Study On The Law Of Gas Desorption In Coal Under Different Positive Pressure

Coal seam gas content is the basic data to evaluate and predict gas reserves and gas emission,and it is the main index of regional prediction.The accurate determination of coal seam gas content is closely related to the safety production of coal mine.The key technology for accurate determination of coal seam gas content is fast sampling at fixed point and calculation of gas loss in the process of sampling.The compressed air sampling can realize fast sampling at fixed point in long borehole,which is characterized by the rapid discharge of coal sample from the bit through the drill pipe under the action of compressed air.However,when calculating the gas loss in the sampling process,the method still adopts the gas desorption law of granular coal under atmospheric pressure,and its rationality is questionable.In order to obtain the gas desorption law of coal in the process of pressure air pressure sampling and accurately calculate the gas loss in the process of pressure air exhaust powder sampling,the evolution law of pressure air pressure along the length of borehole under the conditions of different initial air pressure and different borehole length is studied by numerical simulation method.It is obtained that the pressure loss at the bit is significantly greater than that at the middle and rear of the drill pipe,and that in the drill pipe The back pressure loss is relatively fixed,that is,the wind pressure shows a linear attenuation trend with the length of the borehole,and the relationship between the medium pressure wind pressure and the length of the borehole is established.On this basis,the adsorption equilibrium pressure is 0.5MPa,1.0MPa,1.5MPa and 2.0MPa by using "simulation test system of gas emission characteristics of coal core in different environment coring process" The results show that under the same adsorption equilibrium pressure,the larger the positive pressure is,the smaller the gas desorption capacity and desorption speed are in the same period of time,and the positive pressure environment has a significant inhibition effect on the gas desorption of coal Compared with suction,positive pressure restrains 30%-62%of gas desorption;the desorption data under normal pressure is regressed by using the empirical formula of desorption under normal pressure,and it is found that Wang Youan's,BOT's,Venter's and usjinov's formulas can better describe the gas desorption process under positive pressure.Using the data of gas desorption under constant positive pressure in the laboratory,the gas loss in the process of positive pressure sampling is calculated.Compared with the gas loss calculated by the method under normal pressure,it is found that the loss based on the experimental data is 51%-128%smaller than that of the method,and with the increase of sampling wind pressure,the increase of drilling hole and the increase of adsorption pressure,the loss based on the experimental data is relatively small The larger the method is,the error of the calculation of the gas loss of the compressed air sampling may lead to the misjudgment of the underground gas content measurement,which may cause the hidden danger of the safety in the coal mine production.