Numerical Simulation and Theoretical Study of the Effect of Fracturing Strata on Multi-Stage Fracturing in the Roof of an Outburst Coal Seam

Multi-stage fracturing is one of the key techniques that use boreholes instead of tunnels and can effectively transform a coalbed methane (CBM) reservoir and increase its permeability. However, due to the lack of systematic and in-depth research on the use of multi-stage fracturing for gas drainage, the development of multi-stage fracturing is slow, and there are many problems, which to some extent restrict the application of multi-stage fracturing in low permeable outburst coal seams. In this study, the No. 21 coal seam in the Zhongmacun Coal Mine was taken as the research object, and the lithologies of the roof and floor of the No. 21 coal seam and the in-situ stress were tested. Then, three types of fracturing strata were numerically simulated to study the effect of fracturing strata on the fracture initiation, propagation and morphological characteristics induced by multi-stage fracturing in the roof of an outburst coal seam. The influence of the brittleness index of fractured strata on the formation of a fracture network was analyzed theoretically. The results indicate that the sequence in decreasing order of the fracturing range and failure mode is the medium sandstone model, the sandy mudstone model, and the mudstone model. It is useful to form a fracture network when the fractured strata in the roof of the outburst coal seam is a medium sandstone or siltstone. By comparing the brittleness index of the roof and floor lithologies of the No.21 coal seam in the Zhongmacun Coal Mine, the proper strata for the multi-stage fracturing in the roof of the outburst coal seam was selected. The results of this study provide a theoretical basis and data support for selecting proper strata for multi-stage fracturing in the roof of an outburst coal seam.