Weakening and fracturing mechanism of mechanical properties of coal and rock caused by supercritical CO2

Coal bearing strata contain a large amount of gasbearing coal resources，and efficient exploitation of gasbearing coal resources is of great significance to the development and utilization of unconventional gas resources and energy revolution. The development of coal seams superimposed on other rock layers in coal bearing strata increases the difficulty of extracting gasbearing coal. In this paper，the longterm saturation effect of supercritical CO 2（ScCO 2) on sandstone and coal samples in coal bearing strata and their mechanical variation characteristics was carried out in the laboratory，and the distribution characteristics of cracks after hydraulic fracturing，liquid CO 2 fracturing and ScCO 2 fracturing in coal with CT scanning are studied. The fracturing characteristics and crack propagation mechanism of coal samples and sandstone during liquid CO 2 and ScCO 2 fracturing are revealed. The results show that:① The weakening of mechanical properties of coal by ScCO2 is stronger than that of roof sandstone. The uniaxial compressive strength of sandstone is decreased by 9.10%，21.97% and 48.93% on average with the ScCO 2 saturation time increase from 15，30 to 60 days，and the elastic modulus is also decreased by 12.70%，18.82% and 27.19% on average accordingly. After saturation in ScCO 2 for 20 days，the uniaxial compressive strength of weakly caking coal，1/3 coking coal，lean coal and anthracite is decreased by 65.25%，45.24%，56.02% and 67.94%，and the elastic modulus is also decreased by 65.87%，52.50%，57.14% and 65.91%，respectively.② The main factors leading to the reduction of mechanical strength of coal are the surface energy of coal decreases due to the expansion and deformation of coal after CO 2 adsorption，the extraction of organic matter by ScCO 2 and the ScCO 2 enters into the tiny pores and fissures of coal that increases the pore pressure of coal，and the dissolution of coal minerals by carbonic acid formed by CO2 combined with water.③ ScCO2 fracturing is better than hydraulic fracturing in coal bearing strata. ScCO2 can greatly reduce the mechanical strength of coal and rock mass，thus reducing the fracture pressure. CO 2 molecules can enter the fracture tips of coal and rock mass，accelerating their damage and destruction and forming complex fracture network structures with multiple bifurcations，high tortuosity and long extension distances.