Complex interactions between coal maceral fractions, thermal maturity, reaction kinetics, fractal dimensions and pore-size distributions: Implications for gas storage

Characterizing pore-size distributions (PSD) of coals is required to identify optimum zone for gas recovery and suitable sites to store carbon dioxide (CO₂) or hydrogen. Micropore and mesopore PSD characteristics vary with differences in thermal maturity, maceral fractions, and as this study novelly identifies, bulk-rock reaction kinetic distributions. Five coal samples from the Damodar Coal Province (India) associated with a wide range of thermal maturity and petrology are evaluated using optical microscopy, N₂ and CO₂ low pressure gas adsorption analysis, and single-heating rate and multi-heating rate Rock-Eval pyrolysis. The results reveal distinctive relationships between pore volumes, specific surface area (SSA), fractal dimensions, reaction kinetic distributions, and S2 pyrogram features that differ for the micropore and mesopore PSDs. The most thermally mature coals exhibit the highest micropore volumes, SSA and fractal dimensions but the PSD characteristics are also influenced by maceral fractions, particularly liptinite/vitrinite ratio. Difference in PSD characteristics are observed between the micropores and mesopores. The complexity of the kinetic distributions increases with thermal maturity. Clear relationships exist between the weighted average standard deviation of activation energies and micropore fractal dimensions and SSA. Similar but less well-defined relationships exist between single-heating rate transformation fraction temperature ranges and micropore fractal dimensions and SSA. The significance of these results is that it opens the possibility to use Rock-Eval analysis to estimate coal PSD characteristics. Once calibrated with low-pressure gas adsorption (LPGA) analysis, Rock-Eval pyrograms could be generated more rapidly and at lower costs on multiple samples to better delineate the best coal zones for gas recovery and gas storage. No published studies have previously identified or addressed relationships between reaction kinetics distributions and PSD.