Abstract
In order to enhance gas extraction from unloaded coal seam by drilling borehole in the floor roadway, the mechanism of stress relief improving permeability by protective coal seam mining was analyzed. Based on the multiphysics field theory, the hydraulic-mechanical coupling model of gas extraction in the unloading coal seam was established, and the gas extraction process by drilling borehole in the floor roadway in the overburden of Panyi Coal Mine 1551 (1) panel was simulated. The influence of different drilling arrangements on the gas extraction effect was analyzed. The results show that the permeability of protected coal seams is characterized by zoning and can be divided into the permeability-enhanced zone, the permeability-reduced zone, and the original permeability zone according to the stress state of coal seam. Under the condition of uniform borehole distribution, the gas pressure decreased slowly in the permeability-reduced zone and is still greater than 0.74 MPa after 180 days of extraction, and there is a large extraction blind area in the protected panel. Under the condition of nonuniform borehole distribution arrangement according to the characteristics of permeability zoning, the effective extraction area can almost cover the protected panel, and the blind extraction area is reduced by 91.22% when compared to uniform borehole distribution. These can provide a reference for unloading gas extraction under similar conditions.
Highlights
For coal seams with low gas permeability and high gas, gas extraction measures must be taken before mining to reduce the coal seam gas content
Reasonable arrangement of boreholes is key to the efficient gas extraction of coal seam [3, 4]
Rough experimental research, Levine [24] has found that the coal strain caused by gas adsorption accords with the equation of Langmuir form, which can be expressed as follows: εs εmaxpg, P50 + pg where εmax is the maximum adsorption strain and P50 is the adsorption pressure constant, which refers to the gas pressure when strain is 50% of the maximum adsorption strain (Pa)
Summary
For coal seams with low gas permeability and high gas, gas extraction measures must be taken before mining to reduce the coal seam gas content. Drilling borehole in the floor roadway is one of the most commonly used methods in the protected coal seam gas extraction. Wang et al [7] studied the evolution law of fracture of coal and rock strata in the upper protective layer and considered that gas extraction should consider the factors of spatial and temporal effects of the evolution of the mining fracture. Wang et al [8] and Liu [9] tested the permeability change of the protected coal seam after protective layer mining and established the coupling dynamic model of the coal rock mass deformation and gas flow. Wang et al [14] and Hu et al [15] studied the gas emission and the pressure relief law of the protected layer in the mining face of the close and steep inclined and steep slope protection layer. Based on the multiphysics eld theory, the hydraulicmechanical coupling model of gas extraction in the unloading coal seam was established, and the gas extraction process by drilling borehole in the oor roadway in the overburden of Panyi Coal Mine 1551 (1) panel was simulated. e in uence of di erent drilling arrangements on the gas extraction e ect was analyzed
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