Quantitative Assessment of Coal Seam Permeability Improvement Around an Enlarged Borehole in Gas Extraction Engineering: A Method for Field Application

Understanding the permeability distribution is an important basis for the design of hydraulic strengthened gas extraction. However, the existing methods can not directly determine the nonuniform coal seam permeability after hydraulic flushing. The numerical simulation is commonly used to analyze permeability distribution, but the geostress and coal mechanical parameters are necessary for the previous simulation method, which are difficult to accurately obtain. To grasp the coal seam permeability improvement in hydraulic flushing engineering, the equivalent permeability and radius (EPR) method was proposed. In the EPR method, the equivalent permeability and equivalent permeability-enhancement radius, which are determined by field measurement and by simulation of gas migration models respectively, are used to simplify and equivalently express the coal seam permeability distribution after hydraulic flushing. The coal stress and mechanical parameters are not needed in the EPR method. The permeability and gas migration models of coal seam after the hydraulic flushing engineering were established and solved in the finite element software to verify the reliability of the EPR method. The gas flow rate, gas extraction amount, and gas pressure were compared between equivalent permeability and nonuniform permeability coal seams. Results show that the gas drainage effects of coal seams under the two permeability expressions are consistent, which demonstrates that the EPR method is accurate enough to value the permeability-improvement effect. This paper aims to propose a new method for field application to quantitatively evaluate the coal seam permeability improvement around an enlarged borehole, which is of great importance for hydraulic flushing enhanced-gas extraction engineering.