Study On Mechanism Of Controlling Compound Dynamic Disaster By Displacement-Water Injection In Gas Coal Seam

The compound dynamic disasters of rockbursts and coal and gas outbursts are an atypical new type of dynamic disaster with complex causes,and the effect on mine safety increases with the increase of mining depth.At present,the research on compound dynamic disasters has made some progress,but the systematic research on compound dynamic disasters has not been seen,especially the research with targeted prevention and control methods.Based on this,a systematic study on the classification,mechanism,occurrence conditions,and energy release of compound dynamic disasters is conducted,the prevention and control ideas of compound dynamic disasters are analyzed,and the method of preventing and controlling compound dynamic disasters by displacement-water injection is proposed,and experimental and theoretical studies on the method are conducted.The main contents of the research are as follows:(1)Based on the main body of energy release,the types of rockbursts and compound dynamic disasters are divided,and then the mechanical analysis model of various types of rockbursts and compound dynamic disasters is established to analyze occurrence mechanism,occurrence conditions,energy release,and prevention ideas.To prevent and control compound dynamic disasters,the method of displacement-water injection is proposed to reduce gas energy and increase the critical stress of rockburst,and a displacement-water injection test device is designed.(2)In order to analyze the problems existing in the traditional coal seam water injection and the influence of water on the methane desorption pattern,an experiment on the influence of coal seam water injection on the methane desorption pattern was conducted.The results show that the desorption rate of the coal sample with water injection decreases in a fluctuating manner.And the desorption amount varies gently with only a slow growth stage and a stop stage,but the promotion of water injection on gas desorption is greater than the inhibition of water,which makes the water injection discharge ratio of coal samples with water injection greater than the natural desorption ratio of coal samples without water injection;a small amount of gas is displaced in coal samples under short-time water injection,and the methane content of coal samples does not change much.The displacement gas generated by water injection will enter the less wet area,which increases the risk of compound dynamic disasters.(3)The experiment of coal seam water injection to displace methane,carbon dioxide,and nitrogen was carried out to analyze the pattern of coal seam water injection to displace different adsorbed gases through the experimental device of displacement-water injection and to discuss the factors affecting the pattern of displacement.The results show that the amount of gas displacement increases as the adsorption equilibrium pressure and displacement pressure rise.The growth slope of the gas displacement amount in the water-gas mixed discharge stage is carbon dioxide,methane,and nitrogen from large to small,which is positively related to the solubility of gas in displacement liquid;the time displacement rate is positively related to the water flow rate and the adsorption equilibrium pressure;and the water displacement rate is positively related to the adsorption equilibrium pressure.The time displacement rate and water displacement rate of different adsorbed gases in the stable stage are carbon dioxide,methane,and nitrogen,which turn from large to small and have a linear relationship with the gas solubility in the displacement liquid.When the displacement water flow rate is similar,the displacement time increases with the increase of gas solubility in the displacement fluid.The water injection discharge ratio of methane and carbon dioxide by water injection is close to or greater than the natural desorption ratio of coal samples under the same conditions,indicating that water injection can promote the emission of methane and carbon dioxide by water.The comparative analysis of displacement patterns of different adsorbed gases shows that there is a consistent relationship between gas solubility and the change of variables in displacement patterns.The correlation between gas sorption and displacement pattern is weak and is not a major factor in the different displacement patterns of different gases.(4)In order to determine the gas solubility as an important factor to control the displacement pattern,the experiment of methane displacement by ethanol solution injection was carried out and compared with the methane displacement by water injection in coal seams.The results show that with the increase of gas solubility in the displacing fluid,the displacement amount of methane in the experiment of displacing methane with ethanol solution injection increases faster,the displacement time is shorter,and the displacement ratio is high.It is proven that the gas solubility in the displacement fluid is an important factor determining the displacement pattern and that using a high-solubility displacement fluid can improve displacement efficiency.(5)According to the migration patterns of water and gas in porous mediums and the migration patterns of dissolved gas in water,the mechanism of displacing adsorbed gas by coal seam liquid injection is analyzed,and four types of displacement modes are obtained:driving-displacement,water-replacement displacement,gas-dissolution displacement,and gas-diffusion-dissolution displacement.Under the effect of the four displacement modes,displacement-water injection can discharge most of the adsorbed gas from the coal body,which in turn reduces the gas energy released when the coal body is destroyed,and combines with the weakening effect of water to achieve the prevention and control of power disasters.(6)The problems of traditional coal seam water injection are analyzed,and the integrated method of "pumping-injection" is proposed for the prevention and control of compound dynamic disasters.Field tests are conducted,which show that displacement-water injection can be used to prevent and control compound dynamic disasters.