Experimental study on permeability enhancement of coal seam with high mineral content by acid fracturing

The permeability enhancement is the key to increase the efficiency of gas drainage from low-permeability coal seam. In this paper, the acid fracturing technology is proposed to solve the problem of low permeability of coal seam with high mineral content. The self-developed coal-rock acid fracturing experimental system is used to study the whole process of coal fracturing under different stress conditions and the permeability of coal specimens with varying permeability enhancing methods. The surface characteristics and mineral content of coal specimens before and after acid fracturing were tested by SEM and XRF technologies. The test results show that the effect of coal permeability enhancement by acidizing fracturing is mainly dominated by initial permeability, mineral content, and stress environment. The higher initial permeability, soluble mineral content, and in situ stress difference will lead to greater enhancement of coal permeability after acid fracturing. The fracturing induced fractures extend along the direction of maximum principal stress. Acidification causes the dissolution of Ca and Mg minerals in coal specimens, which provides the priority of fractures for further extension. Therefore, acid fracturing in coal seam is the result of the joint action of hydraulic fracturing (dominant) and acidizing (auxiliary). In this study, acid fracturing increases the permeability of coal specimens by 26.5-28.3 times.