Geochemistry effects of supercritical CO2 and H2O on the mesopore and macropore structures of high-rank coal from the Qinshui Basin, China

The effects of supercritical CO2 (scCO2) on coal pores play a critical role in the geological storage of CO2 and enhanced coalbed methane recovery (CO2-ECBM). To investigate the effects of scCO2 - H2O on the mesopore - macropore structure of high-rank coals, a series of experimental simulations and analyses were performed. Coal samples from the Qinshui Basin, China were exposed to scCO2 and deionized water for 240 h at 45 °C and 10 MPa and at 80 °C and 20 MPa. The influences on the pore structure were analyzed by performing a mercury intrusion capillary pressure (MICP) measurement. Using a field emission scanning electron microscope (FE-SEM) and an advanced mineral identification control system (AMICS), the geochemical effects of scCO2-H2O on the same minerals and the same pore or fracture (nanometer to micrometer scale) before and after the reaction were investigated. Based on the changes of the minerals, pores and fractures, the types, targets, expressions, and scopes of the geochemical effects were explored. The experimental results show that after the injection of scCO2 into the coal samples, the pores and fractures were newly generated or expanded under the interactions of mineral dissolution-precipitation, clay hydration and swelling, coal swelling, and pyrite oxidation with gypsum precipitation. Moreover, the largest improvement of the pore volume was observed in pores larger than 30 μm or smaller than 100 nm, and higher temperatures and pressures were beneficial to the formation of large pores and fractures. The change in the specific surface area (SSA) was mainly caused by the change in pore volume. The corrosion of carbonate minerals showed the most significant contribution to the pore volume of full-scale macropores and mesopores, while most geochemical effects on other minerals had impacts on the mesopores and the macropores smaller than 1 μm. The coal swelling had the most significant effect on the connectivity of the pores in the coal reservoir. •Achieved quantitative characterization on the same mineral and pore-fracture (nanometer to micrometer scale) before and after the reaction.•Summarized the type, mode and scope of geochemical effects on pore-fracture system.•The corrosion of carbonate minerals showed the most significant contribution to the pore volume.•The coal expansion had the most significant effect on the connectivity of the pores in coal reservoir.