Volume 27,Issue 1,2020 Table of Contents
2020, 27(1):1-10.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.001
Abstract:
The CO2 flooding technology is an important technology to enhance oil recovery in low permeability reservoirs and to realize the geological sequestration of CO2 so that it has the wide application prospects in China. In view of the complex geology and development characteristics of the continental low permeability reservoirs in China，the CO2 displacement theory，optimal design of reservoir engineering，injection-production engineering and anticorrosion technology，CO2 recovery and utilization technology and the progress of the field test etc.of SINOPEC are systematically presented. The challenges and technical requirements of the CO2 flooding technology in the low permeability reservoirs are analyzed，and the suggestions on accelerating the development of CO2 flooding technology are put forward from the aspects of the low-cost capture，transportation of CO2，reduction of the miscibility pressure，improvement of the CO2 flooding effect，CO2 storage and monitoring technology，and supercritical CO2 fracturing. It is of the great significance to promote the CO2 flooding and storage technology，improve the reserve producing rate and oil recovery，guarantee the national energy supply and reduce the CO2 emission.
The CO2 flooding technology is an important technology to enhance oil recovery in low permeability reservoirs and to realize the geological sequestration of CO2 so that it has the wide application prospects in China. In view of the complex geology and development characteristics of the continental low permeability reservoirs in China，the CO2 displacement theory，optimal design of reservoir engineering，injection-production engineering and anticorrosion technology，CO2 recovery and utilization technology and the progress of the field test etc.of SINOPEC are systematically presented. The challenges and technical requirements of the CO2 flooding technology in the low permeability reservoirs are analyzed，and the suggestions on accelerating the development of CO2 flooding technology are put forward from the aspects of the low-cost capture，transportation of CO2，reduction of the miscibility pressure，improvement of the CO2 flooding effect，CO2 storage and monitoring technology，and supercritical CO2 fracturing. It is of the great significance to promote the CO2 flooding and storage technology，improve the reserve producing rate and oil recovery，guarantee the national energy supply and reduce the CO2 emission.
2020, 27(1):11-19.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.002
Abstract:
The resources of extra-low permeability reservoirs suitable for CO2 flooding is abundant in Shengli Oilfield，but the application of CO2 flooding faces technical bottlenecks such as being difficult to be miscible and low sweep efficiency due to formation characteristics of deep burial depth，low abundance，strong heterogeneity and high miscible pressure. The geology，fluid mechanics，reservoir engineering theory and methods are applied comprehensively. By combination of the physical experiments and numerical simulation，the evaluation system of adaptability，technologies of laboratory experiment and schedule optimization technology series of reservoir engineering of CO2 flooding for enhancing oil recovery are formed， and the injection-production technologies of CO2 flooding are further established. The field test shows that CO2 has good capability of injection and oil increment is obvious. The cumulative oil increment is 6.9×104 t at cumulative CO2 injection of 30.7×104 t in pilot test area in Gao89-1 Block. The oil recovery is 18.6% in the central well block with enhanced oil recov? ery of 9.7%. The advanced injection of CO2 is 1.9×104 t，and the formation pressure recoveries from 17 MPa to 33.7 MPa in the Fan142-7-X4 well group. The miscible flooding is formed and the corresponding oil well produces fluid by natural flow exploitation. The daily oil production of single well is kept stable at 5-6 t/d，which is much higher than 1 t/d before CO2 injection.
The resources of extra-low permeability reservoirs suitable for CO2 flooding is abundant in Shengli Oilfield，but the application of CO2 flooding faces technical bottlenecks such as being difficult to be miscible and low sweep efficiency due to formation characteristics of deep burial depth，low abundance，strong heterogeneity and high miscible pressure. The geology，fluid mechanics，reservoir engineering theory and methods are applied comprehensively. By combination of the physical experiments and numerical simulation，the evaluation system of adaptability，technologies of laboratory experiment and schedule optimization technology series of reservoir engineering of CO2 flooding for enhancing oil recovery are formed， and the injection-production technologies of CO2 flooding are further established. The field test shows that CO2 has good capability of injection and oil increment is obvious. The cumulative oil increment is 6.9×104 t at cumulative CO2 injection of 30.7×104 t in pilot test area in Gao89-1 Block. The oil recovery is 18.6% in the central well block with enhanced oil recov? ery of 9.7%. The advanced injection of CO2 is 1.9×104 t，and the formation pressure recoveries from 17 MPa to 33.7 MPa in the Fan142-7-X4 well group. The miscible flooding is formed and the corresponding oil well produces fluid by natural flow exploitation. The daily oil production of single well is kept stable at 5-6 t/d，which is much higher than 1 t/d before CO2 injection.
2020, 27(1):20-28.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.003
Abstract:
The carbon capture，utilization and storage（CCUS）are important options to reduce CO2 emissions in a deeper way in response to climate change. In the past decade，the important progress has been made in CCUS technology and industry，especially in China. Based on the research and practical experience of CCUS in recent years，the situation of CCUS technology types and CCUS development deployment in China are described. The development history and project situation of enhanced oil recovery CCUS（CCUS-EOR）technologies in key global regions，as well as different CO2 displacement types and characteristics，and CCUS-EOR supporting technologies with Chinese characteristics are analyzed. The main influencing factors of CCUS-EOR sustainable development are analyzed systematically. On this basis，the development conditions of CO2 flooding in China and the United States are compared，and the difficulties in promoting CO2 flooding technology in China are pointed out. Finally，from the aspects of development basis，main tasks，priority areas，project operation，policies and regulations，etc.，specific suggestions for the sustainable development of China’s CCUS-EOR industry are put forward to provide necessary decision-making references for CCUS policy-making departments.
The carbon capture，utilization and storage（CCUS）are important options to reduce CO2 emissions in a deeper way in response to climate change. In the past decade，the important progress has been made in CCUS technology and industry，especially in China. Based on the research and practical experience of CCUS in recent years，the situation of CCUS technology types and CCUS development deployment in China are described. The development history and project situation of enhanced oil recovery CCUS（CCUS-EOR）technologies in key global regions，as well as different CO2 displacement types and characteristics，and CCUS-EOR supporting technologies with Chinese characteristics are analyzed. The main influencing factors of CCUS-EOR sustainable development are analyzed systematically. On this basis，the development conditions of CO2 flooding in China and the United States are compared，and the difficulties in promoting CO2 flooding technology in China are pointed out. Finally，from the aspects of development basis，main tasks，priority areas，project operation，policies and regulations，etc.，specific suggestions for the sustainable development of China’s CCUS-EOR industry are put forward to provide necessary decision-making references for CCUS policy-making departments.
