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A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection
Journal of Petroleum Science and Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection Zhengbin Wu a,n, Huiqing Liu a, Zhanxi Pang a, Yalong Wu a, Xue Wang a, Dong Liu b, Min Gao a a b
MOE Key Laboratory of Petroleum Engineering in China University of Petroleum, Beijing 102249, PR China Bohai Petroleum Institute, Tianjin Branch, CNOOC China Limited, Tianjin 200452, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 March 2016 Received in revised form 12 August 2016 Accepted 19 August 2016
Hot water flooding is an important method for the development of heavy oil. But due to water channeling and low enthalpy, oil recovery by hot water flooding is very limited. In this paper, a two-dimensional visual experiment is conducted to intuitively study foam flowing characteristics and blocking mechanisms in porous media, as well as the microscopic mechanisms of enhancing heavy oil recovery by foam after hot water flooding. The experiment visually reproduces foam generation in porous media and the processes of hot water flooding and foam flooding. The results show that foam flows along the channels with lower flow resistance in oil layer and can effectively improve the sweep efficiency. Compared with pure hot water flooding, the ultimate sweep efficiency increases from 40.5% to 70.7% after foam injection. Moreover, foam can effectively displace the residual oil caused by hot water flooding. The ultimate oil recovery of thermal foam flooding is 67.54%, 31.30% higher than that of hot water flooding. This paper can provide reference for the study for foam and thermal foam flooding in heavy oil recovery. & 2016 Elsevier B.V. All rights reserved.
Keywords: Heavy oil Hot water flooding Foam flooding Visual experiment Mechanisms
1. Introduction Thermal recovery methods such as cyclic steam stimulation (CSS), steam flooding, in-situ combustion are the major thermal recovery methods for the development of heavy oil (Closmann and Seba, 1983; Heel et al., 2008; Wu et al., 2011; Zhao et al., 2013). But with the increase of CSS cycles, oil production of a single well gradually decreases, and steam-oil ratio (SOR) increase dramatically in the later stage, resulting in the low economic benefit (Buger and Sahuquet, 1972; Tewari et al., 2011). As a result of viscosity fingering and steam override in heavy oil reservoirs, steam breakthrough usually appears earlier in steam flooding projects, leading to low sweep efficiency in both vertical and areal directions, and causing low thermal efficiency. In order to make thermal recovery project more profitable, it needs to take measures to make full use of the residual heat energy after steam injection process (Liu, 1998). The further development of the mature oil reservoirs has been an interactive and a broad subject (Abu El Ela et al., 2013). Hot water flooding is a typical alternate thermal recovery method for steam injection (Fournier, 1965; Wang et al., 2011a, n
Corresponding author. E-mail address: [email protected] (Z. Wu).
2011b; Alajmi et al., 2009; Vinsome 1974). Hot water flooding is actually an immiscible displacement process of crude oil by hot water and cold water (Dong et al., 2011). The initial purpose of injecting hot water is to increase injection ability of water wells instead of enhancing oil recovery. Hot water in reservoir on one hand can transfer heat to oil layer and increase reservoir temperature. On the other hand, it can implement reservoir energy and displace crude oil to wellbore. Compared with conventional water flooding, the EOR mechanisms of hot water flooding for heavy oil include decreasing crude oil viscosity to reduce mobility ratio, improving relative permeability and preventing the formation of oil strata with high viscosity, etc. (Abass and Fahmi, 2013; Lu et al., 2013; Lv et al., 2003). There are two significant problems with the application of hot water flooding in heavy oil reservoirs. One is that hot water cannot carry enough heat into reservoir because of its low enthalpy. The other is water channeling caused by reservoir heterogeneity and the density difference between hot water and crude oil. Hot water tends to flow to higher-permeable formation with lower flow resistance. Water channeling is easily formed between the injection and production wells as a consequence of viscosity fingering. The two disadvantages will cause earlier water breakthrough in production wells and have a negative effect on the thermal recovery for heavy oil. One of the feasible methods of enhancing oil recovery after hot water flooding in heavy oil reservoir is to add surfactant that can
http://dx.doi.org/10.1016/j.petrol.2016.08.023 0920-4105/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Trapped gas
Flowing bubble Glass bead Water
Fig. 1. Schematic diagram of foam propagation in homogenous model.
create foams in porous media (Lv et al., 2007; Lu et al., 2003; Muijs et al., 1988; Isaacs et al., 1994; Bagheri and Clark, 2015; Friedmann et al., 1991). Foam in porous media is a dispersive system that gas bubble disperses in liquid film, which is called lamellae. Lamellae snap-off is one of the main mechanisms of foam generation (Roof, 1970; Kovscek et al., 2007). Some long bubbles can also have a function of blocking. The distribution of foam in porous media is shown in Fig. 1 (Gauglitz et al., 2002). In general, foam is created by the co-injection of gas and surfactant solution. Gas bubbles dispersed in liquids cannot exist stable without surfactants. Foam preferentially enters high-permeable zone of formation to block big pores and diverts steam or hot water into low-permeable layer by its high apparent viscosity, thus expanding sweep efficiency of hot fluids injection process (Eson, 1983; Casteel and Djabbarah, 1988; Patzek, 1996). Surfactant maintains the stability of foam and makes a contribution to the reduction of oil/water interfacial tension (IFT) and the variation of reservoir wettability, hence improving oil displacement efficiency (Jamaloei et al., 2011; Liu et al., 2011; Kumar and Mandal, 2016). As a common non-condensate gas used in petroleum industry, nitrogen can implement reservoir pressure and maintain heat left by steam or hot water. In addition, nitrogen can be dissolved in crude oil to form miscible phase under high pressure, which is helpful to decrease heavy oil viscosity and improve oil mobility. What's more, the capillary pressure which usually acts as the main factor that traps the crude oil in pores will be zero when miscible phase is created. Therefore, the oil will be pushed out of the formation and recovery increases. One of the successful hot water foam EOR projects is conducted in
Fig. 3. Viscosity-temperature curve of the oil.
