Multifractal Characteristics of Pore Structure Evolution in Thermally Damaged Coal-Seam Roof Sandstone Under O2 and CO2 Atmospheres: Implications for Underground Coal Gasification

This study investigated how high-temperature reactive atmospheres associated with underground coal gasification (UCG) differentially modify the pore system of roof sandstone. Roof sandstone from the Shanghai Miao mining area, Inner Mongolia, was conditioned to simulate formation-water saturation and heat-treated at 200 °C, 400 °C, 600 °C, and 800 °C under O2 or CO2 atmospheres. Low-field nuclear magnetic resonance (LF-NMR) T2 spectra, pore-size classification, multifractal analysis, and X-ray diffraction (XRD) were combined to characterize pore-structure evolution. Thermal treatment shifted the pore composition from mesopore-dominated to micropore- and small-pore-dominated. Under O2, NMR porosity increased continuously from 8.41% to 13.94%; the small-pore fraction increased with temperature, and the dominant T2 components expanded into adjacent pore-size domains. Under CO2, porosity peaked at 11.25% at 600 °C and decreased to 9.36% at 800 °C; micropores remained dominant, while medium and long T2 components contracted markedly at high temperature. The normalized T2 probability measures exhibited typical multifractal behavior under all treatment conditions. Under O2, the singularity-spectrum width Δα ranged from 1.464 to 2.803 and H from 0.947 to 0.958, showing greater variability than under CO2 (Δα = 1.627 − 1.776; H = 0.947 − 0.950). Porosity was strongly correlated with positive-order multifractal parameters and D2 (r = 0.82 − 0.90), indicating that pore-volume expansion can be decoupled from the evolution of cross-scale heterogeneity. XRD showed that the raw sandstone was composed mainly of quartz (62.8%) and kaolinite (29.3%). Kaolinite and siderite diffraction peaks were not detected in representative samples treated at 600–800 °C, and the relative proportions of quartz and feldspar phases were reconfigured, providing mineralogical constraints on pore evolution. Two distinct thermal-damage pathways were identified: continuous pore accumulation with scale expansion under O2 and medium-temperature pore expansion, followed by high-temperature reorganization under CO2. These findings provide a quantitative basis for structural-state identification and risk zoning of UCG roof strata.