Micro–Macro Performance Response Characteristics of Reservoirs in Water‐Bearing Coal Seams After Supercritical CO 2 Injection

To explore the evolution pattern of physical properties after injecting supercritical CO 2 (ScCO 2 ) into water‐containing coal reservoirs, under the temperature and pressure conditions of 35°C and 9 MPa (in the state of ScCO 2 ), a micro‐reactive experiment of coal–water–CO 2 was carried out using a high‐pressure reactor, and a macroscopic permeability test was conducted using a true triaxial coal permeability instrument to achieve cross‐scale correlation analysis of micro‐mechanism and macro‐response. The micro‐experiment results showed that after treatment with the ScCO 2 deionized water mixture, carbonate minerals in the coal underwent significant dissolution (the CaO content in ash is reduced by 27%–51%), the pore volume of the coal increased by 5.76%, and the accessibility of micropores was significantly improved. The quantitative multiview scanning electron microscopy analysis revealed that the density of fracture surfaces and the average opening aperture increased by approximately 63% and 62%, respectively, confirming the substantial transformation of the microscopic pores and fractures by the ScCO 2 –water–coal reaction. The permeability test results showed that the stable value of water‐phase effective permeability after the first water injection was 0.09 mD. After the injection of ScCO 2 , the stable value of the CO 2 gas‐phase effective permeability (0.20 mD) increased by approximately 122% compared with the stable value after water injection. The water‐phase effective permeability of the second injection reached a peak of 1.52 mD, increasing by approximately 660% relative to the first injection's peak, and then stabilized at 0.33 mD, an increase of approximately 266% over the first injection's stable value. Comprehensive studies have shown that under ScCO 2 reservoir conditions, alternating injection of ScCO 2 and water can significantly enhance the permeability of the coal body, and the mechanism is the result of the expansion of pore/fracture channels by carbonate mineral dissolution, the drying contraction of the coal matrix, and the synergistic extraction of ScCO 2 with organic matter. This result provides a laboratory‐scale mechanistic reference for CO 2 sequestration and permeability enhancement technologies in deep water‐bearing coal seams.