Drilling large diameter cross-measure boreholes to improve gas drainage in highly gassy soft coal seams

• The effects of increasing cross-measure borehole diameter on the radial permeability and the effective influence zone radius are discussed.
• A new method and a special equipment system for drilling large diameter cross-measure boreholes are developed.
• The stress redistribution interactions between boreholes are studied to evaluate the optimal distance between two cross-measure boreholes.
• The effective influence zone of the large diameter cross-measure borehole is tested in Pingdingshan coalfield, China.

Reducing gas content via cross-measure boreholes is one of the primary gas control technologies in China, where most outburst-threat coal seams are soft and highly gassy. Regardless of the significant costs associated with drilling boreholes, the gas drainage rate remains low because of the low permeability of the soft coal seam and the small influence zone of a single borehole. In this paper, the effect of increasing borehole diameter on coal seam permeability is discussed and a new method for drilling large diameter cross-measure boreholes by using the water-jet technique is proposed. Numerical modeling results indicate that the plastic zone and the effective influence zone of one borehole expand as borehole diameter increases, and the interaction between adjacent boreholes is strengthened. The field test shows that when the borehole diameter is 1.0 m, the effective influence zone radius reaches 4 m which is 2.67 times larger than that of an ordinary borehole. After using the new method, the number of cross-measure boreholes per hundred meters and the length of cross-measure boreholes per meter can reduce by 32.5% and 42.9%, respectively. In addition, the gas drainage rate reaches 52.1%, and the monthly excavation length of coal roadway increases from 50–70 m to 109 m.