Research on Development Characteristics of Micro-Fractures in a Soft Coal Seam Based on the Water-Injection Effect

The development characteristics of micro-fractures in the soft coal seam, which are often neglected in water-injection and exploitation of coalbed methane, are more complex compared to the hard coal seam. To investigate the influences of micro fractures on the water-injection effects in soft coal seams, our research took the dust control through water injection in a soft coal seam with soft roof and floor as the object. By using an optical microscope, a scanning electron microscope (SEM), and an X-ray diffractometer (XRD), the development characteristics of micro fractures in the soft coal seam were obtained. The relationship between the development characteristics of micro fractures and the wetting effect of the coal was analyzed by studying the change of water content in coal samples after water injection in the coal seam. The research results showed that micro fractures were well developed, the fractures had poor directivity and connectivity, and some fractures were filled with minerals. In addition, the fractures were mainly 0~10 im wide, and the linear density was more than 6 fractures per millimeter. Fracture width and linear density positively correlated with the water content of coal after water injection, while clay minerals in coal mass exhibited a negative correlation with the water content.