2020, 27(1):29-35.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.004
Abstract:
The supercritical CO2 foam can effectively reduce the mobility of CO2，increase the plugging strength，and inhibit the channeling of CO2 in fractured tight cores. By evaluating eight kinds of foaming agents under conditions close to the reservoir，a foaming agent with the best stability is selected. The core percolation characteristics of the supercritical CO2 foam flooding under the different gas-liquid ratios，fracture openings and injection modes are studied，and the law of the supercritical CO2 foam flooding after water flooding and gas flooding are analyzed. The results show that the foaming agent HY-2 has the best stability when the mass fraction is 0.5%；when the gas-liquid ratio is equal to 1.0，it has the best plugging effect on the fractured tight cores and the good adaptability to the cores with the fracture openings of 39.80-82.67 μm. The simultaneous injection of gas and liquid is more conducive to improving the plugging effect of the supercritical CO2 foam flooding. There is an increase in oil recovery of more than 20% with supercritical CO2 foam flooding compared to water flooding or gas flooding. Therefore，the supercritical CO2 foam flooding under certain conditions has a significant effect on improving oil recovery of the fractured tight reservoirs.
The supercritical CO2 foam can effectively reduce the mobility of CO2，increase the plugging strength，and inhibit the channeling of CO2 in fractured tight cores. By evaluating eight kinds of foaming agents under conditions close to the reservoir，a foaming agent with the best stability is selected. The core percolation characteristics of the supercritical CO2 foam flooding under the different gas-liquid ratios，fracture openings and injection modes are studied，and the law of the supercritical CO2 foam flooding after water flooding and gas flooding are analyzed. The results show that the foaming agent HY-2 has the best stability when the mass fraction is 0.5%；when the gas-liquid ratio is equal to 1.0，it has the best plugging effect on the fractured tight cores and the good adaptability to the cores with the fracture openings of 39.80-82.67 μm. The simultaneous injection of gas and liquid is more conducive to improving the plugging effect of the supercritical CO2 foam flooding. There is an increase in oil recovery of more than 20% with supercritical CO2 foam flooding compared to water flooding or gas flooding. Therefore，the supercritical CO2 foam flooding under certain conditions has a significant effect on improving oil recovery of the fractured tight reservoirs.
2020, 27(1):36-44.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.005
Abstract:
The minimum miscibility pressure（MMP）of the CO2-crude oil system is a key factor affecting the development effect of CO2 flooding. When the reservoir pressure is lower than the original saturation pressure，the part solution gas dissolved in the oil will be separated from the oil with the development of oil reservoir. Thus，the composition of oil phase and its high-pressure physical properties will also change，which will affect the MMP of CO2-crude oil system. The MMP obtained from the original formation oil sample is no longer applicable. Based on the slim tube experiment and multi-component numerical simulation，the MMPs of crude oil samples at different reservoir pressures in eight typical blocks of an oilfield in western China are systematically studied. Compared with other oilfields in China，the C1 molar content of oil samples from various reservoirs in the study area is higher，ranging from 31.12% to 51.69%，with an average of 43.25%；while the molar content of C2-C6 is lower，ranging from 8.0% to 18.48%，with an average of only 11.3%. The experimental and simulation results show that CO2 can be miscible with the oil samples from eight typical blocks under at original reservoir pressures，but the MMPs of CO2 flooding in various blocks have significant differences，ranging from 17.60 MPa to 41.18 MPa. After the reservoir pressure is lower than the original saturation pressure，the MMPs in CO2 flooding of typical reservoir oil samples tend to decrease slightly. With further decrease of degassed pressure，the molar content of C1N2 in the oil phase decreases，the molar contents of C7+and C24+ increase，and the changes of C2 and C3+ are minor. C1 is the main component of the separated gas at different pressures. The intermediate component increases sharply only at the last degassing pressure. CO2-crude oil miscible zone occurs at the swept front of the injected CO2 close to the injection side. The miscible zone becomes wider with the displacement.
The minimum miscibility pressure（MMP）of the CO2-crude oil system is a key factor affecting the development effect of CO2 flooding. When the reservoir pressure is lower than the original saturation pressure，the part solution gas dissolved in the oil will be separated from the oil with the development of oil reservoir. Thus，the composition of oil phase and its high-pressure physical properties will also change，which will affect the MMP of CO2-crude oil system. The MMP obtained from the original formation oil sample is no longer applicable. Based on the slim tube experiment and multi-component numerical simulation，the MMPs of crude oil samples at different reservoir pressures in eight typical blocks of an oilfield in western China are systematically studied. Compared with other oilfields in China，the C1 molar content of oil samples from various reservoirs in the study area is higher，ranging from 31.12% to 51.69%，with an average of 43.25%；while the molar content of C2-C6 is lower，ranging from 8.0% to 18.48%，with an average of only 11.3%. The experimental and simulation results show that CO2 can be miscible with the oil samples from eight typical blocks under at original reservoir pressures，but the MMPs of CO2 flooding in various blocks have significant differences，ranging from 17.60 MPa to 41.18 MPa. After the reservoir pressure is lower than the original saturation pressure，the MMPs in CO2 flooding of typical reservoir oil samples tend to decrease slightly. With further decrease of degassed pressure，the molar content of C1N2 in the oil phase decreases，the molar contents of C7+and C24+ increase，and the changes of C2 and C3+ are minor. C1 is the main component of the separated gas at different pressures. The intermediate component increases sharply only at the last degassing pressure. CO2-crude oil miscible zone occurs at the swept front of the injected CO2 close to the injection side. The miscible zone becomes wider with the displacement.
2020, 27(1):45-49.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.006
Abstract:
The CO2 flooding technology has developed rapidly at home and abroad because it has the advantages of high efficiency，energy conservation and emission reduction. However，due to the high minimum miscible pressure of CO2 flooding in the bench-bar sandstones reservoirs of Shengli Oilfield，the miscible displacement is difficult to carry out and the oil displacement efficiency is low. As a result，a chemical system combining the double effects of synergism and solubilizing ability is designed，which can significantly reduce the minimum miscible pressure between CO2 and crude oil，and improve the effect of immiscible displacement. Firstly，the synergist DYJ-13 and the solubilizer S6 are screened by measuring the extraction amount of crude oil in CO2 and the dissolution amount of CO2 in crude oil. Furthermore，these two chemical agents are combined，and the effect of different proportions on the capability of crude oil extraction from CO2 is systematically investigated. The results show that with the increase of the mass fraction of DYJ-13，the synergistic factor increases and then decreases，and the solubilization factor do not change much，thus the optimal chemical system of DYJ-13∶S6=3∶7 is determined. Finally，the experiment of long slim tube displacement is carried out，and the minimum miscible pressure between crude oil and CO2 in the test area decreases from 31.65 MPa to 24.60 MPa，with a reduction of 22%，after adding the combined chemical system with a mass fraction of 3%. The developed chemical system has high application potential，and it is recommended to carry out single well tests.
