Study Of The Methods For Determining The Permeability Of Coal Seam Based On The Gas Pressure Build-up Curves

The permeability coefficient in coal seam is the measure of the difficulty degree of gas flow in coal seam, and it is the important technical parameters for studying gas drainage, forecasting gas outburst danger and developing the coal bed methane. Gas pressure buildup curve is determining by using gas pressure buildup curve automatic measurement technique. By analyzing the characteristics of gas pressure recovery process, the difference of fluid flow in coal reservoir and oil-gas reservoir is obtained. Based on these, through analyzing and summarizing gas flow law, the differential equation about gas flow is obtained from the basic equations of gas flow. The differential equation is solved by using Polubarinova-Kochina method, Postman transformantion and others methods. The computational formula of permeability coefficient in coal seam is obtained by analyzing mathematical model of gas pressure buildup curve using semilogarithm method. Finally, the accuracy and reliability of pressure buildup curve method and simplified method are verified by comparing with the radial flow method in the field application. The results showed that:(1)The gas pressure automatic measurement meter is more accurate to record gas pressure recovery process than the traditional pressure measuring method, and can monitor gas pressure recovery process continuously and dynamically, which has small time interval and isn't affected by human factors;(2)In initial phase, the growth speed of gas pressure is slow. And gas flow is unstable and has gas absorption phenomenon as well. In rising phase, the rate of gas radial flow and the speed of gas pressure buildup are steady. Gas folw is affected little by borehole diaturbance zone. So it is accurate and reliable using the gas pressure recovery process of this phase;(3)Coal reservoir and oil-gas reservoir have great difference in fluid composition, occurrence state, reservoir characteristic, fracture system, fluid flow characteristic, fluid flow equations aspect and so on. So the differential equation of fluid flow in oil-gas reservoir and pressure buildup curve cannot be used to describe the gas flow in coal seam;(4)The mathematical model of gas pressure buildup curve is established, and the basic solution is deduced. The calculation formula of permeability coefficient in coal seam is deduced based on the mathematical model of gas pressure buildup curve. Then, the steps of determining method of gas pressure buildup curve are introduced. For avoiding determining the gas flow of borehole, the simplified method of permeability coefficient in coal seam is deduced by using gas pressure buildup curve;(5)Though field application, the average absolute error of pressure buildup curve method is 30.66%, and simplified methods' is 37.64% at the condition of low permeability coefficient. At the seam time, the trend of determination results is same compared with the radial flow method, which proves the two methods is feasible in theory aspect. The error is acceptable for determining permeability coefficient in coal seam.