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 CO₂ 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 CO₂, including CO₂ development parameters, core properties, and fractures. Additionally, the effect of C₄EO₃PO₆ (a polyether surfactant) on enhancing extraction efficiency of CO₂ 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 CO₂ 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 CO₂ within the rock matrix. After three CO₂ 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. C₄EO₃PO₆ effectively reduces the interfacial tension and minimum miscibility pressure between CO₂ and shale oil, significantly enhancing the extraction efficiency of CO₂. With the addition of C₄EO₃PO₆, 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 CO₂ injection.