The CO2 flooding technology has developed rapidly at home and abroad because it has the advantages of high efficiency，energy conservation and emission reduction. However，due to the high minimum miscible pressure of CO2 flooding in the bench-bar sandstones reservoirs of Shengli Oilfield，the miscible displacement is difficult to carry out and the oil displacement efficiency is low. As a result，a chemical system combining the double effects of synergism and solubilizing ability is designed，which can significantly reduce the minimum miscible pressure between CO2 and crude oil，and improve the effect of immiscible displacement. Firstly，the synergist DYJ-13 and the solubilizer S6 are screened by measuring the extraction amount of crude oil in CO2 and the dissolution amount of CO2 in crude oil. Furthermore，these two chemical agents are combined，and the effect of different proportions on the capability of crude oil extraction from CO2 is systematically investigated. The results show that with the increase of the mass fraction of DYJ-13，the synergistic factor increases and then decreases，and the solubilization factor do not change much，thus the optimal chemical system of DYJ-13∶S6=3∶7 is determined. Finally，the experiment of long slim tube displacement is carried out，and the minimum miscible pressure between crude oil and CO2 in the test area decreases from 31.65 MPa to 24.60 MPa，with a reduction of 22%，after adding the combined chemical system with a mass fraction of 3%. The developed chemical system has high application potential，and it is recommended to carry out single well tests.
2020, 27(1):50-56.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.007
Abstract:
There are plenty of dead-end pores in low permeability reservoirs，in which crude oil is hardly initiated by water flooding. As a result，oil in dead-end pores is a kind of residual oil after water flooding. The microscopic dead-end model is made by glass etching. Laboratory experiments are conducted to simulate the reservoir development，including water flooding，CO2 immiscible flooding and CO2 miscible flooding under the condition of saturated oil. With the help of microscopic and visual system，the initiating characteristics of the residual oil in dead-end pores by different displacement methods are observed. Experimental results show that water could only enter the shallow area of the dead-end pores，and residual oil is hard to be initiated. Moreover，supercritical CO2 could penetrate into the dead-end pores，and parts of residual oil is initiated and driven out along the glass wall，and enters into major channel after CO2 immiscible flooding. Furthermore，the supercritical CO2 could displace the residual oil in deep pores at the dead end，and all of the residual oil could be displaced with the increase of the CO2 pore volume after CO2 miscible flooding. The analysis indicates that extremely limited residual oil in dead-end pores can be displaced with the help of pressure change and fluid compressibility by water flooding. However，the supercritical CO2 will exchange some components of the residual oil continuously，and enters into the dead-end pores through fluid mass transfer，and then a large amount of residual oil is displaced. Therefore，the supercritical CO2 not only has a high oil displacement efficiency in the live pores swept by conventional displacement fluid，but also could displace the residual oil in the dead-end pores. In other words，supercritical CO2 could reduce the residual oil saturation and enhance oil recovery.
There are plenty of dead-end pores in low permeability reservoirs，in which crude oil is hardly initiated by water flooding. As a result，oil in dead-end pores is a kind of residual oil after water flooding. The microscopic dead-end model is made by glass etching. Laboratory experiments are conducted to simulate the reservoir development，including water flooding，CO2 immiscible flooding and CO2 miscible flooding under the condition of saturated oil. With the help of microscopic and visual system，the initiating characteristics of the residual oil in dead-end pores by different displacement methods are observed. Experimental results show that water could only enter the shallow area of the dead-end pores，and residual oil is hard to be initiated. Moreover，supercritical CO2 could penetrate into the dead-end pores，and parts of residual oil is initiated and driven out along the glass wall，and enters into major channel after CO2 immiscible flooding. Furthermore，the supercritical CO2 could displace the residual oil in deep pores at the dead end，and all of the residual oil could be displaced with the increase of the CO2 pore volume after CO2 miscible flooding. The analysis indicates that extremely limited residual oil in dead-end pores can be displaced with the help of pressure change and fluid compressibility by water flooding. However，the supercritical CO2 will exchange some components of the residual oil continuously，and enters into the dead-end pores through fluid mass transfer，and then a large amount of residual oil is displaced. Therefore，the supercritical CO2 not only has a high oil displacement efficiency in the live pores swept by conventional displacement fluid，but also could displace the residual oil in the dead-end pores. In other words，supercritical CO2 could reduce the residual oil saturation and enhance oil recovery.
2020, 27(1):57-61.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.008
Abstract:
The CO2-crude oil miscible zone has an important influence on the development effect of CO2 flooding. At present，no reasonable miscible zone characterization method has been proposed. Through the theoretical analysis and component numerical simulation，the formation mechanism and characterization method of CO2-crude oil miscible zone are systematically studied. The results show that the formation of the miscible zone is dominated by the vaporization-condensation between CO2 and crude oil，and the evolution after the formation of the miscible zone is mainly affected by the hydrodynamic dispersion. The isoline with the oil-gas interfacial tension equal to 0.1 dyn/cm is taken as the trailing edge of the miscible zone，and the isoline with the CO2 content in the oil phase equal to 0.2 is taken as the leading edge of the miscible zone.The width of the miscible zone is equal to the ratio of the area of the dimensionless miscible zone to the length of the envelope of the leading edge of the miscible zone. As the cumulative injection volume increases，the width of the miscible zone gradually increases but the growth rate gradually decreases before the leading edge of the miscible zone breaks. After the leading edge of the miscible zone breaks through and before the trailing edge of the miscible zone breaks，the width of the miscible zone decreases rapidly and then stabilizes. Based on the accurate characterization of the miscible zone，it can provide a basis for the regulation of the miscible zone to improve the development effect of CO2 flooding.
The CO2-crude oil miscible zone has an important influence on the development effect of CO2 flooding. At present，no reasonable miscible zone characterization method has been proposed. Through the theoretical analysis and component numerical simulation，the formation mechanism and characterization method of CO2-crude oil miscible zone are systematically studied. The results show that the formation of the miscible zone is dominated by the vaporization-condensation between CO2 and crude oil，and the evolution after the formation of the miscible zone is mainly affected by the hydrodynamic dispersion. The isoline with the oil-gas interfacial tension equal to 0.1 dyn/cm is taken as the trailing edge of the miscible zone，and the isoline with the CO2 content in the oil phase equal to 0.2 is taken as the leading edge of the miscible zone.The width of the miscible zone is equal to the ratio of the area of the dimensionless miscible zone to the length of the envelope of the leading edge of the miscible zone. As the cumulative injection volume increases，the width of the miscible zone gradually increases but the growth rate gradually decreases before the leading edge of the miscible zone breaks. After the leading edge of the miscible zone breaks through and before the trailing edge of the miscible zone breaks，the width of the miscible zone decreases rapidly and then stabilizes. Based on the accurate characterization of the miscible zone，it can provide a basis for the regulation of the miscible zone to improve the development effect of CO2 flooding.
2020, 27(1):62-68.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.009
Abstract:
After depleted production of high-pressure and low-permeability reservoirs，the formation energy is insufficient，which leads to the increase of gas production because of oil degassing. Therefore it is feasible to reinject the produced gas from economical and technical aspects. In order to clarify the microscopic oil displacement mechanism of reinjected natural gas flooding after the depleted production of this kind of reservoir，the factors influencing natural gas flooding is analyzed under high-temperature and high-pressure conditions so as to reveal the influence of depleted production and water flooding on natural gas flooding effect. Using self-developed high-temperature and high-pressure micro-visualization device，the microscopic gas displacement experiments under various conditions are carried out on the glass-etched porous medium simulation model. The results indicate that natural gas flooding can reduce the Jamin effect caused by degassing of oil during depleted production.Under the target reservoir conditions，the gas flooding mechanism is dominated by displacement，and the dissolution extraction is supplemented. Water barrier prevents the contact between gas and oil.In unswept area of natural gas，the residual oil is difficult to be produced because of water barrier，in swept area，natural gas can peel off water film gradually to contact with oil，and displace oil by dissolution and extraction. Natural gas flooding in high-pressure and low-permeability reservoirs can further enhance oil recovery after depleted production.
