Study On Gas Migration Law By Downward Cross-layer Drilling On The Condition Of Water-enriched Roofs

For controlling the coal and gas outburst of coal seam group,the bottom seam gas is usually pre pumped to the bottom layer by the floor rock roadway,and the coal seam is exploited as the lower protective layer.However,the gas pressure in the lower coal seam of the outburst coal seam group is higher,its outburst risk is often higher than the upper coal seam,and the drilling construction is difficult and the pore forming rate is low.Therefore,the top layer of the top layer rock roadway can be used to pre draw the top coal seam gas under the roof rock roadway,and it is used as the upper protective layer to protect the lower coal mining.However,because of the complex hydrogeological conditions in the mine,most of the coal seam roof has aquifers.In the process of coal seam gas extraction,the roof water invades the pores of the coal seam through drilling and forms the local water saturation area around the borehole.In the water invasion coal seam,the gas in the coal seam will dissolve into the pore water under the effect of pore pressure,and then,under the influence of the concentration gradient,in the saturated water coal body region and the borehole water.There are obvious differences between the migration process of gas in the water saturated coal seam area and the general coal seam gas migration.It is of great significance to study the migration process and its influencing factors to understand the gas migration characteristics of the water invasion coal seam and to guide the gas extraction of the water invasion coal seam.Through theoretical analysis,experimental research and numerical simulation,the diffusion and migration characteristics of gas in water and saturated water coal body are discussed,and a mathematical model of gas migration is established.Finally,the accuracy of the model is verified by the 726 working face of the Qing Dong coal mine.The results can provide some theoretical guidance for the gas extraction of the coal body.The specific contents are as follows:Experiments were conducted to determine the solubility of gas under different conditions of temperature,pressure,and salt solution.The study shows that under low temperature conditions,the effect of temperature on the solubility of gas is weak.In the experimental temperature range,the gas solubility decreases slightly with the increase of temperature.Different from the influence of temperature,the solubility of gas in water is significantly affected by the experimental equilibrium pressure,and increases with the increase of equilibrium pressure.The diffusion coefficient of gas in pure water under different temperature and pressure was measured by pressure decay method.The experimental results show that the diffusion coefficient of gas in water is greatly affected by the initial pressure,which is in line with the exponential decay relationship,that is,it gradually increases with the increase of the initial pressure,but the increase range decreases.In addition,the equilibrium pressure of the gas is linearly related to the initial pressure,that is,the higher the pressure of the gas charged into the diffusion chamber,the higher the pressure during the diffusion equilibrium.The diffusion coefficient of gas in water is related to temperature,and it gradually increases with the increase of experimental temperature,which is a linear relationship.Experiments were carried out to determine the diffusion coefficient of gas in coal saturated with water under different temperature and pressure and its influencing factors.The results show that the porosity and curl of the water-saturated coal sample have a certain influence on the effective diffusion coefficient of gas.With the increase of the degree of curl of the water-saturated coal sample,the diffusion path of gas in pore water is more tortuous and its effective diffusion.The effect of experimental temperature and pressure on the effective diffusion coefficient of gas in water saturated coal samples is basically the same as before.In addition,through the fitting data,the influence weight of the experimental temperature and initial pressure on the effective diffusion coefficient of gas is analyzed,which conforms to the formula: ln? =-109.490 + 15.547ln? + 0.045ln?,which shows that the influence weight of the two is obviously different,and the influence of temperature on the diffusion coefficient of gas in water is obviously greater than that of the pressure.A model for gas migration in water-intrusion coal seams was constructed.The COMSOL numerical simulation software was used to predict the gas drainage volume and the gas pressure relief effect.By comparing the actual gas drainage results of Qingdong Mine,the rationality of the numerical model was verified.The results show that the gas pressure decreases with the increase of the diffusion coefficient and increases with the radius of the water saturation region.The influence of coal seam permeability on gas pressure is more complicated: the high permeability coal seam has better overall drainage effect,but in the low permeability coal seam,the drainage effect of the surrounding coal body is only good.