Coal seam porosity and fracture heterogeneity of macrolithotypes in the Hancheng Block, eastern margin, Ordos Basin, China

Abstract

Heterogeneity in cleats and pore size distribution within coal macrolithotypes (bright, semi-bright, semi-dull, and dull coals, determined from the overall relative luster and percentage of bright components) impact coalbed methane (CBM) extraction. However, few studies have investigated the characteristics of macrolithotypes across multiple length scales. Here, the macrolithotype description was performed for the No. 3, 5, and 11 coal seams in the Hancheng Block of Ordos Basin. The cleats were characterized by optical microscopy and X-ray computed tomography (CT) while pores and their connectivity were determined from mercury intrusion porosimetry, N2 and CO2 adsorption, and low field nuclear magnetic resonance (NMR) of water saturated cores evaluated both cleats and pores. The CH4 adsorption capacity of coal samples scraped from the surface of the macrolithotype layers was also determined. Dull lithotypes (semi-dull and dull) were dominant, followed by bright lithotypes (bright and semi-bright) in these seams. The bright lithotype coal has a higher microfracture density (averaging 339 microcleats per 9cm2, μm–mm), better connectivity, and a larger fractal dimension (averaging 1.7) from optical microscopy. The dull lithotype has a lower average microfracture density (272 microcleats per 9cm2) and a smaller fractal dimension (~1.6). This higher connectivity observation was consistent with X-ray CT observations and low field NMR of the water saturated cores (and the transition upon water removal from the fractures and larger pores). Also the ease of mercury injection from cores also supported higher connectivity in the bright lithotypes. Both the semi-bright and semi-dull lithotypes have equivalent pore proportions micro- to macropores and good apparent connectivity at the nm–μm scale; despite mineral occlusions. Based on the CH4 adsorption capacities of the macrolithotypes, the bright coal has the greatest gas adsorption capacity, due to the largest smaller micropores (<2nm) and a higher specific surface area. These observations aid in understanding the complex behavior observed in CBM extraction and evaluating the impacts of heterogeneity and scale on the degassing behavior.