After depleted production of high-pressure and low-permeability reservoirs，the formation energy is insufficient，which leads to the increase of gas production because of oil degassing. Therefore it is feasible to reinject the produced gas from economical and technical aspects. In order to clarify the microscopic oil displacement mechanism of reinjected natural gas flooding after the depleted production of this kind of reservoir，the factors influencing natural gas flooding is analyzed under high-temperature and high-pressure conditions so as to reveal the influence of depleted production and water flooding on natural gas flooding effect. Using self-developed high-temperature and high-pressure micro-visualization device，the microscopic gas displacement experiments under various conditions are carried out on the glass-etched porous medium simulation model. The results indicate that natural gas flooding can reduce the Jamin effect caused by degassing of oil during depleted production.Under the target reservoir conditions，the gas flooding mechanism is dominated by displacement，and the dissolution extraction is supplemented. Water barrier prevents the contact between gas and oil.In unswept area of natural gas，the residual oil is difficult to be produced because of water barrier，in swept area，natural gas can peel off water film gradually to contact with oil，and displace oil by dissolution and extraction. Natural gas flooding in high-pressure and low-permeability reservoirs can further enhance oil recovery after depleted production.
2020, 27(1):69-74.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.010
Abstract:
With the development of CO2 capture technology，CO2 reinjection to the formation may improve oil recovery. Surfactants that could form stable foam with CO2 are selected，and static physical properties and plugging ability of the foam system are evaluated. The influence of various injection methods on the displacement efficiency is analyzed. Compound anionic-nonionic surfactant system（S6-1）is designed as foaming agent with foaming volume of 213 mL and half-life period of 1 112 s，which is better than other surfactants. The CO2 foam system has the characteristics of temperature resistance，salt resistance，low adsorption loss and excellent plugging performance. When the rational gas-liquid ratio is 1∶1 and mass fraction of the foaming agent is 0.5%，resistance factor of the CO2 foam system is stable. Enhanced oil recovery（EOR）is the highest when alternate injection of CO2 foam system and water，and the ultimate recovery reaches to 60%. The CO2 foam system shows good plugging performance and good residual resistance in subsequent water flooding.
With the development of CO2 capture technology，CO2 reinjection to the formation may improve oil recovery. Surfactants that could form stable foam with CO2 are selected，and static physical properties and plugging ability of the foam system are evaluated. The influence of various injection methods on the displacement efficiency is analyzed. Compound anionic-nonionic surfactant system（S6-1）is designed as foaming agent with foaming volume of 213 mL and half-life period of 1 112 s，which is better than other surfactants. The CO2 foam system has the characteristics of temperature resistance，salt resistance，low adsorption loss and excellent plugging performance. When the rational gas-liquid ratio is 1∶1 and mass fraction of the foaming agent is 0.5%，resistance factor of the CO2 foam system is stable. Enhanced oil recovery（EOR）is the highest when alternate injection of CO2 foam system and water，and the ultimate recovery reaches to 60%. The CO2 foam system shows good plugging performance and good residual resistance in subsequent water flooding.
2020, 27(1):75-80.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.011
Abstract:
Problems like small swept volume and short effective period of CO2 huff and puff for single horizontal well in the fault block reservoirs with edge water. Laboratory physical simulation experiments is conducted to optimize CO2 injection volumes of huff and puff for a horizontal well group，and to analyze the performance of water controlling and oil increasing of CO2 huff and puff in the fault block reservoirs with edge water. A 3D physical model of a horizontal well group is designed to model the process of CO2 huff and puff for a horizontal well group. Changes of soaking pressure with injection volumes of huff and puff are analyzed and production performance during production period are discussed. Combined with physical properties of CO2 and high-temperature and high-pressure physical properties of crude oil and CO2 ，the impact of CO2 injection volume on CO2 huff and puff for the horizontal well group in the fault block reservoirs with edge water is understood and the CO2 injection volume under the experimental conditions is determined. Experimental results show that when the CO2 injection volume is increased from 0.07 PV to 0.14 PV，the decline of composite water cut of well groups drops from 0.72% to 5.93%，and increase of oil production rises from 31.4 mL to 148.7 mL. Increase of oil production through CO2 huff and puff is obvious by enhancing injection volumes. When CO2 injection volume is increased to 0.14 PV，the recovery increase is up to 22.36%，but cumulative gas production is up to 8 050 mL at standard conditions，and the gas utilization efficiency drops. The main mechanism of water controlling and oil increasing by CO2 is that the decrease of CO2 isothermal compressibility at certain pressure makes it possible to inject gas with relatively low energy loss and enlarge swept volume. Meanwhile，the crude oil intends to be more flowable with the increase of CO2 composition in the crude oil.The swept efficiency of the injected gas may be improved with the increase of injection volumes.
Problems like small swept volume and short effective period of CO2 huff and puff for single horizontal well in the fault block reservoirs with edge water. Laboratory physical simulation experiments is conducted to optimize CO2 injection volumes of huff and puff for a horizontal well group，and to analyze the performance of water controlling and oil increasing of CO2 huff and puff in the fault block reservoirs with edge water. A 3D physical model of a horizontal well group is designed to model the process of CO2 huff and puff for a horizontal well group. Changes of soaking pressure with injection volumes of huff and puff are analyzed and production performance during production period are discussed. Combined with physical properties of CO2 and high-temperature and high-pressure physical properties of crude oil and CO2 ，the impact of CO2 injection volume on CO2 huff and puff for the horizontal well group in the fault block reservoirs with edge water is understood and the CO2 injection volume under the experimental conditions is determined. Experimental results show that when the CO2 injection volume is increased from 0.07 PV to 0.14 PV，the decline of composite water cut of well groups drops from 0.72% to 5.93%，and increase of oil production rises from 31.4 mL to 148.7 mL. Increase of oil production through CO2 huff and puff is obvious by enhancing injection volumes. When CO2 injection volume is increased to 0.14 PV，the recovery increase is up to 22.36%，but cumulative gas production is up to 8 050 mL at standard conditions，and the gas utilization efficiency drops. The main mechanism of water controlling and oil increasing by CO2 is that the decrease of CO2 isothermal compressibility at certain pressure makes it possible to inject gas with relatively low energy loss and enlarge swept volume. Meanwhile，the crude oil intends to be more flowable with the increase of CO2 composition in the crude oil.The swept efficiency of the injected gas may be improved with the increase of injection volumes.
