Multiscale Assessment of Transformation in Pore System of Shale during Combustion: An Insight into Poromechanical Response and Sorption Dynamics

Thermal stimulation is emerging as a promising method for enhancing shale gas recovery by changing shale matrice's poromechanical and dynamic sorption behaviors. This study delves into the transformation of physio-morpho-nanopores within Barren measure shale matrices in the Jharia coal field under elevated thermal stimulation, aiming to elucidate the impact of nearby coal fires on shale intrinsic properties and to verify the applicability of coal mine fire for thermal stimulation of shales. Employing a comprehensive multiple-multiscale analysis, we integrated low-pressure N2 and CO2 adsorption, SEM analysis, X-ray micro CT, X-ray diffraction, and thermogravimetric analysis. This method allowed for meticulous assessment of both quantitative and qualitative nanopore characteristics and tracked their evolution during combustion. We found shale samples without any heat treatment exhibited a total 3.58% representative pore volume, primarily distributed in a subplanar morphology along bedding planes, resulting in an effective connected porosity of 2.40%. Both specific surface area (SSA) and total pore volume showed a positive correlation with temperature. The SSA of shale samples exhibited a jump of 211% at 250 °C and 166% at 500 °C with respect to that of unheated samples, accompanied by a substantial expansion in the pore volume by 673%. Initially, micro- and mesopore volumes were prevalent at lower temperatures but gradually declined with an increase in temperature, while macropore volumes became more prominent at higher temperatures. Elevated temperature led to an increased fractality in shale samples, resulting in complex pore surfaces with more fractures and microcracks, particularly evident at 500 °C. SEM images revealed increasing pore surface complexity and throat connectivity with increasing temperature, indicating an enhancement in gas storage and recovery potential. Thermal stimulation improved pore connectivity, surface area, and volume, crucial for enhancing permeability in deep shale formations, offering a safer alternative to hydraulic fracturing for augmenting permeability and improving shale gas recovery and carbon sequestration.