Characterization of coalbed methane reservoirs at multiple length scales: A cross-section from southeastern Ordos Basin, China

Coalbed methane (CBM) reservoirs are notoriously difficult to characterize for the existence of heterogeneity at several length scales. These length scales affect processes of desorption at the grain scale (<nm), diffusion at the micropore scale (nm), gas seepage at the mesopore (μm) and cleat (mm) scales, and production at scales of meters to kilometers. We report pore properties for 14 coal sublayers from three continuous coal seams (numbers 3, 5, and 11) from southeastern Ordos Basin, China. Semi-bright and semi-dull coals are the main coal types. Characterization and analysis across the scales is by X-ray computed topography (CT), mercury intrusion porosimetry (MIP), N₂ adsorption/desorption [Brunauer-Emmett-Teller (BET)], and nuclear magnetic resonance (NMR). Proximate and maceral group composition analyses show that the ash yield correlates negatively with the vitrinite content. CT scanning results reflect that strong heterogeneity is developed in the coal between scanned slices at scales of only ∼0.5 mm in thickness, which is coincident with the difference between proximate and maceral groups of each sublayer (∼10 cm scale). A total of 14 samples are classified into three types of mercury intrusion/extrusion curves, indicative of pores with a well-developed adsorption-seepage structure, with poor connection, and alternately those that are adsorption-dominated. The proportion of macro (>1000 nm) and meso (100-1000 nm) pores shows an apparent decrease with an increase in coal burial depth, which is also confirmed by transverse relaxation time (T₂) spectra from NMR analyses. The water saturating the macropores is generally removable, while the water in the mesopores is only partially removable, and the abnormal increase in centrifuged T₂ amplitudes reflects poor connectivity between pores. Three kinds of N₂ adsorption/desorption (BET) curves are recovered and interpreted to be slit-like/plate-like pores, narrow slit-like pores, and ink-bottle (narrow throat and wide body) pores (∼10 nm). Results, using the same source samples throughout, show strong heterogeneity at the microscopic scale for the pore distribution characteristics, even for a single coal seam, and emphasize the utility of using multiple methods of characterization to infer heterogeneity and the textures and connectivity of pore structures.