Evolution of Geomechanical and Pore Structure in Thermally Altered Coal and Shale: Significance for Subsurface CO2 Storage

With increasing global consumption of energy and the urgent need to reduce climate change effects, coal bed methane and shale gas are gaining attention as unconventional energy sources and possible underground CO₂ storage reservoirs. For such sources, the organic matter maturation process is governed by regional geothermal evolution. However, in some cases, thermal metamorphism due to igneous intrusions further alters their chemical and mechanical behaviour; which in turn affects their hydrocarbon generation and storage capacity. Despite the significance, studies on mechanical and petrophysical properties of such thermally altered formations remain understudied, particularly in the Indian context.

This study reduces this gap by comparing chemical, petrophysical, and nano-mechanical properties of both thermally affected and unaffected coals and shales from the Raniganj Basin. Nanoindentation along with Rock-Eval, petrography, XRD, and gas adsorption analyses bring together a constructive overview regarding the impact of intrusive heating on chemical composition, mineralogy, adsorption behaviour and mechanical properties.

These findings, in comparison to unaltered counterparts, reveal significant thermal modification in intrusion-affected samples, as indicated by reduced hydrocarbon index, elevated thermal maturity, quartz enrichment, improved adsorption capacity, and significantly higher Young’s modulus. Enhanced pore volume and mechanical strength of heat-affected samples are attributed to increased aromatic carbon caused by aliphatic chain collapse and hydrocarbon expulsion, forming devolatilization vacuoles and micropores.  

This observation provides important insights into the profound effects of igneous intrusions on coal and shale, highlighting their effects on the CO₂ storage potential and hydrocarbon generation potential of thermally altered basins.