Simulation study of factors affecting CO2 Huff-n-Puff process in tight oil reservoirs

CO2 injection is one of the effective enhanced oil recovery (EOR) methods in conventional oil reservoirs, which has been widely applied in the tight oil reservoirs. Although there are some efforts on the investigation of engineering parameters such as fracture half-length, fracture conductivity, and fracture height, the effects of CO2 molecular diffusion, nanopore confinement, and stress-dependent deformation on CO2-EOR effectiveness are poorly understood. In this paper, we modified the conventional phase-equilibrium model to accurately compute the phase behavior in shale nanopores. Subsequently, we investigated the minimum miscibility pressure (MMP) of CO2-EOR process under different pore sizes. Afterwards, we evaluated the well performance of CO2 Huff-n-Puff process by fully capturing physics of CO2 molecular diffusion and nanopore confinement. The impacts of matrix permeability, CO2 injection rate and stress-dependent deformation mechanisms on the CO2 Huff-n-Puff process are examined in detail. The results show that CO2 diffusion and nanopore confinement play an important role in improving the oil recovery factor, which should be correctly implemented in reservoir simulation model. Additionally, matrix permeability, CO2 injection rate, and stress-dependent deformation mechanisms significantly influence the CO2 Huff-n-Puff process. This work enables operators to optimize the CO2 Huff-n-Puff process to improve the efficiency of CO2-EOR and CO2 storage in tight oil reservoirs.