. When the agitation transfers from water up to oil, its action on oil attenuates successively. Under this weak disturbance, scaly crude oil can be emulsified in alkaline solution. 2.2.3. Sample Cell. The solution and oil are added to the sample cell, which can be kept at a constant temperature with an external circulating water bath. 2.2.4. Cell Holder. When the cell holder is adjusted, the sample cell can be fixed at an appropriate site, so that its center is taken up by the stirring rod. The testing procedure is as follows: first 40 mL of oil displacement agent solution is poured into the sample cell. Then, the stirring rod is equipped, making sure that the stirring rod points to the center of the sample cell vertically and the rotator is under the surface of the solution. Then, 10 mL of oil is transferred slowly to the sample cell. When the solution is heated to the preset temperature (55 °C), the agitator is started. If the oil can be dispersed thoroughly into the solution within 10 min, another test is needed at a rotation rate reduced by 25 rpm; otherwise, the rotation rate is increased by 25 rpm, and a new test is started. The rotation rate is continuously changed until Rmin is found. Figures 2 and 3 give the variation of oil and water phases during the measurement of Rmin for 0.2 and 1.0 wt % NaOH/oil. It can be found that oil droplet formed during agitation for the 0.2 wt % NaOH/oil system was smaller than that of the 1.0 wt % NaOH/oil system at the beginning of emulsification. In Figure 3c, some oil attached on the inner wall, which indicated the inner wall became partially oil-wet. 2.3. Measurement of IFT. At 55 °C, dynamic IFT values between Binnan heavy-oil and alkaline solutions with different compositions were measured by an American Texas-500 spinning drop IFT apparatus with image acquisition and analysis software developed by our lab.
2.1. Materials. Two heavy-oil samples were used. One is from well Shang10-49 of the Binnan reservoir in the Shengli oilfield, and the other is from well Zhuang106-15-X18 of the Zhuangxi reservoir in the Shengli oilfield. Their basic properties are shown in Table 1. In micromodel experiments, Zhuangxi oil was selected to investigate the mechanism of alkaline flooding for its relatively low viscosity. In other experiments, Binnan oil was used. In addition, the aqueous phase was the brine containing 0.5 wt % NaCl. Chemicals applied in this study, such as NaOH, NaBO2, Na2CO3, diethylamine, and NaCl, were all analytical-grade reagents. 2.2. Emulsification Test. Two kinds of emulsification tests were shown in this paper. One set of tests were performed in a 20 mL glass test tube. At first, 10 mL of aqueous phase (with a certain concentration of alkali and sodium chloride) and 10 mL of heavy oil were added to a test tube successively, and then the test tube was stilled at 55 °C for 30 min. After that, the test tube was shaken up and down quickly 50 times and investigated the type of emulsion by the dilution method. In addition, to characterize the emulsifying capability of alkali, the minimum rotation rate (abbreviated as Rmin) for a specified volume of oil to be dispersed thoroughly in the solution of an oil displacement agent in a certain time was measured by the self-designed instrument.9 This instrument contains four parts: agitator, stirring rod, sample cell, and cell holder, as shown in Figure 1. 2.2.1. Agitator. Physica Rheolab MC1 made by Physica Messtechnik Gmbh in Germany was used as the agitator for its stable and adjustable rotation rate. (8) Arhuoma, M.; Yang, D.; Dong, M.; Li, H.; Idem, R. Energy Fuels 2009, 23 (12), 5995–6002. (9) Ge, J.; Wang, D.; Zhang, G.; Jiang, P.; Liu, H. Acta Pet. Sin. 2009, 25 (5), 690–696.
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Figure 3. Variation of oil and water phases during Rmin measurement for the 1.0 wt % NaOH/Binnan oil system.
2.4. Sandpack Flood Studies. The sandpacks used for flood test are about 19.40 cm in length and 2.35 cm in inner diameter. They were packed as follows: At first, quartz sand with 100-200 and 80-100 mesh was blended with a weight ratio of 3:1, and then they were added to the tube in several increments. In each step, after the sand was poured in, some deionized water was added to moisten the sand and then the tube was slightly vibrated. During this process, the water surface should be kept above the sand surface to avoid the existence of air. The sandpack displacement was conducted at 55 °C. Steps are as follows: (1) At first, the sandpack is saturated with brine solution, and then the permeability is measured and the porosity is calculated. (2) Binnan heavy oil is injected into the sandpack until the water cut is less than 2 vol %, and then the oil saturation can be obtained. (3) Water flooding is conducted until the oil cut is less than 2 vol %, and then a chemical slug of 0.5 pore volume (PV) is injected, followed by an extended waterflooding until the oil production becomes negligible (oil cut < 2 vol %). Unless specified otherwise, the injection rate of brine solution and chemical slug is set at 0.5 mL/min. 2.5. Micromodel Test. The steps for the micromodel test are as follows: (1) the micromodel is evacuated; (2) the micromodel is saturated with brine (0.5 wt % NaCl); (3) Zhuangxi oil is injected; (4) chemical flooding at 0.003 mL/min is conducted, and all of the dynamic processes are videotaped; (5) the collected images are analyzed. To observe the phenomena during the flooding easily, 0.05 wt % eosin is added to the aqueous solution.
