Progress and prospect of gas extraction technology by underground gas injection displacement for increasing flow in low-permeability coal seam in China

Most coal mining areas in China have a low coal seam permeability. Gas extraction technology by gas injection displacement for increasing flow is one of effective methods to improve the efficiency of gas extraction in low-permeability coal seams. In this study, the gas occurrence characteristics of low-permeability coal seam in China were reviewed and classified. Five characteristics were summarized, including high resource reserves, low gas saturation, coexistence of multiple reservoir pressures, low-permeability, and complex partitioning. Based on the method of difference confidence, grey correlation and confidence interval calculation, the applicability of coal seam gas in China to the gas extraction technology by underground gas injection displacement for increasing flow was analyzed. Six direct factors affecting the effect of gas injection displacement were obtained. The influence degree was as follows: Platts coefficient > Gas pressure > Permeability > Ash content > Gas content > Langmuir pressure constant. A screening and evaluation method of coal seam parameters was established which was suitable for the gas drainage technology by underground gas injection displacement for increasing flow. According to the different mechanisms of increasing gas flow in gas injection displacement, the gas flow increasing mechanism of gas injection displacement was clarified from two aspects: gas factor increasing flow(carry CH₄ with filling energy to increase flow, promote CH₄ desorption by sharing pressure to increase flow, promote CH₄ diffusion by diluting to increase flow, and competitive adsorption to increase CH₄ flow) and energy injection modification increasing flow(enlarge pores with filling energy to increase CH₄ flow, enlarge pores with promoting desorption to increase CH₄ flow, expand pores with promoting desorption to increase CH₄ flow, and damaged pore structure/spread fracture network by pulse gas flow to increase CH₄ flow). The influence of gas source and process parameters on the gas injection displacement effect was summarized. A brief introduction to the key technology and equipment, application mode and application effect of the underground gas extraction technology by gas injection displacement for increasing flow. The future development trend of the underground gas extraction technology by gas injection displacement for increasing flow in China was prospected, including multidisciplinary deepening the theory of gas extraction by gas injection displacement for increasing flow, multimethod collaborative the technology of gas extraction by gas injection displacement for increasing flow, precision of the parameters of gas extraction by gas injection displacement for increasing flow, intellectualization of the equipment of gas extraction by gas injection displacement for increasing flow, coordination of injection-pumping-excavation-mining connection layout in coal seam.