Block A of Xinjiang Oilfield in China (Wu et al., 2015). Compared with pure water flooding, hot water foam flooding massively lowered down the water cut and enhanced oil displacement efficiency from 29.8% to 53%. In addition, Block Jin90 of Liaohe Oilfield in China conducted hot water foam flooding pilots in 19-141 well group (Yuan et al., 2004). Results showed that from September 1996 to September 1999, the cumulative oil production was 3.54 104 t and the cumulative oil increment was 2.14 104 t. From January 2004 to December 2004, also in Jin 45-19-141 well group, hot water foam flooding was conducted again and brought a profit of 3.13 million dollars and the rate of output and input was 2.65 (Wang, 2006). To explore the EOR mechanisms of foam flooding after hot water flooding for heavy oil, a series of experiments are performed. Inspired by the studies of several researchers on visualizing the sweep efficiency improvement by chemical flooding (Pei et al., 2011, 2013; Wang et al., 2011a, 2011b; Guillen et al., 2012), in this paper, hot water flooding process is firstly carried out to display the macroscopic distribution of remaining oil in a visual model filled with glass beads. Then, foam is injected into this model. In this process, foam generation and migration in porous media are intuitively reproduced, which vividly reflects foam seepage characteristics and blocking mechanism. The more important is to make a comparison of sweep efficiency and oil recovery prior and posterior foam injection. Finally, the EOR mechanisms of foam flooding after hot water flooding for heavy oil are summarized. This work is of significance for the selection of foam systems and the implementation for foam projects.
(a) Side elevation diagram
(b) Porous media area
(c) Top view diagram
Fig. 2. Structure diagram of the visualization model. 1 – nut; 2 – model holder; 3 – silicone pad; 4 – quartz glass; 5 – porous media; 6 – glass beads; 7 – draining trench; 8 – tape; 9 – injection pot; 10 – production pot. (a) Side elevation diagram. (b) Porous media area. (c) Top view diagram.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 4. Schematic diagram of visual experiment. Table 1 Physical property of the visualized model and the injection parameters of fluid. Parameter Total pore volume (mL)
Initial oil saturation (%)
Injection flow rate (mL/min)
Foam quality
Value
100
0.2
0.8
22.4
Fig. 6. Half-time vs. temperature.
Fig. 5. Maximum foaming volume vs. temperature.
2. Experimental methods and setup 2.1. Experimental equipment and materials A visual experimental study of foam flowing and blocking in porous media is conducted. The experimental device is composed of the visual physical model which can resist a pressure of 2.5 MPa and a temperature of 300 °C, injection system, data acquisition and measuring system. The injection system includes a pump with a constant pressure and a varied injection rate, an oil tank, a water tank, a surfactant tank, a nitrogen tank, a six-port valve and several two-port valves. Data acquisition and measuring system includes a high-definition camera, a flat lamp, pressure sensors,
Fig. 7. Schematic of the injection syringe.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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(a) 0.12PV
(c) 1.448PV
(b) 0.98PV Inllet
Outlet
Fig. 8. Process of hot water flooding.
(a) 1.76P PV
(b) 2.42PV Inllet
(c)) 3.94PV Outlet
Fig. 9. Process of thermal foam flooding.
Fig. 10. Emulsion in the effluent.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 11. Water cut and oil production rate vs. injected volume.
Fig. 12. Oil recovery and pressure drop vs. injected volume.
temperature sensors, data acquisition device, measuring cylinders and a computer. Physical model includes a model holder, two pieces of heat-resistant quartz glass and a constant-temperature oven. The visual model consists of two glasses, one of which has four holes for installing simulative wells. In this paper, the visual model simulates 1/4 five-spot well pattern. The space between two glasses is filled with two layers of glass beads with a diameter of 0.84 mm. The size of the quartz glass is 25 cm 25 cm, and that of the viewable area is 20 cm 20 cm, as shown in Fig. 2. Porosity and permeability of the model are tested as 0.38 and 2.01 mm2, respectively after the model is completed. During the process of foam injection, pipeline and simulative wells are heated with an electronic device, the temperature of which is in consistent with that of foam. The oil used in the experiment is lubricating oil produced by Mobile, and is brown under standard condition. The viscositytemperature curve of the oil is shown in Fig. 3. 2.2. Experimental methods and procedures 2.2.1. Thermal stability of bulk-foam and surfactant optimization Considering the temperature of hot-water in the visual displacement experiments, it is necessary to test the surfactant stability against high temperature. Foaming ability and stability are, respectively, reflected by foaming volume (Vm) and half-time (t1/2). The former is defined as the maximum volume of foam for a certain foaming agent solution shearing for several minutes at a certain temperature, and the latter is defined as the time that foam takes to decrease to half of its initial volume at the same temperature. In this part, three kinds of surfactant solutions, named CSL (sodium dodecyl sulfate), DRfoam (alpha olefin sulfonate) and HFA
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(Dodecyl dimethyl betaine), with the mass concentration of 0.5% are tested of foaming volume and half-time at 20 °C, 30 °C, 50 °C, 70 °C, 90 °C, respectively. The experimental apparatus include automatic mixer, visual reaction oven, glass rod, 1000-mL breaker, and stopwatch. During the experiment, 100-mL surfactant solution is injected into the reaction oven which is then kept at the experimental temperatures for three hours. Then, the surfactant solution is stirred by the automatic mixer at a rotating speed of 1400 r/min for five minutes. Afterwards, the foaming volume and half-time are measured at one temperature. The reaction oven is set at another temperature to test the corresponding foaming volume and half-time. Consequently, the foaming volume and half-time of the other two surfactants at different experimental temperatures are obtained with the previous method. 2.2.2. Visualized oil displacement experiments High oil viscosity usually brings on high displacement pressure, which is disadvantageous to the visual model. The experimental temperature is set as 90 °C and the oil viscosity is 63.1 mPa s. The experimental procedures are as follows. (1) Connect the experimental device according to Fig. 4. (2) Inject distilled water into visual model by the ISCO pump until liquid is stable at the outlet. (3) Inject simulative oil into the visual model until oil cut at the outlet is about 100%. Then, the model is left alone in an oven at 90 °C for 24 h to make the oil distribute uniformly. (4) Hot water flooding is conducted until the water cut at the outlet is about 95%. (5) After hot water flooding, the oil layer is swept at a certain degree, and thermal foam flooding is conducted to investigate the variation of sweep area in oil layer. Throughout the visual experiment, images are collected by an image acquisition system to compare the variation of sweep areas prior and posterior foam injection. Meanwhile, the whole process of water channeling in hot water flooding for heavy oil development is also tracked, as well as displacement pressure difference, liquid production, water production, etc. The physical property of the visualized model and the injection parameters of the fluid are listed in Table 1.