2020, 27(1):81-88.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.012
Abstract:
CO2 puff and huff cannot achieve commercial oil productivity for the ultra-deep and low-permeability heavy oil reservoirs at present. The thermal recovery assisted by viscosity reducer and CO2 faces the issues of poor performance of oil viscosity reduction，low productivity of single well and large difficulty of reservoir development. CO2 solubilizing and viscosity reducing system is developed according to molecular system design and experimental verification.In this technology，chemical viscosity reduction and CO2 recovery for the heavy oil are combined to reduce oil viscosity and enhance heavy oil mobility through viscosity reduction effect of chemicals itself and the enhanced ability of CO2 dissolved in oil. The developed CO2 solubilizing and viscosity reducing system can increase the solubility of CO2 by 7 times，and the viscosity reduction ratio is up to 99.2%，which effectively solves the problem of narrow range of oil viscosity reduction for the ultra-deep and low-permeability heavy oil reservoirs. The field application results show that the average daily oil production of the test well is higher than 2.5 times of the original daily oil production，which achieved a good performance.
CO2 puff and huff cannot achieve commercial oil productivity for the ultra-deep and low-permeability heavy oil reservoirs at present. The thermal recovery assisted by viscosity reducer and CO2 faces the issues of poor performance of oil viscosity reduction，low productivity of single well and large difficulty of reservoir development. CO2 solubilizing and viscosity reducing system is developed according to molecular system design and experimental verification.In this technology，chemical viscosity reduction and CO2 recovery for the heavy oil are combined to reduce oil viscosity and enhance heavy oil mobility through viscosity reduction effect of chemicals itself and the enhanced ability of CO2 dissolved in oil. The developed CO2 solubilizing and viscosity reducing system can increase the solubility of CO2 by 7 times，and the viscosity reduction ratio is up to 99.2%，which effectively solves the problem of narrow range of oil viscosity reduction for the ultra-deep and low-permeability heavy oil reservoirs. The field application results show that the average daily oil production of the test well is higher than 2.5 times of the original daily oil production，which achieved a good performance.
2020, 27(1):89-92.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.013
Abstract:
It is difficult to carry out water flooding in volatile oil reservoirs due to the sufficient formation energy and the high initial pressure. CO2 flooding has been implemented in this type of reservoir for its excellent oil displacement characteristics. However，the higher dissolved gas oil ratio and the higher methane content contribute to a higher minimum miscible pressure（MMP）of CO2 flooding，which decreases the displacement performance. Dynamic miscibility characteristics and the injection timing of CO2 injection for volatile oil reservoirs are studied through laboratory experiment and numerical simulation. The results show that the characteristics of releasing methane and decreasing CO2 MMP is evident in the CO2 flooding after a depletion production of volatile oil reservoirs. That is to say，methane partially releases from the crude oil as the formation pressure decreases，which is favorable to the decreasing of CO2 MMP. The extent of decreasing CO2 MMP is closely related to the crude oil type and initial dissolved gas-oil ratio. When the crude oil is closer to the condensate oil，a higher dissolved gas-oil ratio results in a lager decreasing degree of MMP. On the contrary，when the crude oil is closer to black oil，a lower dissolved gas-oil ratio results in a smaller degreasing degree of MMP. The CO2 injection timing after the depletion of volatile oil reservoirs is proposed according to the dynamic miscibility mechanism. It is found that as the dissolved gas-oil ratio increases，the injection timing and the formation energy supplement decrease. Otherwise，the injection timing and the formation energy supplement increase.
It is difficult to carry out water flooding in volatile oil reservoirs due to the sufficient formation energy and the high initial pressure. CO2 flooding has been implemented in this type of reservoir for its excellent oil displacement characteristics. However，the higher dissolved gas oil ratio and the higher methane content contribute to a higher minimum miscible pressure（MMP）of CO2 flooding，which decreases the displacement performance. Dynamic miscibility characteristics and the injection timing of CO2 injection for volatile oil reservoirs are studied through laboratory experiment and numerical simulation. The results show that the characteristics of releasing methane and decreasing CO2 MMP is evident in the CO2 flooding after a depletion production of volatile oil reservoirs. That is to say，methane partially releases from the crude oil as the formation pressure decreases，which is favorable to the decreasing of CO2 MMP. The extent of decreasing CO2 MMP is closely related to the crude oil type and initial dissolved gas-oil ratio. When the crude oil is closer to the condensate oil，a higher dissolved gas-oil ratio results in a lager decreasing degree of MMP. On the contrary，when the crude oil is closer to black oil，a lower dissolved gas-oil ratio results in a smaller degreasing degree of MMP. The CO2 injection timing after the depletion of volatile oil reservoirs is proposed according to the dynamic miscibility mechanism. It is found that as the dissolved gas-oil ratio increases，the injection timing and the formation energy supplement decrease. Otherwise，the injection timing and the formation energy supplement increase.
2020, 27(1):93-100.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.014
Abstract:
In the process of gas flooding development of tight reservoirs，gas channeling usually leads to poor development performance or even shutdown. There was no mature theoretical method to calculate the percolation characteristic parameters after gas injection in the tight reservoirs. Based on the principle of percolation mechanics，the characterization functions between oil-gas-water three-phase saturations and relative permeability are established by using the Stone model. By analyzing the function relation among the dynamic parameters，the percolation characteristics parameters and relative permeability，the saturation-water cut-gas-oil ratio（GOR）relation charts are established. With the help of these charts，the percolation characteristic parameters are reversed by using dynamic development parameters. The dynamic development parameters include water cut and GOR，and the percolation characteristic parameters include the fluidity of underground mixed fluid，average reservoir permeability and average permeability of gas channeling channel. This method is applied to perform the dynamic reversion of reservoir percolation characteristics parameter in the well group with gas channeling. The results show that the inversion results are consistent with the characteristics of single-well gas channeling and production dynamic behavior. The average permeability of gas channeling wells during production is much lower than that of wells without gas channeling. The higher the degree of gas channeling is，the greater the average permeability of gas channeling channel will become，which indicates that there are obvious fractures or gas channeling channels in the layer.
In the process of gas flooding development of tight reservoirs，gas channeling usually leads to poor development performance or even shutdown. There was no mature theoretical method to calculate the percolation characteristic parameters after gas injection in the tight reservoirs. Based on the principle of percolation mechanics，the characterization functions between oil-gas-water three-phase saturations and relative permeability are established by using the Stone model. By analyzing the function relation among the dynamic parameters，the percolation characteristics parameters and relative permeability，the saturation-water cut-gas-oil ratio（GOR）relation charts are established. With the help of these charts，the percolation characteristic parameters are reversed by using dynamic development parameters. The dynamic development parameters include water cut and GOR，and the percolation characteristic parameters include the fluidity of underground mixed fluid，average reservoir permeability and average permeability of gas channeling channel. This method is applied to perform the dynamic reversion of reservoir percolation characteristics parameter in the well group with gas channeling. The results show that the inversion results are consistent with the characteristics of single-well gas channeling and production dynamic behavior. The average permeability of gas channeling wells during production is much lower than that of wells without gas channeling. The higher the degree of gas channeling is，the greater the average permeability of gas channeling channel will become，which indicates that there are obvious fractures or gas channeling channels in the layer.
