Multiscale Pore-Fracture Structure Characteristics of Deep Coal Reservoirs in the Eastern Margin of the Ordos Basin, China

The pore-fracture structure of deep coal deposits is highly important for the potential evaluation, investigation, and utilization of deep coalbed methane resources. This study used methods such as low-pressure CO2 adsorption, low-temperature N2 adsorption, high-pressure mercury intrusion porosimetry, scanning electron microscopy, and optical microscopy to describe the pore-fracture structure of deep coal reservoirs at multiple scales and to discuss the development features, complexity, and influence on permeability of the pore-fracture structure of coal reservoirs. The results showed that there were significant differences in the pore volume and specific surface area (SSA) of the coal specimens with respect to the distribution of pore diameters. The micropore volume and SSA accounted for the largest proportions (85.93% and 98.63%, respectively). The more moisture and fixed carbon content there were in coal, the larger the micropore volume was. The higher the yields of ash and volatile matter were, the smaller the micropore volume was. The larger the pore radius in coal was, the greater the fractal dimension was. Besides, within their respective pore size sections, as the fractal dimension increased, the pore volume gradually decreased. As the vitrinite content increased, the fracture aperture and surface density gradually increased. As the fracture aperture increased, the fracture fractal dimension decreased, while the fracture tortuosity increased. Compared with shallow coal seams, the fracture aperture of deep coal seams showed a decreasing trend, while the pore volume showed an increasing trend.