Insights into fractures and minerals in subbituminous and bituminous coals by FESEM-EDS and X-ray μ-CT

Fractures are the main pathways for fluid transport, which constrain the coalbed methane (CBM) reservoir permeability. Permeability is one of the key petrophysical parameters for CBM production. The fractures in the low and middle rank coals usually filled with minerals, which can significantly reduce the reservoir permeability. In this study, X-ray μ-CT combined with field emission scanning electron microscopy (FESEM) together with energy dispersive spectrometry (EDS) were used to quantitatively evaluate the features of fractures and minerals of FK sample (Ro,max 0.65%) and ML sample (Ro,max 1.49%) including morphology, complexity, volume and 3D structure, and its impacts on permeability. The results showed that fractures are well developed as observed in the samples FK and ML by the 2D cross section of X-ray μ-CT. Dolomite and kaolinite are common in the samples FK and ML as confirmed by FESEM images with EDS. Moreover, the 3D fracture structure including the open fracture and mineral filled fracture were reconstructed. The quantitative structural parameters (e.g. length, width, area and volume) of the fractures for these two samples were acquired by the commercial software Avizo 9.0.1. The middle rank coal of sample ML has larger fracture density (181 fractures per mm3) than that of the low rank coal of sample FK (104 fractures per mm3). Based on the reconstructed 2D and 3D fracture structure, the fracture complexity was evaluated through fractal dimension. And the impacts of mineralization in coals on petrophysical properties were evaluated, which show that the connectivity of fractures, porosity and permeability were greatly improved after demineralization. Therefore, this study may have implications for enhanced CBM recovery through demineralization.