Study on the permeability model and in-situ testing of coal seams using repeatedinjection pressure drop method

Permeability of coal seams is an important indicator for assessing the difficulty of gas seepage and extraction, and its accurate determination holds significant significance. In light of the existing issues with long testing cycles, unstable results, and imperfect models, this research focuses on developing a rapid and accurate method for determining coal seam permeability and the corresponding computational model. Based on the radial unsteady-state gas flow control equation within coal seams and the volume flow equation for gas under different pressure differentials within the coal seam, we have established an injection pressure drop calculation model that utilizes the entire pressure drop dataset for measuring coal seam permeability. The COMSOL numerical simulation software, equipped with the Darcy flow module, is employed to solve the model. To address variables affecting the pressure drop curve in field engineering design, a univariate analysis of the length of the pressure measuring gas chamber is conducted. According to simulation results, the gas chamber length for pressure measurement in boreholes can be designed as 2.0 meters. Field arrangements are made based on the numerical simulation results, and an underground repeated gas injection pressure drop test system is established. This system, in conjunction with coal seam gas occurrence conditions and tunnel conditions, facilitates the drilling of two cross-seam boreholes for each of the two test points. These boreholes are injected with compensatory gas twice, each time exceeding the coal seam gas pressure, to conduct in-situ permeability tests. The testing periods are approximately 6 days and 17 days for the first and second rounds, respectively, with the second round having higher injection pressures. Combined with theoretical derivation, it was verified that the Reynolds numbers of coal seam gas remained within the linear Darcy flow regime during the gas injection pressure drop process, and gas flow within the coal seam adheres to Darcy's law, thus satisfying the assumptions of the calculation model. When compared to traditional methods for calculating coal seam permeability, the results indicate that this method aligns closely with the radial flow rate method, effectively meeting the requirements of practical engineering applications. The testing results of the repeated gas injection pressure drop method are stable and reliable, offering the advantage of rapid determination.