Figure 4. pH of different alkaline solutions (all containing 0.5 wt % NaCl).
Figure 5. Dynamic IFT curves of compound alkali/heavy oil.
3. Results and Discussion 3.1. Alkalis Used in Flooding. In this paper, three alkalis, including NaOH, diethylamine, and the mixture of NaBO2 and Na2CO3, were selected in the flood tests. As reported in recent years, NaBO2 is an alkaline agent that can be used in water with a high concentration of calcium and magnesium ions.10 Our experiments showed that, when NaBO2 and Na2CO3 were mixed by a mass ratio of 9:1, the compound exhibited good emulsifying capability for Binnan heavy oil. For convenience, the mixture of the two alkalis is simply called compound alkali in the later sections of this paper. As shown in Figure 4, for the solutions containing the same concentration of alkali, the pH value of NaOH solutions is the highest, followed successively by diethylamine and compound alkali. 3.2. IFT Measurement. Figures 5-7 show the IFT between the heavy-oil and different alkaline solutions (all containing 0.5 wt % NaCl). It can be seen that three sets of curves exhibit similar trends. When the alkaline concentration is low, the dynamic IFT increases gradually from a low value to more than 0.1 mN/m with time; however, when the alkaline concentration
Figure 6. Dynamic IFT curves of NaOH/heavy oil.
is above a certain value, the dynamic IFT shows little change with time and the equlibirum value is 0.1-0.01 mN/m. The reason may be that, after a certain concentration is exceeded, the alkaline solution can provide enough OH- at the interface of oil and water; thus, there will be enough active agents that are generated by OH- and petroleum acids to decrease the IFT. 3.3. Emulsion of Alkali. Emulsification of alkali includes which type of emulsion (W/O or O/W) forms when alkali contacts heavy oil and whether the heavy oil can be dispersed easily by alkaline solutions. As shown in Figure 8, the type of emulsion tends to be O/W when the alkaline concentration is at a low level and W/O emulsion occurs when the alkali concentration is at a high level. Emulsifying power is in inverse proportion to the minimum emulsification rotation speed Rmin. A low Rmin indicates that this system has a good emulsifying power. As shown in
(10) Flaaten, A. K.; Nguyen, Q. P.; Zhang, J.; Hourshad, M.; Pope, G. A. SPE J. 2010, 184–196.
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saturation ranges from 80 to 92%, and the oil recovery by waterflooding is about 35%. The relationship of the alkaline concentration versus incremental recovery is plotted in Figure 10. The overall trend is that incremental oil recovery increases with the increase of the alkaline concentration. Within the range of 0.1-0.9 wt %, NaOH exhibits the best performance, followed by diethlyamine and compound alkali. When the alkaline concentration is above 1.0 wt %, incremental oil recovery for the compound alkali increases sharply but that for diethlyamine exhibits a slight decrease. It can be inferred that two factors dominate the performance of alkaline flooding. One is the alkaline concentration, which has also been reported by Dong et al.3 and Arhuoma et al.,11 and the other is alkaline type. When the concentration is above 0.5 wt %, the interface activity of NaOH solutions is less effective than that of diethylamine solutions (Figures 6 and 7) and the emulsifying capacity of NaOH solutions is also weaker than that of diethylamine solutions (Figure 9), but NaOH exhibits better performance in the flood test than diethylamine and compound alkali at the same mass concentration. Therefore, the basicity has an important influence on the performance of alkaline flooding. Pressure changes during different alkaline flooding are shown in Figures 11-13. It is shown that the pressure response of chemical flooding is weak when the alkaline concentration is low, while the flooding with a high alkaline concentration had a strong pressure response. In combination with Figure 10, it can also be inferred that a higher incremental recovery is often accompanied with a higher increase in the pressure drop after the chemical injection and the incremental recovery is less than 10% for some tests when there is no obvious pressure response after the injection of chemical agents. From the results of sandpack flooding tests, we can come to the conclusion that the alkaline concentration is another important factor for alkaline flooding of heavy oil. Because of viscous fingering caused by the adverse mobility ratio between oil and water, heavy oil is easily bypassed. Thus, some water channels may appear in the porous medium at the end of waterflooding. The pressure rising after the chemical injection indicates that diversion of the displacement agent occurs. Some studies credited this phenomenon to the formation of more viscous W/O emulsion between chemical solution and heavy oil, which improves the mobility ratio and delays viscous fingering. In addition, another series of experiments were performed to verify the relationship between oil recovery and emulsion type. At first, the effect of salinity on the type of emulsion was investigated. Phase tests indicated that when the concentration of compound alkali was 1.0 wt %, W/O emulsion tended to be formed no matter the concentration of NaCl in alkaline solution. However, when the concentration of compound alkali was kept at 0.2 wt %, the emulsion type depended upon the concentration of NaCl in alkaline solution. When the concentration of NaCl was above 0.5 wt %, W/O emulsion occurred. Otherwise, O/W emulsion may appear. Comparative analysis was conducted. As shown in Figure 14, phase behavior tests mentioned above did not fully correspond with the flooding results. When the concentration of
Figure 7. Dynamic IFT curves of diethylamine/heavy oil.
