Characterization of Pore Structure and Its Relationship with Methane Adsorption on Medium-High Volatile Bituminous Coal: An Experimental Study Using Nuclear Magnetic Resonance

Abstract

A number of studies have used the nuclear magnetic resonance (NMR) technique to analyse pore characteristics and to discuss the influencing mechanisms of pore structure on methane adsorption. However, there are few studies on the dynamic characteristics of methane adsorption over time under the same temperature and pressure conditions, especially by using the cylindrical coal samples. In this study, scanning electron microscopy (SEM), mercury injection porosimetry (MIP), isothermal adsorption and NMR techniques were carried out on the four medium-high volatile bituminous coal samples from Shanxi Province, China. The simulation of methane adsorption was carried out with the custom adsorption instruments. Based on the experimental results and the Hodot pore size classification standard, the pore size distribution of the samples was analysed. In addition, the influence of nanopore structure and water content on methane adsorption was discussed. The results show that the T2 relaxation diagram of the four coal samples has a bimodal-triple peak, which reflects the complexity of the pore structure. Due to the clay minerals filling microfractures in the sample HX, the connectivity of the nanopores is reduced, in addition there is an obvious gap between the peaks in the relaxation diagram. After calculation of the T2 relaxation diagrams of the coals, the results can be converted into the pore size distribution map. The pores in the four samples are mainly composed of the macropores, followed by the mesopores, and the ratio of micropores and transition pores is relatively small. At Sw (saturated in 5% brine for 24 h) and Sir (dried at 333 K for 3 h) conditions, the adsorption capacity of the four samples presented a positive correlation with the effective porosity and the ratio of micropores, and presented a negative correlation with the ratio of mesopores, while the macropores contribute less to the adsorption. Compared with samples at Sw conditions, the adsorption capacity of the samples at Sir conditions shows an overall increasing trend, which is approximately 1.6 times that of the samples at Sw conditions on average. When a large amount of liquid water invades the nanopores and fractures, the water occupies the adsorption space of the methane due to the wettability effects and capillary pressure, which reduces the adsorption capacity.