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CO2 assisted steam flooding in late steam flooding in heavy oil reservoirs
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 46, Issue 6, December 2019 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2019, 46(6): 1242–1250.
RESEARCH PAPER
CO2 assisted steam flooding in late steam flooding in heavy oil reservoirs XI Changfeng1, 2,*, QI Zongyao1, 2, ZHANG Yunjun1, 2, LIU Tong1, 2, SHEN Dehuang1, 2, MU Hetaer3, DONG Hong3, LI Xiuluan1, 2, JIANG Youwei1, 2, WANG Hongzhuang1, 2 1. State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of Exploration & Development, Beijing 100083, China; 2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China; 3. Research Institute of Petroleum Exploration & Development, PetroChina Xinjiang Oilfield Company, Karamary 834000, China
Abstract: To improve the oil recovery and economic efficiency in heavy oil reservoirs in late steam flooding, taking J6 Block of Xinjiang Oilfield as the research object, 3D physical modeling experiments of steam flooding, CO2-foam assisted steam flooding, and CO2 assisted steam flooding under different perforation conditions are conducted, and CO2-assisted steam flooding is proposed for reservoirs in the late stage of steam flooding. The experimental results show that after adjusting the perforation in late steam flooding, the CO2 assisted steam flooding formed a lateral expansion of the steam chamber in the middle and lower parts of the injection well and a development mode for the production of overriding gravity oil drainage in the top chamber of the production well; high temperature water, oil, and CO2 formed stable low-viscosity quasi-single-phase emulsified fluid; and CO2 acted as a thermal insulation in the steam chamber at the top, reduced the steam partial pressure inside the steam chamber, and effectively improved the heat efficiency of injected steam. Based on the three-dimensional physical experiments and the developed situation of the J6 block in Xinjiang Oilfield, the CO2 assisted steam flooding for the J6 block was designed. The application showed that the CO2 assisted steam flooding made the oil vapor ratio increase from 0.12 to 0.16 by 34.0%, the oil recovery increase from 16.1% to 21.5%, and the final oil recovery goes up to 66.5% compared to steam flooding after perforation adjustment. Key words: heavy oil reservoir; three-dimensional physical simulation experiment; steam flooding; CO2 assisted steam flooding; steam chamber; steam (CO2) chamber overriding gravity drainage
Introduction At present, heavy oil reservoirs are mainly developed by thermal production technologies, including steam huff and puff, steam flooding, steam assisted gravity drainage technology and fire flooding etc.[14] Steam flooding is an important replacement technology of steam huff and puff for heavy oil reservoirs. However, problems such as low oil-steam ratio, high water cut and steam channeling often occur in the later stage of steam flooding, and with the economic benefits turning extremely poor, the steam flooding has to be stopped. Therefore, it is urgent to find new ways of reservoir development[56]. In view of the problems existing in steam flooding technology, gas foam, high-temperature gel and other steam flooding profile control technologies have been proposed at home and abroad[710]. Since CO2 dissolved in heavy oil can expand oil volume, reduce the oil viscosity and interfacial
tension[1113], it can be used as the main auxiliary agent for steam assisted recovery of heavy oil reservoirs[1417]. Bagci et al.[18] studied the influence of continuous injection of CO2 on heavy oil recovery through one-dimensional physical simulation experiment. The experimental results showed that CO2 assisted steam flooding increased recovery by about 15.6% than steam flooding, and the gas-vapor ratio was the key parameter affecting the ultimate recovery. Tao et al.[19], Ouyang et al.[20], Li et al.[21], Sun et al.[22] conducted in-depth experimental researches and numerical calculations on the migration mechanism of CO2 and heavy oil. Jha et al.[2329] discussed the impact of CO2 injection on enhancing oil recovery, and conducted laboratory experiments and numerical simulation studies on the interaction between supercritical CO2 and superheated steam, proving that CO2 injection was an effective measure to improve recovery of heavy oil reservoirs. But the researches about CO2 assisted recovery in heavy oil
Received date: 18 Dec. 2018; Revised date: 05 Aug. 2019. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05012-002). https://doi.org/10.1016/S1876-3804(19)60277-6 Copyright © 2019, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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reservoirs in China and abroad before mostly focused on the lab experiment and theoretical research of interaction mechanism between CO2 and steam, or profile control technical measures, few three-dimensional physical simulation research was carried out according to actual reservoir conditions by similar proportion method in order to apply simulation results directly to extended reservoir production. The researches were more theoretical, but lack of sustainability, systematicness and pertinence, and couldn’t solve the fundamental problems of steam channeling and oil recovery enhancement in late steam flooding stage. There is no effective replacement development mode and technology in the later stage of steam flooding[3031]. In this study, J6 block in Xinjiang oilfield was taken as a typical example, according to the specific reservoir conditions and production technology, based on the principle of similar scale, the three-dimensional physical simulation experiments on steam flooding, CO2 assisted steam flooding, CO2 foam assisted steam flooding under the condition of full section perforation, and steam flooding, CO2 assisted steam flooding experiments under lower half part perforation of lower section were carried out, and CO2 assisted steam flooding development technology was put forward in the late stage of steam flooding. Then, the CO2 assisted steam flooding was designed and applied based on the results of three-dimensional physical simulation experiment and the development status of steam flooding in block J6 of Xinjiang oilfield.
1. Three-dimensional physical simulation experiment of CO2 assisted steam flooding after steam flooding Aiming at the problems of steam channeling and low oil-steam ratio in the late stage of steam flooding in block J6
Fig. 1. Table 1.
of Xinjiang oilfield, the development effects of steam flooding, CO2 assisted steam flooding, CO2 foam assisted steam flooding under the condition of full oil layer perforation, and steam flooding and CO2 assisted steam flooding under the condition of perforating only the lower half of the lower oil layer were studied by physical modeling. 1.1.
The equipment
The steam flooding in block J6 of Xinjiang oilfield adopts the inverse nine-point area well pattern, so the three-dimensional physical simulation experimental model took a quarter of the inverse nine-point area well pattern (Fig. 1). In order to study the impact of perforation location on development effect, 8 wells were set up in the model, that is, 2 wells were set up for each well location to model the cases of perforating the whole oil layer and the lower half of the lower oil layer respectively. According to the actual reservoir conditions, the filling sand model was designed, and the model parameters were designed according to the similar proportion method (Table 1)[14]. The model had an upper oil layer, a middle physical interlayer and lower oil layer, which respectively represent the J3q22-1 (2-1 small layer of the second oil layer group in upper Jurassic Qigu Formation) small layer, physical interlayer and J3q22-2 small layer in the oilfield. 1.2.