3. Results and discussions 3.1. Static performance of foaming agents The results of evaluation on the static performances of foaming agents at different temperatures are shown in Fig. 5 and Fig. 6. Fig. 5 presents the maximum foaming volume and Fig. 6 shows the half-time. Results show that the foaming volume and half-time of CSL are both maximum at high temperatures. Consequently, surfactant CSL is selected for the following displacement experiments. Surfactant is a substance that is active or highly enriched at the surface or interface between phases. However, if surfactant is adsorbed on the rock surface, it would be separated from liquid phase and would lose its main function as a foam generator. Surfactant adsorption to glass beads is tested with an injection syringe of 120 mL as shown in Fig. 7. The injection syringe is filled with glass beads to form porous media, and then surfactant solution with the concentration of 0.5% by weight is slowly injected into the syringe for 2 h at the rate of 0.5 mL/min. The weight of the glass beads in the syringe is 195.28 g. Afterwards, dry the syringe and weigh it with an electronic micro-balance. Result shows that the weight of the syringe before and after surfactant solution injection is 218.06 g and 218.12 g. Therefore, the adsorption of surfactant to glass beads is 0.31 mg/g, which indicates that the surfactant adsorption to glass beads is so slight that can be ignored.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 13. The lamellae snap-off mechanism of foam generation.
Fig. 14. Long bubbles in porous media.
3.2. Discussion on the plane sweep efficiency In order to intuitively understand the blocking characteristics and the mechanisms of enhancing oil recovery of foam in porous media, variation of areal sweep efficiency and dynamic variation of oil displacement are studied in the experiment with the visual model.
3.2.1. The process of hot water injection Macroscopic images of hot water (the blue liquid) injection process are shown in Fig. 8. It can be seen that hot water first flows along the direction that the pressure gradient is maximum on account of the mobility difference between hot water and oil, as well as the pressure propagation in the visual model. During the injection process, hot water is continuously heating the oil on both sides of the flowing path. The viscosity of oil decreases while flow ability increases. As a result, oil is carried out by the following hot
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 15. Foam blocking in porous media.
Fig. 16. Schematic diagram of foam blocking in porous media.
water, and the sweep efficiency is expanding constantly. It can be seen from the images that oil saturation is lower at the inlet, which means oil displacement efficiency is higher at near the injector. Due to the obvious viscosity fingering caused by the viscosity difference between oil and water, a lot of remaining oil is stranded on both sides of the main flow channel which has a strong inhibitory effect on the generation of other flow channels. As a consequence, inter-well communication is easily produced during hot water injection process due to different injection and production working systems. It would make hot water unevenly heat the areas around the wells and dash along the direction of inter-well communication, resulting in low sweep efficiency and low thermal efficiency. The experimental results show that the ultimate sweep efficiency of hot water flooding is 40.5%. 3.2.2. The process of thermal foam flooding Macroscopic images of thermal foam flooding process are shown in Fig. 9. The experimental temperature is 90 °C. Because the foaming agent solution is prepared with distilled water, so the color of main flow channel becomes shallow after thermal foam injection. It can be seen that foam first flows into the channels caused by the previous hot water flooding. Foam has greater apparent viscosity, and the flow resistance increases with the increase of injected volume. As the flow resistance in big pores exceeds that in small pores, foam diverts to flow into the small pores that have lower permeability. Moreover, with the continuous injection of foam, emulsions appear by the effect of surfactant and residual oil. The immobilized residual oil may also migrate forward with the following fluid due to foam collapse. As a result, the macroscopic sweep efficiency gradually increases and the ultimate value is 65.7%. Fig. 10(a) is the comparison diagram of effluent and original. The green is displacement water and the brown is
emulsion. After displacing for a long time, the oil changes to dark brown. Fig. 10(b) clearly presents large quantities of emulsions existing in the effluents, which is beneficial for the displacement of oil and contributes to the improved oil recovery. 3.3. Dynamic performance of displacement process Fig. 11 presents the variation of water cut and oil production rate during the displacement process. Fig. 12 reflects pressure drop variation at the inlet and oil displacement efficiency during the displacement process. Fig. 11 shows that in the initial period of hot water flooding, oil production rate remains 0.2 mL/min. Water free production period ends when the injection volume reaches 0.14 PV. From this point, oil production rate decreases while water cut increases rapidly. Hot water injection process ends as the injection volume reaches 1.52PV, and the ultimate oil recovery percentage of hot water flooding is 36.24%. At the beginning of thermal foam flooding, oil production rate increases greatly from 0.04 mL/min to 0.1 mL/min, and water cut decreases to 60.67%. With the further injection of foam, oil production rate gradually declines, while water cut gradually increases. Water cut reaches 95% when injection volume is 4.19 PV, at which point the ultimate oil recovery percentage is 67.54%, 31.30% higher than that of hot water flooding. Fig. 12 shows that pressure drop rapidly increases to 258 kPa at the initial period of hot water flooding. Then pressure drop slowly but not sharply decreases to 120 kPa after water breakthrough because of the relatively lower liquid injection rate in the experiments. After thermal foam injection, displacement pressure difference increases and finally becomes stable at about 394 kPa, indicating that foam has good blocking ability in porous media.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 17. Microscopic images of thermal foam flooding.
