Reconstruction and degradation mechanism of three-dimensional CT cracks in coal under the cycle impact of liquid CO₂ high-temperature steam

To solve the problem of long melting time of pore ice in coal seams frozen by liquid CO₂, a technology of cold and hot cycling impact on coal seam for penetration enhancement was proposed using liquid CO₂ high-temperature steam. This paper used the CT scanning testing technology to analyze the three-dimensional pore structure parameters and degradation mechanism of anthracite under cold and hot cycling impact. The research results indicated that: ① the three-dimensional pores of the coal under the cold and hot cycle impact of liquid CO₂ high-temperature steam continued to extend, gradually forming through cracks. The number, surface area, volume, and maximum slice porosity of the three-dimensional pores were all related to the index of cold and hot cycle impact times; ② According to the Hodott pore classification method and the equivalent diameter calculation formula, it was found that in the early stage of cold and hot cycling impact, the proportion of seepage pore volume increased, and the three-dimensional pore degradation manifested as crack penetration. In the later stage of cold and hot cycling impact, the proportion of adsorption pore volume increased, and the three-dimensional pore degradation manifested as the generation of large number of new pores inside the coal. Further analysis shows that: ① the Fractal dimension of seepage pore radius of coal body impacted by liquid CO₂ high temperature steam cold and hot cycle increased, the fractal characteristics of pores increased, the surface roughness increased, and the thermal storage performance decreased; ② The lg r in the pore radius classification model increased with the increase of the number of cold and hot cyclic impacts, indicating a significant expansion of pore size degradation; ③ Based on the characteristics of pore mechanical damage in coal under cold and hot cycling impact, a three-dimensional pore damage model of coal under liquid CO₂ high-temperature steam cold and hot cycling impact was summarized; ④ The projection method was used to obtain the slice damage rate and define the three-dimensional pore damage amount of coal under the cold and hot cycle impact of liquid CO₂ high-temperature water vapor. When subjected to 12 cycles of cold and hot impact, the three-dimensional pore damage was 48.55.