Water Injection Into Coal Seams for Outburst Prevention: The Coupling Effect of Gas Displacement and Desorption...

Gassy coal seams generally have low permeability and dry coal bodies, which are susceptible to coal and gas outburst hazards in the process of mining. Water injection into coal seams can significantly alter the gas release rate and flow behavior. However, water has dual effects on coal seams: gas displacement and water-locking, and the coupling mechanism of these two effects is not clear in the whole process of coal seam water injection. By measuring high-pressure gas adsorption isotherms and gas diffusion initial velocity, it was found that both the Langmuir adsorption constant a and gas diffusion initial velocity ΔP decrease with the increase of water content, which would reduce outburst risks. Through the self-developed integrated experimental device of "gas adsorption + water injection displacement + gas desorption″, the changing rules of gas displacement amount, desorption amount, and water lock amount under different water injection conditions were studied. The results show that when the water injection ratio increases from 6 to 25%, the gas displacement would increase from 0.62 to 1.16 mL/g, with an increase of 87.09%. Also, at the same time, the gas desorption capacity would decrease from 4.86 to 4.05 mL/g after pressure relief, with a decrease of 16.67%. The amount of water-locking increased from 0.11 to 0.38 mL/g. The effect of water injection to control coal and gas outburst occurs in two different water injection stages. In the process of water injection, water plays a major role in gas displacement, which is conducive to reducing the gas content of the coal seam. After the completion of water injection, the static water pressure remaining in the coal seam can reduce the gas emission rate. The combination of these two effects can effectively reduce the risk of outbursts.