The process
During the experiment, the initial steam injection speed of steam flooding stage was 80 mL/min (water equivalent), and the steam injected was superheated steam from vapor generator, when it is injected into model, the steam quality was more than 85%, and the later steam injection speed was appropriately adjusted according to the experimental process. Under
Appearance and well pattern diagram of filling sand model in 3D physical modeling experiment.
Designed parameters of the model.
Model Reservoir prototype Physical model
Geometry 60 m×60 m×40 m 30 cm×30 cm×20 cm
Thickness Upper layer Physical interlayer Lower layer 16 m 4 m. 20 m. 8cm 2 cm 10 cm
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Permeability/103 μm2 Upper layer Physical interlayer Lower layer 2600 200 1200 2600 200 1200
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the condition of adding CO2 gas, the sum volume of injected steam and CO2 was basically the same as that of steam injected under the same temperature and pressure condition of steam flooding. 1.2.1.
Physical modeling experiment of full perforation
The full perforation physical simulation experiment can be divided into three stages: steam flooding, CO2 assisted steam flooding, and CO2 foam assisted steam flooding. During the experiment, all steam injection wells and production wells adopted full perforation of all the oil layers, which was consistent with the oilfield perforation mode, and there was no adjustment during the experiment. 1.2.2. Physical modeling experiment of perforating adjustment after steam flooding The physical modeling experiment process of perforating adjustment after steam flooding is mainly divided into two stages: steam flooding and CO2 assisted steam flooding. In the steam flooding stage, the injection wells and production wells took the whole oil layer perforated manner. After steam channeling, when the water cut reached 95%, the steam flooding stage ended. Then, the injection wells and production wells were adjusted the perforation section, with only the lower half part of the lower oil layer was perforated for CO2 assisted steam flooding.
2. 2.1.
The analysis and discussion Physical modeling experiment of full perforation
In the steam flooding stage, the initial steam injection rate was 80 mL/min. The results of steam flooding and combined steam flooding under the condition of the whole oil layer perforated are shown in Table 2 and the temperature field is shown in Fig. 2. The experiment included three stages. The first stage is steam flooding stage. It can be seen that in the case of whole oil layer perforated, the gas adsorption layer was mainly the upper oil layer and steam channeled quickly along the upper oil layer and entered the production well. Subsequently, the produced liquid showed high temperature and high water cut, and the steam flooding was forced to stop. The second stage was the mixed injection mode of steam and CO2. The development effect was improved because CO2 can improve oil displacement efficiency and thermal utilization efficiency. But steam (gas) channeling soon occurred, and CO2 assisted steam flooding couldn’t be carried out effectively. In order to effectively suppress steam (gas) channeling, CO2 foam assisted steam flooding was adopted in the third stage of Table 2.
the experiment. In this stage, CO2 foam slug and steam were injected alternately. In the first cycle, 0.05 PV (pore volume multiple) CO2 foam slug was injected, followed by 0.25 PV steam. In the second cycle, a CO2 foam slug of 0.10 PV was injected, and then 0.25 PV steam was injected. In the third cycle, 0.15 PV CO2 foam slug was injected, and then 0.25 PV steam was injected. It can be seen that with the development of steam chamber, in order to seal the upper steam chamber and prevent steam producing from the upper part of perforation interval, the amount of CO2 foam injected increased gradually, and the plane sweep efficiency increased, but the vertical sweep efficiency increased little (Fig. 2d), so CO2 foam did not effectively improve development effect and economic benefit became worse and worse. Apparently, it is difficult to enhance the vertical producing degree of reservoir simply by injecting foam. It should be noted that the CO2 foam-agent used in the experiment is the best high-temperature resistance foam-agent that can be obtained after repeated screening and evaluation by the author’s project team. At the temperature of over 250 C, the resistance factor of the foam can still reach more than 20[32]. In conclusion, CO2 assisted steam flooding and CO2 foam assisted steam flooding can enhance reservoir sweep volume after steam channeling within a certain range. But for the reservoir with a large vertical thickness of oil layer, they cannot fundamentally solve the problem of vertical sweep volume of reservoir and fail to produce the lower reservoir effectively. 2.2. Physical modeling experiment of perforation adjustment after steam flooding After steam flooding and perforation adjustment, the recovery effects of CO2-assisted steam flooding are shown in Table 3, and the temperature field is shown in Fig. 3. From Table 2, Table 3, Fig. 2a and Fig. 3a, it can be seen that in steam flooding stage, the actual production performance and experimental effect are basically consistent because the experimental conditions are similar. After the perforation section was adjusted to the lower half part of the lower oil layer, CO2-assisted steam flooding was carried out smoothly without steam (gas) channeling. No foam was needed for injection and production profile control. The time of this stage was 4.5 h, which means the production can be kept stable for a long time in this manner. The stage recovery percent was 61% and the oil/steam ratio was 0.32. During the experiment process, the steam chamber expanded gradually from the upper part of production wells downward, and the recovery rate
Results of steam flooding and combined steam flooding under the condition of whole oil layer perforated.
Displacement mode Steam flooding CO2 assisted steam flooding CO2 foam assisted steam flooding2 Total
Stage recovery/% 22.5 7.2 10.1 39.8
Stage water cut/% 95.53 94.28 95.10
Steam injection pore volume multiple 1.25 0.26 0.75 2.26
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CO2 (foam) injection pore volume multiple 0 0.07 0.3 0.3
Stage oil steam ratio 0.11 0.12 0.09 0.10
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Fig. 2. Table 3.
Temperature fields of steam flooding and combined steam flooding under the condition of whole oil layer perforated.
Effect of CO2-assisted steam flooding with perforation adjustment after steam flooding.
