A systematic review of CO2 injection for enhanced oil recovery and carbon storage in shale reservoirs

Global shale oil is abundant in geological reserves, but there are widespread problems of difficult exploitation and unsatisfactory oil recovery. CO2 injection in shale reservoirs can achieve effective exploitation of shale reservoirs and alleviate the current energy shortage. In this paper, investigations on shale oil in recent years were summarized and analyzed. Firstly, the reservoir characteristics and fluid properties of several shale reservoirs and their effects on CO2-enhanced oil recovery (CO2-EOR) were analyzed, including porosity, permeability, mineral composition, pore structure, geomechanical characteristics and organic matter properties of shale reservoirs, as well as the density, viscosity, gas-oil ratio and composition of shale oil. Subsequently, multiple mechanisms of CO2-EOR in shale reservoirs were revealed, including swelling, diffusion, viscosity reduction, extraction, miscibility, and competitive adsorption. The effects of temperature, pressure, CO2 injection amount, oil composition and reservoir heterogeneity on different mechanisms were analyzed. The effects of nanopore confinement and CO2 injection on the fluid phase were also analyzed. Then, the CO2 storage forms of structural space, bound space, dissolution, and mineralization were introduced. And the effects of temperature, pressure, reservoir heterogeneity and formation water salinity on various forms of CO2 storage were summarized. Furthermore, the research methods of CO2 injection technique through molecular simulation, physical simulation, mathematical model, and reservoir numerical simulation were introduced. Finally, the reasonable prospect of CO2 injection technique for shale oil reservoirs was proposed based on the current technical problems. The purpose of this paper is to provide a theoretical basis for EOR and carbon geological storage of shale reservoirs by using the CO2 injection technique.