Fig. 18. Oil-water interfacial tension vs. surfactant concentration. Table 2 Comparison of hot water flooding and foam flooding.
Hot water flooding Foam flooding
Ev (%)
ED (%)
ER (%)
40.5 70.7
89.28 95.5
36.16 67.54
3.4. Microscopic mechanism of foam flooding 3.4.1. Foam blocking in porous media Foam is a dispersed system in porous media and influences
flow pattern of other fluids. In general, lamellae snap-off is considered as one of the main mechanisms of foam generation in porous media (Owete and Brigham, 1987; Hardy, 1986; Ransohoff and Radke, 1988), which is illustrated in Fig. 13. When a gas bubble penetrates a pore throat, the curvature of liquid film increases (Fig. 13(a)), as well as the relevant capillary pressure. As the front of liquid film passes through the throat, both the film curvature and relevant capillary pressure are decreased because of the extension of gas-liquid interface (Fig. 13(a) and (b)), which leads to a liquid phase pressure gradient. As a result, gas bubble collapses and new bubbles generate (Fig. 13(d)). Foam generation by snapoff is an interative process because pores and throats repeatedly appear in porous media. Meanwhile, long bubbles will be created as another blocking mechanism of foam when the injection rate is below the critical value (as shown in Fig. 14). During the process of foam migration in porous media, gas bubble fronts invade into pore space filled with liquid and deform to create long bubbles that will stay in pore space if capillary pressure or pressure gradient is not great enough (Fig. 14(b)), which holds up the flow channels of gas, resulting the decrease of gas phase relative permeability and the increase of gas bubble flow resistance. Since the generated foams occupy the big pores, the flow channel of hot water is blocked and the following injected fluid is diverted to the unswept region, hence expanding sweep efficiency of the oil layer, as is shown in Fig. 15. It can be seen from Fig. 15 (a) and (b) that fluid changes flow direction from A to B because of the blocking of foams (Fig. 16).
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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3.4.2. Micro-mechanism of thermal foam flooding For a certain reservoir, oil recovery percentage (ER) depends on sweep efficiency (Ev) and oil displacement efficiency (ED). Namely,
ER = EV⋅ED
(1)
The mechanisms of thermal foam flooding to enhance oil recovery mainly include increasing macroscopic sweep efficiency and microscopic oil displacement efficiency of oil layer, and the latter is shown in Fig. 17. The area marked in pink circle (Fig. 17(a)– (c)) is residual oil generated by hot water flooding. Here we can see that after hot water flooding, there is still plenty of residual oil existing in oil layer (Fig. 17(a)). During the process of thermal foam injection, the water stayed in mainstream channels is pushed by foam and flows to the residual oil (Fig. 17(b)). But most water still bypasses the residual oil and cannot carry it out. When foam arrives and contacts with the residual oil (Fig. 17(c)), the latter begins to move and gradually create emulsion, which is carried out by the following fluid. This is because the surfactant at the liquid film can decrease interfacial tension between oil and water as shown in Fig. 18, and improve the wettability of the reservoir (Hou et al., 2015; Mirchi et al., 2015). Therefore, under the disturbance of foam, the residual oil gradually separates from the pore space. Moreover, the following fluid has greater flooding ability due to the dissolution of surfactant. As a result, residual oil is displaced out of the pore space which is then occupied by the following foam and water (Fig. 17(d)). 3.5. Discussion of recovery mechanisms
(1) The effect of foam flooding mainly lies in two aspects: profile control in injection wells (as shown in Fig. 9) and water cut reduction in production wells (as shown in Fig. 11). Foam has greater apparent viscosity and blocks higher permeability pores, thus decreasing the mobility of fluid in high-permeability areas. As a result, the following hot water and foam are diverted to the lower permeability areas. With the constant injection of foam, the injection profile gradually becomes uniform. What's more, foam can effectively inhibit water channeling and make the displacing front relatively uniform between the injection and production wells. (2) Surfactant that flows into the oil layer can largely decrease oilwater interfacial tension (as shown in Fig. 18), thus increasing oil mobility. In addition, surfactant can increase displacement efficiency of hot water (as shown in Fig. 14). What's more, lamellae generated by nitrogen and surfactant can decrease the mobility of hot water and gas in reservoir. (3) As a non-condensable gas, nitrogen can maintain heat in wellbore and reduce heat loss in reservoir due to its low thermal conductivity. In addition, the injection of foam can raise bottom-hole pressure, leading to the increase of displacement pressure difference to recovery heavy oil. Combined with the experimental results, foam can further enhance heavy oil recovery after hot water flooding (as shown in Table 2).
4. Conclusion A 2D visualized model is adopted in this article to investigate the EOR mechanisms of foam flooding for heavy oil after hot-water flooding. Based on the experimental results, several conclusions are obtained as follows:
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(1) The experiment visually reproduces foam generation in porous media. Lamellae snap-off is the main mechanism of foam generation and long bubbles can block high-permeable channels to a certain extent. (2) During the hot water flooding process, hot water channeling is generated between the injection and production wells of the five-spot well pattern, causing limited sweep efficiency and plenty of residual crude oil. (3) Foam firstly enters the flow channels caused by hot water flooding and occupies pore space and diverts the following displacing fluid to unswept areas to improve sweep efficiency. Thermal foams can also effectively wash out residual oil and increase oil displacement efficiency. The ultimate oil recovery of thermal foam flooding is 67.54%, 31.30% higher than that of hot water flooding.
Acknowledgements This study is supported by National Natural Science Foundation of China (No. 51274212 and No. 51474226).
