Article pubs.acs.org/EF
Visualization Study on Plugging Characteristics of TemperatureResistant Gel during Steam Flooding Yazhou Wang,†,‡ Huiqing Liu,† Zhanxi Pang,*,† and Min Gao† †
Key Laboratory of Petroleum Engineering of the Ministry of Education, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ Binnan Oil Production Plant, Shengli Oilfield of Sinopec, Dongying, Shandong 256600, People’s Republic of China ABSTRACT: Steam channeling is a serious problem during steam flooding in heavy oil reservoir. In this paper, an improvement measure was introduced to enhance oil recovery by injecting temperature-resistant gel during steam flooding in heavy oil reservoirs. A two-dimensional (2D) visualization physical model was designed to study the characteristics of viscous fingering and steam channeling during steam flooding. After steam channeling, temperature-resistant gel was injected into the 2D model. Then, the macro- and microscopic mechanisms of improving the development effect were analyzed after gel injection during steam flooding. The experimental results showed that steam channeling was easily formed between the injection well and the production well, resulting from a large viscosity difference of oil−steam or serious heterogeneity of formation, which led to a lower sweep efficiency. Gel first flowed into channeling paths and occupied large pores between the particles to make injected steam bypass mainstream channels and further improved oil recovery. The results showed that the final sweep efficiency after gel injection can reach 82.69%, which was 35.91% higher than steam flooding. The ultimate oil recovery was 60.45%, which was 15.17% higher than steam flooding.
1. INTRODUCTION
In this paper, a few of the sand-pack experiments were conducted to evaluate the plugging performance of temperatureresistant gel in porous media. Then, a two-dimensional (2D) visualization experiment was used to observe the process of oil displacement by steam injection during different stages, including steam flooding, temperature-resistant gel injection, and subsequent steam flooding. Finally, on the macroscopic and micro- levels, the EOR mechanisms of gel were summarized according to the results of 2D visualization experiments.
Presently, heavy oil and bitumen have become important resources in the petroleum supply for the world. However, heavy oil could not be recovered by traditional ways as a result of high viscosity under reservoir conditions.1 Thermal recovery methods, such as cyclic steam stimulation, steam flooding, steamassisted gravity drainage, and in situ combustion, are the most effective enhanced oil recovery (EOR) technologies for heavy oil.2,3 However, the serious heterogeneity and the large mobility difference between steam and oil easily result in viscous fingering and steam override.4,5 Gel injection is an effective technology to block water channeling between injection wells and production wells.6−9 It is also an effective method to improve sweep efficiency and enhance oil recovery through plugging steam channeling during steam injection.10−14 The most common polymers used in the petroleum industry are cross-linked with a specific cross-linker to produce suitable gels.15 If an inorganic cross-linker is used to prepare the gel, the gel has a short gelation time at above 60 °C.16,17 However, an organic cross-linker makes the gel always have good heat stability at high temperatures, even up to 150 °C.18,19 Some researchers also developed new temperature-resistant gels to plug water channeling or steam channeling.20,21 Dai et al. conducted a threedimensional physical experiment to study the gel profile control in heavy oil reservoirs.22 The results showed that the gel can effectively resist the path of steam channeling from expanding, which improved steam injection profile and enhance oil recovery. However, they only gave the blocking ability and the oil displacement effect of the gel in a high-salinity reservoir. All of the results were not used to explain how the gel increased sweep efficiency and improved oil recovery factor in heavy oil reservoirs. © XXXX American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. 2.1.1. Plugging Experiment. The experimental system is shown in Figure 1. The system includes a constant temperature oven, an ISCO pump, a sand-pack (100 cm in length and 3.8 cm in inner radius), two intermediate containers, pressure sensors, etc. The system can be used to measure the plugging performance of temperature-resistant gel in porous media. For the sand pack, the permeability is 2.15 μm2 and the porosity is 0.37. During these experiments, 0.2 pore volume (PV) temperature-resistant gel solution was first injected into the sand pack. After gel formation in the sand pack, steam flooding was carried out to determine the relationships between the pressure difference and the PV of steam injection. The experimental procedures are as follows. First, the temperature of the experimental system was adjusted to 90 °C, and then the ISCO pump was employed to inject formation water into the sand pack until a stable pressure difference that was called the fundamental pressure difference. Second, the temperature-resistant gel solution of 0.2 PV was injected into the sand pack by the ISCO pump. Third, the inlet and outlet of the sand pack were closed at least for 24 h to ensure the gel solidification in the sand pack. Then, the steam generator was used to continuously inject 200 °C of steam Received: May 4, 2016 Revised: August 8, 2016
A
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Schematic of the plugging experiment.
Figure 2. Schematic of the 2D visualization experiment.