Displacement mode Steam flooding CO2-assisted steam flooding Total
Oil recovery/ % 20.1 61.0 81.1
Stage water cut/ % 95.2 75.1
increased from 20.1% to 81.1%, forming an efficient development mode. (1) The development mode of lateral expansion of steam chamber in the middle and lower part of injection well and overriding gravity drainage of steam chamber in the top layer of production well has been established. The injection rate ratio of CO2 to steam can be adjusted in a wide range. Because the perforation section of the steam injection well was adjusted to the lower half of the lower oil layer, the middle and lower parts of the formation showed signs of steam absorption in the initial stage of CO2-assisted steam flooding, and the steam channeling in the upper reservoir was inhibited to a certain extent (Fig. 3b). However, steam and CO2 quickly
Steam injection pore volume multiple 1.10 1.62 2.72
CO2 injection pore volume multiple 0 0.66 0.66
Stage oil steam ratio 0.12 0.32 0.24
overlapped the upper oil layer, and the steam chamber expanded completely in the upper oil layer (Fig. 3c), and then expanded to the lower oil layer. Generally speaking, the steam chamber near the injection well has the characteristics of overall lateral expansion and overlap development in the upper oil layer. The steam chamber near the production well follows the law of vertical development from top to bottom, and the steam chamber near the production well has a larger vertical distance from the perforated section at the bottom of the reservoir, so the steam will not be produced from the upper reservoir section. The steam chamber can continuously expand vertically downward along the production well, making full use of the energy of injected steam. For production
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Fig. 3.
Temperature field of CO2-assisted steam flooding after steam flooding and perforation adjustment.
wells, the expansion front of steam chamber is a stable high temperature oil and water belt controlled by the vapor-liquid interface, which is similar to the control mechanism of steamliquid interface in steam-assisted gravity drainage (SAGD) production process[33]. There are fundamental differences between the long-term stable production of high-temperature liquid in production wells during CO2-assisted steam flooding and the rapid steam channeling of steam in conventional steam flooding, which results in shutdown or transition to intermittent steam flooding, thus forming a gravity drainage mode under the condition of vertical well pattern. Based on the above characteristics of steam chamber development and stable production process, it is not necessary to use CO2 foam for profile control and flooding. The injection parameters during CO 2 -assisted steam flooding need to match the consumption capacity of the steam chamber. After adjusting the perforation section, when the injection rate was changed from the initial steam injection rate of 80 mL/min to "65 mL/min of steam injection rate + 200 mL/min of CO2 injection rate (measurement value under standard condition)", and the injection lasted 15 min, high
temperature steam channeling occurred easily in the production wells, the steam chamber was hard to control, and the water cut fluctuated around 85%. When the injection parameters were adjusted to "50 mL/min steam injection rate + 250 mL/min CO2 injection rate", the production wells produced high temperature fluid stably, and the steam chamber developed steadily. This shows that the injection rate matches the maximum capacity of team chamber consumption. Under this condition, the injection lasted for 65 min, and the water cut was stable at about 75%. In order to verify the reasonable injection ratio parameter of CO2 and steam, the injection parameters were adjusted further to study the development of steam chamber and production performance of production wells. When the injection rate of CO2 was increased by adjusting the injection parameters to "steam injection rate of 50 mL/min + CO2 injection rate of 380 mL/min" (this rate has reached the maximum value of CO2 injection metering equipment), the steam chamber and production wells maintained a stable state with water content of about 65%, which lasted for 133 min. Then the injection parameters were adjusted again to “steam injection rate of 30 mL/min + CO2
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Fig. 4. Production dynamic curve of CO2-assisted steam flooding after steam flooding and perforation adjustment. ① Steam flooding at steam injection rate of 80 mL/min; ② CO2-assisted steam flooding at steam injection rate of 65 mL/min and CO2 injection rate of 200 mL/min; ③ CO2-assisted steam flooding at the steam injection rate of 50 mL/min and the CO2 injection rate of 250 mL/min for; ④ CO2-assisted steam flooding at the steam injection rate of 50 mL/min and the CO2 injection rate of 380 mL/min; ⑤ CO2-assisted steam flooding at the steam injection rate of 30 mL/min and CO2 injection rate of 380 mL/min.
Fig. 5.
Experimental production of CO2 emulsified foam oil.
injection rate of 380 mL/min”. Under this condition, the steam chamber expanded steadily, and the production well produced stably, with a water content of about 70%. But there was intermittent gas slug in the production wells in this period. The analysis shows that there was free gas phase CO2 produced, which lasted for 100 min, and the whole CO2-assisted steam flooding experiment ended. The production dynamic curve of CO2-assisted steam flooding after steam flooding and perforation adjustment is shown in Fig. 4. The experiments show that the injection rate of CO2 has a large adjustable range. When the ratio of CO2 injection rate to steam injection rate ranges from 5:1 to 13:1, the steam chamber can expand and develop steadily. From the whole experimental process, in the initial stage, the injection rate of CO2 should be low and the injected steam mainly forms stable chamber. In the rear stage, the injection rate of CO2 can be further increased when the steam
chamber develops to a certain volume. (2) High temperature water, oil, and CO2 formed stable low-viscosity quasi-single-phase emulsified fluid. When the ratio of CO2 and steam ranges from 5:1 to 13:1, the production wells can produce the stable emulsion fluid. The primary analysis indicates this emulsion fluid has characteristics of low pseudo viscosity and large expansion coefficient. The ratio of liquid and CO2 is about 1:6, its water cut is about 60%–70%. The emulsion fluid may formed when CO2 moves from steam chamber to production wells, especially passing through the interface between steam chamber and low liquid. (3) CO2 acts as heat insulation at the top of the steam chamber, reduces the steam partial pressure in the steam chamber, and effectively improves the thermal efficiency of injected steam. Unlike the near-oil drainage mode of SAGD development[3435], in the process of steam flooding, steam needs longdistance migration. Especially in gravity-stable steam flooding, during the steam chamber expansion, the steam needs to migrate the length of the whole well spacing, so the heat loss to the top caprock is more than 30%, in this case, CO2 can act as thermal insulation of the upper part of the reservoir. In addition, CO2 reduces the partial pressure of steam in the steam chamber and improves the latent heat utilization ratio of steam[36]. The comparative experiments under the same conditions show when 2.3 PV steam is injected without the assistance of CO2 after perforation adjustment, the recovery degree can reach 60%, but the oil-steam ratio is only 0.19.