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Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection Zhengbin Wu a,n, Huiqing Liu a, Zhanxi Pang a, Yalong Wu a, Xue Wang a, Dong Liu b, Min Gao a a b
MOE Key Laboratory of Petroleum Engineering in China University of Petroleum, Beijing 102249, PR China Bohai Petroleum Institute, Tianjin Branch, CNOOC China Limited, Tianjin 200452, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 March 2016 Received in revised form 12 August 2016 Accepted 19 August 2016
Hot water flooding is an important method for the development of heavy oil. But due to water channeling and low enthalpy, oil recovery by hot water flooding is very limited. In this paper, a two-dimensional visual experiment is conducted to intuitively study foam flowing characteristics and blocking mechanisms in porous media, as well as the microscopic mechanisms of enhancing heavy oil recovery by foam after hot water flooding. The experiment visually reproduces foam generation in porous media and the processes of hot water flooding and foam flooding. The results show that foam flows along the channels with lower flow resistance in oil layer and can effectively improve the sweep efficiency. Compared with pure hot water flooding, the ultimate sweep efficiency increases from 40.5% to 70.7% after foam injection. Moreover, foam can effectively displace the residual oil caused by hot water flooding. The ultimate oil recovery of thermal foam flooding is 67.54%, 31.30% higher than that of hot water flooding. This paper can provide reference for the study for foam and thermal foam flooding in heavy oil recovery. & 2016 Elsevier B.V. All rights reserved.
Keywords: Heavy oil Hot water flooding Foam flooding Visual experiment Mechanisms
1. Introduction Thermal recovery methods such as cyclic steam stimulation (CSS), steam flooding, in-situ combustion are the major thermal recovery methods for the development of heavy oil (Closmann and Seba, 1983; Heel et al., 2008; Wu et al., 2011; Zhao et al., 2013). But with the increase of CSS cycles, oil production of a single well gradually decreases, and steam-oil ratio (SOR) increase dramatically in the later stage, resulting in the low economic benefit (Buger and Sahuquet, 1972; Tewari et al., 2011). As a result of viscosity fingering and steam override in heavy oil reservoirs, steam breakthrough usually appears earlier in steam flooding projects, leading to low sweep efficiency in both vertical and areal directions, and causing low thermal efficiency. In order to make thermal recovery project more profitable, it needs to take measures to make full use of the residual heat energy after steam injection process (Liu, 1998). The further development of the mature oil reservoirs has been an interactive and a broad subject (Abu El Ela et al., 2013). Hot water flooding is a typical alternate thermal recovery method for steam injection (Fournier, 1965; Wang et al., 2011a, n
Corresponding author. E-mail address: [email protected] (Z. Wu).
2011b; Alajmi et al., 2009; Vinsome 1974). Hot water flooding is actually an immiscible displacement process of crude oil by hot water and cold water (Dong et al., 2011). The initial purpose of injecting hot water is to increase injection ability of water wells instead of enhancing oil recovery. Hot water in reservoir on one hand can transfer heat to oil layer and increase reservoir temperature. On the other hand, it can implement reservoir energy and displace crude oil to wellbore. Compared with conventional water flooding, the EOR mechanisms of hot water flooding for heavy oil include decreasing crude oil viscosity to reduce mobility ratio, improving relative permeability and preventing the formation of oil strata with high viscosity, etc. (Abass and Fahmi, 2013; Lu et al., 2013; Lv et al., 2003). There are two significant problems with the application of hot water flooding in heavy oil reservoirs. One is that hot water cannot carry enough heat into reservoir because of its low enthalpy. The other is water channeling caused by reservoir heterogeneity and the density difference between hot water and crude oil. Hot water tends to flow to higher-permeable formation with lower flow resistance. Water channeling is easily formed between the injection and production wells as a consequence of viscosity fingering. The two disadvantages will cause earlier water breakthrough in production wells and have a negative effect on the thermal recovery for heavy oil. One of the feasible methods of enhancing oil recovery after hot water flooding in heavy oil reservoir is to add surfactant that can
http://dx.doi.org/10.1016/j.petrol.2016.08.023 0920-4105/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Trapped gas
Flowing bubble Glass bead Water
Fig. 1. Schematic diagram of foam propagation in homogenous model.
create foams in porous media (Lv et al., 2007; Lu et al., 2003; Muijs et al., 1988; Isaacs et al., 1994; Bagheri and Clark, 2015; Friedmann et al., 1991). Foam in porous media is a dispersive system that gas bubble disperses in liquid film, which is called lamellae. Lamellae snap-off is one of the main mechanisms of foam generation (Roof, 1970; Kovscek et al., 2007). Some long bubbles can also have a function of blocking. The distribution of foam in porous media is shown in Fig. 1 (Gauglitz et al., 2002). In general, foam is created by the co-injection of gas and surfactant solution. Gas bubbles dispersed in liquids cannot exist stable without surfactants. Foam preferentially enters high-permeable zone of formation to block big pores and diverts steam or hot water into low-permeable layer by its high apparent viscosity, thus expanding sweep efficiency of hot fluids injection process (Eson, 1983; Casteel and Djabbarah, 1988; Patzek, 1996). Surfactant maintains the stability of foam and makes a contribution to the reduction of oil/water interfacial tension (IFT) and the variation of reservoir wettability, hence improving oil displacement efficiency (Jamaloei et al., 2011; Liu et al., 2011; Kumar and Mandal, 2016). As a common non-condensate gas used in petroleum industry, nitrogen can implement reservoir pressure and maintain heat left by steam or hot water. In addition, nitrogen can be dissolved in crude oil to form miscible phase under high pressure, which is helpful to decrease heavy oil viscosity and improve oil mobility. What's more, the capillary pressure which usually acts as the main factor that traps the crude oil in pores will be zero when miscible phase is created. Therefore, the oil will be pushed out of the formation and recovery increases. One of the successful hot water foam EOR projects is conducted in
Fig. 3. Viscosity-temperature curve of the oil.
