Temperature-Induced Pore Structure Evolution in Shale: Implications for Underground Coal Gasification Applications

Underground coal gasification (UCG) offers a viable approach for extracting deep-seated coal deposits with minimal surface disruption. The thermomechanical behavior of adjacent rock formations, particularly shale, which typically acts as a ceiling or floor rock, has a significant impact on the success of UCG operations. This study examines the pore structure evolution of shale samples at increased temperatures from room temperature to 800 °C, approximating the thermal range experienced during UCG procedures. The primary goal is to understand how high-temperature exposure changes the porosity and microstructure of shale, altering gas movement, confinement, and overall system stability.

Shale samples were collected from Jharia Basin, India, and were heated in a muffle furnace at gradually increasing temperatures. The pore properties were assessed by Low-Pressure Gas Adsorption (LPGA), Helium Pycnometry, and Scanning Electron Microscopy (SEM). SEM imaging showed considerable microcrack formation and intergranular pore growth at temperatures above 300 °C. LPGA data showed a shift from microporous to meso- and macroporous materials as temperature increased, implying gradual pore coalescence. The Helium Pycnometer results verified a temperature-dependent increase in apparent porosity, which corresponded well to the observed physical degradation. The findings show a non-linear rise in total porosity and considerable microstructural disintegration of shale at high temperatures, which can improve gas flow paths but may expose the confining layers' stability. These thermal changes are critical to UCG operations because they affect both gas recovery efficiency and subsurface safety. The work sheds light on the thermal behavior of shale under UCG-relevant conditions, emphasizing the importance of complete thermomechanical studies in site selection and operational planning for UCG projects.

Keywords: Underground Coal Gasification, LPGA, Permeability, temperature, Porosity.