Effect of liquid CO2 phase change on pores and fractures in coal: An experimental study

The evolution characteristics of pores and fractures in coal after liquid carbon dioxide (CO₂) phase change are important factors that determine the permeability increase effect. Therefore, it is critical to correctly understand the influences of liquid CO₂ phase change on pores and fractures in coal. The changes of adsorption and desorption isotherm, pore size, pore volume, and specific surface area of fractured coal and fractured coal were compared by low temperature liquid nitrogen adsorption experiment. In addition, a scanning electron microscope was adopted to observe fracture characteristics of fractured and unfractured coal samples and analyze changes in the connectivity and fracture development. Experimental results show that the fractured coal samples exhibit better hysteresis loops and a larger proportion of gas desorption than the unfractured ones. Fractured coal samples contain more developed pores and fractures compared with unfractured ones, and their fragmentation degree, pore diameter, fracture width, and connectivity of pores and fractures are also better. Besides, the closer the samples from the fracturing boreholes are, the better the fracturing effect. This indicates that liquid CO₂ phase change can effectively enhance the gas transport capacity in pores and fractures in coal. The research results provide a solid basis for the better application of liquid CO₂ phase-change fracturing to the prevention of coal and gas outburst disasters and the realization of efficient gas extraction in deep coal seams.