Experimental investigation of coal seepage characteristics of a "ring zone" structure around a gas extraction borehole

• Minimum pore size is a prerequisite for influencing the pore pressure gradient. • The non-Darcy flow factor is linearly related to the inverse of the grain size. • The mixing resistance is related to the gas influx flow rate. The pore and fracture network of the fractured coal around the gas pre-drainage borehole is extremely complex, and the " ring zone " structure of the coal around the borehole is also the necessary channel for gas convergence. In order to measure the permeability parameters of the coal in the annular zoning structure around the borehole, an " ring zone " coal seepage test was conducted. The seepage characteristics of coal under different pore diameters and combinations were analyzed. It was found that the difference in pore diameter leads to a reduction in the contact area between the fluid and the permeation skeleton, which intensifies the non-Darcy flow phenomenon. The enhancement effect of coal fragmentation on permeability and the weakening effect of the layered interface between the fracture zone and the broken zone on permeability were pointed out. Furthermore, it was further studied and demonstrated that the difference between the gas inrush velocity and the mainstream velocity in the borehole causes complex mixed flow phenomena, including backflow, stagnation, and swirling, which increase the flow resistance. This study provides a theoretical basis for optimizing borehole design, which is of great significance for promoting the sustainable development of the coal industry.