Investigating Hierarchical Gas Confinement in High-Rank Coal through Small-Angle Neutron Scattering

Hierarchical gas confinement in porous coal material is critical for gas adsorption and storage in underground coal seams. Small-angle neutron scattering (SANS) with contrast-matching method has been employed to quantify the possible gas confinement in macro-/meso-/micropores on high-rank anthracite with the forms of powder and thin sections. Model-free and data-fitting methods were used to estimate the hierarchical scattering contrast, the absolute difference between the scattering length densities (SLDs) of the solid matrix and fluid in pores, under different pressure and gas injection conditions. The scattering contrast generally reduces in macro-/mesopores but increases in micropores as the pressure increases during CD₄ injection, suggesting a high degree of densification of methane in micropores. However, the cases for CO₂ injection are complex, which could be because of multiple factors, including matrix compression and sorption-induced swelling, pore accessibility, the structure of the confined fluid, and the local structure of the matrix. This study uniquely differentiates the gas confinement in macro-/meso-/micropores of high-rank coal based on the model-free method considering pore accessibility effect and data-fitting method with and without considering effects of polydispersity and background. Care should be made to estimate scattering background, which influences estimating the scattering contrast in micropores based on the data-fitting method.