Multifractal Evolution Characteristics of Multiscale Fracture-Pore Structures in Coal under CO2 Phase-Transition Fracturing

Abstract To deeply analyze the multifractal evolutionary characteristics of multiscale fracture-pore structures in coal subjected to CO2 phase transition fracturing (CO2–PTF), CO2–PTF experiments on coal samples were carried out in this study. Multiple multiscale fracture-pore characterization techniques, including computed tomography (CT) scanning, high-pressure mercury intrusion (HPMI), low-temperature N2 adsorption (LTNA), and low-temperature CO2 adsorption (LTCA), were adopted to systematically investigate the variations in fracture-pore structural parameters of coal after CO2–PTF treatment. Moreover, classical multifractal theory was introduced to explore the multifractal responses of the fracture–macropore–mesopore–micropore system in CO2–PTF coal, and to uncover the evolution of local heterogeneity within the multiscale fracture-pore structural system. The results indicate that CO2–PTF induces fracture generation and expansion-transformation effects in coal, causing the volume, surface area, and maximum length of the fractures in coal to increase significantly by 2626.55%, 833.12%, and 115.93%, respectively. This leads to a pronounced increase in the heterogeneity of fracture distribution across low-probability, high-probability, and overall zones. For macropore–mesopore structures, CO2–PTF has a pore enlargement-conversion effect, which enhances the heterogeneity of macropore distribution in low-probability, high-probability, and overall zones, yet reduces the heterogeneity of mesopore distribution over these three regions. Notably, CO2–PTF imposes no obvious transformation effect on micropore structures, which arises from the chemical properties of micropores. Such chemical characteristics of micropores limit the effective transformation and stimulation of coal reservoirs via CO2–PTF. Accordingly, future studies should focus on green chemical stimulation technologies for coal reservoirs.