Transformation effect of multi-scale pores induced by true triaxial supercritical carbon dioxide fracturing in high-rank coal

Supercritical carbon dioxide (ScCO₂) fracturing can not only promote the fracture propagation of coal reservoir, but also cause significant pore transformation. To investigate this transformation effects of fracturing parameters and stress conditions on the multi-scale pores in coal, a true triaxial ScCO₂ fracturing simulation was carried out, and the pore morphology and structure variations of the micropores (<10 nm), transition pores (10–100 nm), mesopores (100–1000 nm), macropores (1000–10000 nm), and megapores (>10000 nm) in coal after ScCO₂ fracturing were analyzed. The results show that ScCO₂ had no obvious effect on the pore shape, but it reduced the complexity and enhanced the connectivity of the pore structure. After ScCO₂ fracturing, the total volume and specific surface area of the pores increased by 114 % and 385 %, respectively, especially regarding the transformation of the megapores and micropores. The transition pores and macropores experienced a general promotive effect, while obvious propagation and merging occurred in the mesopores. With increasing stress difference, the dominant direction of the pore transformation changed from the direction of the maximum horizontal stress to the direction of the vertical stress. With increasing injection flow, the pore transformation scope increased, but the high stress difference and high injection flow were not conducive to increasing the pore volume. The effective transformation of pores induced by ScCO₂ fracturing can enable the smooth desorption and migration of coalbed methane in the low permeability reservoir, which creates the necessary conditions for its long-term and efficient extraction.