A computational method of critical well spacing of CO2 miscible and immiscible concurrent flooding

Abstract

Abstract Based on the theory of non-Darcy seepage, a mathematical model for CO2 miscible and immiscible concurrent flooding considering changes of oil viscosity and threshold pressure gradient of oil and CO2 is established. A computational method of critical well spacing of CO2 miscible and immiscible concurrent flooding in ultra-low permeability reservoirs is deduced by solving the distribution of CO2 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 CO2 seepage area to the critical well spacing is the largest, the contribution of length in CO2-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 CO2 seepage area and CO2-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 CO2-Oil effective mass transfer area in miscible affected area is bigger than that of immiscible affected area.