Technical Parameters of Hydraulic Punching in a Typical Coal Seam and an Investigation of Outburst Prevention Effect: A Case Study in the Machi Mine, China

The Dengfeng coalfield belongs to the typical “three soft” coal seam and has a high gas content ranging from 10 to 13 m³/t, and the coal seam gas content reaches a maximum 13.69 m³/t. Because of its coal seam occurrence, no protective layer can be mined. The conventional regional pre-pumping measures have several problems. For example, the construction of a pre-pumping borehole is difficult, and it easily causes a coal and gas outburst. The low permeability and rheological characteristics of the coal seam lead to the collapse of the borehole and seriously affects the drainage effect. The influence radius of the drainage borehole is small, which results in a larger quantity of new drilling boreholes and significantly increases the operational cost. Compared to the conventional regional pre-pumping measures, floor roadway hydraulic flushing rapid outburst technology can effectively overcomes these issues. Hydraulic flushing uses water to flush the borehole, and the newly formed fractures and the release of free gas greatly reduce the elastic potential and gas expansion energy of coal and rock, which thus significantly improve the permeability of the coal and improve the coal seam degasification efficiency for outburst risk control. In this paper, a hydraulic punching method is utilized for coal seam stimulation in the Machi mine. Reasonable technical parameters of hydraulic punching were obtained: the water pressure of punching and breaking coal is 8–15 MPa, and the equivalent diameter of the hydraulic punching nozzle should be < 7.47 mm. After the implementation of the hydraulic punching measures, the gas extraction capacity of the coal around the borehole is greatly improved. For 90 days gas extraction after stimulation, the gas content of the 11,061 lane decreased from 8.42–12.12 m³/t to 3.09–5.75 m³/t, relieving pressure and increasing permeability of “three soft” coal and simultaneously eliminating the outburst danger of the coal seam.