Comparative Study of Pore Structure Response in Longmaxi Marine Shale and Yanchang Continental Shale under the Combined Action of CO2 and Slickwater

Supercritical CO2 (ScCO2) + slickwater hybrid fracturing technology exhibits significant potential in enhancing shale gas recovery and CO2 geological sequestration. However, the retention of CO2 and slickwater may induce alterations in the shale's pore structure postfracturing, exhibiting distinct variations between marine and continental formations. The marine shales from Longmaxi Formation and continental shales from Yanchang Formation were selected as samples in this study. Low-temperature nitrogen adsorption (LN(2)A), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), X-ray diffraction (XRD), and fractal theory were employed to analyze the variations in the mineral composition and pore structure resulting from ScCO2-slickwater treatment. This study compares the changes in pore structure of two different sedimentary environment shales after CO2-slickwater treatment, identifies the regularity of pore structure changes in shales, and analyzes the reasons for the differences in pore changes between the two. The results show that after CO2-slickwater treatment, the total specific surface area (TSSA) and total pore volume (TPV) of Longmaxi marine shale increase, with more micro- and mesopores but fewer macropores, while the trend in Yanchang continental shale is the opposite. This phenomenon may be influenced by mineral composition and original pore structure. Additionally, scanning electron microscopy confirms that both marine and continental shales undergo significant dissolution after CO2-slickwater treatment, with some rock surfaces covered by a thin film and residual polyacrylamide (PAM) adsorbed inside or around the pores. Due to differences in mineral composition, marine shale primarily exhibits mineral dissolution reactions, while continental shale mainly shows mineral precipitation reactions, leading to a greater amplitude of pore structure parameter changes in marine shale. These changes in the pore structure strongly affect fluid flow and storage in shale reservoirs, which is of great significance for shale gas development and carbon dioxide geological storage.