Enhancing Permeability and Gas Extraction in Low-Permeability Coal Seams: A Study on Gas-Liquid Two-Phase Jet Technology.

Water jet technology causes severe "water lock" in downward boreholes, hindering gas extraction. To address this limitation, a gas-liquid two-phase jet technology was proposed. This study employs an integrated methodology to investigate coal damage characteristics under water jet impact, gas jet debris discharge characteristics, and engineering applications of gas-liquid two-phase jet technology. The main conclusions drawn from the study are as follows: (1) Water jet-induced coal damage progresses through three stages: initial no-damage, damage accumulation, and complete damage stages. Damage initiates locally, expands structurally, and forms cavities until full disintegration. (2) Larger nozzle diameters improve the coal-breaking efficiency, increasing both erosion diameter and depth, with vertical damage zones exceeding horizontal ones, recommending 3.5 mm nozzles for depth. Standoff distance directly governs erosion depth: erosion depth remains stable within the 5-15 mm optimal range but decreases significantly in the diffusion zone. Jet velocity shows staged effects: 200-400 m/s forms shallow craters, while 400-800 m/s creates a deep cavity with linear depth-velocity correlation, demonstrating longitudinal penetration dominance. (3) Gas jets enable effective debris transport in water-bearing conditions, showing zonal dynamics: bubble-induced particle mobility at the bottom, enhanced three-phase vortex in the middle, and efficient upward transport due to the gas-liquid lifting forces in the upper section near the orifice. (4) Under gas jet action, debris and bubble-liquid films form a complex interaction system, where particles exhibit distinct helical reciprocating motion while encapsulated. The bubble-liquid film critically controls debris transport by reshaping flow structures. (5) Field tests confirm that gas-liquid jets sustain higher gas concentrations versus conventional methods. While the high-pressure water jets effectively expand the coal damage zones around boreholes, the gas jets markedly improve coal seam permeability, thereby synergistically enhancing gas desorption and migration capabilities.