Mechanisms of CO ₂ Preinjection Fracturing-Flooding for Enhancing Oil Recovery in Tight Oil Reservoirs

In order to solve the problem of energy replenishment in tight reservoirs, fracturing-flooding has been carried out in some oilfields in China. Similar to CO 2 prefracturing technology, injecting a CO 2 slug before fracturing-flooding can activate the bedding and natural fractures. In this study, variations in oil–water interfacial tension (IFT), rock mineral composition, wettability, and mechanical properties during CO 2 injection under different conditions were analyzed. Then, a series of core-scale fracturing-flooding experiments were performed to evaluate the effects of CO 2 PI on fracturing pressure, oil recovery factor, and microscopic remaining oil distribution. The results indicate that the CO 2 preinjection and soaking (CO 2 PIS) fracturing-flooding process can reduce both the fracturing pressure and the pressuring time compared to conventional fracturing-flooding. However, the total oil recovery factor from the CO 2 PIS test was the lowest, primarily due to water channeling through the generated microfractures. Notably, the CO 2 PIS process significantly improved oil recovery from micropores to 17% and lowered the limit of pore size for oil mobilization from 0.045 to 0.013 μm compared with conventional fracturing-flooding. Subsequently, an imbibition stage was introduced after fracturing to take full advantage of the extended microfracture network. With the help of the imbibition process, the oil recovery of the CO 2 PIS fracturing-flooding process increased by 18.9%, and the lower mobilization pore size limit was further reduced from 0.013 to 0.007 μm, demonstrating a substantial expansion in the range of effectively accessible pores.