3D characterization and quantitative evaluation of pore-fracture networks of two Chinese coals using FIB-SEM tomography

To more finely describe the physical basis of coalbed methane (CBM) accumulation and flow mechanisms, two different rank coals (Ro,m of 0.4% and 0.85%) from the southern Junggar Basin of northwest China were systematically investigated to characterize pore-fracture spaces and their nanoscale connectivity in three dimensions (3D) using focused ion beam scanning electron microscopy (FIB-SEM) tomography. We reconstructed the 3D pore morphology of two different rank coals, quantify the pore size/volumes distribution and developed a novel pore network extraction (PNE) model. Samples of subbituminous coal (SC) and high-volatile bituminous coal (HBC) presented significant pore spaces for CBM adsorption, with total voxel numbers of 28,944 and 83,866, respectively; both samples predominantly consisted of closed-pore diameters of 10-50 nm. The numbers, volumes, and areas of pores (< 100 nm) were less in SC than in HBC, revealing that the SC adsorption capability was weaker than HBC. The porosities acquired by FIB-SEM were 26.06 and 42.85% for the samples SC and HBC, respectively, which could indicate the storage capability of the SC reservoir was lower than that of the HBC reservoir. The SC and HBC samples revealed a significant connectivity for CBM seepage, with a connected pores proportion of ~ 100% in pores > 300 nm. The SC sample allowed pore space connections throughout, whereas the HBC sample partially displayed pore space connections in a 3D pore network. It was found that a throat size of 400-500 nm could possibly dominate coal permeability in the SC sample; the numbers of throats (fractures) decreased with rising throat size. These results show that the gas accumulation capacity of the SC reservoir are lower than that of the HBC, whereas the fluid flow capability of the SC reservoir is higher than that of the HBC reservoir. These findings may be scientifically significant for lucubrating nanoscale pores to affect CBM storage and seepage. This paper comments on the reduction of the gas sorption capability and mass transport rate in CBM development, and we provide recommendations for relevant future work.