Size Distribution and Fractal Characteristics of Coal Pores through Nuclear Magnetic Resonance Cryoporometry

Characterization of the coal pore structure plays a critical role in the adsorption and flow of coalbed methane (CBM) during CBM exploitation. The accuracy of conventional techniques is relatively low, especially for micropores. Nuclear magnetic resonance cryoporometry (NMRC), as a new technique that is used to detect the pore structure of porous media, has been applied to many fields. However, it is rarely used for CBM reservoirs. In this study, the pore size distribution (PSD) and fractal characteristics of semianthracites and anthracites are investigated through NMRC, routine NMR and low-temperature nitrogen adsorption methods. The results show that the PSD obtained from NMRC is divided into three types, which are mainly affected by the metamorphic degree of the selected coals (coal rank). Type I PSD from NMRC shares a high consistency with that yielded by NMR. The comparison between PSD from NMRC and NMR shows that the NMR method yields a higher pore volume for adsorption pores than that of NMRC due to the presence of skeleton information and paramagnetic impurities. The fractal result of coal pores from NMRC indicates that the transition pores and mesopores are more complex than the micropores. Moreover, the results from NMRC represent a more accurate pore structure for the same coal sample compared with NMR. The relationships between pore volume, permeability, Langmuir volume and pore fractals has also been established, which proves that, as a new method, NMRC is of great significance in characterizing the petrophysical properties of CBM reservoirs.