Permeability Enhancement and Methane Drainage Capacity of Tree-type Boreholes to Stimulate Low-permeability Coal Seams

Abstract

Low permeability coal makes methane control in underground coal mines difficult. Drilling tree-type boreholes is a new method for improving the permeability of coal seams to stimulate cross-measure boreholes drainage. This paper presents an analysis of the permeability enhancement and methane drainage capacity of tree-type boreholes based on data from a coal mine in China. After applying the proposed method, numerical models show that a large area of enhanced permeability is created between the tree’s sub-boreholes. Compared with conventional drainage programs, tree-type boreholes increase the average rate of methane extraction by a factor of 13.9 and the effective influence radius of the borehole by a factor of 4.9. The simulated methane production rates from both types of boreholes are verified against field data from the Shoushan mine, China. This paper presents a quantitative analysis of the relationships between different tree-type borehole properties and methane drainage performance. Longer and more numerous sub-boreholes result in logarithmic drainage increases in the enhanced-permeability areas. Orienting the initial tree-type borehole angle parallel to the direction of maximum horizontal stress maximizes borehole stimulation. During drainage, the effective influence radius of tree-type boreholes expands logarithmically with sub-borehole length; cumulative methane production increases linearly with the length and number of sub-boreholes. These results can provide reliable scientific guidance for optimizing the layout of tree-type boreholes in field test.