Effects of supercritical carbon dioxide on macroscopic seepage and microstructure of anthracite under thermo-mechanical coupling

This study conducted high-temperature and high-pressure CO 2 in-situ extraction and permeability testing of coal bodies. In addition, an in-depth investigation was performed on the cracking characteristics and microstructure of coal samples subjected to in-situ CO 2 treatment. The results indicate: Firstly, under different temperatures of supercritical CO 2 (ScCO 2 ), the permeability of the coal exhibits distinctly different correlations with the duration of ScCO 2 action. As the action time increases, the permeability of the coal gradually decreases at low temperatures (<100 °C), while it gradually increases at high temperatures (>160 °C). Secondly, under the same action duration, the permeability of the coal increases with the rising temperature of ScCO 2 . The temperature of 160 °C is considered a reasonable temperature for ScCO 2 displacement extraction of coalbed methane (CBM). At high temperatures, thermal cracking of the coal and the extraction of organic matter by ScCO 2 , which has lower viscosity and higher wettability, significantly influence the permeability of the coal. Then, as the temperature of ScCO 2 increases, the number of internal fractures, pore volume, and specific surface area of the coal also gradually increase. In the fracture structure, micro-fractures (100 μm < fracture length <500 μm) are predominant, while in the pore structure, macropores (pore diameter >1000 nm) are significantly more developed compared to medium and small pores. Finally, when the temperature of ScCO 2 exceeds 160 °C, thermal cracking of the coal forms extensive micro-fractures. The pore structure transitions from a semi-closed to an open and interconnected structure, which greatly facilitates the extraction of CBM. Through a comprehensive analysis of the macroscopic seepage characteristics and microstructure of the coal under in-situ ScCO 2 treatment, it was found that across the entire temperature range, the variation patterns of the number of micro-fractures and macropores inside the coal body are consistent with the changes in permeability. The permeability of the coal is primarily influenced by micro-fracture structure and the structure of larger pores. Based on these findings, industrial implications are proposed, including high-pressure hydraulic fracturing to modify coal reservoirs, injecting ScCO 2 at temperatures above 160 °C to displace CBM, and high-pressure CO 2 injection for geological sequestration. This study is of significant importance for optimizing CBM extraction and CO 2 geological sequestration.