Gas Extraction Efficiency in Soft Coal Seams through Hydraulic Dilation and Hydraulic Flushing Technology

In response to the challenge of low gas extraction efficiency in soft coal seams within high-gas mines, this study introduces a new technology known as Hydraulic Dilation and Hydraulic Flushing (HDHF). By pretreating the coal seam with low-pressure fluid to induce plastic dilation and subsequently employing hydraulic flushing to expand the fracture network, the HDHF technology significantly improves the permeability and gas extraction efficiency of soft coal seams. Grounded in the theory of coal seam stress release, this study establishes a coupled model of gas flow and geomechanics. Through numerical simulations, the applicability of HDHF technology in soft coal seams is analyzed, and the mechanism of permeability enhancement is elucidated. The results demonstrate that (1) Utilizing the hydraulic dilation and hydraulic flushing technology to enhance permeability in coal seams with varying solidity coefficients, the stress release zone is largest in soft coal. The enhanced permeability areas for Case 1, Case 2, and Case 3 are 1.67, 5.18, and 9.58 m2, respectively. (2) The HDHF technology significantly reduces coal seam gas pressure and enhances gas flow velocity. In the initial stage of extraction, the rate of gas pressure decline is 1.81 times that of conventional hydraulic flushing, with permeability increasing by a factor of 6.6. The gas extraction output under HDHF technology is markedly higher than that of conventional hydraulic flushing, with higher extraction efficiency and shorter required extraction time. (3) To enhance the permeability of soft coal seams, prestress treatment is essential to elevate the recovery rate and permeability during the initial extraction phase, establishing a higher baseline and thereby prolonging the attenuation period. (4) Owing to the pretreatment of the coal seam, no low-permeability zones formed between the boreholes, and the permeability and gas flow velocity at the distal end of the coal seam were significantly enhanced. The initiation of microfractures leads to a plastic dilation state in the coal seam. Coal particles that spall off fall between the fractures. Under the action of hydraulic flushing, shear displacement occurs between the fractures. This effectively prevents coal and gas outburst disasters, addresses issues such as coal dust blockage and stress concentration between boreholes, and ensures safe and efficient mine production.