Study on CO 2 –Water Co-Injection Miscible Characteristics in Low-Permeability Near-Critical Volatile Oil Reservoir

Low-permeability near-critical volatile reservoirs are characterized by light oil, complex fluid phase, and strong reservoir inhomogeneity, etc. Purely injecting CO 2 will create a series of problems, such as serious gas channeling, low sweep efficiency, and low oil recovery. Therefore, in this paper, through a combination of experiments and simulations and in the process of studying the problem from simple to complex, we carried out phase equilibrium experiments for CO 2 -near-critical volatile oil and CO 2 -near-critical volatile oil-formation water, as well as experiments for minimum miscible pressure of slim-tube with pure CO 2 and CO 2 –water co-injection to the comparative study of the miscible characteristics and displacement oil efficiency between pure CO 2 injection and CO 2 –water co-injection. It provides an important reference for improving oil recovery by CO 2 –water co-injection in low-permeability near-critical volatile reservoir. The results of CO 2 -near-critical volatile oil/CO 2 -near-critical volatile oil-formation water phase equilibrium experiments show that the saturation pressure, density, and gas–oil ratio of the system increase, and the viscosity decreases with the increase in CO 2 injection. In the three-phase system of CO 2 -near-critical volatile oil-formation water, the CO 2 content in the flash gas of crude oil, gas–oil ratio, and gas–water ratio are negatively correlated with the water saturation. The results of slim-tube experiments and simulations on the miscible characteristics and displacement oil efficiency of pure CO 2 injection and CO 2 –water co-injection show that the recovery degree of crude oil under CO 2 –water co-injection is higher than that of pure CO 2 injection, and the CO 2 dissolved transition section in oil and gas is shorter and the gas breakthrough time is later under CO 2 –water co-injection, which effectively increases the sweep efficiency and improves the degree recovery of crude oil. When CO 2 –water co-injection, the ratio of water is higher, the later the gas–oil ratio rises, the later the CO 2 breakthrough, and the higher the degree of crude oil recovery. It indicates that when CO 2 –water co-injection, the ratio of water is higher, the more CO 2 is dissolved in water, which effectively inhibits the occurrence of gas channeling and increases the sweep area, thus improving the degree recovery of crude oil. The research results of this paper provide an experimental basis and theoretical foundation for CO 2 –water co-injection for enhanced crude oil recovery in low-permeability near-critical volatile reservoirs.