Beyond liquid density assumptions: A novel SANS-based approach to quantify adsorbed methane and adsorption-induced coal microstructure alterations

This study investigates methane storage and its adsorption-induced structural changes in coal under various pressures and temperatures through small-angle neutron scattering (SANS). We introduce a direct SANS-based method to quantify micropore accessibility and demonstrate that rising external pressure not only boosts methane adsorption but also triggers detectable microstructural compaction in the organic matrix. A novel scattering model incorporating mechanical constraints is proposed to capture the interplay of micropore deformation and adsorbed methane density. Results reveal pronounced pressure amplification in nano-confined pores, up to 27-fold, yet the true adsorbed-phase density never exceeds 0.305 g/mL, substantially lower than the liquid-methane value routinely assumed. By incorporating the SANS-derived density, we accurately convert Gibbs surface excess to absolute adsorption, revealing significant underestimations (≤10 % at field pressures) that arise from using liquid density approximations. Moreover, sorption-induced contractions of aromatic lamellae and the evolving micropore geometry underscore the dynamic role of microstructure in regulating gas adsorption and transport. These findings demonstrate the need to refine density corrections in sorptive gas adsorption in coal for improving assessments of coalbed methane recovery, carbon sequestration, and outburst risk. Extending the proposed methods to diverse carbonaceous media will establish a more comprehensive, pressure-dependent framework for accurate gas-storage predictions.