Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Jie CHI, Binshan JU, Guangzhong LYU, Xing ZHANG, Jiabei WANG Abstract Based on the theory of non-Darcy seepage, a mathematical model for CO 2 miscible and immiscible concurrent flooding considering changes of oil viscosity and threshold pressure gradient of oil and CO 2 is established. A computational method of critical well spacing of CO 2 miscible and immiscible concurrent flooding in ultra-low permeability reservoirs is deduced by solving the distribution of CO 2 concentration in miscible flooding area with the mass transfer-diffusion-absorption equation and solving the saturation equation of immiscible affected area with characteristic line method. A critical well spacing example is built in the F142 and G89 reservoir blocks and the results show: (1) The critical well spacing increases with gas injection pressure, while decreases with gas injection speed; (2) The contribution of length in pure CO 2 seepage area to the critical well spacing is the largest, the contribution of length in CO 2 -Oil effective mass transfer area and immiscible affected area is secondary, the contribution of pure oil area is the least, the gap of length between pure CO 2 seepage area and CO 2 -Oil effective mass transfer area and immiscible affected area decreases with gas injection speed and it increases with the decreasing of gas injection speed, meanwhile, the law is more significant; (3) Pressure drop gradient of miscible affected area is significantly different from that of immiscible affected area and pressure drop gradient of CO 2 -Oil effective mass transfer area in miscible affected area is bigger than that of immiscible affected area.