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Fractures and heterogeneity commonly exist in ultra-low permeability reservoirs, which will cause serious gas channeling phenomena and lead to poor oil recovery during CO2immiscible flooding. CO2water alternating gas (WAG) flooding combined with gas channeling control treatments are recognized as effective approaches to enlarge the sweep efficiency of CO2. Performance of gel systems were evaluated to screen out a high-strength gel which can be utilized as fracture blocking agent. Then, performance of small molecule amines were evaluated to screen out a kind of small molecule amine which can be utilized as rock-matrix blocking agent. Performance evaluation results showed that high-strength gel with 8% modified starch + acrylic amide, 0.05% crosslinking agent, 0.15% initiator and 0.15% stabilizer could be served as fracture blocking agent, while etheylenediamine was proposed as rock-matrix blocking agent which could react with CO2 and generate solid particles to improve the heterogeneity of the reservoirs. To examine gas channeling control effect in actual reservoir conditions, a 3D radial flow model with complex fractures and heterogeneity was designed according to the actual oilfield, and five-spot pattern was employed in the model. Waterflooding, continuous CO2 flooding, and gas channeling control using high-strength gel, etheylenediamine and CO2 WAG flooding were conducted successively in the 3D radial flow model. After a series of treatments, the oil recovery was enhanced by 15.09%, which was significant for CO2 immiscible flooding in the field application. The gas channeling along major fracture and microfractures of the model was effectively controlled by high-strength gel, while fluid diversion could occurred after ethylenediamine injection due to the solid particles which could mitigate gas channeling along relatively high permeability areas of the rock-matrix, and the gas channeling phenomena can be further mitigated by the mobility control ability of CO2 WAG flooding.

Reservoir heterogeneity and natural fractures greatly affect CO 2 flooding efficiency in ultra-low permeability oil reservoirs. CO 2 water alternating gas (WAG) flooding and the combinations of continuous CO 2 flooding and gas channeling treatments are recognized as effective approaches to improve CO 2 sweep efficiency. It is of major interest to propose the novel gas channeling treatments and conduct a feasibility study of different CO 2 -EOR technologies. Heterogeneous cores with different permeability ratios and fracture model were utilized in the laboratory experiments to simulate different types of heterogeneity in the oil reservoirs. Continuous CO 2 flooding, CO 2 WAG flooding, continuous CO 2 flooding+one-stage gas channeling control, and continuous CO 2 flooding+two-stage gas channeling control were conducted after water flooding in homogeneous and heterogeneous cores and fracture model. Ethylenediamine and modified starch gel were proposed as blocking agents to mitigate gas channeling. During the experiments, the CO 2 flooding efficiency was evaluated through the oil recovery increment, fluid mobility control and the changes of producing pressure drop. Experimental results show that producing pressure drop during continuous CO 2 flooding decreases rapidly with an increase in permeability ratio. Continuous CO 2 flooding cannot displace much of remaining oil in low permeability layers due to high mobility of CO 2 and serious heterogeneity. WAG flooding can effectively control the fluid mobility and improve CO 2 flooding efficiency when the permeability ratio is less or equal to 30. Plenty of remaining oil can be displaced by combining continuous CO 2 flooding and gas channeling treatments. When the permeability ratio is less or equal to 100, ammonium carbamate, the reaction product of injected ethylenediamine and CO 2 can block off high permeability layer and impel the injecting gas into low permeability layer. The two-stage injection of modified starch gel and etheylenediamine can significantly mitigate the gas channeling within high-capacity gas channel model and fracture model during continuous CO 2 flooding, and the incremental oil recovery could be more than 20%. Pilot test of CO 2 flooding was operated in Northwest China and concentrations of CO 2 are monitored in six production wells. Two kinds of construction plans for gas channeling treatments have been preliminary designed to control the gas channeling in the pilot.