Figure 3. Structure of the 2D visualization model. into the sand pack. In this process, the pressure difference between the inlet and outlet of the sand pack was recorded continuously, which was called the resistance pressure difference. The ratio of the resistance
pressure difference and the fundamental pressure difference was called as the resistance factor of temperature-resistant gel at a certain temperature. B
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 4. Appearance of the 2D visualization model.
Table 1. Ingredients of the Temperature-Resistant Gel temperature (°C)
FT (wt %)
AM (wt %)
90
3.0−3.5
2.5−3.0
cross-linker (wt %) control agent (wt %) solution viscosity (mPa·s) 0.05−0.1
0.0025−0.025
salinity (mg/L)
gelation time (h)
apparent viscosity (mPa s)
oily sewage saline water distilled water
48500 48500 0
6.0 5.5 5.3
33619 37526 39437
gelation time (h) apparent viscosity (mPa·s) 3−6
30000−50000
important part, which includes two quartz glass plates and a stainless frame. As shown in Figure 3, a layer of glass beads are first pasted on the inner side of one glass plate. Then, the two pieces of quartz glass plates are bonded by a temperature-resistant seal adhesive. As shown in Figure 4, the stainless frame is used to fasten the 2D model. A quarter of the five-point well pattern can be simulated to analyze the characteristics of steam channeling and the plugging performance of temperatureresistant gel in porous media. Two layers of 40-mesh glass beads are laid between the two glass plates that are sealed with temperature-resistant seal adhesive. The porosity is 0.38, and the permeability is 2.01 μm2. For the experimental parameters, the highest temperature is 200 °C, the highest affordable pressure is 3 MPa, the range of injection flux is from 0.1 to 0.5 mL/min, the size of the visual scope is 20 × 20 cm, and the size of glass beads is 40-mesh (0.38 mm).
Table 2. Salt-Resistant Performance of the TemperatureResistant Gel water sample
5−20
2.1.2. Two-Dimensional Visualization Experiment. The experimental apparatus mainly includes: high definition camera, macro lens, 2D visualization physical model, ISCO pump, steam generator, stopwatch, pressure gauge, and some measuring cylinders, as shown in Figure 2. Among them, the 2D visualization physical model is the most
Figure 5. Gelation state at different temperatures.
Figure 6. Resistance ability of gel versus PV during water flooding. C
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. Curves of production performance versus PV in difference stages.
Figure 8. Macroscopic images of sweep efficiency during steam flooding. D
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 9. Macroscopic images of sweep efficiency during gel injection. At 90 °C, three water samples, such as oily sewage, saline water, and distilled water, were used to simulate the influence of water type on the salt-resistant performance of gel in porous media. The experimental results are listed in Table 2. In the different water sample, the gelation times were roughly equal to each other and the apparent viscosities were all above 33 000 mPa·s. Therefore, the temperature-resistant gel was characterized by a perfect salt-resistant performance under real reservoir conditions. Then, the temperature-resistant gel was dissolved in saline water to analyze the influence of the temperature on the gelation performance. The temperatures were chosen as 90, 150, 200, and 250 °C. The gel solution was put into the high-temperature oven for 24 h, and then the gelation state was observed, as shown in Figure 5. The results showed that the gel kept a good gelation state when the temperature was lower than 200 °C. However, when the temperature was over 250 °C, the gel system was wholly carbonized after being heated for 24 h. Therefore, the highest affordable temperature can be chosen at 200 °C. 2.2.2. Reservoir Fluids. The oil sample from an actual oilfield of China is characterized by the density of 0.978 g/cm3 and the viscosity of 18 749.0 mPa s at reservoir temperature. The total salinity of formation water is about 48 500 mg/L. 2.3. Experimental Procedures. The procedures of the 2D visualization experiment are as follows: (1) The experimental system
Figure 10. Microscopic images when gel migrates in porous media. 2.2. Experimental Fluids. 2.2.1. Temperature-Resistant Gel. According to the current reservoir temperature and the salinity of formation water in an actual reservoir from China, a kind of nonionic temperature-resistant gel is chosen to conduct these experiments. The ingredients of the temperature-resistant gel are listed in Table 1. E
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 11. Macroscopic images of sweep efficiency during subsequent steam flooding after gel injection. (3) The ISCO pump was used to inject formation water into the visualization model at the rate of 0.2 mL/min until a stable state of water production at the outlet. (4) The oil sample was injected into the visualization model at the rate of 0.2 mL/min using the ISCO pump until the oil content reached 100% at the outlet. (5) The temperature of the steam generator was set to 200 °C. The ISCO pump and steam generator were used to inject steam into the visualization model at a constant rate of 0.2 mL/min (cold water equivalent). The steam temperature was at 200 °C, and the steam quality was 0.8. (6) When water cut at the outlet reached 95% during steam flooding, the temperature-resistant gel was injected into the visualization model at the rate of 0.2 mL/min. The total volume of gel solution was designed by considering 0.2 times steam swept volume. (7) After gel solution solidified completely, the ISCO pump was used to inject steam into the visualization model again at a constant rate of 0.2 mL/min until water cut reached 95% again at the outlet.