3. Field application of CO2 assisted steam flooding after steam flooding 3.1.
Geologic profile of J6 Block in Xinjiang Oilfield
Nine typical steam flooding well groups in the J6 block of Xinjiang Oilfield were selected for the CO2-assisted steam
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flooding field test. The reservoir in this test area has a temperature of 20 C and crude oil viscosity of 7000–12 000 mPas, and is a typical heavy oil reservoir. The layer series of development of J6 block is J3q22-1+J3q22-2, which is a set of braided river delta front sediments. Among them, the sand body of J3q22-1 small layer has an average porosity of 30.3%, an average permeability of 2623×103 μm2, an average oil saturation of 78%, a thickness of 12 to 23 m and 16 m averagely. The J3q22-2 small layer sand body has an average porosity of 28.2%, average permeability of 1200×103 μm2, average oil saturation of 73%, a thickness of 13 to 26 m and 20 m on average. There is a set of stable argillaceous sandstone, sandy conglomerate and mudstone in local part between the J3q22-1 small layer and the J3q22-2 small layer, with a thickness of 1 to 6 m and 4 m on average. The argillaceous sandstone and sandy conglomerate have a permeability of 100×103 to 800×103 μm2, 200×103 μm2 on average, which theoretically can’t block vapor flow[37]. 3.2. Development situation of J6 Block in Xinjiang Oilfield The steam flooding of J6 block adopts inverse-nine point well pattern. From January 1st, 1989, steam huff-and-puff has been implemented for nearly 10 years, and then the conventional steam flooding has been carried out until January 1st, 2018. Affected by steam overlap and reservoir heterogeneity, the thermal breakthrough of steam occurred first at the top of the oil layer, and high-temperature steam or hot water was directly produced from the upper perforation section of the production well. Inefficient steam circulating layer was formed on the top of the J3q22-1 oil layer, then the volume of the underground steam hardly grew anymore. Intermittent steam injection and intermittent oil production have to been adopted in this field to maintain production at low efficiency[38].Then the heat utilization efficiency declined dramatically, and the steam flooding was basically futile. According to the data analysis and numerical simulation of four coring wells in the nine well groups of the test area (ranging from 35 to 50 m from the original injection steam well), it is found that the area near the steam injection well has higher steam sweeping range and larger drop of oil saturation; whereas in the area far from the steam injection well, the steam channeling path develops along the top of the reservoir, and the oil saturation of oil layer below the steam channel dropped little. In general, the steam chamber has been formed at the top of the oil layer in local part, but it is the steam channel of the production well due to the perforation of the whole oil layer. After steam breakthrough, the steam chamber cannot expand effectively, resulting in the mainly hot water flooding of the oil layer with low displacement efficiency, and oil-vapor gravity differentiation of the whole oil reservoir[39]. 3.3.
oil-steam ratio was 0.07, and the recovery percent was 45%. Due to steam channeling, using the original perforation mode and steam injection mode can’t produce effectively anymore. According to the above physical simulation experiment results and the optimized design results, the perforation was adjusted, and both the production wells and the steam injection wells were only perforated in the lower half of the J3q22-2 layer. According to the design principle of injection-production parameters of conventional steam flooding, the steam injection rate of J6 block was designed at 200 t/(km2md), and the daily volume of steam injection of single well should reach 80 t. Based on the development mode of gravity drainage under the condition of vertical well pattern, the CO2-assisted steam flooding was optimized by numerical simulation. The designed daily steam injection rate of single well was 50 t, the steam quality of injected steam at bottom hole should be more than 65%, and the daily CO2 injection rate was 1.0 t (500 m3). The following three schemes were compared: (1) Steam flooding with the whole reservoir section perforated at the injection rate of 50 t/d; (2) steam flooding after adjustment of perforation at the injection rate of 50 t/d; (3) CO2-assisted steam flooding after the adjustment of perforation. The predicted development effects of these development schemes are shown in Fig. 6 and Table 4. It can be seen from Fig. 6 and Table 4 that the scheme 1 is not feasible; after adjustment of perforation, the CO2-assisted steam flooding had better effect than steam flooding, with the oil-steam ratio increasing by 34.0% from 0.119 to 0.160, the stage recovery percent increasing from 16.1% to 21.5%, and the final recovery rate reaching 66.5%. By adjusting the perforation position and injecting CO2 for assistance, the steam chamber can be effectively expanded to the lower part of the reservoir, resulting in the successful gravity drainage under the condition of vertical well pattern (Fig. 7). The simulation results show that the CO2-assisted steam flooding after perforation adjustment resulted in the lateral expansion of the steam chamber in the middle and lower parts of the steam injection well and the overlap gravity drainage of the top steam chamber of the production well. The high-temperature water, oil and CO2 can form a stable emulsified pseudomonophasic fluid. CO2 acts as a heat insulator at the top of the steam chamber, reducing the partial steam pressure, effectively
Field application design and effects
From 2018, the nine steam-flooding well groups in the test area were basically in a state of shutdown before the flooding conversion. The total daily oil production was 10 t, the daily oil production per well was generally lower than 0.5 t, the
Fig. 6. Daily oil production curves of different development schemes.
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Table 4.
Predicted development effects of different development schemes.
Scheme Whole reservoir perforation steam flooding Steam flooding after perforation adjusted CO2-assisted steam flooding after perforation adjusted
Cumulative steam injection/104t 112.06 112.06
Cumulative CO2 Cumulative oil Cumulative Enhanced injection/104t production/104 m3 oil steam ratio recovery/% 0 3.92 0.035 4.7 0 13.40 0.119 16.1
112.06
2.67
16.68
0.160
21.5
the oil layer is mainly driven by hot water with low oil displacement efficiency. When the perforation is adjusted to the lower half of the lower oil layer after steam flooding, the gravity drainage mode under the vertical well pattern can be realized, the steam chamber can fully expand, and the oil recovery can be greatly improved. After adjusting the position of perforation, the CO2-assisted steam flooding can result in the lateral expansion of steam chamber in the middle-lower part of the steam injection well, and the overlap gravity drainage of the top steam chamber in the production well. High temperature water, oil and CO2 can form stable emulsified pseudomonophasic fluid. The CO2 can act as heat insulator at the top of the steam chamber, reduce the partial pressure of steam and effectively enhance the thermal efficiency of the injected steam. CO2-assisted steam flooding technology has been successfully implemented in the field, providing an effective replacement technology for improving the development performance and oil recovery of heavy oil reservoirs in the later stage of steam flooding.
References [1] Fig. 7. Steam chamber development before and after CO2 assisted steam flooding.
improving the thermal efficiency of the injected steam enabling the full expansion of the steam chamber, thereby greatly improving oil recovery. The test plan was implemented on October 31st, 2017, and the effect was remarkable. By September 1st, 2019, the daily oil production in the test area increased from 6 t to 53 t, and the oil-steam ratio increased from 0.04 to 0.12. The maximum daily oil production and oil-steam ratio is expected to reach 70 t and 0.16 respectively on December 31st, 2019. The test area will produce stably for 6 years, and eventually have an enhanced recovery of 20%. The success of this technology will provide an effective replacement technology for improving the development effect and oil recovery of heavy oil reservoirs in the late stage of steam flooding.
