Effect of Supercritical Carbon Dioxide Fracturing on Shale Pore Structure

Supercritical carbon dioxide (SC-CO2) fracturing technology has the potential for shale reservoir stimulation. Most studies have predominantly focused on the fracture morphology of shales after SC-CO2 fracturing, while the alterations in shale pore structure have rarely been investigated. Here, CO2 adsorption, liquid nitrogen (N2) adsorption, and mercury intrusion porosimetry (MIP) tests were used to quantitatively characterize the changes in the pore shape, volume, and area as well as fractal characteristics of shales fractured by water and SC-CO2. The results show that the changes in micro-, meso-, and macropores are controlled by the injection pressure, axial-confining pressure, and infiltration range of SC-CO2 and water. However, both hydraulic fracturing and SC-CO2 fracturing do not alter the dominance of the plate-shaped and slit-type pores in the shales. For samples away from the induced fracture, the extent of SC-CO2 infiltration is greater than that of water, which is documented by the increase in total CO2 adsorption, cumulative intrusion, incremental/cumulative pore volumes of macropores, and porosity. After hydraulic/SC-CO2 fracturing, the proportions of micropores and mesopores reduce sharply, while the proportion of macropores increases significantly, reaching above 70%. Both hydraulic/SC-CO2 fracturing operations result in more regular pore structures and smoother pore surfaces for meso- and macropores near the induced fractures. However, by comparing the average fractal dimension of the samples near the induced fractures after hydraulic/SC-CO2 fracturing, it is found that the treatment of SC-CO2 makes the mesopores structure more complex and the pore surface of mesopores and macropores rougher.