A predictive model for coal permeability evolution under thermal-mechanical-chemical coupling effects

Abstract

Coal permeability evolution under thermal-mechanical-chemical coupling plays a critical role in underground coal thermal treatment (UCTT) and underground coal gasification (UCG). Unlike existing models limited to low-temperature applications, this study presents a novel permeability evolution model that integrates key mechanisms including thermal expansion, thermal dehydration, effective stress variation, gas adsorption/desorption, pyrolysis, gasification, and fracture generation or closure. The model also incorporates a three-stage deterioration process for coal moduli and introduces two key parameters, fracture formation strength and fracture number ratio, to characterize fracture development led by thermal and stress-induced damage, or shear deformation. This model is validated using triaxial permeability test data and 3D micro-CT images of bituminous coal, demonstrating good predictive capability. Key findings include: (1) A U-shaped permeability evolution during the dehydration stage (25–100 °C), with thermal expansion reducing permeability at 25–35 °C, followed by an increase due to thermal dehydration from 35 to 100 °C. (2) A three-phase permeability evolution in the 100–600 °C range: Phase I features rapid permeability increase driven by fracture generation; Phase II shows permeability decline due to coal softening; and Phase III sees permeability increase again due to pyrolysis and gasification. (3) Elevated pore pressures intensify shear-induced fracture closure, offsetting the relatively modest effects of volumetric strain, thus reducing overall permeability. This model enhances the mechanistic understanding of permeability evolution under thermal-mechanical-chemical conditions and provides a robust predictive tool to support the optimization of UCTT and UCG technologies.