Structure evolution law of coalbed methane reservoirs under different initial pressure CO2 phase change fracturing conditions

As a new coal bed fracturing technology, CO2 phase transition fracturing (CPTF) has received much attention due to its advantages of safety and high efficiency. However, little research has been conducted to investigate the fracture behavior of CPTF in coal seams under different CO2 release pressure conditions. To address this research gap, we performed fracture tests on coal rock under different initial pressure conditions using the developed CPTF experimental system. Then, scanning electron microscopy, low-pressure nitrogen adsorption, and mercury intrusion porosimetry testing techniques were used to investigate the fracture characteristics and microstructural evolution of coal rocks under different initial pressures of CPTF. The results show that with increasing initial pressure, the number of macroscopic fractures and the degree of fragmentation of the coal after CPTF fracturing increased significantly, the number of microscopic fractures and pores in the fractured coal samples increased, the N-2 adsorption capacity and the amount of mercury intrusion of the coal samples increased to a greater extent, and the visible porosity increased from 52.47% of the raw coal to 63.88%, 64.31%, 68.48%, 63.64%, and 62.83%, and the proportion of macroporosity increased from 24.31% to 28.48%, 31.73%, 26.55%, and 34.38%. This research will contribute to a fuller understanding of the potential of CPTF as a technique for improving the pore and fracture structure of coalbed methane reservoirs.