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RESEARCH PAPER
CO2 assisted steam flooding in late steam flooding in heavy oil reservoirs XI Changfeng1, 2,*, QI Zongyao1, 2, ZHANG Yunjun1, 2, LIU Tong1, 2, SHEN Dehuang1, 2, MU Hetaer3, DONG Hong3, LI Xiuluan1, 2, JIANG Youwei1, 2, WANG Hongzhuang1, 2 1. State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of Exploration & Development, Beijing 100083, China; 2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China; 3. Research Institute of Petroleum Exploration & Development, PetroChina Xinjiang Oilfield Company, Karamary 834000, China
Abstract: To improve the oil recovery and economic efficiency in heavy oil reservoirs in late steam flooding, taking J6 Block of Xinjiang Oilfield as the research object, 3D physical modeling experiments of steam flooding, CO2-foam assisted steam flooding, and CO2 assisted steam flooding under different perforation conditions are conducted, and CO2-assisted steam flooding is proposed for reservoirs in the late stage of steam flooding. The experimental results show that after adjusting the perforation in late steam flooding, the CO2 assisted steam flooding formed a lateral expansion of the steam chamber in the middle and lower parts of the injection well and a development mode for the production of overriding gravity oil drainage in the top chamber of the production well; high temperature water, oil, and CO2 formed stable low-viscosity quasi-single-phase emulsified fluid; and CO2 acted as a thermal insulation in the steam chamber at the top, reduced the steam partial pressure inside the steam chamber, and effectively improved the heat efficiency of injected steam. Based on the three-dimensional physical experiments and the developed situation of the J6 block in Xinjiang Oilfield, the CO2 assisted steam flooding for the J6 block was designed. The application showed that the CO2 assisted steam flooding made the oil vapor ratio increase from 0.12 to 0.16 by 34.0%, the oil recovery increase from 16.1% to 21.5%, and the final oil recovery goes up to 66.5% compared to steam flooding after perforation adjustment. Key words: heavy oil reservoir; three-dimensional physical simulation experiment; steam flooding; CO2 assisted steam flooding; steam chamber; steam (CO2) chamber overriding gravity drainage
Introduction At present, heavy oil reservoirs are mainly developed by thermal production technologies, including steam huff and puff, steam flooding, steam assisted gravity drainage technology and fire flooding etc.[14] Steam flooding is an important replacement technology of steam huff and puff for heavy oil reservoirs. However, problems such as low oil-steam ratio, high water cut and steam channeling often occur in the later stage of steam flooding, and with the economic benefits turning extremely poor, the steam flooding has to be stopped. Therefore, it is urgent to find new ways of reservoir development[56]. In view of the problems existing in steam flooding technology, gas foam, high-temperature gel and other steam flooding profile control technologies have been proposed at home and abroad[710]. Since CO2 dissolved in heavy oil can expand oil volume, reduce the oil viscosity and interfacial
tension[1113], it can be used as the main auxiliary agent for steam assisted recovery of heavy oil reservoirs[1417]. Bagci et al.[18] studied the influence of continuous injection of CO2 on heavy oil recovery through one-dimensional physical simulation experiment. The experimental results showed that CO2 assisted steam flooding increased recovery by about 15.6% than steam flooding, and the gas-vapor ratio was the key parameter affecting the ultimate recovery. Tao et al.[19], Ouyang et al.[20], Li et al.[21], Sun et al.[22] conducted in-depth experimental researches and numerical calculations on the migration mechanism of CO2 and heavy oil. Jha et al.[2329] discussed the impact of CO2 injection on enhancing oil recovery, and conducted laboratory experiments and numerical simulation studies on the interaction between supercritical CO2 and superheated steam, proving that CO2 injection was an effective measure to improve recovery of heavy oil reservoirs. But the researches about CO2 assisted recovery in heavy oil
Received date: 18 Dec. 2018; Revised date: 05 Aug. 2019. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05012-002). https://doi.org/10.1016/S1876-3804(19)60277-6 Copyright © 2019, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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reservoirs in China and abroad before mostly focused on the lab experiment and theoretical research of interaction mechanism between CO2 and steam, or profile control technical measures, few three-dimensional physical simulation research was carried out according to actual reservoir conditions by similar proportion method in order to apply simulation results directly to extended reservoir production. The researches were more theoretical, but lack of sustainability, systematicness and pertinence, and couldn’t solve the fundamental problems of steam channeling and oil recovery enhancement in late steam flooding stage. There is no effective replacement development mode and technology in the later stage of steam flooding[3031]. In this study, J6 block in Xinjiang oilfield was taken as a typical example, according to the specific reservoir conditions and production technology, based on the principle of similar scale, the three-dimensional physical simulation experiments on steam flooding, CO2 assisted steam flooding, CO2 foam assisted steam flooding under the condition of full section perforation, and steam flooding, CO2 assisted steam flooding experiments under lower half part perforation of lower section were carried out, and CO2 assisted steam flooding development technology was put forward in the late stage of steam flooding. Then, the CO2 assisted steam flooding was designed and applied based on the results of three-dimensional physical simulation experiment and the development status of steam flooding in block J6 of Xinjiang oilfield.
1. Three-dimensional physical simulation experiment of CO2 assisted steam flooding after steam flooding Aiming at the problems of steam channeling and low oil-steam ratio in the late stage of steam flooding in block J6
Fig. 1. Table 1.
of Xinjiang oilfield, the development effects of steam flooding, CO2 assisted steam flooding, CO2 foam assisted steam flooding under the condition of full oil layer perforation, and steam flooding and CO2 assisted steam flooding under the condition of perforating only the lower half of the lower oil layer were studied by physical modeling. 1.1.
The equipment
The steam flooding in block J6 of Xinjiang oilfield adopts the inverse nine-point area well pattern, so the three-dimensional physical simulation experimental model took a quarter of the inverse nine-point area well pattern (Fig. 1). In order to study the impact of perforation location on development effect, 8 wells were set up in the model, that is, 2 wells were set up for each well location to model the cases of perforating the whole oil layer and the lower half of the lower oil layer respectively. According to the actual reservoir conditions, the filling sand model was designed, and the model parameters were designed according to the similar proportion method (Table 1)[14]. The model had an upper oil layer, a middle physical interlayer and lower oil layer, which respectively represent the J3q22-1 (2-1 small layer of the second oil layer group in upper Jurassic Qigu Formation) small layer, physical interlayer and J3q22-2 small layer in the oilfield. 1.2.