2020, 27(1):101-106.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.015
Abstract:
During the development of CO2 flooding in low permeability reservoirs，the most serious problem is gas channeling. At present，there are no quantitative gas channeling identification standards，and the existing calculation methods of gas channeling volume are not practical. The dynamic variation characteristics of CO2 content in the produced gas are analyzed，and the types of gas channeling are divided into the obvious gas channeling and the weak gas channeling. The calculation method of gas breakthrough time is determined，and the identification and evaluation criteria for gas channeling are established. On this basis，gas channel volumes from the injection side and the production side are calculated respectively by using the principle of material conservation. In order to eliminate the influence of single well fluid production ratio or pattern shape factor on the gas channel volume，the gas channel volume is corrected by the gas channel volume intersection diagram. The calculation method of gas channel volume has strong operability and practicability according to the results of gas channeling wells in a tight reservoir. On one hand，this method realizes the qualitative evaluation on gas channeling type and gas channeling degree. On the other hand，this method realizes the quantitative calculation of gas channeling volume and gas channeling horizontal area. It can provide theoretical basis for reasonable design of plugging measures and dosage of plugging agent in the later development stage.
During the development of CO2 flooding in low permeability reservoirs，the most serious problem is gas channeling. At present，there are no quantitative gas channeling identification standards，and the existing calculation methods of gas channeling volume are not practical. The dynamic variation characteristics of CO2 content in the produced gas are analyzed，and the types of gas channeling are divided into the obvious gas channeling and the weak gas channeling. The calculation method of gas breakthrough time is determined，and the identification and evaluation criteria for gas channeling are established. On this basis，gas channel volumes from the injection side and the production side are calculated respectively by using the principle of material conservation. In order to eliminate the influence of single well fluid production ratio or pattern shape factor on the gas channel volume，the gas channel volume is corrected by the gas channel volume intersection diagram. The calculation method of gas channel volume has strong operability and practicability according to the results of gas channeling wells in a tight reservoir. On one hand，this method realizes the qualitative evaluation on gas channeling type and gas channeling degree. On the other hand，this method realizes the quantitative calculation of gas channeling volume and gas channeling horizontal area. It can provide theoretical basis for reasonable design of plugging measures and dosage of plugging agent in the later development stage.
2020, 27(1):107-112.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.016
Abstract:
Determining a reasonable amount of CO2 injection is critical to the reservoir engineering design and dynamic adjustment of CO2 flooding reservoirs. Considering the characteristics of CO2 flooding，the process of CO2 flooding is divided into three zones：CO2 flow zone，CO2-oil mixed zone and oil flow zone. Based on this model，a calculation model for the reasonable injection amount of CO2 flooding is established. Combining FICK law and laboratory experimental results，the key parameters of the model are calculated and the model is solved. The CO2 flooding charts of injection-production quality ratio with pressure level under different water production rate are established. It is found that，while maintaining a certain formation pressure，the underground volume of produced oil，CO2 and water is equal to the sum of the pore volume occupied by CO2 in the pure CO2 flow zone and the expansion volume of oil in the CO2-oil mixed zone. In the CO2-oil mixed zone，the diffusion rate of CO2 of miscible flooding is first fast and then slow. With the increase of pressure level，the injection-production quality ratio increases gradually，but the increasing trend became slower. The higher the water production rate，the lower the injection-production quality ratio. The reliability of this study is verified by combining with field test data. The reasonable injection amount of CO2 flooding reservoirs can be conveniently and accurately calculated with the research results，and which can guide the design of CO2 flooding scheme and dynamic tracking control.
Determining a reasonable amount of CO2 injection is critical to the reservoir engineering design and dynamic adjustment of CO2 flooding reservoirs. Considering the characteristics of CO2 flooding，the process of CO2 flooding is divided into three zones：CO2 flow zone，CO2-oil mixed zone and oil flow zone. Based on this model，a calculation model for the reasonable injection amount of CO2 flooding is established. Combining FICK law and laboratory experimental results，the key parameters of the model are calculated and the model is solved. The CO2 flooding charts of injection-production quality ratio with pressure level under different water production rate are established. It is found that，while maintaining a certain formation pressure，the underground volume of produced oil，CO2 and water is equal to the sum of the pore volume occupied by CO2 in the pure CO2 flow zone and the expansion volume of oil in the CO2-oil mixed zone. In the CO2-oil mixed zone，the diffusion rate of CO2 of miscible flooding is first fast and then slow. With the increase of pressure level，the injection-production quality ratio increases gradually，but the increasing trend became slower. The higher the water production rate，the lower the injection-production quality ratio. The reliability of this study is verified by combining with field test data. The reasonable injection amount of CO2 flooding reservoirs can be conveniently and accurately calculated with the research results，and which can guide the design of CO2 flooding scheme and dynamic tracking control.
2020, 27(1):113-119.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.017
Abstract:
This paper focuses on the unknown problems of the well testing interpretation method for CO2 flooding in low permeability oil reservoir. Based on the percolation theory of multi-region composite model，an improved model of well testing interpretation is developed，which is solved by the numerical differential method. In this model，the threshold pressure gradient，pressure sensitivity effect and fluid dynamic property during CO2 flooding in the low permeability oil reservoir are considered. Meanwhile，Simultaneous Perturbation Stochastic Approximation（SPSA）optimum searching algorithm is conducted to fit pressure curve and pressure derivative curve simultaneously. Finally，automatic matching and parameter interpretation method of well testing for CO2 flooding in the low permeability oil reservoir is proposed. The variation of interpretation parameters obtained from practical well testing curves is analyzed. The results show that consideration of low permeability characteristics would have an obvious influence on the interpretation of reservoir permeability，and the interpreted permeability after optimization is increased by 37.97% compared with the initial value. On the contrary，the interpreted pressure transmitting coefficient and compressibility variation index are decreased by 16.94% and 21.97% respectively from the original interpretations，which are considered to be influenced seriously by fluid heterogeneity. The applications reveal that the proposed interpretation method is easier to operate，and has good feasibility.
This paper focuses on the unknown problems of the well testing interpretation method for CO2 flooding in low permeability oil reservoir. Based on the percolation theory of multi-region composite model，an improved model of well testing interpretation is developed，which is solved by the numerical differential method. In this model，the threshold pressure gradient，pressure sensitivity effect and fluid dynamic property during CO2 flooding in the low permeability oil reservoir are considered. Meanwhile，Simultaneous Perturbation Stochastic Approximation（SPSA）optimum searching algorithm is conducted to fit pressure curve and pressure derivative curve simultaneously. Finally，automatic matching and parameter interpretation method of well testing for CO2 flooding in the low permeability oil reservoir is proposed. The variation of interpretation parameters obtained from practical well testing curves is analyzed. The results show that consideration of low permeability characteristics would have an obvious influence on the interpretation of reservoir permeability，and the interpreted permeability after optimization is increased by 37.97% compared with the initial value. On the contrary，the interpreted pressure transmitting coefficient and compressibility variation index are decreased by 16.94% and 21.97% respectively from the original interpretations，which are considered to be influenced seriously by fluid heterogeneity. The applications reveal that the proposed interpretation method is easier to operate，and has good feasibility.
