Experimental and numerical studies on CO2 injectivity in low permeability oil reservoirs

Introduction: Contrary to the traditional recognition that CO 2 has large injectivity (the ratio of CO 2 injection volume to pressure drop) in low permeability oil reservoirs to keep the formation pressure at a high level, the CO 2 injection rate usually cannot attain the set value. It is essential to study the factors that influence CO 2 injectivity and propose the optimal strategies to improve the CO 2 injectivity. Methods: Therefore, in this study, several core samples collected from low permeability oil reservoirs are used to experimentally investigate the influences of CO 2 injection rate, formation permeability, pressure and water saturation on CO 2 injectivity, and the corresponding pressure drop, oil and gas production are examined. To determine the primary factor that influences the CO 2 injectivity, orthogonal experimental design (ODE) and numerical simulations are utilized. In addition, to improve CO 2 injectivity, the techniques of mini-fracturing and radial perforation are presented, and the threshold values for these two parameters are determined. Result and discussion: The results demonstrate that according to the magnitude of the extent that influences CO 2 injectivity, the rank for the above factors is CO 2 injection rate, reservoir pressure, formation water saturation and permeability. The oil recovery is mainly influenced by CO 2 injection rate and formation permeability, and the influences of reservoir pressure and water saturation on oil recovery are smaller. The threshold values for radial perforation and fracture half-length are 25 m and 50m, respectively, which can provide some guidance for the strategies that should be taken to improve the CO 2 injectivity and recover more oil from low permeability oil reservoirs.