Assessment of miscibility effect for CO2 flooding EOR in a low permeability reservoir

H59 block in Jilin Oilfield was selected as a pilot site for CCS (carbon capture and storage) project. The block is of a light oil reservoir with average permeability of 3.5 mD and well-developed natural fractures. CO2 separated from a nearby natural gas reservoir was injected into the oil block during six years operation, with nearly 0.26 million tons of CO2 (0.32 HCPV) has been injected. In order to evaluate the miscibility effect of CO2 flooding, injection and well production data were analyzed, including injection pressure, bottom hole flowing pressure (BHFP) of producer wells, CO2 breakthrough time and produced oil and gas compositions. Reservoir pressure distribution during CO2 injection was figured out based injection pressure and BHFP using the Kriging interpolation method, and the varying miscible region was illustrated in pressure contours. The analysis results show that CO2 injection can significantly increase oil production, but the size of the miscible region can be greatly reduced after early CO2 breakthrough, which can cause BHFP and oil production declining. In comparison with the experimental data of core flooding with CO2 at different modes, the produced oil composition data indicate that CO2 flooding in the field was more likely in near miscible or immiscible modes, though the injection pressure was higher than the MMP, which can be attributed to unexpected early CO2 breakthrough and low permeability nature of the reservoir.