Objective Underground coal gasification (UCG) represents a key technology for the clean utilization of coal resources. However, during the UCG process, thermal damage to surrounding rocks under thermal effects directly affects the stability and engineering safety of gasification channels.

Methods To address the thermal damage to surrounding rocks during UCG, this study investigated the silty mudstones and coarse-grained sandstones in the coal seam roof of the Xishanyao Formation, Baikouquan area, Xinjiang. Through high-temperature uniaxial compression experiments, combined with characterization methods like X-ray diffraction (XRD) and scanning electron microscopy (SEM), this study delved into the dynamic evolutionary patterns of rock mechanical properties and the macro-micro damage differentiation of both rock types under thermal effects. Moreover, this study quantified the critical threshold of thermal damage and analyzed the synergistic damage-inducing mechanisms.

Results and Conclusions  In terms of rock mechanical properties, the silty mudstones experienced thermal strengthening (25 ℃ to <400 ℃), thermal damage (400‒600 ℃), and thermal sintering (>600 ℃ to ≤800 ℃) stages, with peak stresses showing an increase of 36.24%, a decrease of 45.99%, and a surge of 75.20%, respectively. In contrast, the coarse-grained sandstones experienced the dehydration weakening (25 ℃ to <200 ℃), thermal expansion-induced compaction (200 ℃ to <400 ℃), and thermal damage (400‒800 ℃) stages, with peak stresses decreasing by 50.74%, 45.16%, and 52.78%, respectively. At 400‒600 ℃, the elastic moduli of the silty mudstones and coarse-grained sandstones significantly decreased to 58.87% and 69.42%, respectively, and their damage variables surged to 0.59 and 0.69, respectively. Concerning the controlling mechanisms in the three evolutionary stages, the silty mudstones were subjected to the synergistic effects of thermal evaporation and expansion, the synergistic destruction of thermal cracking and mineral phase transitions, and the sintering reaction process, sequentially. In contrast, for the coarse-grained sandstones, the controlling mechanisms in the three stages were dominated by thermal evaporation, thermal expansion, and thermal cracking - mineral phase transition in sequence. A temperature of 400℃ was identified as the initial threshold for the synergistic destruction by thermal cracking and mineral phase transitions (e.g., α–β phase transition of quartz, and kaolinite dehydroxylation) of both rock types. Beyond this threshold, damage to the transition zone of surrounding rocks accelerated. Therefore, in engineering practice, the temperatures of key roofs with sealing capacities in oxidation zones must be controlled below 400℃ to mitigate the rock instability risk. Overall, this study determines the thermal damage threshold and rock mechanical differentiation mechanisms of surrounding rocks under UCG-induced thermal effects in the Baikouquan area, providing key temperature-sensitive parameters and stability control criteria for the safe design of gasification channels.