CO2 Front Migration Law and Injection-Production Optimization Based on Incomplete-Miscible Displacement

At present, CO2 displacement has become one of the most effective ways to develop low-permeability reservoirs, and these were characterized by immiscible interfaces and easy gas channels. In order to clarify the front migration law of low-permeability reservoirs under CO2 displacement, the concepts of pressure front, diffusion front, and interface front were proposed. A conceptual model was established to describe the front migration characteristics and parameter changes of different miscible degrees during displacement. It is worth mentioning that the quantitative characterization of miscibility based on the coupling relationship between the front and the pressure in this work is different from the previous characterization of miscibility by a single pressure parameter, which is more in line with the development characteristics. The results showed that with the increase of the CO2 injection, three fronts migrated from the injection well to production well and the pressure front was the fastest followed by the diffusion front and the interface front. With the decrease of the miscible degree, the pressure front receded faster, the width of the miscible band narrowed by about 11 m, and the degree of miscibility decreased significantly. Based on the front migration law, the injection-production process was optimized as alternating water and gas injection, five-point well pattern, well spacing of 450 m, water and gas slug ratio of 1:1, and slug size of 0.04 HCPV. This work provided some technical guidance for the efficient development of CO2 flooding in low-permeability reservoirs.