Study On The Influence Of Gas On Water Seepage Characteristics During Coal Seam Water Injection

Coal seam water injection is one of the means to prevent and control coal and gas outburst in coal mining face.However,it is difficult to inject water in some mines where gas pressure is relatively high and permeability is low.If water injection pressure is simply raised to solve this problem,local gas pressure may be too high and the danger of coal and gas outburst will be increased.Therefore,the role of gas should be emphasized in the process of coal seam water injection.From this point of view,this paper studies the influence of gas on water seepage characteristics during coal seam water injection,which is also very important to optimize the parameters of coal seam water injection and improve the effect of coal seam water injection.In this paper,the permeability and imbibition characteristics of raw coal and briquette coal with different coal tar content are firstly analyzed through experiments,and the preparation scheme of briquette coal for subsequent experiments is determined accordingly.Then,the experiment of the influence of water on gas adsorption and desorption is carried out,and the correction coefficient of the influence of water on gas adsorption is obtained and applied to the construction of permeability model.The water-gas two-phase relative permeability evolution model is established and verified by experiments.Combined with the actual situation of coal seam water injection,the water-gas two-phase seepage model of loading coal containing gas was constructed and simulated.Water-gas two-phase seepage experiment has been carried out in the laboratory,and coal seam water injection field test has been carried out in Lvjiatuo Coal Mine.The comparison of experimental results and numerical simulation results verify the applicability of the seepage model in the process of coal seam water injection.Firstly,coal samples were taken in No.12 coal seam of Lvjiatuo Coal Mine,and standard raw coal samples and briquette coal with tar content of 0%,1.7%,3.3%,4.9%,6.4%,7.9%respectively were prepared.The permeability and imbibition characteristics of raw coal samples and briquette coal samples were tested by using the water-gas two-phase seepage system and the imbibition experimental platform by electrical resistance method independently developed by the research group.The results show that when the gas pressure is constant,the permeability decreases gradually with the increase of effective stress,and the permeability and effective stress satisfies exponential function.With the prolongation of imbibition time,the imbibition height increases gradually,the permeability and effective stress satisfies power function.The permeability and the speed of imbibition of briquette decreases with the increase of coal tar content.The permeability and the speed of imbibition of raw coal are between the briquette coal with tar content of 6.4%and 7.9%,but it is closer to briquette coal with tar content of 6.4%,so briquette coal with tar content of 6.4%is selected as the experimental coal sample.Water will affect the gas adsorption and desorption ability of coal.The gas adsorption experiments of coal samples with different water content were carried out.The results showed that under the same adsorption equilibrium pressure,with the increase of water content,the ability of coal to adsorb gas gradually decreased,and the correction coefficients of water influence on gas adsorption capacity of coal samples were obtained and applied to the construction of permeability model.The comparative experiments of gas desorption amount between natural desorption of coal sample and water injection process have been carried out.The results show that under the same conditions of axial pressure,confining pressure and gas pressure,the gas desorption amount and desorption rate of coal sample in water injection process are larger than that of natural desorption coal sample,indicating that water injection promotes gas desorption.On the basis of considering the effects of coal stress,gas adsorption affected by water content,water adsorption,pore pressure and temperature,the coal porosity model and permeability model were established.According to the characteristics of water-gas coupled seepage in coal seam water injection process,the relative permeability model of water-gas two-phase seepage is established.The relative permeability test of water-gas two-phase was carried out,the rationality of the relative permeability model was verified by comparing the theoretical values with the measured values.Based on the construction of the governing equations of coal deformation,gas flow and water flow in the process of coal seam water injection,a water and gas two phase model of loading coal containing gas is established,and the boundary conditions and initial conditions of the seepage model are determined.Combining with the coal sample size and experimental conditions of laboratory experiments,the numerical simulation is carried out.The results show that the wetting range of coal samples varies greatly in unit time at the beginning of water injection.With the extension of time,the wetting rate decreases gradually.There is logarithmic functional relationship between wetting range and water injection time.The simulation results provide reference for the setting of water injection pressure and drilling hole position in the process of coal seam water injection.The water-gas two-phase seepage experiment of coal with gas was carried out on the water-gas two-phase seepage system.The relationship between wetting range and injection time of coal samples under different gas pressures and water injection pressures was obtained.The experimental results were basically in agreement with the numerical simulation results,which verified the applicability of the model in the process of water injection.Finally,the coal seam water injection test was carried out at 5424Z working face of No.12 coal seam in Lvjiatuo Coal Mine of Kailuan Group.The results show that with the extension of water injection time,the wetting range of coal body increases gradually,but the increasing rate decreases gradually,and the relationship between wetting range and water injection time satisfies logarithmic function.At the same time,on the basis of the water-gas two-phase seepage model of loading coal containing gas established in this paper,the numerical simulation is carried out in combination with the field test conditions.The simulation results are basically consistent with the test results,which verifies the applicability of the model in the process of coal seam water injection.