Pore-scale distribution of primary water in deep coal reservoirs: Implications for methane recovery and CO2 sequestration

The distribution of formation water in the pore system of deep-coal reservoirs significantly influences natural gas adsorption, transport, and its production in coalbed methane (CBM). However, previous studies primarily relied on simulation or experiments of water core flooding, neglecting the initial state of the samples, specifically formation (primary) water distribution. Therefore, to fill this knowledge gap, this study characterizes the initial distribution of formation water in coal samples through a comprehensive experimental analysis of both dried and undried (natural as received) coal samples retrieved from two different wells. To do so, small angle neutron scattering (SANS), N₂ and CO₂ physisorption, and contrast-matching SANS (CM-SANS) were employed to assess pore structure and accessibility to toluene under ambient conditions. Results indicate that formation water that is occupying pore spaces, reduces toluene accessibility in natural samples compared to dried ones. Furthermore, formation water is predominantly occupying pores of 1–10 nm in size, with water saturation inversely correlated with the pore size. Water saturation is also influenced by clay minerals and the organic matter content of the samples, particularly the vitrinite maceral group in comparison to inertinite. Collectively, these findings provide critical insights for better optimization of coalbed methane recovery and assessing coal reservoirs for CO₂ sequestration through a better understanding of water-hydrocarbon-pore interactions.