Study On Accurate Calculation Method Of Gas Loss By Positive Pressure Reverse Circulation Sampling

Coal Coal seam gas content is an indispensable and important parameter for coal seam mining and gas prevention.Positive pressure reverse circulation sampling is the latest sampling method for direct determination of coal seam gas content,which can ensure the purity of sampling,take coal samples with large depth in a short time,and improve the sampling efficiency.However,in the sampling process,the coal sample is desorbed in the positive pressure desorption environment,but the calculation of gas loss is based on the gas desorption data of the coal sample under atmospheric pressure.The desorption environments of the two are different,so the reliability of loss calculation is not clear.Therefore,this paper makes experimental research and theoretical analysis on the gas desorption law in the process of positive pressure reverse circulation sampling and gas content determination.From the difference of gas desorption characteristics and the calculation error of gas loss under positive pressure or atmospheric pressure desorption environment,the accuracy of the calculation result of current gas loss is revealed,According to the gas desorption law of coal samples in the positive pressure desorption environment,a more accurate calculation technology and method of gas loss in positive pressure reverse circulation sampling are explored.The main conclusions of this paper are as follows:(1)The gas desorption characteristics of coal samples under positive pressure are different from those under normal pressure.The gas desorption speed under normal pressure is greater than that under positive pressure;With the increase of sampling desorption environmental pressure,the positive pressure desorption rate of coal sample decreases gradually,and the desorption rate of atmospheric gas increases gradually.(2)The laboratory simulation test shows that for the calculation of gas loss in positive pressure reverse circulation sampling,the deviation between the calculated value of √t method and the real value is the largest,up to more than 300%;The gas loss calculated by power function method is the closest to the real value,but the average error rate is also more than 58%.It can be seen that at present,the error of gas loss in the process of back extrapolating positive pressure and reverse circulation sampling with atmospheric gas desorption data is large.(3)The existing empirical formula of gas desorption can better characterize the gas desorption law of coal samples under constant positive pressure environment.Among them,the fitting effect of uskinov formula is the best and the stability is the best.Based on this,a gas desorption measuring device suitable for positive pressure reverse circulation sampling is developed.With the help of this device,the gas desorption of coal samples can be carried out under the conditions similar to the internal environmental pressure of drill pipe in the sampling process,so as to ensure that the measured gas desorption law of coal samples is the same as that of coal samples in the sampling process,and the gas desorption curve is continuous,so as to realize the accurate calculation of gas loss of positive pressure reverse circulation sampling coal samples.(4)For the above gas desorption measuring device,the power function method is recommended to calculate the gas loss.Laboratory comparative experiments show that compared with the traditional positive pressure reverse circulation sampling method,the gas loss calculated by this device is more accurate,and the error between the calculated value and the measured value of gas loss is less than 10%.