Multi-scale pore characterization of vitrain and durain in low rank coal

Coal is composed of multiple macroscopic compositions, and its complex pore-fracture system determines the adsorption and desorption capability of coalbed methane (CBM). In this paper, the multi-scale method were used to finely characterize the full pore size distribution of vitrain and durain, the differences of micropore and effective porosity between them were emphatically analyzed. The results indicated that the pore specific surface area (SSA) and pore volume (PV) of vitrain exceed those of durain via full pore size distribution analysis, primarily attributable to the influence of the super-micropore (0.6 nm ~ 0.85 nm). For pore characteristics affecting methane diffusion and seepage, the effective porosity ratio ranges from 9.8% to 35.1%, all of which are less than 50%, reflecting that much pores in coal reservoirs are closed pores. The effective porosity, full-scale average pore size and pore connectivity of durain are all superior to those of the corresponding coal samples of vitrain. These characteristics indicate that the pores of durain are more conducive to fluid migration. This provides a profound understanding for the efficient development of CBM in low-rank coal.