2020, 27(1):120-125.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.018
Abstract:
In order to understand the pressure response characteristics during the CO2 flooding process and determine the location of the displacement front，a well testing interpretation method for the CO2 flooding displacement front is studied.Considering the effects of CO2 flooding mechanism，start-up pressure gradient，and heterogeneity characteristics of the lowpermeability reservoirs，a multi-component well testing model for CO2 flooding wells in low permeability reservoirs is set up. Based on the unstructured grid generation technique and the finite volume method，the nonlinear algebraic equations are solved by the Newton-Raphson method. According to the characteristics of the flow pattern，the well testing curves are divided into five sections：the wellbore storage section，transitional section，radial flow section of pure CO2 zone，flow section of CO2 swept zone and flow section of CO2 unswept zone. By tracing the effect of fluid property changes on the well testing curves，the effect of CO2 concentration changes on the pressure is further studied，and a method for determining the flooding displacement front is established. The field application practice shows that this method can be used to analyze the location of the displacement front，predict the advancement of the front，and provide an effective means for understanding the development law and optimizing the development scheme of CO2 flooding in low-permeability reservoirs.
In order to understand the pressure response characteristics during the CO2 flooding process and determine the location of the displacement front，a well testing interpretation method for the CO2 flooding displacement front is studied.Considering the effects of CO2 flooding mechanism，start-up pressure gradient，and heterogeneity characteristics of the lowpermeability reservoirs，a multi-component well testing model for CO2 flooding wells in low permeability reservoirs is set up. Based on the unstructured grid generation technique and the finite volume method，the nonlinear algebraic equations are solved by the Newton-Raphson method. According to the characteristics of the flow pattern，the well testing curves are divided into five sections：the wellbore storage section，transitional section，radial flow section of pure CO2 zone，flow section of CO2 swept zone and flow section of CO2 unswept zone. By tracing the effect of fluid property changes on the well testing curves，the effect of CO2 concentration changes on the pressure is further studied，and a method for determining the flooding displacement front is established. The field application practice shows that this method can be used to analyze the location of the displacement front，predict the advancement of the front，and provide an effective means for understanding the development law and optimizing the development scheme of CO2 flooding in low-permeability reservoirs.
2020, 27(1):126-133.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.019
Abstract:
In order to clarify the characteristics of front and displacement efficiency of CO2 flooding in Fan142 Block，Zhenglizhuang Oilfield，a numerical simulation method for CO2 flooding and the optimization for well open sequence for oil wells are studied. A fully-implicit reservoir numerical simulator for CO2 flooding is developed by using the multiphase percolation theory and the numerical solution technique. Combining the geological/production data analysis and oil reservoir simulation，the distribution of CO2 and the displacement front of the block are studied during the pressure build-up stage by continuous CO2 injection. It is found that an obvious fingering phenomenon occurs at the CO2 flooding front. After 2 years of injection，the front has reached Well F142-6-2. The shortest distances from the front to Well F142-6-3 and F141-1-1 are 52 m and 45 m，respectively. The best scheme is that wells F142-8-3 and F142-7-3 in the low pressure area are first opened and the remaining 4 wells are opened for production after 1 year. The cumulative production for 5 years of this scheme is 1.34% higher than that of the scheme that the wells are opened at the same time，and the increase of oil production is 5 407 t. By comparing the oil recoveries，geological sequestration rates and replacement rates of CO2，it is indicated that the sequence of well opening have an significant effect on the displacement efficiency.
In order to clarify the characteristics of front and displacement efficiency of CO2 flooding in Fan142 Block，Zhenglizhuang Oilfield，a numerical simulation method for CO2 flooding and the optimization for well open sequence for oil wells are studied. A fully-implicit reservoir numerical simulator for CO2 flooding is developed by using the multiphase percolation theory and the numerical solution technique. Combining the geological/production data analysis and oil reservoir simulation，the distribution of CO2 and the displacement front of the block are studied during the pressure build-up stage by continuous CO2 injection. It is found that an obvious fingering phenomenon occurs at the CO2 flooding front. After 2 years of injection，the front has reached Well F142-6-2. The shortest distances from the front to Well F142-6-3 and F141-1-1 are 52 m and 45 m，respectively. The best scheme is that wells F142-8-3 and F142-7-3 in the low pressure area are first opened and the remaining 4 wells are opened for production after 1 year. The cumulative production for 5 years of this scheme is 1.34% higher than that of the scheme that the wells are opened at the same time，and the increase of oil production is 5 407 t. By comparing the oil recoveries，geological sequestration rates and replacement rates of CO2，it is indicated that the sequence of well opening have an significant effect on the displacement efficiency.
2020, 27(1):134-139.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.020
Abstract:
The rationality of injection-production ratio is important to the production performance of gas flooding. According to concepts and research results of the material balance principle，Darcy’s law，fractional flow theory，oil production rate multiplier due to gas flooding；and“oil wall”description of gas flooding，and by considering the quantified factors including gas injection dissolution，reservoir fluid expansion，mineralization and solidification of injected gas，reservoir pressure-sensitive effect，natural fracture drainage and dry layer suction，liquid exchange between gas injection areas and external environment，and the composition of produced gas oil ratio，the formula of injection-production ratio based on underground two-phase volume of the produced oil-water（2P formula）and underground three-phase volume of produced oil-water-gas（3P formula）are proposed，respectively，and the calculation method of daily gas injection rate is proposed as well. The new method is applied to study the injection-production ratio change of representative CO2 flooding projects in extralow permeability oil reservoirs. It is found that it is feasible to allocate injection rate according to the injection-production ratio estimated by the 3P formula after continuous gas injection（CGI），and to allocate injection rate of water-alternatinggas（WAG）injection according to the injection-roduction rate estimated by 2P formula.
The rationality of injection-production ratio is important to the production performance of gas flooding. According to concepts and research results of the material balance principle，Darcy’s law，fractional flow theory，oil production rate multiplier due to gas flooding；and“oil wall”description of gas flooding，and by considering the quantified factors including gas injection dissolution，reservoir fluid expansion，mineralization and solidification of injected gas，reservoir pressure-sensitive effect，natural fracture drainage and dry layer suction，liquid exchange between gas injection areas and external environment，and the composition of produced gas oil ratio，the formula of injection-production ratio based on underground two-phase volume of the produced oil-water（2P formula）and underground three-phase volume of produced oil-water-gas（3P formula）are proposed，respectively，and the calculation method of daily gas injection rate is proposed as well. The new method is applied to study the injection-production ratio change of representative CO2 flooding projects in extralow permeability oil reservoirs. It is found that it is feasible to allocate injection rate according to the injection-production ratio estimated by the 3P formula after continuous gas injection（CGI），and to allocate injection rate of water-alternatinggas（WAG）injection according to the injection-roduction rate estimated by 2P formula.
