Deep Coal Reservoirs’ Ultimate Methane Sorption Capacity: Constraints Imposed by Multiscale Pore Networks and Fractal Geometry

Abstract

Investigating the controlling mechanism of heterogeneous pore structure at multiple scales in deep coal reservoirs on methane adsorption capacity is a key scientific issue for the accurate evaluation of the resource capacity of deep coal gas reservoirs. Herein, ten cores (burial depth: 1888.2–2276.9 m) retrieved from the eastern flank of the Ordos Basin were characterized via LP-CO2, LT-N2, MIP, and isothermal methane sorption tests. The primary objective was to elucidate the quantitative links between pore geometry, fractal scaling, and ultimate gas storage capacity. Findings demonstrate that as burial depth rises, total pore volume tends to grow; however, micro- and mesopore volume fractions shrink while macroporosity expands, suggesting that deeper coal formations possess markedly enlarged free-gas storage capacity. The micropore fractal dimension (DV) and mesopore spatial fractal dimension (DF2) show negative correlations with the Langmuir volume (VL), whereas the macropore fractal dimension (DS) and mesopore surface fractal dimension (DF1) exhibit positive correlations with VL. These observations imply that surface roughness and spatial connectivity across variable pore-size regimes differentially modulate the methane sorption potential of coal. Specifically, an excessively high micropore fractal dimension can cause pore-throat narrowing and an increase in closed micropores, thereby reducing effective adsorption space. Conversely, an elevated macropore fractal dimension can improve pore connectivity, protect the micropore adsorption system, and enhance adsorption capacity. This research offers a scientific foundation for the precise characterization and optimized exploitation of deep CBM reservoirs.