Study On The Method Of Solving Coal Seam Permeability Based On Borehole Gas Pressure Build-up Theory

The permeability of coal bed is an important indicator to measure the difficulty of gas flow in coal seams and benefits the right evaluation of gas drainage capability,which could be used to select the suitable technique to increase the permeability and drainage effect.Based on the theory of borehole gas pressure recovery,this paper starts from the basic equation of gas flow,considering the similarity and difference between the pressure build-up of coal gas and oil and gas storage the pressure recovery test,and the gas flow model has been set up to which will be appropriate to the underground of coal mine.The equation is properly simplified,and the analytical solution is obtained by means of mathematical methods such as Bozeman transform.On this basis of this,a new method for calculating the permeability of coal bed is derived,that is linear segment analysis and pressure derivative analysis.In this paper,The starting point of radial flow in the pressure build-up curve could be designated rationally using the double and half logarithmic pressure build-up characteristic curve with the analytic method of half logarithmic straight line,then the least squares method is used to calculate the slope of the midline segment accurately and the correlation formula of the straight line segment analysis method is used to calculate the Permeability coefficient of coal seam.By calculating the arithmetic mean of the pressure derivative term on the horizontal line of the double logarithmic pressure derivative characteristic curve,the permeability coefficient of the coal seam is also calculated by the correlation formula of the pressure derivative method.So the permeability of coal bed by the borehole gas pressure build-up curve is practical.After the verification and comparison with the result of in-situ examples by the radial flow method,the relative error of the five examples used the straight line segment analysis method is between 12.5 % and 30.7% and the average error is 18.8%.and the relative error using the pressure derivative method is between 5%-55.1%,and the average error is 31.9%,such an error is acceptable for engineering calculations,It is proved that the method of solving the permeability of coal bed is feasible based on the theory of borehole gas pressure recovery.