In-situ testing method of the permeability coefficient in a coal seam based on the finite volume method and its application

The permeability coefficient in a coal seam is an important index to evaluate the efficiency of gas drainage. Accurately testing the permeability coefficient is of great significance for gas disaster prevention and gas resource utilization. The radial flow (RF) method, which is the main method for determining the permeability coefficient in Chinese coal mines, simplifies the gas flow control equation by transforming it from a nonlinear partial differential (NPD) equation to a linear one. Such a transformation changes the quantitative relationship between gas content and gas pressure, and ultimately results in the relatively small permeability coefficient calculated by the RF method. In view of the problem, based on the finite volume method and the golden section search method, a novel finite volume flow (FVF) method was proposed in this study to determine the permeability coefficient. The FVF method solves the reverse solution problem facing the NPD equation. In order to verify the FVF method, a field experiment was performed in Jiulong Coal Mine. The results show that the FVF and RF methods are both of good stability, that is, the permeability coefficients calculated by the two methods both approach stable values. However, the FVF method is superior to the RF method in predictability, operability and expansibility. Taking the data of #1 borehole on the 20th day as an example, the prediction error rate for the RF method is 1.56 times that for the FVF method. Therefore, the FVF method is more suitable for determining the permeability coefficient than the RF method. In addition, the sensitivities of seven test parameters were calculated to evaluate their effects on the permeability coefficient. Based on the findings, the equipment for permeability coefficient testing was improved. The research results lay the foundation for further promoting the accuracy of permeability coefficient measurement.