3. RESULTS 3.1. Plugging Performance. Using the model of a single sand pack, formation water was first injected into the sand pack until a steady state to obtain the fundamental pressure difference and then 0.2 PV gel solution was injected into the sand pack again
Figure 12. Curve of sweep efficiency during different stages. was exactly equipped according to Figure 2. Then, high-pressure nitrogen (3 MPa) was injected into the system to test leakage of the experimental apparatus. (2) The temperature of the entire experimental system was maintained at 50 °C, the original reservoir temperature. F
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. Statistic Results of Sweep Efficiency and Oil Recovery Factor during Different Stages stage
time (min)
PV
sweep efficiency (%)
incremental sweep efficiency (%)
oil recovery factor (%)
incremental recovery factor (%)
steam flooding gel injection subsequent steam flooding
57 8 58
1.35 0.15 1.32
46.78 49.55 82.69
2.77 33.14
45.28 47.96 60.45
2.68 12.49
at 90 °C. After the gel solidified completely, the formation water was injected into the sand pack again to flush the temperatureresistant gel to obtain the relationships between the residual resistance factor and injection volume of formation water. The PV was chosen as 0.5, 1, 2, 3, 4, 5, 7, and 10 PV. The experimental results are shown in Figure 6. The residual resistance factor gradually decreased with the PV of water increasing, but it was still more than 120 when the injected water was 10 PV. The results showed that solidification gel located in the path of water channeling was good at anti-flush ability. 3.2. Visualization Analysis. Figure 7 shows the curves of oil production, water cut, and pressure difference versus PV of injected fluids during a process of the 2D visualization experiment. The results showed that the oil production was maintained at about 0.2 mL/min at the beginning of steam flooding. When the PV of steam injection reached 0.15 PV, the water-free period was over and then oil production sharply decreased. When the PV of steam injection was 1.35 PV, steam flooding came to an end. Water cut began to be over 95%, and oil recovery of steam flooding is 45.28%. After the gel solution was injected into the model, oil production increased significantly from 0.01 to 0.04 mL/min and the water cut declined to 82.18 from 96.24%. In the process of subsequent steam flooding, oil production decreased smoothly. When the PV of steam injection was 2.55 PV, water cut came to 96.31% and oil recovery reached 60.45%, which was 15.17% higher than steam flooding. The pressure difference reached 0.354 MPa at the beginning of steam flooding. Then, the pressure difference decreased quickly and remained at about 0.109 MPa. After gel solution was injected into the sand pack, the pressure difference increased largely, reached 0.256 MPa, then gradually declined, and was finally maintained at about 0.182 MPa. 3.2.1. Stage of Steam Flooding. The macroscopic images of the steam flooding process are as shown in Figure 8. As a result of the large mobility difference between oil and steam or condensate water, steam first breaks through along the direction of the maximum pressure gradient to form a path of steam channeling. Heavy oil is mainly located in the both sides of steam channeling. Heavy oil is continually heated by injected steam and hot water to result in a large reduction of the viscosity of heavy oil and an obvious improvement of the flowing capacity. In this process, the swept zone of steam and hot water was gradually expanding and the displacement efficiency of the entrance was higher than the other zones. The fingering phenomenon of steam and hot water was obvious, resulting from the viscosity difference of oil−water, which made a lot of remaining oil on both sides of the mainstream line. 3.2.2. Stage of Gel Injection. The macroscopic images of the gel injection process are shown in Figure 9. The temperatureresistant gel was dyed blue. The gel solution also flowed forward along the path of steam channeling, but its viscosity was slightly higher than that of the water; therefore, it moved forward and slightly increased the swept area. After the gel solution was injected, the inlet and outlet of the 2D visualization model was shut in to make the temperature-resistant gel solidify.
Figure 13. Microscopic distribution image of remaining oil during steam flooding.
Figure 14. Microscopic distribution images of remaining oil during subsequent steam flooding after gel injection.
