Pore structure characterization of middle- and high-ranked coal reservoirs in northern China

This study aimed to quantitatively characterize pore structure of middle- and high-ranked coal reservoirs in northern China. A total of 51 coal samples from the three coalbed methane (CBM) exploration areas including Anhe, Huaibei, and Huainan coalfields in northern China were investigated using proximate analysis, coal petrology, low-temperature nitrogen adsorption, mercury intrusion porosimetry, nuclear magnetic resonance (NMR), and x-ray computed tomography. The results show that the coalification of the Anhe and Huaibei coalfields is greatly influenced by igneous intrusion, with the maximum vitrinite reflectance (R-o,R-max) ranging from 1.41% to 2.90% and from 0.59% to 2.12%, respectively. In contrast, the Huainan coalfield has a narrow range of coal rank, with the R-o,R-max ranging from 0.59% to 0.87%. Of the coal samples studied, those from the Anhe coalfield have the greatest vitrinite content (average 83.81%) and the largest micropores, corresponding to the highest methane adsorption capacity and relatively low pore connections. In the Huaibei coalfield, coals are characterized by a better pore connectivity than those in the Anhe and Huainan coalfields, thus possessing the highest permeability and the largest potential saturation of movable fluid. Coals in the Huainan coalfield are characterized by a low incremental porosity and an isolated trimodal distribution of NMR transverse relaxation time spectra, corresponding to weak methane adsorption and seepage capacities. In general, the variation in pore size distribution in middle- and high-ranked coals is complex. Specifically, the relative abundance of micropores in low-ranked coals has a broad range of values ranging from 10.65% to 53.34%, suggesting that the change of coal structure is irregular, and the pore distribution is relatively scattered when the R-o,R-max is lower than 1.0%. The micropore content generally increases while the transition pore content decreases gradually with increasing coal rank when the R-o,R-max is higher than 1.0%. The helium porosity, porosity of movable water, air permeability, NMR permeability, and potential saturation of the movable fluid first increase and then decrease with increasing R-o,R-max, with the inflection point occurring at R-o,R-max ranging from 1.1% to 1.9%. With respect to pore and fracture development, the coal reservoirs of the Huaibei and Anhe coalfields with R-o,R-max ranging from 1.1% to 1.9% are at the best target maturity level of the three areas for CBM exploitation. This study reveals the complex variation in the pore structure evolution of middle- and high-rank coals and the significant influence of coalification on the physical properties of CBM reservoirs.