Distribution of coal seepage resistance around extraction boreholes: an experimental study

Pore structure change was one of the important factors affecting the coal seepage resistance around gas extraction boreholes. In order to study the evolution characteristics of the coal seepage resistance in the crushed and fractured zones around the borehole. Impact of initial pore size on internal pore pressure within crushed coal samples under triaxial compression was investigated, using the steady-state seepage method. Additionally, the influence of specimen layering interfaces on pore structure property parameters during the permeation process was analyzed. Following conclusions were obtained in this study. The percolation state of single pore size specimen with same percolation velocity experienced gradual transition from laminar flow to turbulent flow as the pore size increased, and the percolation resistance state also changed from viscous-resistance dominance to inertial-resistance. The minimum pore size of the double pore size specimen was a prerequisite for the change of pore pressure gradient. The average particle size of coal consisting of double pore sizes ranging from 0. 1 to 1. 025 mm, and the pore-throat ratio was from 2 to 4. The permeability pressure demonstrated a quadratic curve-like behavior, increasing with the increase of the pore-throat ratio. The change of the pore size greatly impacted the permeability, and the gas penetration through the coal body was enhanced with the increase of the initial pore size. The non-Darcy flow factor of single-pore-size specimens and double-pore-size combined specimens expressed linear relation with the inverse of the particle size and the inverse of particle size difference, respectively. Moreover, the non-Darcy percolation effect became apparent as the non-Darcy flow factor increased, which indicated that the growth trend of non-Darcy flow β factor was influenced by its pore size. Sudden increase of the pore size led to a reduction of the relative contact area between the fluid and the permeable skeleton, then the non-Darcy flow β factor could be further increased. Therefore, favorable consistency, as gas mitigated, could be pronounced between the fragmentation degree of the coal around the borehole and the permeability of the coal seam. Quantitative description of geometric pore parameters of the specimen could be established by combining permeability and non-Darcy influence factor, so as to reveal the influence of pore structure changes on seepage resistance. © 2024 China University of Mining and Technology. All rights reserved.