Influence of supercritical CO2 and its aqueous solution on the seepage characteristics of the niutitang shale

Abstract

The influence of supercritical CO₂ and its aqueous fluids on shale permeability is crucial during supercritical CO₂-enhanced shale gas extraction and CO₂ geological storage in shale gas reservoirs. The research on the effects of supercritical CO₂ on shale permeability, particularly under formation temperature and pressure conditions with and without water, remains insufficiently explored. Therefore, this study conducted one-month immersion experiments involving supercritical CO₂-shale, deionized water-shale, and supercritical CO₂-deionized water-shale interactions at 80 °C and 15 MPa.The influence of supercritical CO₂ on the seepage characteristics of shale under both water-bearing and water-absent conditions was investigated and revealed by examining changes in shale mineral composition and microscopic pore structures. The results indicated that permeability increased by 10 times following deionized water immersion, by 28 % after supercritical CO₂ immersion, and by 86 times following supercritical CO₂+deionized water immersion. Shale minerals experienced varying degrees of chemical dissolution depending on the immersion fluid, with the supercritical CO₂+deionized water solution resulting in the most significant dissolution, far exceeding the effects of the other two fluids. The change in plane porosity was most pronounced following immersion in the supercritical CO₂+deionized water solution, which corresponded to the highest observed permeability increase. Therefore, in the process of supercritical CO₂-enhanced shale gas extraction and carbon sequestration, the combined influence of supercritical CO₂ and formation water on the seepage characteristics of shale formations should be given special attention.