Evolution characteristics on coal fractures induced with CO₂ phase transition fracturing based on CT scanning

This study conducted the CO₂-PTF coal experiment to further reveal the fracturing transformation mechanism of CO₂ phase transition fracturing(CO₂-PTF)coal. According to the CT scanning and 3D fracture reconstruction, we analyzed the fracture structure parameters of coal before and after CO₂-PTF, and clarified the evolution characteristics of the three-dimensional fracture structure of coal induced by CO₂-PTF. The research results indicated that after CO₂-PTF, the total number of fractures in the coal sample decreased, while the total volume and surface area of fractures increased. The CO₂-PTF generated fracture expansion and transformation effects where the small-scale fractures were expanded and transformed into larger scale fractures under the CO₂-PTF pressure. The number, volume, and surface area of fractures of less than 1 000 μm in length were significantly reduced, while the volume and surface area of fractures of longer than 1 000 μm in length were significantly increased. The expansion and connection between fractures caused a decrease in their quantity. CO₂-PTF improves the connectivity of the three-dimensional fracture in coal and is conducive to gas migration and production. This study offers new insights into and evaluation method for the effect of CO₂-PTF, and could provide references for the research on fracture evolution characteristics in other unconventional natural gas reservoirs and their modifications.