Study on the Strength Parameters and Permeability Evolution Law during Loading Process of Coal Measure Sandstone after Thermal Treatments: Implications for Syngas Leakage during Underground Coal Gasification

Underground coal gasification (UCG) is a promising technology that converts underground coal into syngas for clean energy production. The mechanical behavior and permeability of the gasification channel-surrounding rock are critical to the safety of UCG. This study conducts triaxial compression tests on coal-measured sandstone subjected to 25–1000 °C. The microstructure, strength parameters, and permeability evolution of thermally damaged rocks are analyzed. The results show that (1) when the temperature exceeds 400 °C, the fractal dimension of microcracks significantly increases, the strength decreases, and the stress level corresponding to rock volume expansion under load decreases as the temperature rises; (2) within the temperature range of 25–400 °C, the thermal expansion of rock particles makes the permeation channels close. However, the expansion of the crack volume will trigger a sudden increase in permeability under load. After exceeding 800 °C, the rock volume gradually compresses and then expands, new cracks emerge, and the permeability decreases before gradually increasing. (3) The permeability model based on piecewise functions can be used to analyze the evolution process during loading, which is in good agreement with the evolution process of the loading crack, which reveals the mechanism of fluid flow triggered by the loading-induced fracture of thermally damaged rocks. The results provide a reference for the analysis of the mechanical properties and seepage characteristics of the gasification channels surrounding rock in UCG to increase the safety and stability.