CO2 mobility control and sweep efficiency improvement using starch gel or ethylenediamine in ultra-low permeability oil layers with different types of heterogeneity

Reservoir heterogeneity and natural fractures greatly affect CO2 flooding efficiency in ultra-low permeability oil reservoirs. CO2 water alternating gas (WAG) flooding and the combinations of continuous CO2 flooding and gas channeling treatments are recognized as effective approaches to improve CO2 sweep efficiency. It is of major interest to propose the novel gas channeling treatments and conduct a feasibility study of different CO2-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 CO2 flooding, CO2 WAG flooding, continuous CO2 flooding+one-stage gas channeling control, and continuous CO2 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 CO2 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 CO2 flooding decreases rapidly with an increase in permeability ratio. Continuous CO2 flooding cannot displace much of remaining oil in low permeability layers due to high mobility of CO2 and serious heterogeneity. WAG flooding can effectively control the fluid mobility and improve CO2 flooding efficiency when the permeability ratio is less or equal to 30. Plenty of remaining oil can be displaced by combining continuous CO2 flooding and gas channeling treatments. When the permeability ratio is less or equal to 100, ammonium carbamate, the reaction product of injected ethylenediamine and CO2 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 CO2 flooding, and the incremental oil recovery could be more than 20%. Pilot test of CO2 flooding was operated in Northwest China and concentrations of CO2 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.