Figure 8. Relations between the alkaline concentration and emulsion types.
Figure 9. Minimum emulsification rotation speed Rmin as a function of the alkaline concentration.
Figure 9, the alkaline solutions exhibit much stronger emulsifying power than the brine water without alkali. However, Rmin shows different trends with the increase of the alkaline concentration. For NaOH and compound alkali, Rmin gradually increases with the increase of the alkaline concentration, but for diethylamine, Rmin shows a little decrease. Therefore, when the concentration is low, the emulsifying power of three alkalis is similar, but when the concentration is above 0.2 wt %, diethylamine presents the best performance. 3.4. Performance of Alkaline Flooding in Sandpacks. To evaluate the result of alkaline flooding for heavy oil, 25 flooding tests were performed in the sandpacks and incremental oil recoveries were obtained. Table 2 shows the parameters of the sandpacks, chemical slug compositions, and flooding results. The porosity of the sandpacks is generally about 36.6-47.5%, and the water permeability is about 1.0-2.6 μm2. The initial oil
(11) Arhuoma, M.; Yang, D.; Dong, M.; Idem, R. Proceedings of the Canadian International Petroleum Conference (CIPC); Calgary, Alberta, Canada, June 16-18, 2009; Paper 2009-053.
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Table 2. Summary of Alkaline Flooding for Binnan Heavy Oil core label
porosity (%)
permeability (md)
initial oil saturation (%)
formula of alkaline solution
oil recovery by waterflooding (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
36.6 38.8 43.1 47.2 46.5 38.0 45.7 44.4 42.2 43.4 42.2 41.4 43.3 41.4 42.2 43.0 43.4 44.6 43.1 43.2 43.7 47.5 46.0 42.3 42.2
2215 1309 1112 1337 1645 1509 1263 1910 1071 1537 1537 1911 1286 1645 1645 2193 1861 2021 2143 1497 1653 2571 1553 1407 1381
83.3 81.7 93.0 91.3 86.2 89.2 88.9 94.3 90.5 90.3 94.2 93.2 93.2 91.9 91.4 91.2 88.5 87.1 87.7 92.2 93.9 86.4 90.5 90.0 90.0
0.2% compound alkali þ 0.5% NaCl 0.6% compound alkali þ 0.5% NaCl 0.8% compound alkali þ 0.5% NaCl 0.9% compound alkali þ 0.5% NaCl 1.0% compound alkali þ 0.5% NaCl 1.2% compound alkali þ 0.5% NaCl 0.8% NaCl þ 0.2% compound alkali 0.8% NaCl þ 0.2% compound alkali 1.0% NaCl þ 0.2% compound alkali 1.2% NaCl þ 0.2% compound alkali 0.8% NaCl þ 1.0% compound alkali 1.0% NaCl þ 1.0% compound alkali 1.2% NaCl þ 1.0% compound alkali 1.5% NaCl þ 1.0% compound alkali 0.1% NaOH þ 0.5% NaCl 0.3% NaOH þ 0.5% NaCl 0.4% NaOH þ 0.5% NaCl 0.5% NaOH þ 0.5% NaCl 0.8% NaOH þ 0.5% NaCl 1.0% NaOH þ 0.5% NaCl 0.2% diethylamine þ 0.5% NaCl 0.5% diethylamine þ 0.5% NaCl 0.8% diethylamine þ 0.5% NaCl 1.0% diethylamine þ 0.5% NaCl 1.2% diethylamine þ 0.5% NaCl
41.3 33.3 32.8 34.8 31.4 31.0 37.6 32.7 38.4 34.4 30.8 32.6 34.9 32.3 30.9 36.4 36.0 34.0 40 33.4 28.7 32.8 38.4 31.6 38.3
Figure 10. Influence of the alkaline concentration on the displacement efficiency.
Figure 12. Pressure changes during NaOH flooding.
