Research On Mechanism And Application Of High-pressure Water Infusion To Displace Gas In Permeability Coal Seam

High-pressure water infusion is one of the key techniques to improve the effect of gas extraction in low permeability coal seam.In this paper,the research background is low permeability coal seam in Changcun Coal Mine and Xiadian Coal Mine of Lu’an Group.Field investigation,theoretical analysis,laboratory experiment,numerical simulation and field test are adopted.The mechanism of gas-water seepage of water infusion in coal seam is revealed.The comprehensive influence of high-pressure water infusion on coal gas desorption is obtained.The effect of water infusion on gas drainage and its main influencing factors are confirmed.Finally,the application test of high-pressure water infusion to improve gas extraction was carried out,and good results have been achieved.On the basis of analyzing the process of coal seam water infusion and gas extraction,the single phase seepage mathematical model of coal seam water infusion and gas extraction were established.The law of Interface motion between water and gas under piston displacement was systematic analyzed.The water saturation distribution equation of plane radial percolation under non-piston displacement was derived.The mathematical model of two-phase percolation of water infusion to displace gas was established.Basing on self-made adsorption-infusion-desorption experimental device,the effect of water infusion on gas displacement and replacement desorption were proved,and water infusion can increase the total gas desorption of gas-bearing coal.The total gas desorption capacity of CHC coal and XD coal increased respectively about 0.90-1.36m3/t and 0.46-1.20m3/t.In the whole process of high-pressure water infusion,the gas extraction amount includes not only the free gas amount in the coal pore,but also the gas adsorbed amount by water replacement desorption and the gas desorbed amount by increasing the permeability of coal seam.Using the coupling of VOF model of Fluent software and porous medium model,promoting gas extraction by water infusion and its influence factors was simulated based on multiphase porous flow theory.It was found that the gas flow of drainage hole decreased,increased and decreased with time before infusion,during infusion and after infusion,respectively.The opportunity,time,pressure,layout,space between boreholes and the way of water infusion were the 6 main factors which influence the effect of gas extraction by water infusion.It can achieve the best effect when intermittent water injection was carried out for 10 days under the pressure of 8MPa water injection according to the way of one injection and one pumping and arranging the drilling holes at 5m after 20 days of the methane drainage.Field test results show that the prue gas flow quality of adjacent drainage hole increased significantly during and after water infusion,and increased by about 475%and 105%.The water injection effect period was over 35 days.Compared with conventional extraction area,the residual gas content of water injection decrease by 7.4～11.1%,and the moisture content increase by 3～6 times.The high-pressure water infusion by alternative arrangement of water injection holes and extraction holes can achieve remarkable gas extraction effect in single low permeability coal seam.