Abstract
Deep coal seams are characterized by large stress, high gas pressure, and low permeability. The gas disaster threatens the safe production of coal mine seriously. Gas extraction by crossing-seam boreholes from floor roadway (GECMBFR) can reduce the pressure and content of coal seam gas, which is the main measure to prevent gas disaster. Considering the Klinkenberg effect, governing equations of gas adsorption/desorption-diffusion, gas seepage, and stress fields within the coal seam are established to form the seepage-stress coupling model. The governing equations are embodied into a finite element driven software to numerically simulate gas migration and fluid-solid coupling law in coal seam. On this basis, the process of gas extraction under different borehole spacings and diameters is simulated. The effects of these two key parameters on coal seam gas pressure, gas content, and gas permeability were analyzed. The borehole spacing and diameter were determined to be 5 m and 0.09 m, respectively. Combined with the actual situation of a mine, the process of gas extraction from floor roadway with different cross-sectional schemes, ordinary drilling boreholes and punching combined drilling boreholes, is comparatively analyzed. The results show that the gas extraction effect by ordinary drilling boreholes is lower than that of the punching combined drilling boreholes, and the extraction is uneven and makes it difficult to meet the standard. Hydraulic punching was carried out, and coal was washed out of the borehole, which expanded the contact area between the borehole wall and coal seam. The coal seam around the punching borehole is unloaded, which improves coal permeability and accelerates gas migration towards the borehole, thus promoting the efficiency of gas extraction. It is more reasonable to use punching combined drilling borehole scheme when implementing the GECMBFR technology.
Highlights
As the mining depth increases, gas pressure and content in coal seam increase. This may lead to gas accidents such as coal and gas outburst and gas explosion, which seriously threaten the safety production of coal mine [1, 2]
Considering the occurrence environment of coal seam gas, we put forward the following assumptions [17,18,19]: (1) coal seam is an elastic continuous medium, which contains both fractures and porous matrix; (2) gas adsorbs on the surface of both pores and fractures, and gas transports in the space of pores and fractures in the state of free; (3) with the process of gas extraction, gas first desorbs from the surface of pores and diffuses into fractures and seepages into boreholes from the fractures, obeying the Langmuir adsorption law, Fick’s law, and Darcy’s law, respectively; and (4) gas is a kind of ideal gas
(5), (8), and (12)–(16), the seepage-stress coupling model is obtained, and the partial differential equations can be embedded into software by programming to simulate the law of gas extraction by drilling through floor roadway
Summary
As the mining depth increases, gas pressure and content in coal seam increase. This may lead to gas accidents such as coal and gas outburst and gas explosion, which seriously threaten the safety production of coal mine [1, 2]. Gas extraction by borehole drilling from floor roadway can effectively reduce coal seam gas and eliminate gas disaster risk [3]. Scholars have studied the interaction relationship between coal and gas They consider that the change of geostress and gas pressure in the coal seam will cause coal deformation and change of porosity and permeability. The Klinkenberg effect is often ignored, resulting in large errors in borehole design and gas extraction calculation [15, 16]. In this study, considering the action of the Klinkenberg effect, the seepage-stress coupling model for gas extraction from coal seam is established by combining the controlling equations of gas diffusion field in coal matrix, gas seepage field in fractures, and stress field within the coal seam. The research results provide guidance for optimizing the borehole schedule of gas extraction
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