Block A of Xinjiang Oilfield in China (Wu et al., 2015). Compared with pure water flooding, hot water foam flooding massively lowered down the water cut and enhanced oil displacement efficiency from 29.8% to 53%. In addition, Block Jin90 of Liaohe Oilfield in China conducted hot water foam flooding pilots in 19-141 well group (Yuan et al., 2004). Results showed that from September 1996 to September 1999, the cumulative oil production was 3.54 104 t and the cumulative oil increment was 2.14 104 t. From January 2004 to December 2004, also in Jin 45-19-141 well group, hot water foam flooding was conducted again and brought a profit of 3.13 million dollars and the rate of output and input was 2.65 (Wang, 2006). To explore the EOR mechanisms of foam flooding after hot water flooding for heavy oil, a series of experiments are performed. Inspired by the studies of several researchers on visualizing the sweep efficiency improvement by chemical flooding (Pei et al., 2011, 2013; Wang et al., 2011a, 2011b; Guillen et al., 2012), in this paper, hot water flooding process is firstly carried out to display the macroscopic distribution of remaining oil in a visual model filled with glass beads. Then, foam is injected into this model. In this process, foam generation and migration in porous media are intuitively reproduced, which vividly reflects foam seepage characteristics and blocking mechanism. The more important is to make a comparison of sweep efficiency and oil recovery prior and posterior foam injection. Finally, the EOR mechanisms of foam flooding after hot water flooding for heavy oil are summarized. This work is of significance for the selection of foam systems and the implementation for foam projects.
(a) Side elevation diagram
(b) Porous media area
(c) Top view diagram
Fig. 2. Structure diagram of the visualization model. 1 – nut; 2 – model holder; 3 – silicone pad; 4 – quartz glass; 5 – porous media; 6 – glass beads; 7 – draining trench; 8 – tape; 9 – injection pot; 10 – production pot. (a) Side elevation diagram. (b) Porous media area. (c) Top view diagram.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 4. Schematic diagram of visual experiment. Table 1 Physical property of the visualized model and the injection parameters of fluid. Parameter Total pore volume (mL)
Initial oil saturation (%)
Injection flow rate (mL/min)
Foam quality
Value
100
0.2
0.8
22.4
Fig. 6. Half-time vs. temperature.
Fig. 5. Maximum foaming volume vs. temperature.
2. Experimental methods and setup 2.1. Experimental equipment and materials A visual experimental study of foam flowing and blocking in porous media is conducted. The experimental device is composed of the visual physical model which can resist a pressure of 2.5 MPa and a temperature of 300 °C, injection system, data acquisition and measuring system. The injection system includes a pump with a constant pressure and a varied injection rate, an oil tank, a water tank, a surfactant tank, a nitrogen tank, a six-port valve and several two-port valves. Data acquisition and measuring system includes a high-definition camera, a flat lamp, pressure sensors,
Fig. 7. Schematic of the injection syringe.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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(a) 0.12PV
(c) 1.448PV
(b) 0.98PV Inllet
Outlet
Fig. 8. Process of hot water flooding.
(a) 1.76P PV
(b) 2.42PV Inllet
(c)) 3.94PV Outlet
Fig. 9. Process of thermal foam flooding.
Fig. 10. Emulsion in the effluent.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 11. Water cut and oil production rate vs. injected volume.
Fig. 12. Oil recovery and pressure drop vs. injected volume.
temperature sensors, data acquisition device, measuring cylinders and a computer. Physical model includes a model holder, two pieces of heat-resistant quartz glass and a constant-temperature oven. The visual model consists of two glasses, one of which has four holes for installing simulative wells. In this paper, the visual model simulates 1/4 five-spot well pattern. The space between two glasses is filled with two layers of glass beads with a diameter of 0.84 mm. The size of the quartz glass is 25 cm 25 cm, and that of the viewable area is 20 cm 20 cm, as shown in Fig. 2. Porosity and permeability of the model are tested as 0.38 and 2.01 mm2, respectively after the model is completed. During the process of foam injection, pipeline and simulative wells are heated with an electronic device, the temperature of which is in consistent with that of foam. The oil used in the experiment is lubricating oil produced by Mobile, and is brown under standard condition. The viscositytemperature curve of the oil is shown in Fig. 3. 2.2. Experimental methods and procedures 2.2.1. Thermal stability of bulk-foam and surfactant optimization Considering the temperature of hot-water in the visual displacement experiments, it is necessary to test the surfactant stability against high temperature. Foaming ability and stability are, respectively, reflected by foaming volume (Vm) and half-time (t1/2). The former is defined as the maximum volume of foam for a certain foaming agent solution shearing for several minutes at a certain temperature, and the latter is defined as the time that foam takes to decrease to half of its initial volume at the same temperature. In this part, three kinds of surfactant solutions, named CSL (sodium dodecyl sulfate), DRfoam (alpha olefin sulfonate) and HFA
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(Dodecyl dimethyl betaine), with the mass concentration of 0.5% are tested of foaming volume and half-time at 20 °C, 30 °C, 50 °C, 70 °C, 90 °C, respectively. The experimental apparatus include automatic mixer, visual reaction oven, glass rod, 1000-mL breaker, and stopwatch. During the experiment, 100-mL surfactant solution is injected into the reaction oven which is then kept at the experimental temperatures for three hours. Then, the surfactant solution is stirred by the automatic mixer at a rotating speed of 1400 r/min for five minutes. Afterwards, the foaming volume and half-time are measured at one temperature. The reaction oven is set at another temperature to test the corresponding foaming volume and half-time. Consequently, the foaming volume and half-time of the other two surfactants at different experimental temperatures are obtained with the previous method. 2.2.2. Visualized oil displacement experiments High oil viscosity usually brings on high displacement pressure, which is disadvantageous to the visual model. The experimental temperature is set as 90 °C and the oil viscosity is 63.1 mPa s. The experimental procedures are as follows. (1) Connect the experimental device according to Fig. 4. (2) Inject distilled water into visual model by the ISCO pump until liquid is stable at the outlet. (3) Inject simulative oil into the visual model until oil cut at the outlet is about 100%. Then, the model is left alone in an oven at 90 °C for 24 h to make the oil distribute uniformly. (4) Hot water flooding is conducted until the water cut at the outlet is about 95%. (5) After hot water flooding, the oil layer is swept at a certain degree, and thermal foam flooding is conducted to investigate the variation of sweep area in oil layer. Throughout the visual experiment, images are collected by an image acquisition system to compare the variation of sweep areas prior and posterior foam injection. Meanwhile, the whole process of water channeling in hot water flooding for heavy oil development is also tracked, as well as displacement pressure difference, liquid production, water production, etc. The physical property of the visualized model and the injection parameters of the fluid are listed in Table 1.
