Objective and Method For broken soft and low-permeability coal seams, gas drainage achieved through permeability enhancement and pressure relief by cross-seam hydraulic caving in coal seam floor faces problems including serious borehole collapse in the drilled coal seam intervals, low construction efficiency, and poor gas drainage performance. To address these problems, this study proposed a high-efficiency drilling - punching - hole protection integrated gas drainage technology, which enabled hydraulic caving, permeability enhancement, and pressure relief. Furthermore, this study developed a drilling - punching - hole protection integrated caving device. Through laboratory experiments conducted under pressure conversion and central channels equipped with controllable opening/closing mechanisms, this study selected the optimal scheme for central channel sealing and determined the optimal hydraulic parameters of the hydraulic caving device. Using large through-hole drill rods and openable/closable drill bits in combination, this study compared the construction efficiency and gas drainage performance of the integrated technology with those of the traditional drilling-punching technique.
Results and Conclusions The drilling - punching - hole protection integrated caving device allowed for the shift from low-pressure drilling to high-pressure caving under a pressure of 3 MPa. A drilling flow rate of ≥ 100 L/min and a flow rate for high-pressure caving of ≥150 L/min could be guaranteed when the flow channel diameter was ≥ 4 mm and the nozzle diameter for caving was ≥ 3.5 mm. Compared to the original hydraulic punching and caving technology, the drilling - punching - hole protection integrated technology increased the average placing rate of screen pipe from 87.1% to 97.5%. Most specifically, this technology increased the average hole protection rate in the drilled coal seam intervals from 55.9% to 98.2%. After gas drainage for 30 days, the daily pure gas flow from a borehole and that from an equivalent 100-m-long drilled coal seam interval increased to 3.8 times and 4.5 times, respectively, and the cumulative pure gas flow increased to 1.69 times and 2.01 times, respectively, compared to the original technology. In addition, the new technology requires only one round of drilling throughout the whole construction process. Compared to the original technology, the integrated technology yielded coal punching efficiency of up to 8.35 times and doubled overall operational efficiency. The drilling – punching – hole protection integrated technology allows for integrated construction of the full process consisting of drilling, hydraulic punching, and hole protection. This technology effectively addresses the challenges of existing punching and caving construction for broken soft and low-permeability coal seams, including low efficiency, cumbersome operation process, and the phenomenon that the screen pipe cannot be placed without lifting drilling tools after caving. Therefore, the proposed technology can effectively improve gas drainage performance through hydraulic caving, permeability enhancement, and pressure relief.
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