The microscopic pore crude oil production characteristics and influencing factors by DME-assisted CO₂ injection in shale oil reservoirs

CO2 injection into shale oil reservoirs can effectively enhance recovery while simultaneously realizing geological storage. However, shale oil recovery by CO2 injection remains low. In this paper, classification and evaluation standards for shale oil reservoirs were established. Based on this, comparative experiments between pure CO2 flooding and dimethyl ether (DME)-assisted CO2 flooding were conducted, supplemented by nuclear magnetic resonance testing. The microscopic production characteristics of shale oil under different injection methods were analyzed. The effects of injection pressure, gas injection volume, and DME concentration on oil recovery were studied, and the potential of DME-assisted CO2 flooding was explored. The results show three types of shale samples: I, II, and III, with their corresponding physical properties, pore-throat structures, and percolation capacities decreasing in the same order. With the addition of DME, the CO2 solubility increased, while the produced oil viscosity significantly decreased. DME-assisted CO2 flooding has higher recovery compare with CO2 flooding. The increase is more significant as pressure increases. The increase in ultimate recovery was the largest for type III samples. In addition, regardless of whether pure CO2 flooding or DME-assisted CO2 flooding was applied, a higher injection pressure and injection volume mean higher recovery. The increase by DME-assisted CO2 flooding is much greater than that by pure CO2 flooding. During the miscible stage, the recovery is greater than that during the immiscible stage. The recovery increased with increasing DME molar concentration and then slowed down; therefore, the recommended molar concentration of DME in the mixed gas is 20 mol%. The research results provide new methods and ideas for enhancing shale oil recovery and realizing geological storage by CO2 injection.