CO ₂ Adsorption Storage Alteration by Hydrocarbon and Water in the Chang 7 Lacustrine Oil-Bearing Shales, Ordos Basin, China: Implications for CO ₂ Sequestration

Although multiphase pore fluids strongly influence the CO 2 adsorption of shale, quantitative characterization methods and predictive models capable of assessing their contributions to adsorption capacity during geological CO 2 storage remain limited. In this study, CO 2 and N 2 adsorption experiments were performed on shale samples following the sequential removal of pore fluids to evaluate the effects of fluid type and content on CO 2 adsorption. The adsorption capacity of shale increased in the sequence: original (all fluids retained) < hexane-extracted (heavy oil and water remained) < DCM-extracted (only water remained) < oven-dried (no oil or water). Reduction of pore fluids enhanced the level of CO 2 adsorption by increasing the specific surface area, pore volume, and isosteric heat. Following the removal of light and heavy oils, additional CO 2 adsorption primarily occurred on the surfaces of clay minerals and kerogen. The isosteric heat increased from 7.347 to 9.089 and 10.083 kJ/mol after the elimination of light and heavy oils, respectively, but showed a negligible change after water removal. The effect of light oil on the CO 2 adsorption was approximately 1.5 times stronger than that of heavy oil. During shale oil extraction, the amount of CO 2 sequestered increased, with oil displacement with adsorption serving as the dominant sequestration mechanism under all fluid conditions. These results provide new insights into the distribution of pore fluids in shale nanopores and improve the quantitative assessment of the CO 2 storage potential in fluid-bearing shale formations.