Experimental Investigation of the Geochemical Interactions between Supercritical CO2 and Shale: Implications for CO2 Storage in Gas-Bearing Shale Formations

Interactions between injected CO2 and shale formation during the process of CO2 sequestration with enhancing shale gas recovery (CS-EGR) may alter the physical and chemical properties of the rock, affecting the efficiency of CO2 storage as well as CH4 production. To better understand these interaction-induced changes in shale properties, two shale samples selected from a marine Longmaxi formation and terrestrial Chang-7 member of the Yanchang formation were first reacted with supercritical CO2 (scCO(2)) in a laboratory batch reactor at 80 degrees C and 15 MPa with different time intervals, and then characterization methods were designed to access the geochemical changes induding optical microscope (OM), X-ray diffraction (XRD), element analysis (EA), low-pressure gas adsorption (LPGA), and Fourier transform infrared spectroscopy (FTIR). The results indicate that the nanopore structure system of the two shale samples was significantly changed after scCO(2) shale interaction due to the scCO(2)-induced extraction of hydrocarbons, chemical reactions in minerals, and the swelling effect in clay minerals as well as organic matter. However, after exposure to scCO(2), the variation trend of pore structure parameters between the marine Longmaxi and terrestrial Chang-7 sample was quite different, which was related to the huge discrepancies in terms of mineralogy and geochemical properties between them. For marine Longmaxi sample, the pore surface area and pore volume obviously decreased after a relatively short period of scCO(2) treatment, whereas an opposite trend was observed in a terrestrial Chang-7 sample after long-term scCO(2) treatment. In addition, an obvious decrease in fractal dimensions for marine Longmaxi sample was also observed after scCO(2) exposure, reflecting the degree of pore surface roughness, and pore structure complexity were reduced, whereas the terrestrial Chang-7 sample exhibited an opposite trend. The results contribute to the understanding of the potential factors for the pore-structure evolution during long-terCO(2) storage and the possible effect on the CS-EGR process.