Adsorption and Deformation Behavior of Shale under the Action of Supercritical Carbon Dioxide

Classical adsorption theories fail under supercritical conditions, particularly in describing the CO2 adsorption behavior during supercritical CO2 fracturing. Studies on Longmaxi Formation shale in the Sichuan Basin reveal discrepancies in adsorption models under varying experimental conditions, alongside limited research on adsorption deformation. Therefore, this study employs the gravimetric method using rock samples from the deep shales of the Longmaxi Formation to conduct adsorption experiments at different temperatures. Through blank experiments and buoyancy correction, the volume and mass of the sample containers as well as the true volume of the samples were determined. Subsequently, CO2 adsorption measurements were conducted at multiple pressure points within the 0–35 MPa range. Postequilibrium changes in the pressure, temperature, and weight were recorded to derive the CO2 adsorption isotherm. Experimental data were fitted and analyzed using multiple adsorption models, ultimately establishing an adsorption model applicable to this region. Multiple adsorption models were applied with the modified Langmuir model showing superior accuracy for describing CO2 adsorption in the target area. Simultaneously, helium adsorption experiments were conducted on target shale samples under isothermal and isobaric conditions using the microgravity method. By converting the weight change data from these adsorption experiments into volumetric changes, the deformation caused by carbon dioxide adsorption within the shale was determined. Results show adsorption decreases with temperature, approaching zero at high temperatures (120 °C) and pressures (35 MPa). Adsorption deformation increases with temperature below 10 MPa, while higher pressures show combined temperature and pressure effects, and when pressure increased to 35 MPa, with adsorption deformation remaining virtually zero under high temperatures. Although negligible under high temperature and pressure, adsorption deformation becomes significant at fracture tips during CO2 fracturing, where localized low-temperature and low-pressure zones form. These findings enhance our understanding of shale adsorption characteristics, revealing the adsorption behavior and deformation patterns of supercritical CO2, aiding future gas extraction and storage projects.