Fracture Mechanism of Coal Seams Induced by a Multibranched Borehole Drilling (MBBD) Technique to Enhance Coalbed Methane (CBM) Extraction

The multibranched borehole drilling (MBBD) method employs a self-propelled nozzle (SPN) to artificially create multiple branching boreholes (BBs), which induces fracturing of the coal seams and increases their permeability in order to strengthen the extraction of coal seam gas in underground mines. This novel technology for enhancing coal seam permeability has gradually been applied in China, and there have been promising results. To improve our understanding of the drilling-induced fracture distribution patterns of coal seams in underground mines that are created by using MBBD, we employed a numerical model based on the discrete element method (DEM) that uses the Particle Flow Code (PFC) software. We concluded that, while the stress difference has a minimal effect on the fracture area and fracture amount, it significantly impacts the fracture density and subsequent fracture generation efficiency. It was also found that different drilling sequences and branch borehole lengths (BBLs) result in distinct fracture distribution patterns. Additionally, commencing BB drilling along the minimum principal stress direction yields a higher concentration of fractures, whereas initiating BB drilling to the maximum principal stress direction results in a relatively lower cumulative fracture count with a more uniform distribution pattern of the fractures. This study provides numerical insight for the in situ implementation of MBBD.