The process
During the experiment, the initial steam injection speed of steam flooding stage was 80 mL/min (water equivalent), and the steam injected was superheated steam from vapor generator, when it is injected into model, the steam quality was more than 85%, and the later steam injection speed was appropriately adjusted according to the experimental process. Under
Appearance and well pattern diagram of filling sand model in 3D physical modeling experiment.
Designed parameters of the model.
Model Reservoir prototype Physical model
Geometry 60 m×60 m×40 m 30 cm×30 cm×20 cm
Thickness Upper layer Physical interlayer Lower layer 16 m 4 m. 20 m. 8cm 2 cm 10 cm
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Permeability/103 μm2 Upper layer Physical interlayer Lower layer 2600 200 1200 2600 200 1200
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the condition of adding CO2 gas, the sum volume of injected steam and CO2 was basically the same as that of steam injected under the same temperature and pressure condition of steam flooding. 1.2.1.
Physical modeling experiment of full perforation
The full perforation physical simulation experiment can be divided into three stages: steam flooding, CO2 assisted steam flooding, and CO2 foam assisted steam flooding. During the experiment, all steam injection wells and production wells adopted full perforation of all the oil layers, which was consistent with the oilfield perforation mode, and there was no adjustment during the experiment. 1.2.2. Physical modeling experiment of perforating adjustment after steam flooding The physical modeling experiment process of perforating adjustment after steam flooding is mainly divided into two stages: steam flooding and CO2 assisted steam flooding. In the steam flooding stage, the injection wells and production wells took the whole oil layer perforated manner. After steam channeling, when the water cut reached 95%, the steam flooding stage ended. Then, the injection wells and production wells were adjusted the perforation section, with only the lower half part of the lower oil layer was perforated for CO2 assisted steam flooding.
2. 2.1.
The analysis and discussion Physical modeling experiment of full perforation
In the steam flooding stage, the initial steam injection rate was 80 mL/min. The results of steam flooding and combined steam flooding under the condition of the whole oil layer perforated are shown in Table 2 and the temperature field is shown in Fig. 2. The experiment included three stages. The first stage is steam flooding stage. It can be seen that in the case of whole oil layer perforated, the gas adsorption layer was mainly the upper oil layer and steam channeled quickly along the upper oil layer and entered the production well. Subsequently, the produced liquid showed high temperature and high water cut, and the steam flooding was forced to stop. The second stage was the mixed injection mode of steam and CO2. The development effect was improved because CO2 can improve oil displacement efficiency and thermal utilization efficiency. But steam (gas) channeling soon occurred, and CO2 assisted steam flooding couldn’t be carried out effectively. In order to effectively suppress steam (gas) channeling, CO2 foam assisted steam flooding was adopted in the third stage of Table 2.
the experiment. In this stage, CO2 foam slug and steam were injected alternately. In the first cycle, 0.05 PV (pore volume multiple) CO2 foam slug was injected, followed by 0.25 PV steam. In the second cycle, a CO2 foam slug of 0.10 PV was injected, and then 0.25 PV steam was injected. In the third cycle, 0.15 PV CO2 foam slug was injected, and then 0.25 PV steam was injected. It can be seen that with the development of steam chamber, in order to seal the upper steam chamber and prevent steam producing from the upper part of perforation interval, the amount of CO2 foam injected increased gradually, and the plane sweep efficiency increased, but the vertical sweep efficiency increased little (Fig. 2d), so CO2 foam did not effectively improve development effect and economic benefit became worse and worse. Apparently, it is difficult to enhance the vertical producing degree of reservoir simply by injecting foam. It should be noted that the CO2 foam-agent used in the experiment is the best high-temperature resistance foam-agent that can be obtained after repeated screening and evaluation by the author’s project team. At the temperature of over 250 C, the resistance factor of the foam can still reach more than 20[32]. In conclusion, CO2 assisted steam flooding and CO2 foam assisted steam flooding can enhance reservoir sweep volume after steam channeling within a certain range. But for the reservoir with a large vertical thickness of oil layer, they cannot fundamentally solve the problem of vertical sweep volume of reservoir and fail to produce the lower reservoir effectively. 2.2. Physical modeling experiment of perforation adjustment after steam flooding After steam flooding and perforation adjustment, the recovery effects of CO2-assisted steam flooding are shown in Table 3, and the temperature field is shown in Fig. 3. From Table 2, Table 3, Fig. 2a and Fig. 3a, it can be seen that in steam flooding stage, the actual production performance and experimental effect are basically consistent because the experimental conditions are similar. After the perforation section was adjusted to the lower half part of the lower oil layer, CO2-assisted steam flooding was carried out smoothly without steam (gas) channeling. No foam was needed for injection and production profile control. The time of this stage was 4.5 h, which means the production can be kept stable for a long time in this manner. The stage recovery percent was 61% and the oil/steam ratio was 0.32. During the experiment process, the steam chamber expanded gradually from the upper part of production wells downward, and the recovery rate
Results of steam flooding and combined steam flooding under the condition of whole oil layer perforated.
Displacement mode Steam flooding CO2 assisted steam flooding CO2 foam assisted steam flooding2 Total
Stage recovery/% 22.5 7.2 10.1 39.8
Stage water cut/% 95.53 94.28 95.10
Steam injection pore volume multiple 1.25 0.26 0.75 2.26
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CO2 (foam) injection pore volume multiple 0 0.07 0.3 0.3
Stage oil steam ratio 0.11 0.12 0.09 0.10
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Fig. 2. Table 3.
Temperature fields of steam flooding and combined steam flooding under the condition of whole oil layer perforated.
Effect of CO2-assisted steam flooding with perforation adjustment after steam flooding.
Displacement mode Steam flooding CO2-assisted steam flooding Total
Oil recovery/ % 20.1 61.0 81.1
Stage water cut/ % 95.2 75.1
increased from 20.1% to 81.1%, forming an efficient development mode. (1) The development mode of lateral expansion of steam chamber in the middle and lower part of injection well and overriding gravity drainage of steam chamber in the top layer of production well has been established. The injection rate ratio of CO2 to steam can be adjusted in a wide range. Because the perforation section of the steam injection well was adjusted to the lower half of the lower oil layer, the middle and lower parts of the formation showed signs of steam absorption in the initial stage of CO2-assisted steam flooding, and the steam channeling in the upper reservoir was inhibited to a certain extent (Fig. 3b). However, steam and CO2 quickly
Steam injection pore volume multiple 1.10 1.62 2.72
CO2 injection pore volume multiple 0 0.66 0.66
Stage oil steam ratio 0.12 0.32 0.24
overlapped the upper oil layer, and the steam chamber expanded completely in the upper oil layer (Fig. 3c), and then expanded to the lower oil layer. Generally speaking, the steam chamber near the injection well has the characteristics of overall lateral expansion and overlap development in the upper oil layer. The steam chamber near the production well follows the law of vertical development from top to bottom, and the steam chamber near the production well has a larger vertical distance from the perforated section at the bottom of the reservoir, so the steam will not be produced from the upper reservoir section. The steam chamber can continuously expand vertically downward along the production well, making full use of the energy of injected steam. For production
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Fig. 3.
