Gas sorption and flow capabilities of lignite, subbituminous and high-volatile bituminous coals in the Southern Junggar Basin, NW China

Abstract

Coalbed methane (CBM) resources are abundant in the Southern Junggar Basin in NW China, encompassing a total volume of 3.54 × 1012 m3. However, the gas sorption and flow capabilities of lignite, subbituminous and high-volatile bituminous coals (LSBC, Ro, m <1.0%) in targeted research remain ambiguous. The pore-fractures characteristics of coals (pore size/volume distribution, pore surface areas, fracture geometry, low-field nuclear magnetic resonance (NMR) porosity, permeability, and fractal dimension) are investigated to elaborate the gas adsorption and transport capabilities based on laboratory measurements, including optical microscopy, scanning electron microscopy (SEM), low-temperature (77 K) N2 adsorption/desorption (LTNA), mercury intrusion porosimetry, low-field NMR and a CH4 isotherm adsorption analysis. The SEM images show that the coal pores are primary intergranular pores, residual plant tissue pores and gas outburst pores. Pores that have varying pore widths (3.7–22.5 nm), pore surface areas (0.05–17.23 m2/g) and pore volumes (0.19–18.07 × 10−3 mL·g−1) are divided into five types of seepage pores (pore size >102 nm) and four types of adsorption pores (pore size <102 nm) based on mercury intrusion porosimetry and LTNA. The coal pores consist of transition pores and macropores with diameter of 10–102 nm and > 103 nm, followed by pores with diameter of 102–103 nm and <10 nm. The fractal investigation shows that coals with Langmuir volumes of 15–23 m3/t on a dry ash-free basis are considerably affected by the fractal dimension D1, with a P/Po of 0–0.5. Gases in the coal are concentrated on adsorbing at the pore surface in pores with 2–10 nm and 10–50 nm diameters. The coal permeability is closely related to the coal’s maceral composition (e.g., the ratio of vitrinite to inertinite, V/I) and existing micro-fractures. V/I has a positive influence on the macropores contents and a negative influence on mesopores contents. The NMR results show that the areas of T2 (>2.5 ms) distribution (0.71–14.50 × 103 ms) and movable fluid porosity (1.47–20.34%) significantly affect the coal permeability (0.001–15.584 mD), exhibiting power function relationships. The T2 (0.5–2.5 ms) distribution with slight addition after centrifuging probably owed to the retention water onto internal movable pore surfaces. Therefore, those conclusions can have significant implications for LSBC coalbed methane exploration and development.