Effect of Supercritical CO2–Water–Shale Interaction on Pore Evolution and Oil Composition

Abstract

Supercritical CO₂ (ScCO₂) soaking significantly impacts pore structure evolution and oil composition in tight shale oil reservoirs. This study conducted static soaking experiments under actual reservoir temperature and pressure, and the brine and deionized water were used as control fluids. The variations in pore structures and properties are characterized using scanning electron microscopy (SEM), X-ray diffraction, low-field nuclear magnetic resonance core analysis system, low-temperature nitrogen adsorption, and gas chromatography in shale samples before and after ScCO₂ soaking. Results show ScCO₂ soaking enhances pore connectivity, particularly in microscale and mesoscale pores, by dissolving carbonates and feldspar, increasing pore volume (+30%), surface area (+4.9%), and average pore diameter (+9%). SEM images reveal fracture expansion and new pore formation due to mineral dissolution and precipitation during ScCO₂ soaking. Wettability analysis shows a shift from water-wet to CO₂-wet conditions, with increased contact angles for deionized water (+18.8%) and brine (+40.5%) after 15 days. An increasing trend in the hydrophobicity of the shale matrix can be observed. Gas chromatography indicates a rise in C15–C30 fractions and heavy hydrocarbons, increasing the shale oil’s average molecular weight to 298.36 g/mol. These findings provide new insights into the feasibility of using CO₂ to enhance oil recovery and CO₂ sequestration in shale reservoirs. Enhancing pore connectivity and altering wettability properties can potentially improve shale oil development.