Experimental and Molecular Insights into Organic Matter Wettability: Implications for Hydrocarbons Recovery, and Gas Geo-Storage in Source Rocks

Wettability in organic-rich samples such as coal controls the fluid distribution, accessible pore spaces, and flow, impacting hydrocarbon recovery and geo-storage processes. This study combines experimental measurements and characterization with molecular dynamics (MD) simulations to investigate the complex interplay between organic matter maturity, its surface chemistry and its impact on wettability toward hydrocarbons, water and gas-phase fluids. Coal samples of varying maturity from the Sohagpur (SP), Jharia (JB), and Rajmahal (RJB) basins were characterized using Raman spectroscopy, surface energy analysis, XRD, FTIR, and zeta potential measurements. The results of contact angle measurements reveal that less matured coal (e.g., RJB), with a TOC of approximately 65%, remained water-wet (contact angle <50°). Whereas, the most matured coal sample (JB) of TOC 75% exhibited a contact angle up to 130° with CO₂, depicting strongly gas-wet at similar conditions. On the other hand, increased hydrophobicity with maturity was observed in the case of toluene. MD simulations using kerogen models (types I-A, II-A, II-D, and III-A) quantified fluid-surface interaction energies and self-interaction strengths, shows a maturity-dependent increase in hydrocarbon affinity and reduced water adhesion. Notably, CO₂ exhibited the strongest interaction with mature kerogen (interaction energy ≈−142 kcal/mol), surpassing that of water and CH₄, thus confirming its superior storage potential and displacement capability. The study further shows that H₂ has minimal interaction with both kerogen and itself, explaining its mobility but limited adsorption in nanoporous coal matrices. These integrated findings offer mechanistic insight into wettability evolution in source rocks, directly informing site-specific design of enhanced hydrocarbon recovery, CO₂ sequestration, and H₂ geo-storage strategies in organic-rich reservoirs.