Temperature field of CO2-assisted steam flooding after steam flooding and perforation adjustment.
wells, the expansion front of steam chamber is a stable high temperature oil and water belt controlled by the vapor-liquid interface, which is similar to the control mechanism of steamliquid interface in steam-assisted gravity drainage (SAGD) production process[33]. There are fundamental differences between the long-term stable production of high-temperature liquid in production wells during CO2-assisted steam flooding and the rapid steam channeling of steam in conventional steam flooding, which results in shutdown or transition to intermittent steam flooding, thus forming a gravity drainage mode under the condition of vertical well pattern. Based on the above characteristics of steam chamber development and stable production process, it is not necessary to use CO2 foam for profile control and flooding. The injection parameters during CO 2 -assisted steam flooding need to match the consumption capacity of the steam chamber. After adjusting the perforation section, when the injection rate was changed from the initial steam injection rate of 80 mL/min to "65 mL/min of steam injection rate + 200 mL/min of CO2 injection rate (measurement value under standard condition)", and the injection lasted 15 min, high
temperature steam channeling occurred easily in the production wells, the steam chamber was hard to control, and the water cut fluctuated around 85%. When the injection parameters were adjusted to "50 mL/min steam injection rate + 250 mL/min CO2 injection rate", the production wells produced high temperature fluid stably, and the steam chamber developed steadily. This shows that the injection rate matches the maximum capacity of team chamber consumption. Under this condition, the injection lasted for 65 min, and the water cut was stable at about 75%. In order to verify the reasonable injection ratio parameter of CO2 and steam, the injection parameters were adjusted further to study the development of steam chamber and production performance of production wells. When the injection rate of CO2 was increased by adjusting the injection parameters to "steam injection rate of 50 mL/min + CO2 injection rate of 380 mL/min" (this rate has reached the maximum value of CO2 injection metering equipment), the steam chamber and production wells maintained a stable state with water content of about 65%, which lasted for 133 min. Then the injection parameters were adjusted again to “steam injection rate of 30 mL/min + CO2
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Fig. 4. Production dynamic curve of CO2-assisted steam flooding after steam flooding and perforation adjustment. ① Steam flooding at steam injection rate of 80 mL/min; ② CO2-assisted steam flooding at steam injection rate of 65 mL/min and CO2 injection rate of 200 mL/min; ③ CO2-assisted steam flooding at the steam injection rate of 50 mL/min and the CO2 injection rate of 250 mL/min for; ④ CO2-assisted steam flooding at the steam injection rate of 50 mL/min and the CO2 injection rate of 380 mL/min; ⑤ CO2-assisted steam flooding at the steam injection rate of 30 mL/min and CO2 injection rate of 380 mL/min.
Fig. 5.
Experimental production of CO2 emulsified foam oil.
injection rate of 380 mL/min”. Under this condition, the steam chamber expanded steadily, and the production well produced stably, with a water content of about 70%. But there was intermittent gas slug in the production wells in this period. The analysis shows that there was free gas phase CO2 produced, which lasted for 100 min, and the whole CO2-assisted steam flooding experiment ended. The production dynamic curve of CO2-assisted steam flooding after steam flooding and perforation adjustment is shown in Fig. 4. The experiments show that the injection rate of CO2 has a large adjustable range. When the ratio of CO2 injection rate to steam injection rate ranges from 5:1 to 13:1, the steam chamber can expand and develop steadily. From the whole experimental process, in the initial stage, the injection rate of CO2 should be low and the injected steam mainly forms stable chamber. In the rear stage, the injection rate of CO2 can be further increased when the steam
chamber develops to a certain volume. (2) High temperature water, oil, and CO2 formed stable low-viscosity quasi-single-phase emulsified fluid. When the ratio of CO2 and steam ranges from 5:1 to 13:1, the production wells can produce the stable emulsion fluid. The primary analysis indicates this emulsion fluid has characteristics of low pseudo viscosity and large expansion coefficient. The ratio of liquid and CO2 is about 1:6, its water cut is about 60%–70%. The emulsion fluid may formed when CO2 moves from steam chamber to production wells, especially passing through the interface between steam chamber and low liquid. (3) CO2 acts as heat insulation at the top of the steam chamber, reduces the steam partial pressure in the steam chamber, and effectively improves the thermal efficiency of injected steam. Unlike the near-oil drainage mode of SAGD development[3435], in the process of steam flooding, steam needs longdistance migration. Especially in gravity-stable steam flooding, during the steam chamber expansion, the steam needs to migrate the length of the whole well spacing, so the heat loss to the top caprock is more than 30%, in this case, CO2 can act as thermal insulation of the upper part of the reservoir. In addition, CO2 reduces the partial pressure of steam in the steam chamber and improves the latent heat utilization ratio of steam[36]. The comparative experiments under the same conditions show when 2.3 PV steam is injected without the assistance of CO2 after perforation adjustment, the recovery degree can reach 60%, but the oil-steam ratio is only 0.19.