3. Results and discussions 3.1. Static performance of foaming agents The results of evaluation on the static performances of foaming agents at different temperatures are shown in Fig. 5 and Fig. 6. Fig. 5 presents the maximum foaming volume and Fig. 6 shows the half-time. Results show that the foaming volume and half-time of CSL are both maximum at high temperatures. Consequently, surfactant CSL is selected for the following displacement experiments. Surfactant is a substance that is active or highly enriched at the surface or interface between phases. However, if surfactant is adsorbed on the rock surface, it would be separated from liquid phase and would lose its main function as a foam generator. Surfactant adsorption to glass beads is tested with an injection syringe of 120 mL as shown in Fig. 7. The injection syringe is filled with glass beads to form porous media, and then surfactant solution with the concentration of 0.5% by weight is slowly injected into the syringe for 2 h at the rate of 0.5 mL/min. The weight of the glass beads in the syringe is 195.28 g. Afterwards, dry the syringe and weigh it with an electronic micro-balance. Result shows that the weight of the syringe before and after surfactant solution injection is 218.06 g and 218.12 g. Therefore, the adsorption of surfactant to glass beads is 0.31 mg/g, which indicates that the surfactant adsorption to glass beads is so slight that can be ignored.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 13. The lamellae snap-off mechanism of foam generation.
Fig. 14. Long bubbles in porous media.
3.2. Discussion on the plane sweep efficiency In order to intuitively understand the blocking characteristics and the mechanisms of enhancing oil recovery of foam in porous media, variation of areal sweep efficiency and dynamic variation of oil displacement are studied in the experiment with the visual model.
3.2.1. The process of hot water injection Macroscopic images of hot water (the blue liquid) injection process are shown in Fig. 8. It can be seen that hot water first flows along the direction that the pressure gradient is maximum on account of the mobility difference between hot water and oil, as well as the pressure propagation in the visual model. During the injection process, hot water is continuously heating the oil on both sides of the flowing path. The viscosity of oil decreases while flow ability increases. As a result, oil is carried out by the following hot
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 15. Foam blocking in porous media.
Fig. 16. Schematic diagram of foam blocking in porous media.
water, and the sweep efficiency is expanding constantly. It can be seen from the images that oil saturation is lower at the inlet, which means oil displacement efficiency is higher at near the injector. Due to the obvious viscosity fingering caused by the viscosity difference between oil and water, a lot of remaining oil is stranded on both sides of the main flow channel which has a strong inhibitory effect on the generation of other flow channels. As a consequence, inter-well communication is easily produced during hot water injection process due to different injection and production working systems. It would make hot water unevenly heat the areas around the wells and dash along the direction of inter-well communication, resulting in low sweep efficiency and low thermal efficiency. The experimental results show that the ultimate sweep efficiency of hot water flooding is 40.5%. 3.2.2. The process of thermal foam flooding Macroscopic images of thermal foam flooding process are shown in Fig. 9. The experimental temperature is 90 °C. Because the foaming agent solution is prepared with distilled water, so the color of main flow channel becomes shallow after thermal foam injection. It can be seen that foam first flows into the channels caused by the previous hot water flooding. Foam has greater apparent viscosity, and the flow resistance increases with the increase of injected volume. As the flow resistance in big pores exceeds that in small pores, foam diverts to flow into the small pores that have lower permeability. Moreover, with the continuous injection of foam, emulsions appear by the effect of surfactant and residual oil. The immobilized residual oil may also migrate forward with the following fluid due to foam collapse. As a result, the macroscopic sweep efficiency gradually increases and the ultimate value is 65.7%. Fig. 10(a) is the comparison diagram of effluent and original. The green is displacement water and the brown is
emulsion. After displacing for a long time, the oil changes to dark brown. Fig. 10(b) clearly presents large quantities of emulsions existing in the effluents, which is beneficial for the displacement of oil and contributes to the improved oil recovery. 3.3. Dynamic performance of displacement process Fig. 11 presents the variation of water cut and oil production rate during the displacement process. Fig. 12 reflects pressure drop variation at the inlet and oil displacement efficiency during the displacement process. Fig. 11 shows that in the initial period of hot water flooding, oil production rate remains 0.2 mL/min. Water free production period ends when the injection volume reaches 0.14 PV. From this point, oil production rate decreases while water cut increases rapidly. Hot water injection process ends as the injection volume reaches 1.52PV, and the ultimate oil recovery percentage of hot water flooding is 36.24%. At the beginning of thermal foam flooding, oil production rate increases greatly from 0.04 mL/min to 0.1 mL/min, and water cut decreases to 60.67%. With the further injection of foam, oil production rate gradually declines, while water cut gradually increases. Water cut reaches 95% when injection volume is 4.19 PV, at which point the ultimate oil recovery percentage is 67.54%, 31.30% higher than that of hot water flooding. Fig. 12 shows that pressure drop rapidly increases to 258 kPa at the initial period of hot water flooding. Then pressure drop slowly but not sharply decreases to 120 kPa after water breakthrough because of the relatively lower liquid injection rate in the experiments. After thermal foam injection, displacement pressure difference increases and finally becomes stable at about 394 kPa, indicating that foam has good blocking ability in porous media.
