Practice shows that CO₂/N₂-ECBM is an effective technology to enhance coalbed methane. However, there are few field tests in which the technology is applied to enhance the gas drainage in underground coal mines, and the effect is uncertain. In this study, firstly, the reasons for the decrease of gas drainage efficiency in the exhaustion period were analyzed based on the theory of fluid mechanics. Secondly, the mechanism of N₂ injection to enhance coal seam gas drainage (N₂-ECGD) was discussed: with the gradual decrease of gas pressure in the drainage process, coal seam gas enters a low-pressure state, the driving force of flow is insufficient, and the drainage enters the exhaustion period. The nitrogen injection technology has triple effects of “promoting flow”, “increasing permeability” and “replacing”. Thirdly, the numerical simulations of the nitrogen pressure on drainage effect were carried out based on the fully coupled model. The results show that the higher the nitrogen pressure, the greater the displacement effect between injection and drainage boreholes, the larger the effective range. Finally, a field test of N₂-ECGD was carried out in the Liu Zhuang coal mine in Huainan Coalfield, China. The results show that N₂ injection can significantly enhance the gas flow rate and CH₄ flow rate in the drainage boreholes, and the coal seam gas content decreased 39.73% during N₂ injection, which is about 2.6–3.3 times that of the conventional drainage. The research results provide an important guidance for promoting the application of N₂-ECGD in underground coal mines.

Directional long drilling on the roof is an effective gas control measure in the goaf, but there is little research on the stability of the surrounding rock. In this study, the geological conditions of the #4 coal seam in the Tingnan Coal Mine, Shaanxi Province, China taken as the application background, and the deformation characteristics of boreholes under four typical coal and rock conditions were first analyzed based on the Universal Distinct Element Code (UDEC) numerical simulation. Secondly, the stress, strain, and plastic deformation of the rock surrounding the borehole with different diameters were carried out using the Fast Lagrangian Analysis of Continua 3D (FLAC 3D). The effect of the casing on the stability of the borehole was also simulated. The results showed that the borehole stability of coal and mudstone was lower than that of fine-grained sandstone and coarse-grained sandstone. The larger the borehole diameter, the lower the stability. The borehole tended to be unstable, especially when the diameter was 160 mm and 200 mm. Traditional pipes can provide some protection, but for large boreholes, the protection is poor. Based on the above research, uniaxial compression tests were carried out on various internal support tubes, such as ‘line-shaped’, ‘Y-shaped’, and ‘cross-shaped’. The results showed that the cross-shaped pipe had the highest compressive strength, which was 4–5 times that of the other types of protective pipe and had a good protective effect. The research results can provide reliable technical support for the protection of directional boreholes on roofs through strata and have important implications for the popularization and application of the directional long borehole technique.

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 R _hyd. At 1.4 MPa injection pressure P _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.