Dynamic evolution process and application of gas displacement by water injection in gas-bearing coal bodies

•The dynamic evolution law of water-flooding gas seepage characteristics under different mechanical environments is revealed.•The reasons for large hydrodynamic pressure losses and poor desorption effects of adsorbed gas are expounded.•A step-by-step water-injection gas-displacement extraction technology suitable for low permeability coal seams is proposed. We carried out a large-scale water flooding and gas displacement seepage experiment to explore high-efficiency drainage of water-flooded coal seams. We also investigated the complex problem of multiphase seepage in the process of water flooding in coal seams. We revealed the dynamic evolution law of water flooding and gas displacement seepage characteristics under different mechanical environments. We then proposed a step-by-step water flooding and gas extraction technology suitable for low-permeability coal seams. Our results showed that the dynamic seepage process of gas driven by water injection in a gas-bearing coal includes four stages: Initial water injection displacement, gas–water two-phase seepage, gas–water integrated displacement, and fracture-pore water saturation. Based on the characteristics of the experimental curve, we found that water injection into a coal seam can improve the permeability of the coal, but the second stage has a long fracture-initiation time, resulting in a large loss of hydrodynamic pressure. The third stage has a short gas-displacement time, resulting in poor desorption of adsorbed gas. To address this, a technology for enhancing gas extraction by step-by-step water injection and flooding was developed. Compared with traditional water injection technology, the average concentration and pure flow of single hole extraction increased by 1.5 and 5.1 times, respectively. The effect of coal mine water injection is significantly improved.