Study on the Feasibility of Enhancing Gas Extraction Efficiency Based on Hydraulic Flushing Combined with Gas Injection Displacement (HF-GID): Model Development and Numerical Simulation.

Abstract

As an effective coal seam permeability enhancement technology, hydraulic flushing (HF) has been applied on a large scale in some Chinese mining areas. However, with mining depth extension and increasingly complex coal seam geology, some coal seams still enter the extraction depletion period after HF. Realizing secondary enhanced extraction of low-permeability coal seams is urgent. Thus, this study investigates feasibility based on hydraulic flushing combined with gas injection displacement (HF-GID) synergistic extraction, focusing on feasibility in the No. 3 coal seam of Shanxi's Qinshui Coalfield via numerical simulation. First, a multifield coupling GID model (covering coal deformation, CH₄/N₂ migration, and permeability evolution) and an HF model (based on dual-porosity media effective stress and high-pressure water jet coal-breaking) were established. Then, four working conditions (OD, HF, GID, and HF-GID) were simulated, leveraging No. 3 coal seam properties and drilling layouts. Temporal permeability evolution, stress mechanisms of HF/HF-GID, and impacts on gas extraction (especially content reduction) were analyzed. Results show HF-GID achieved significant permeability enhancement. An elliptical high-permeability zone (with permeability reaching a maximum of 250 times the initial value) forms around the boreholes, characterized by slower growth in the early stage and accelerated growth in the mid-to-late stage and outperforming single technologies. GID alleviates the HF-induced peripheral stress concentration via injection pressure, improving gas migration. HF-GID reached the safety threshold (8 m³/t) in 75 d, outperforming HF (125 d) and GID (93 d) . A synergistic gain factor η was proposed. Under basic conditions, η ranges 0.08-0.42; optimized parameters increase it to 0.33-1.19. η positively correlates with the hydraulic cavitation radius <i>R</i> _hyd. At 1.4 MPa injection pressure <i>P</i> _inj, η peaks, boosting extraction efficiency by 33% compared with other single technologies. HF-GID overcomes the limitations of single technologies through the mechanisms of pressure relief-flow promotion, copermeability enhancement, and accelerated replacement, providing theoretical/technical support for low-permeability coal seam extraction.