Unlocking shale oil: Synergistic mechanisms of CO2 extraction enhanced by fractures and surfactants

The development of unconventional oil and gas resources is currently the primary focus of the petroleum industry. Shale oil has attracted significant attention due to its vast reserves. However, shale oil reservoirs are characterized by low porosity and low permeability, combined with rich organic matter, resulting in low oil recovery. The extraction efficiency of CO 2 is one of the key mechanisms for enhancing oil recovery. In this study, a comprehensive analysis was conducted on the factors influencing the extraction efficiency of CO 2 , including CO 2 development parameters, core properties, and fractures. Additionally, the effect of C 4 EO 3 PO 6 (a polyether surfactant) on enhancing extraction efficiency of CO 2 was evaluated. The results show that the developed micropore and nanopore structure in shale makes oil in micropores, as well as absorbed and adsorbed oil, particularly difficult to mobilize. Under comparable permeability and porosity conditions, the CO 2 extraction recovery for tight sandstone is 2.2 times higher than that for shale, and the mobilization efficiency of free oil is 3.87 times higher than that of absorbed and adsorbed oil. The presence of fractures facilitates the dissolution and diffusion of CO 2 within the rock matrix. After three CO 2 extraction cycles, the extraction recoveries reach 81.6% for the naturally fractured shale core (containing natural fractures) and nearly 100% for both shale block samples and tight sandstone block samples. C 4 EO 3 PO 6 effectively reduces the interfacial tension and minimum miscibility pressure between CO 2 and shale oil, significantly enhancing the extraction efficiency of CO 2 . With the addition of C 4 EO 3 PO 6 , the extraction recovery of the shale core increases by 29.43%, with improvements of 27.25% for oil in micropores and 27.31% for absorbed and adsorbed oil. This work aims to provide an innovative development strategy for enhancing shale oil recovery through CO 2 injection.