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

Abstract

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 nuclear magnetic resonance (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...