Tree-Type Boreholes in Coal Mines for Enhancing Permeability and Methane Drainage: Theory and an Industrial-Scale Field Trial

摘要

Tree-type borehole drilling (TTBD) is a novel stress relief method for stimulating methane drainage from coal in underground coal mines. This paper is a report on studies that analyzed the permeability enhancement capabilities of tree-type boreholes in coal seams and presents a precedent-setting industrial trial of this new method in a 200-m section of a drainage roadway in a coal mine, the Shoushan coal mine in Henan province, China. Detailed simulation results indicated that after coal seams were stimulated using the TTBD method, a large number of fractures were initiated around the tree-type boreholes. These fractures propagated along the direction of maximum horizontal stress and formed highly permeable flow channels for methane transport from the coal into the tree-type sub-boreholes. The fractures gradually became shorter along the length of the sub-boreholes. As the sub-boreholes were shortened, the area fractured and the number of induced fracture decreased rapidly. Conversely, the fracture density and the value of homogeneity coefficient,beta, increased. The industrial trial showed that compared with gas drainage from a conventional borehole in the Ji(15-17)coal seams, the gas production rate from a single tree-type borehole was 4.7 times faster and the percentage of methane in the gas produced was 1.7 times higher. In addition, the borehole drainage attenuation time was 2.5 times longer for the tree-type boreholes. The total volume of gas production produced from 238 tree-type boreholes in a 200-m section of roadway was over 70,000 cubic meters, 1.4 times the methane extracted by 938 conventional boreholes in an adjoining 200-m section of the same roadway. In addition, the number of boreholes required to drain 1.4 times as much gas was 75% smaller. These results have provided support for the appreciation and application of this new technique for improving underground coal mine methane drainage.