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-propellednozzle (SPN) to artificially create multiple branching boreholes (BBs),which induces fracturing of the coal seams and increases their permeabilityin order to strengthen the extraction of coal seam gas in undergroundmines. This novel technology for enhancing coal seam permeabilityhas gradually been applied in China, and there have been promisingresults. To improve our understanding of the drilling-induced fracturedistribution patterns of coal seams in underground mines that arecreated by using MBBD, we employed a numerical model based on thediscrete element method (DEM) that uses the Particle Flow Code (PFC)software. We concluded that, while the stress difference has a minimaleffect on the fracture area and fracture amount, it significantlyimpacts the fracture density and subsequent fracture generation efficiency.It was also found that different drilling sequences and branch boreholelengths (BBLs) result in distinct fracture distribution patterns.Additionally,commencing BB drilling along the minimum principal stress directionyields a higher concentration of fractures, whereas initiating BBdrilling to the maximum principal stress direction results in a relativelylower cumulative fracture count with a more uniform distribution patternof the fractures. This study provides numerical insight for the insitu implementation of MBBD.

Automated summary

The multibranched borehole drilling (MBBD) method uses a self-propelled nozzle (SPN) to create multiple branching boreholes (BBs) in underground mines, which increases the permeability of coal seams and improves the extraction of coal seam gas. This technology has been applied in China with promising results.