Study on the Sealing Capability of Overlying Mudstone of Coal Under Thermal Action During Underground Coal Gasification

This study systematically investigates the microstructure, physical properties, and sealing capacity of mudstone under thermal action using scanning electron microscopy (SEM), high-pressure mercury intrusion, breakthrough pressure testing, and diffusion coefficient analysis. Pore-throat structure, porosity, permeability, breakthrough pressure, and diffusion coefficient were measured at 25, 300, 600, 900, and 1200 °C, while additional SEM observations were conducted at 100, 200, 400, and 500 °C. Based on the integration of these discrete measurements and supplementary SEM observations, the evolution of mudstone sealing capability was interpreted as follows. (1) At 25–200 °C, mineral dehydration exposes primary pores, with porosity and permeability increasing gradually while strong sealing capacity is maintained. (2) At 200–400 °C, clay mineral dehydration, thermal expansion, and secondary fracture development increase pore connectivity, resulting in a sharp decline in sealing capability. (3) At 400–1200 °C, mineral structure collapse, pore-fracture connection, and mineral melting further increase porosity and permeability, leading to progressive sealing failure. (4) The threshold temperature for mudstone sealing failure is inferred to be approximately 400 °C. (5) Mudstone affected by temperatures below 200 °C is recommended as an effective caprock for underground coal gasification chambers. This study provides theoretical guidance for caprock selection and safe operation in underground coal gasification engineering.