Numerical Investigation of Competitive Methane Transport in Multiborehole Coal Seam Drainage and Its Implications for Borehole Layout Design

Abstract

Methane drainage efficiency in coal seams is significantly affected by borehole arrangement, as overlapping pressure sinks in multiborehole systems can redistribute gas migration pathways and reduce the effective performance of individual boreholes. To quantify this interference effect, this study proposes a borehole efficiency coefficient (BEC), defined as the ratio of cumulative methane production from a reference borehole under multiborehole drainage to that under single-borehole drainage under identical geological conditions. A multiphysics numerical model is developed to simulate methane desorption, diffusion, and seepage in a dual-porosity coal seam. The model is validated against field data and further compared with a previously published numerical model. Using the validated model, the effects of borehole spacing, initial gas pressure, initial permeability, and layout pattern on interborehole interference are systematically investigated. The results show that competitive methane transport can reduce methane extraction from a reference borehole by up to 22.39% when the borehole spacing is 5 m. BEC increases with borehole spacing, indicating that larger spacing weakens interborehole interference, although the marginal improvement becomes less significant beyond 15 m. In contrast, BEC decreases with increasing initial gas pressure and permeability, suggesting that interference becomes more pronounced under conditions that favor methane flow. Among the three layout patterns considered, the trapezoidal arrangement yields the highest BEC and the weakest interference, demonstrating better drainage performance than the rectangular and rhombic layouts. Analysis of methane migration pathways further reveals that pressure-sink overlap diverts part of the gas flow away from the reference borehole, which explains the observed reduction in single-borehole efficiency. These findings indicate that BEC can serve as a practical indicator for interference-aware borehole layout design in coal seam methane drainage systems.