The Investigation into Carbonated Water as Pre-Fracturing Fluid to Improve Shale Oil Recovery by CT Online Scanning and Imbibition Experiments

Abstract Carbonated water (CW) imbibition is an effective method for enhanced oil recovery, by combining the advantages of CO2 and water. Nevertheless, there are few reports in existing studies on its performance in shale oil reservoirs. This research is specifically focused on these shale oil reservoirs, investigating the variation in countercurrent imbibition distance (CID) and the imbibition effects of using CW as a pre-fracturing fluid. In this study, CT online scanning experiments were conducted with shale cores to investigate the CID of formation water, CW, and fracturing fluids. Simultaneously, we determined and comparatively analyzed the countercurrent imbibition recovery (CIR) of these fluids. Furthermore, imbibition experiments were conducted using a high-temperature and high-pressure visual apparatus to assess the imbibition recovery of formation water, CW, and fracturing fluids. The findings suggest that CW significantly enhances shale oil recovery, with CID and CIR values reaching 1.75 cm and 6.95%, respectively. Compared to formation water and fracturing fluid, the CID of CW increased by 0.75 cm and 0.25 cm, while the CIR rose by 4.77% and 1.90%, respectively. CW achieved the highest imbibition recovery at 29.22%, exceeding that of fracturing fluids (25.18%) and formation water (10.25%). In this research, the application of CW as a pre-fracturing fluid to enhance shale oil recovery is introduced for the first time. Moreover, it quantifies the CID of CW and fracturing fluids in shale matrices, offering a pioneering strategy for the enhanced coordination of fracturing, shut-in, and production processes in shale oil exploitation.