Reasonable layout of drainage boreholes based on anisotropic permeability in coal seam
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Abstract:
Coal seam degasification by in-seam drilling borehole is one of the popular techniques for coal mine methane (CMM) control. Generally, the more borehole numbers for a specific drainage area, the shorter drainage time but the higher drainage cost. In order to resolve the contradiction between borehole numbers and drainage duration, in-situ tests of effective gas drainage radius were conducted continuously in an anisotropic coal seam for 180 days at Jiulishan coal mine in China, and coal permeability was measured associated with bedding and cleat structures, and then gas-solid coupled seepage model was built to simulate the change rules of effective drainage radius in an anisotropy coal seam using finite-element based Comsol package. The results indicate that the coal permeability parallel to the bedding plane in butt cleat direction is 2.6 times as high as that perpendicular to the bedding plane. And effective drainage radius parallel to the bedding plane in butt cleat direction will get much larger than that perpendicular to the bedding plane with the increase of drainage duration and shows exponential relationship with gas pre-extraction time. The numerical results of simulated tests are consistent with those of field tests. Based on numerical results, the ellipse influence area of borehole in an anisotropic coal seam is proposed, and the relativity of effective drainage radius with drainage time is found. On the basis of these findings the proper in-seam borehole arrangements for a specific mining panel at different drainage duration are determined.
