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| Three-dimensional fracture evolution characteristics and transformation effects of coal under CO2 phase transition fracturing |
| ZHANG Zhen1, LIU Gaofeng2*, HE Yongliang1, LIU Huan2, SI Nian2, FENG Kunpeng2 |
| (1. School of Safety and Emergency Management Engineering, Taiyuan University of Science and Technology, Taiyuan, Shanxi 030024, China; 2. School of Resources and Environment, Henan Polytechnic University, Jiaozuo, Henan 454003, China) |
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Abstract To thoroughly analyze the three-dimensional (3D) fracture evolution characteristics and transformation effects of coal under CO2 phase-change fracturing (CO2-PTF), experiments were conducted on CO2-PTF coal at fracturing pressures of 120 MPa and 185 MPa. By integrating computed tomography (CT) scanning with 3D fracture reconstruction techniques, both single fractal and multifractal theories were employed to systematically investigate the geometric and fractal parameter evolution of the 3D fracture structure in coal following CO2-PTF, thereby elucidating the 3D transformation effects of CO2-PTF coal. The results demonstrate that CO2-PTF significantly enhances the fracture volume, porosity, surface area, and maximum fracture length of coal. The fracturing pressure has a considerable positive influence on fracture evolution; as the fracturing pressure increases, the effects on fracture volume, porosity, surface area, and fracture length become more pronounced. The relationship between the evolution of 3D fracture volume and surface area with fracture length further indicates that CO2-PTF coal exhibits both fracture generation and expansion transformation effects. Analysis of the single fractal and multifractal characteristics of the fracture structure reveals that the fractal dimension for fracture volume in coal increases post-CO2-PTF, suggesting greater complexity in the fracture structure. Additionally, the heterogeneity within the low probability, high probability, and overall regions of fracture structure distribution increases, demonstrating significant local concentration and enhanced singularity. Although notable differences exist within the fracture structure, the predominant influence of the low probability region in controlling its heterogeneity diminishes. Further analysis indicates that during the CO2-PTF process, the dynamic high-pressure gas jet stage primarily corresponds to the fracture generation effect, resulting in an increased fractal dimension for fracture volume and greater enhancement of heterogeneity in the low probability, high probability, and overall fracture distribution. Conversely, the subsequent quasi-static high-pressure gas stage predominantly induces the fracture expansion transformation effect, leading to a substantial reduction in the volume and surface area of small-scale fractures, causing the fracture structure to become locally concentrated in high probability regions, thereby exacerbating internal disparities and weakening the low probability region's dominance in controlling fracture structure heterogeneity. These research findings provide a theoretical basis for optimizing CO2-PTF device parameters and enhancing fracturing and permeability improvement in low-permeability coal seams.
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2026, Vol. 45 
