Synergistic ECBM extraction technology and engineering application based on hydraulic flushing combing gas injection displacement in low permeability coal seams

•Hydraulic flushing integrated with gas injection displacement was proposed.•Gas injection displacement can weaken the adverse effects of concentrated stress.•A coupled gas flow model was established to optimize the engineering parameters.•The gas drainage and outburst elimination efficiency are significantly improved. With a continuous increase in mining depth, high gassy coal seams with low permeability increasingly appear. Although hydraulic flushing (HF) is an efficient technology applied widely for enhanced coalbed methane (ECBM) in low permeability coal seams (LPCS), the methods utilized to eliminate the adverse effects of stress concentration (SC) zones generated by HF on gas drainage have not been proposed. Therefore, a synergistic ECBM extraction technology based on HF integrating with gas injection displacement (GD) has been put forward in this paper. The primary role of HF is to provide ample migration space for gases so that the gas injection resistance can be reduced in the following GD process. Accordingly, the GD is implemented to eliminate the negative effect of the SC zones on gas drainage by increasing the pressure gradient, and the permeability enhancement and pressure holding function of nitrogen-rich gas also take effect. Additionally, a coupled gas flow model was developed and validated to describe the synergistic process. The optimal engineering parameters that can provide theoretical reference for engineering application were determined systematically according to the modeling results. The on-site boreholes should be all flushed, and the actual drilling diameter should still be determined according to the geological and technical conditions in the field. Furthermore, low pressure (LP) is the best choice before the effects of HP being further investigated on fracturing coal and inducing outbursts. Meanwhile, the analysis results of the field experiment in the Chensilou coal mine indicate that the proposed technology can effectively promote the adsorbed methane remaining in the SC zones to desorb adequately and then migrate to the gas drainage boreholes. Especially for LPCS, this synergistic ECBM extraction technology integrating HF with GD possesses a great application prospect.