Understanding pore structural complexities of coal is essential in coalbed methane (CBM) enhanced recovery and optimization of CO2 sequestration strategies. Coal’s micropores play a pivotal role in gas adsorption, while its mesopores and macropores facilitate gas migration and recovery. This study investigates the relationship between thermal maturity, maceral composition, and pore structural attributes in five coal samples with progressing thermal maturity from the Raniganj and Jharia Basins, India, using low-pressure nitrogen (N2) and carbon dioxide (CO2) adsorption techniques. A key focus is to derive fractal dimensions from CO2 adsorption data, which effectively captures micropore complexity and heterogeneity, offering critical insights into the coal’s gas storage potential. The results reveal that thermal maturity significantly impacts pore development, with postmature coals exhibiting greater micropore volumes and higher fractal dimensions, indicating higher complexity of the pore surface area and gas storage capacity. The analysis of the CO2 adsorption data proved superior to the N2 ones in characterizing micropores, which contribute significantly in estimating the maximum gas adsorption potential of coal. This study highlights strong correlations between fractal dimensions, maceral composition, and thermal maturity markers obtained from programmed pyrolysis. This work highlights that CO2-derived fractal dimension analysis coupled with organic petrography and the Rock-Eval thermal maturity parameter can be an effective way to understand the surface heterogeneity of micropores in coals and its implications for gas storage.

Extracting gas from unconventional shale reservoirs with low permeability is challenging. To overcome this, hydraulic fracturing (HF) is employed. Despite enhancing shale gas production, HF has drawbacks like groundwater pollution and induced earthquakes. Such issues highlight the need for ongoing exploration of novel shale gas extraction methods such as in situ heating through combustion or pyrolysis to mitigate operational and environmental concerns. In this study, thermally immature shales of contrasting organic richness from Rajmahal Basin of India were heated to different temperatures (pyrolysis at 350, 500 and 650 °C) to assess the temperature protocols necessary for hydrocarbon liberation and investigate the evolution of pore structural facets with implications for CO₂ sequestration in underground thermally treated shale horizons. Our results from low-pressure N₂ adsorption reveal reduced adsorption capacity in the shale splits treated at 350 and 500 ºC, which can be attributed to structural reworking of the organic matter within the samples leading to formation of complex pore structures that limits the access of nitrogen at low experimental temperatures. Consequently, for both the studied samples BET SSA decreased by ∼58% and 72% at 350 °C, and ∼67% and 68% at 500 °C, whereas average pore diameter increased by ∼45% and 91% at 350 °C, and ∼100% and 94% at 500 °C compared to their untreated counterparts. CO₂ adsorption results, unlike N₂, revealed a pronounced rise in micropore properties (surface area and volume) at 500 and 650 ºC (∼30%–35% and ∼41%–63%, respectively for both samples), contradicting the N₂ adsorption outcomes. Scanning electron microscope (SEM) images complemented the findings, showing pore structures evolving from microcracks to collapsed pores with increasing thermal treatment. Analysis of the SEM images of both samples revealed a notable increase in average pore width (short axis): by ∼4 and 10 times at 350 °C, ∼5 and 12 times at 500 °C, and ∼10 and 28 times at 650 °C compared to the untreated samples. Rock-Eval analysis demonstrated the liberation of almost all pyrolyzable kerogen components in the shales heated to 650 °C. Additionally, the maximum micropore capacity, identified from CO₂ gas adsorption analysis, indicated 650 °C as the ideal temperature for in situ conversion and CO₂ sequestration. Nevertheless, project viability hinges on assessing other relevant aspects of shale gas development such as geomechanical stability and supercritical CO₂ interactions in addition to thermal treatment.