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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Fig. 17. Microscopic images of thermal foam flooding.
Fig. 18. Oil-water interfacial tension vs. surfactant concentration. Table 2 Comparison of hot water flooding and foam flooding.
Hot water flooding Foam flooding
Ev (%)
ED (%)
ER (%)
40.5 70.7
89.28 95.5
36.16 67.54
3.4. Microscopic mechanism of foam flooding 3.4.1. Foam blocking in porous media Foam is a dispersed system in porous media and influences
flow pattern of other fluids. In general, lamellae snap-off is considered as one of the main mechanisms of foam generation in porous media (Owete and Brigham, 1987; Hardy, 1986; Ransohoff and Radke, 1988), which is illustrated in Fig. 13. When a gas bubble penetrates a pore throat, the curvature of liquid film increases (Fig. 13(a)), as well as the relevant capillary pressure. As the front of liquid film passes through the throat, both the film curvature and relevant capillary pressure are decreased because of the extension of gas-liquid interface (Fig. 13(a) and (b)), which leads to a liquid phase pressure gradient. As a result, gas bubble collapses and new bubbles generate (Fig. 13(d)). Foam generation by snapoff is an interative process because pores and throats repeatedly appear in porous media. Meanwhile, long bubbles will be created as another blocking mechanism of foam when the injection rate is below the critical value (as shown in Fig. 14). During the process of foam migration in porous media, gas bubble fronts invade into pore space filled with liquid and deform to create long bubbles that will stay in pore space if capillary pressure or pressure gradient is not great enough (Fig. 14(b)), which holds up the flow channels of gas, resulting the decrease of gas phase relative permeability and the increase of gas bubble flow resistance. Since the generated foams occupy the big pores, the flow channel of hot water is blocked and the following injected fluid is diverted to the unswept region, hence expanding sweep efficiency of the oil layer, as is shown in Fig. 15. It can be seen from Fig. 15 (a) and (b) that fluid changes flow direction from A to B because of the blocking of foams (Fig. 16).
Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i
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3.4.2. Micro-mechanism of thermal foam flooding For a certain reservoir, oil recovery percentage (ER) depends on sweep efficiency (Ev) and oil displacement efficiency (ED). Namely,
ER = EV⋅ED
(1)
The mechanisms of thermal foam flooding to enhance oil recovery mainly include increasing macroscopic sweep efficiency and microscopic oil displacement efficiency of oil layer, and the latter is shown in Fig. 17. The area marked in pink circle (Fig. 17(a)– (c)) is residual oil generated by hot water flooding. Here we can see that after hot water flooding, there is still plenty of residual oil existing in oil layer (Fig. 17(a)). During the process of thermal foam injection, the water stayed in mainstream channels is pushed by foam and flows to the residual oil (Fig. 17(b)). But most water still bypasses the residual oil and cannot carry it out. When foam arrives and contacts with the residual oil (Fig. 17(c)), the latter begins to move and gradually create emulsion, which is carried out by the following fluid. This is because the surfactant at the liquid film can decrease interfacial tension between oil and water as shown in Fig. 18, and improve the wettability of the reservoir (Hou et al., 2015; Mirchi et al., 2015). Therefore, under the disturbance of foam, the residual oil gradually separates from the pore space. Moreover, the following fluid has greater flooding ability due to the dissolution of surfactant. As a result, residual oil is displaced out of the pore space which is then occupied by the following foam and water (Fig. 17(d)). 3.5. Discussion of recovery mechanisms
(1) The effect of foam flooding mainly lies in two aspects: profile control in injection wells (as shown in Fig. 9) and water cut reduction in production wells (as shown in Fig. 11). Foam has greater apparent viscosity and blocks higher permeability pores, thus decreasing the mobility of fluid in high-permeability areas. As a result, the following hot water and foam are diverted to the lower permeability areas. With the constant injection of foam, the injection profile gradually becomes uniform. What's more, foam can effectively inhibit water channeling and make the displacing front relatively uniform between the injection and production wells. (2) Surfactant that flows into the oil layer can largely decrease oilwater interfacial tension (as shown in Fig. 18), thus increasing oil mobility. In addition, surfactant can increase displacement efficiency of hot water (as shown in Fig. 14). What's more, lamellae generated by nitrogen and surfactant can decrease the mobility of hot water and gas in reservoir. (3) As a non-condensable gas, nitrogen can maintain heat in wellbore and reduce heat loss in reservoir due to its low thermal conductivity. In addition, the injection of foam can raise bottom-hole pressure, leading to the increase of displacement pressure difference to recovery heavy oil. Combined with the experimental results, foam can further enhance heavy oil recovery after hot water flooding (as shown in Table 2).
4. Conclusion A 2D visualized model is adopted in this article to investigate the EOR mechanisms of foam flooding for heavy oil after hot-water flooding. Based on the experimental results, several conclusions are obtained as follows:
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(1) The experiment visually reproduces foam generation in porous media. Lamellae snap-off is the main mechanism of foam generation and long bubbles can block high-permeable channels to a certain extent. (2) During the hot water flooding process, hot water channeling is generated between the injection and production wells of the five-spot well pattern, causing limited sweep efficiency and plenty of residual crude oil. (3) Foam firstly enters the flow channels caused by hot water flooding and occupies pore space and diverts the following displacing fluid to unswept areas to improve sweep efficiency. Thermal foams can also effectively wash out residual oil and increase oil displacement efficiency. The ultimate oil recovery of thermal foam flooding is 67.54%, 31.30% higher than that of hot water flooding.
Acknowledgements This study is supported by National Natural Science Foundation of China (No. 51274212 and No. 51474226).
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Please cite this article as: Wu, Z., et al., A visual investigation of enhanced heavy oil recovery by foam flooding after hot water injection. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.08.023i