The microscopic images of gel injection are shown in Figure 10. It was clearly seen in the figures that the gel solution entered into the pores originally occupied by condensate water. After gel solidification, the original channeling path was blocked, which made steam and hot water divert to flow into the other zones with higher oil saturation. The reason is that solidified gel belongs to a kind of viscoelastic fluid, which migrates like a piston from a pore into another pore. The effect can largely increase the sweep efficiency in porous media.11 3.2.3. Stage of Subsequent Steam Injection. After gel solidification in the porous media, the macroscopic images of the subsequent steam flooding are shown in Figure 11. The results showed that solidification gel effectively blocked the steam channeling path to make subsequent steam flooding not flow along the original path. The swept area increased significantly after subsequent steam flooding. It indicated that the temperature-resistant gel played a significant role in blocking steam G
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 15. Fundamental mechanisms about increasing microscopic sweep efficiency.
channeling and enhancing oil recovery. Furthermore, there was still a weak migration phenomenon of the gel system in porous media during steam injection. Figure 12 shows the sweep efficiency of the 2D visualization model during different stages, including steam flooding, gel injection, and subsequent steam flooding. During steam flooding, the sweep efficiency first increases sharply and then increases lightly, as shown in Figure 12. Finally, the sweep efficiency becomes 46.78% and the oil recovery factor is 45.28% during the stage of steam flooding, as shown in Table 3. During the gelinjection stage, the sweep efficiency and oil recovery factor both obviously increase. The incremental sweep efficiency is 2.77% and the incremental recovery factor is 2.68% higher than steam flooding, as shown in Table 3. After gel solidification is formed, subsequent steam flooding begins. The sweep efficiency linearly increases before 93 min and then lightly increases to 82.69% at 123 min, as shown in Figure 12. The final oil recovery factor is 60.45%, which is 15.17% higher than steam flooding.
4. ANALYSIS AND DISCUSSION 4.1. Distribution of Remaining Oil. Figure 13 shows the microscopic distribution image during steam flooding. In this figure, the dark area is occupied by heavy oil but the bright yellow area is swept by steam or hot water. The spherical spot is glass beads. The remaining oil in the swept area can be divided into two types. One is the bypassed pocket of oil, and the other is retained oil. The bypassed pocket of oil results from microscopic heterogeneity of the pore structure. Steam always bypasses smaller pores and flows into larger pores, which are shown as zone A in Figure 13. Otherwise, during steam flooding, steam and condensate water cannot move evenly in porous media to make some oil remain in the thinner corner or absorb at the surface of the rock pore. This kind of residual oil is related to the wettability and shape of the pore structure, which are shown as zone B in Figure 13. Figure 14 shows microscopic distribution images of remaining oil during subsequent steam flooding after gel injection. It can be clearly observed that the remaining oil is displaced by subsequent
Figure 16. Fundamental mechanisms about increasing microscopic displacement efficiency.
steam after gel is injected. The pores are occupied by gel that can effectively block the path of lower resistance after steam breakthrough. 4.2. EOR Mechanisms. (1) The temperature-resistant gel increases microscopic sweep efficiency.9 When gel flows into higher permeability formation, it can make other fluids divert to flow into lower permeability or higher oil saturation formation. In the process of water flooding or steam flooding, injected fluids tend to flow into a larger pore to make residual oil remain in a tiny pore. As shown in Figure 15, the gel goes into larger pores that are originally occupied by water to increase the resistance of these pores, which results in injected water or steam diverting to H
DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels flow into thinner pores that are occupied by oil. As a result, the sweep efficiency of injected water or steam is largely increased, and therefore, the ultimate oil recovery will be improved. (2) The viscoelastic effect of gel increases microscopic displacement efficiency.11 Gel belongs to a kind of viscoelastic fluid that can decrease its whole velocity gradient in the capillary. Under the same conditions, the flow rate of water is higher than that of the polymer, whose flow rate is higher than the gel in the capillary, as shown in Figure 16. However, from the wall to the center in the capillary, the lower difference of the velocity gradient makes the migration of the gel like a piston, which is helpful to displace the oil film on the surface of pores in porous media, as shown in Figure 16b.
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5. CONCLUSION (1) The large viscosity difference between oil and steam or water leads to an obvious fingering phenomenon during steam flooding. Steam preferentially flows along the mainstream line between injection wells and production wells to form plenty of remaining oil. Steam channeling has an extremely negative impact on displacing remaining oil in heavy oil reservoirs. (2) The temperature-resistant gel has a strong ability of salt resistance and anti-flush under reservoir conditions. As a result of gel plugging the paths of steam channeling, steam or hot water is diverted to flow into lower permeability formation and a higher oil saturation zone. The final sweep efficiency of subsequent steam flooding after gel injection can reach 82.69%, which is 35.91% higher than steam flooding. The ultimate oil recovery is 60.45%, which is 15.17% higher than steam flooding. (3) The mechanisms of EOR mainly involve the enlargement of sweep efficiency and the improvement of oil displacement efficiency in porous media. The gel can increase microscopic sweep efficiency as a result of higher apparent viscosity in porous media. Furthermore, the gel can largely enhance microscopic displacement efficiency, resulting from the viscoelastic effect in the capillary.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-010-89739827. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The study was supported by the National Natural Science Foundation of China (51104165), the National Science and Technology Major Projects of China (2016ZX05009001), and the Science Foundation of China University of Petroleum, Beijing (2462015YQ0202).
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b01082 Energy Fuels XXXX, XXX, XXX−XXX