3. Field application of CO2 assisted steam flooding after steam flooding 3.1.
Geologic profile of J6 Block in Xinjiang Oilfield
Nine typical steam flooding well groups in the J6 block of Xinjiang Oilfield were selected for the CO2-assisted steam
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flooding field test. The reservoir in this test area has a temperature of 20 C and crude oil viscosity of 7000–12 000 mPas, and is a typical heavy oil reservoir. The layer series of development of J6 block is J3q22-1+J3q22-2, which is a set of braided river delta front sediments. Among them, the sand body of J3q22-1 small layer has an average porosity of 30.3%, an average permeability of 2623×103 μm2, an average oil saturation of 78%, a thickness of 12 to 23 m and 16 m averagely. The J3q22-2 small layer sand body has an average porosity of 28.2%, average permeability of 1200×103 μm2, average oil saturation of 73%, a thickness of 13 to 26 m and 20 m on average. There is a set of stable argillaceous sandstone, sandy conglomerate and mudstone in local part between the J3q22-1 small layer and the J3q22-2 small layer, with a thickness of 1 to 6 m and 4 m on average. The argillaceous sandstone and sandy conglomerate have a permeability of 100×103 to 800×103 μm2, 200×103 μm2 on average, which theoretically can’t block vapor flow[37]. 3.2. Development situation of J6 Block in Xinjiang Oilfield The steam flooding of J6 block adopts inverse-nine point well pattern. From January 1st, 1989, steam huff-and-puff has been implemented for nearly 10 years, and then the conventional steam flooding has been carried out until January 1st, 2018. Affected by steam overlap and reservoir heterogeneity, the thermal breakthrough of steam occurred first at the top of the oil layer, and high-temperature steam or hot water was directly produced from the upper perforation section of the production well. Inefficient steam circulating layer was formed on the top of the J3q22-1 oil layer, then the volume of the underground steam hardly grew anymore. Intermittent steam injection and intermittent oil production have to been adopted in this field to maintain production at low efficiency[38].Then the heat utilization efficiency declined dramatically, and the steam flooding was basically futile. According to the data analysis and numerical simulation of four coring wells in the nine well groups of the test area (ranging from 35 to 50 m from the original injection steam well), it is found that the area near the steam injection well has higher steam sweeping range and larger drop of oil saturation; whereas in the area far from the steam injection well, the steam channeling path develops along the top of the reservoir, and the oil saturation of oil layer below the steam channel dropped little. In general, the steam chamber has been formed at the top of the oil layer in local part, but it is the steam channel of the production well due to the perforation of the whole oil layer. After steam breakthrough, the steam chamber cannot expand effectively, resulting in the mainly hot water flooding of the oil layer with low displacement efficiency, and oil-vapor gravity differentiation of the whole oil reservoir[39]. 3.3.
oil-steam ratio was 0.07, and the recovery percent was 45%. Due to steam channeling, using the original perforation mode and steam injection mode can’t produce effectively anymore. According to the above physical simulation experiment results and the optimized design results, the perforation was adjusted, and both the production wells and the steam injection wells were only perforated in the lower half of the J3q22-2 layer. According to the design principle of injection-production parameters of conventional steam flooding, the steam injection rate of J6 block was designed at 200 t/(km2md), and the daily volume of steam injection of single well should reach 80 t. Based on the development mode of gravity drainage under the condition of vertical well pattern, the CO2-assisted steam flooding was optimized by numerical simulation. The designed daily steam injection rate of single well was 50 t, the steam quality of injected steam at bottom hole should be more than 65%, and the daily CO2 injection rate was 1.0 t (500 m3). The following three schemes were compared: (1) Steam flooding with the whole reservoir section perforated at the injection rate of 50 t/d; (2) steam flooding after adjustment of perforation at the injection rate of 50 t/d; (3) CO2-assisted steam flooding after the adjustment of perforation. The predicted development effects of these development schemes are shown in Fig. 6 and Table 4. It can be seen from Fig. 6 and Table 4 that the scheme 1 is not feasible; after adjustment of perforation, the CO2-assisted steam flooding had better effect than steam flooding, with the oil-steam ratio increasing by 34.0% from 0.119 to 0.160, the stage recovery percent increasing from 16.1% to 21.5%, and the final recovery rate reaching 66.5%. By adjusting the perforation position and injecting CO2 for assistance, the steam chamber can be effectively expanded to the lower part of the reservoir, resulting in the successful gravity drainage under the condition of vertical well pattern (Fig. 7). The simulation results show that the CO2-assisted steam flooding after perforation adjustment resulted in the lateral expansion of the steam chamber in the middle and lower parts of the steam injection well and the overlap gravity drainage of the top steam chamber of the production well. The high-temperature water, oil and CO2 can form a stable emulsified pseudomonophasic fluid. CO2 acts as a heat insulator at the top of the steam chamber, reducing the partial steam pressure, effectively
Field application design and effects
From 2018, the nine steam-flooding well groups in the test area were basically in a state of shutdown before the flooding conversion. The total daily oil production was 10 t, the daily oil production per well was generally lower than 0.5 t, the
Fig. 6. Daily oil production curves of different development schemes.
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Table 4.
Predicted development effects of different development schemes.
Scheme Whole reservoir perforation steam flooding Steam flooding after perforation adjusted CO2-assisted steam flooding after perforation adjusted
Cumulative steam injection/104t 112.06 112.06
Cumulative CO2 Cumulative oil Cumulative Enhanced injection/104t production/104 m3 oil steam ratio recovery/% 0 3.92 0.035 4.7 0 13.40 0.119 16.1
112.06
2.67
16.68
0.160
21.5
the oil layer is mainly driven by hot water with low oil displacement efficiency. When the perforation is adjusted to the lower half of the lower oil layer after steam flooding, the gravity drainage mode under the vertical well pattern can be realized, the steam chamber can fully expand, and the oil recovery can be greatly improved. After adjusting the position of perforation, the CO2-assisted steam flooding can result in the lateral expansion of steam chamber in the middle-lower part of the steam injection well, and the overlap gravity drainage of the top steam chamber in the production well. High temperature water, oil and CO2 can form stable emulsified pseudomonophasic fluid. The CO2 can act as heat insulator at the top of the steam chamber, reduce the partial pressure of steam and effectively enhance the thermal efficiency of the injected steam. CO2-assisted steam flooding technology has been successfully implemented in the field, providing an effective replacement technology for improving the development performance and oil recovery of heavy oil reservoirs in the later stage of steam flooding.
References [1] Fig. 7. Steam chamber development before and after CO2 assisted steam flooding.
improving the thermal efficiency of the injected steam enabling the full expansion of the steam chamber, thereby greatly improving oil recovery. The test plan was implemented on October 31st, 2017, and the effect was remarkable. By September 1st, 2019, the daily oil production in the test area increased from 6 t to 53 t, and the oil-steam ratio increased from 0.04 to 0.12. The maximum daily oil production and oil-steam ratio is expected to reach 70 t and 0.16 respectively on December 31st, 2019. The test area will produce stably for 6 years, and eventually have an enhanced recovery of 20%. The success of this technology will provide an effective replacement technology for improving the development effect and oil recovery of heavy oil reservoirs in the late stage of steam flooding.
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The physical simulation experiments show that the steam chamber formed at the top of the oil layer in local part of the J6 block of Xinjiang Oilfield is the channel of direct steam channeling in the production well, influencing by the general perforation over the whole reservoir. Due to steam channeling, the steam chamber cannot expand and enlarge effectively, so
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