Control of directional long borehole on gas drainage and optimal design: Case study

The Directional Long Borehole (DLB) technique, which has the benefits of low construction costs and high drainage efficiency, will be extensively used in future mining operations for gas drainage and utilization. Choosing the right borehole parameter is critical for improving the drilling stability and drainage efficiency of DLB. This paper begins with research on the gas migration and enrichment law in the mining-induced fissure, and proposes a gas migration channel zoning model. Furthermore, the dynamic process of gas migration in fissure fields is analyzed, as well as the key borehole parameters are identified. The optimal DLB parameters are eventually addressed and presented for the 2205 working face via combined UDEC and COMSOL. Research indicates that: (1) The mining-induced fissure serves as a channel for gas flow, and presents different pore morphology in space, forming a gas migration channel zoning model. The model's permeability increases initially and then decreases from the gob upward, increasing in an O-ring outward diffusion from the gob's center, it can be represented as a rectangular ladder platform with 15 zones. (2) Gas goes through a dynamic process of state change during drainage that involves adsorption/desorption, diffusion, and seepage. The main factors that affect this process are negative pressure, borehole length, borehole diameter, and the location of the borehole in the fissure zone. (3) The optimal parameters of DLB in the 2205 working face, are 2 drill sites, each with 5 sets of 500 m long boreholes, with a diameter of 133 mm, and a pumping negative pressure of 13 kPa, and placed in a layer height of 22 m. DLB provides benefits over the high-level suction highway in terms of construction cost, drainage effectiveness, and timeliness. The findings can be used to guide the design of DLB, enhance gas energy utilization, and prevent gas disasters.