Mineral occurrence and its impact on fracture generation in selected Qinshui Basin coals: An experimental perspective

• Mineral compositions of Qinshui coals were investigated.
• Spatial occurrence of mineral matters was acquired by X-ray CT.
• The primary butt cleats and face cleats were confirmed by the fracture rose diagram.
• Fracture generation was controlled by the mineral occurrence modes.

The effect of mineral matter on the fracture evolution was investigated for Lower Permian and Upper Pennsylvanian coal samples from the Qinshui Basin of China. Optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry (SEM–EDS), and X-ray computed tomography (X-ray CT) were used to investigate the composition and modes of occurrence of minerals within the coals and to characterize the natural and induced fractures present within the samples. The spatial distribution of mineral matter investigated by X-ray CT demonstrates that the variation of mineralization within the cleats/fractures occurred at fine scales, and most of minerals were scattered in coal core samples. The original fractures normally were present as two sets: one set was almost parallel to the bedding plane, and the other was almost perpendicular to the bedding plane. For created fractures, the difference in density between the matrix and mineral matter may weaken the cohesion of coals, and it could explain why the created fractures propagate along the junction of minerals and organic coal matrix. The internal fracture porosities varied from 0.06% to 20.69%, which indicates a strong internal heterogeneity of the coals after a stress is applied. The inclination of the main shear fractures ranges from 55° to 95°. Using energy analysis, the fracture energy appears to represent nearly half of the total dissipated energy, which can be correlated with the fracture density and orientation. The formation and distribution of shear fractures is related to the spatial distribution of mineral matter and the overall mineral content. These investigations serve to characterize hydro-fracturing and pulverized coal generation during coalbed methane development.