Mechanical analysis and numerical simulation of a forming fracture network in the roof of an outburst coal seam by multi-staged fracturing

Multi-staged fracturing in the roof of an outburst coal seam has been applied to more and more coal mines for use in coalbed methane (CBM) pre-drainage in China. The multiple artificial fractures in the roof can traverse the coal–rock interface to communicate outburst coal seams and achieve gas drainage. However, whether a fracture network can form in the roof is closely related to the in-situ stress, mechanical properties of the coal and rock, fracturing pattern, interval distance, fluid flow, and other aspects. Additionally, the mechanism of multi-staged fracturing in CBM reservoirs is not very clear. In this paper, we theoretically studied the mechanical conditions of a forming fracture network in the roof of an outburst coal seam. The optimal interval distances of sequential fracturing and alternating fracturing were obtained based on the horizontal stress-difference coefficient. In addition, we numerically simulated the effects of the minimum horizontal stress and stress ratio on the fracture propagation and fracture network using RFPA2D-Flow software. The research results indicated that the induced stress difference increases at first and then decreases with the increasing of the interval distance. Turning phenomenon of the maximum stress in the local area near multiple fractures should meet the condition that the induced stress difference is bigger than the horizontal stress difference. The optimal interval distance of the forming fracture network depended on the position of the maximum induced stress difference, which was closely related to the fracture height, Poisson’s ratio, and horizontal stress difference, but was unrelated to the net pressure in the fracture. When the horizontal stress difference was constant, the initial pressure and fracture initiation time increased with the increasing of the minimum horizontal stress. The stress ratio of a single fracture was inversely proportional to the fracture width. In addition, the induced stress may make the two stresses become close, which is conducive to forming the fracture network. The research results can provide a theoretical basis and data support for studying the mechanism of the forming fracture network in the roof of an outburst coal seam.