Figure 11. Pressure changes during compound alkali flooding. Figure 13. Pressure changes during diethylamine flooding.
the compound alkali was 1.0 wt %, it exhibited high efficiency in the oil recovery, whereas the incremental oil recovery was less than 10% for 0.2 wt % compound alkali. Even the salinity was high. It could be inferred that, although
the emulsion type did have a certain relation with the performance of the flooding system, it was not the only dominant factor. 6350
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Figure 17. Images of the displacing front during alkaline flooding.
Figure 14. Performances of compound alkali flooding as a function of the NaCl concentration.
Figure 18. Images of alkaline solution breakthrough and obvious breakthrough.
into the crude oil and brown water columns coating the oil film appeared. These water columns could be easily intersected by oil streams and split into smaller water columns or bubbles at the pore joints. Thus, water-oil alternating slug flow at the displacing front occurred. On the basis of the above observations, it is easy to understand the occurrence of W/O emulsion in the inlet of the model in experiments conducted by Bryan et al.6 These W/O emulsions may be water droplets or bubbles generated by alkaline solution penetrating in the crude oil. For alkaline flooding, the slug flow continued for some time from breakthrough to the formation of a continuous channel between the producer and injector, during which lots of oil could be displaced, as shown in Figure 18. However, a continuous water channel would be directly formed once water broke through for water flooding. It can be concluded from the above differences that the channeling of the alkaline solution along the diagonal direction was weakened, owing to the slug flow. In addition, because of the high penetration capacity of alkaline solution, its advancing perpendicularly to the main stream was obviously faster than water. This conclusion can be drawn by comparing Figure 16 to Figure 19. Therefore, the excellent sweep efficiency was attributed to the high penetration capacity of alkaline solution. By micromodel tests, Dong et al.7 also observed the same phenomenon of the penetration of alkali in the oil phase. They proposed that in situ W/O emulsion acted as the main mechanism to enhance oil recovery. However, on the basis of the micromodel study in this paper, it could be concluded that W/O emulsion was just a derived result from the penetration of alkali in the oil phase, which usually occurred at the later stage of alkaline flooding. Although a fluid diversion may be caused by this disperse system, it may not play a leading role. During flooding, the flow rate
Figure 15. Glass-etched models saturated with water and oil.
Figure 16. Swept area during water flooding at different times.
3.5. Microscopic Visual Model Test. To further illustrate the mechanisms of alkaline flooding in improving oil recovery, microscopic visual model tests were carried out. Figure 15 shows images of the glass-etched microscopic model saturated with water and oil. When brine water was injected into the oil-saturated model, it could be observed in Figure 16 that water advanced along the diagonal direction and the flooding could not improve the sweep efficiency greatly once water broke through. Therefore, the low recovery of waterflooding may be mainly attributed to the adverse mobility ratio. A similar test for alkaline flooding was performed in another oil-saturated model, as shown in Figure 17. It can be seen that the flow of alkaline solution in the model was much different from that of brine water. Once the alkaline solution came into contact with crude oil, it would penetrate 6351
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Figure 20. Oil clots difficult to be driven out during alkaline flooding.
4. Conclusions Three types of alkalis were selected in this paper. All three of these alkalis can reduce the oil/water IFT to 10-2 order of magnitude. When the alkaline concentration was above a certain value, the dynamic IFT changed slightly with time. The displacement efficiency of alkaline flooding depended upon the alkaline concentration and type of alkali. Displacement efficiency increased with the increase of the alkaline concentration and basicity. It was relative to the type of emulsion generated by alkaline solutions to a certain degree but independent of the IFT between the alkaline solution and heavy oil. Micromodel tests indicated that the permeation of alkaline solutions in crude oil was the main mechanism to improve sweep efficiency during alkaline flooding and W/O emulsion was just the derived result of alkaline permeation and not the basic reason for EOR by alkaline flooding.
Figure 19. Swept area during alkaline flooding at different times.
of the main stream was always exhibited high. Therefore, the main mechanism to improve heavy-oil recovery by alkaline flooding was the high sweep efficiency, which resulted from the penetration of alkali in the oil phase. This action increased with the increase of basicity or its concentration. It can provide an explanation to the good performance of alkaline flooding with a strong alkali or a high concentration of alkali. For the same concentration of alkali, although the pH value of compound alkali was higher than diethylamine, diethylamine exhibited a better performance in the flood test. This may be attibuted to its higher penetration capacity across the interface for its good compatibility with oil. Although alkaline flooding could enhance the sweep efficiency, it may cause other problems. As shown in Figure 20, there are some cellulated residual oil blocks left in the swept area. Therefore, recovering this kind of residual oil during alkaline flooding is a new work.
Acknowledgment. Financial support by the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (Grant 114016) and the New Century Excellent Talents Awards Program from the Ministry of Education of China (Grant NECT-07-0846) is gratefully acknowledged.
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