A Comparative Analysis of Pore Attributes of Sub-Bituminous Gondwana Coal from the Damodar and Wardha Valleys: Implication for Enhanced Coalbed Methane Recovery

With the increasing drive for geological CO2 sequestration, unconventional resources like shale gas and coalbed methane have gained massive popularity. With a vast resource of bituminous Gondwana coal, India has a good potential for enhanced coalbed methane recovery through CO2 sequestration. Pore attributes and their interconnectivity play a significant role in determining the gas storage capacity and ease of injection/ production. In this study, samples from both opencast and underground measures were collected for a thorough comparison between coal samples of the Damodar and Wardha valleys belonging to the same Gondwana coal measure. Quantitative pore attributes determined using CO2 and N2 low-pressure gas adsorption suggests that the coal samples from Wardha are more adsorptive and thus have more gas storage capacity. The total mesopore volume in Wardha coals determined using the density functional theory model (low-pressure N2 adsorption branch) is higher with coals from opencast mines showing the highest pore volume. Two distinct Frenkel-Halsey-Hill fractal dimensions were observed in the mesopore range. Here, larger mesopores demonstrated higher fractal dimensions, indicating that surface roughness is higher for larger mesopores. The total micropore volume determined using CO2-density functional theory (DFT) (on low-pressure CO2 adsorption isotherm) is higher in the Wardha coals, showcasing a positive correlation with increasing depth and organic matter composition. The adsorption-derived pore attributes were validated using high-resolution microscopic imaging and subsequent image analysis. Elemental analysis of the isolated kerogen indicates an escalation of organic carbon with depth, whereas clay content (primarily kaolinite) shows an opposite trend. This study has elucidated the entire spectrum of micro- to mesopore attributes for these two significant coal basins, illustrating their distinguishing characteristics caused by different depositional settings and diagenesis, despite belonging to the same Gondwana measure.