2020, 27(1):140-145.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.021
Abstract:
Because CO2 flooding can meet the demand of both environmental protection and enhanced oil recovery，it has attracted increasing attention. The reservoir screening and potential evaluation methods of CO2 flooding are the basis of medium and long-term development planning and large-scale field application，but the screening criteria guided by miscibleimmiscible flooding theory are difficult to adapt to China’s continental sandstone reservoirs. Therefore，the miscibility characteristics of CO2 flooding are firstly studied by numerical simulation. The transient miscibility at the initial injection end appears. Afterwards，the miscibility zone moves to the production end，and phenomena including phase separation，interfacial tension reduction at leading edge and interfacial tension rise in the production well appears. The variation characteristics of miscibility degree are analyzed and the boundaries of near-miscibility flooding are proposed. On the basis of existing standards，a new standard for screening CO2 flooding reservoirs is proposed. It is combined with fuzzy evaluation and analytic hierarchy process，and a screening method for CO2 flooding in sandstone reservoirs is proposed and applied to the screening of CO2 flooding reservoirs of SINOPEC. The results show that SINOPEC has 2.262 billion tons of proven reserves suitable for CO2 flooding except for chemical flooding reservoirs and heavy oil reservoirs，of which，489 million tons are miscible flooding，659 million tons are near miscible flooding and 1 114 million tones are immiscible flooding，and the recoverable reserves are expected to increase by 203 million tons.
Because CO2 flooding can meet the demand of both environmental protection and enhanced oil recovery，it has attracted increasing attention. The reservoir screening and potential evaluation methods of CO2 flooding are the basis of medium and long-term development planning and large-scale field application，but the screening criteria guided by miscibleimmiscible flooding theory are difficult to adapt to China’s continental sandstone reservoirs. Therefore，the miscibility characteristics of CO2 flooding are firstly studied by numerical simulation. The transient miscibility at the initial injection end appears. Afterwards，the miscibility zone moves to the production end，and phenomena including phase separation，interfacial tension reduction at leading edge and interfacial tension rise in the production well appears. The variation characteristics of miscibility degree are analyzed and the boundaries of near-miscibility flooding are proposed. On the basis of existing standards，a new standard for screening CO2 flooding reservoirs is proposed. It is combined with fuzzy evaluation and analytic hierarchy process，and a screening method for CO2 flooding in sandstone reservoirs is proposed and applied to the screening of CO2 flooding reservoirs of SINOPEC. The results show that SINOPEC has 2.262 billion tons of proven reserves suitable for CO2 flooding except for chemical flooding reservoirs and heavy oil reservoirs，of which，489 million tons are miscible flooding，659 million tons are near miscible flooding and 1 114 million tones are immiscible flooding，and the recoverable reserves are expected to increase by 203 million tons.
2020, 27(1):146-151.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.022
Abstract:
The CO2 flooding technologies formed from research and practice in Zhongyuan Oilfield are systematically summarized in recent ten years，including CO2 flooding mechanism，suitability screening and evaluation technology，reservoir engineering optimization technology，injection and production engineering technology. The implementation results in the Lower Es1 reservoir of Pucheng Oilfield and Block Hu96 etc. are evaluated and analyzed，and the CO2 flooding experience for different types of reservoirs is summarized. Research results show that the CO2 can drive the residual oil after water flooding because of dissolving in oil spot and oil film for extra-high water cut reservoirs. The CO2 can displace the crude oil in the pore throat above 0.01 μm because of the small interfacial tension between the CO2 and crude oil and the small capillary resistance for the deep reservoir with low permeability. The field practice has proved that the relatively mature CO2 flooding technologies have been developed in the corrosion protection，profile monitoring，stratified injection and mobility control. Application of the CO2 flooding in the deep reservoirs with low permeability has obtained good results，which are in extra-high water cut stage. Because the CO2 flooding performance is not affected by temperature or salinity，it has great potential in the high temperature and high salinity reservoirs，which also provides references for the gas injection technology and practice in other types of reservoirs.
The CO2 flooding technologies formed from research and practice in Zhongyuan Oilfield are systematically summarized in recent ten years，including CO2 flooding mechanism，suitability screening and evaluation technology，reservoir engineering optimization technology，injection and production engineering technology. The implementation results in the Lower Es1 reservoir of Pucheng Oilfield and Block Hu96 etc. are evaluated and analyzed，and the CO2 flooding experience for different types of reservoirs is summarized. Research results show that the CO2 can drive the residual oil after water flooding because of dissolving in oil spot and oil film for extra-high water cut reservoirs. The CO2 can displace the crude oil in the pore throat above 0.01 μm because of the small interfacial tension between the CO2 and crude oil and the small capillary resistance for the deep reservoir with low permeability. The field practice has proved that the relatively mature CO2 flooding technologies have been developed in the corrosion protection，profile monitoring，stratified injection and mobility control. Application of the CO2 flooding in the deep reservoirs with low permeability has obtained good results，which are in extra-high water cut stage. Because the CO2 flooding performance is not affected by temperature or salinity，it has great potential in the high temperature and high salinity reservoirs，which also provides references for the gas injection technology and practice in other types of reservoirs.
CHEN Zuhua
,
WU Gongyi
,
QIAN Weiming
,
WANG Jun
,
MA Tao
,
WANG Haimei
,
ZHENG Yongwang
,
XIONG Xinya
2020, 27(1):152-162.
DOI: 10.13673/j.cnki.cn37-1359/te.2020.01.023
Abstract:
Based on the geological characteristics of“fragmentary，small，low，thin and deep”and the development characteristics of“three low and three high”of the small complex fault block reservoirs in Subei Basin，five development technologies are formed including the laboratory physical simulation experiment and suitability evaluation for CO2 flooding/huff and puff，the scheme design and optimization，miscible identification and development result evaluation for CO2 flooding. In addition，totally 6 supporting technologies of collection，purification，gathering and transportation，injection，oil production and recovery are formed. Under the situation of low oil price and by focusing on the two key factors of cost and benefit，the CO2 EOR method applied in low permeability reservoirs is extended to the medium-high permeability reservoirs，and also developed from single flooding/huff and puff to combination of them，which provides strong technical support for stable and increasing production and effective development in the old fields of Subei Basin.
Based on the geological characteristics of“fragmentary，small，low，thin and deep”and the development characteristics of“three low and three high”of the small complex fault block reservoirs in Subei Basin，five development technologies are formed including the laboratory physical simulation experiment and suitability evaluation for CO2 flooding/huff and puff，the scheme design and optimization，miscible identification and development result evaluation for CO2 flooding. In addition，totally 6 supporting technologies of collection，purification，gathering and transportation，injection，oil production and recovery are formed. Under the situation of low oil price and by focusing on the two key factors of cost and benefit，the CO2 EOR method applied in low permeability reservoirs is extended to the medium-high permeability reservoirs，and also developed from single flooding/huff and puff to combination of them，which provides strong technical support for stable and increasing production and effective development in the old fields of Subei Basin.
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