Effects of geomechanical properties of interburden on the damage-based permeability variation in the underlying coal seam

Abstract

Interaction between multi-layer coal seams is of significant guidance for designing gas drainage boreholes and maximizing the drainage performance. Although extensive investigations on the multi-seam interaction have been conducted from perspectives of rock deformation and variations in mechanics and permeability, sensitivity analysis on the seam interaction is done at a less extent. In this study, a 2D simulation model is established based on data of a coal mine. Weibull statistical distribution is adopted to assign the geomechanical property in the model to reveal rock heterogeneity. Meanwhile, a mathematical model that incorporates coal permeability with the coal damage, mechanics change and gas desorption is developed. This permeability model is implemented into the numerical simulation to investigate the effects of geomechanical properties of interburden on the permeability change in the de-stressed underlying coal seam, as well as the gas drainage performance. Results show that: (1) when assigning the geomechanical property using Weibull statistical distribution, the property homogeneity is positively correlated with Weibull shape parameter, (2) overall, the permeability increases significantly in the de-stressed underlying coal seam, and with the increase in the distance to interburden, the increase ratio decreases rapidly before a relatively slight decline, and (3) the average permeability ratio decreases initially but then rises with the increasing Young's modulus. In contrast, the average permeability ratio and the Poisson's ratio show a positive linear relationship. The gas drainage performance in the underlying coal seam is corresponding to the permeability variation of each geomechanical parameter group and could be verified with each other. In summary, when the interburden possesses smaller Young's modulus and greater Poisson's ratio, the interaction between adjacent seams becomes more intense and the permeability in the underlying coal seam tends to increase more. Therefore, when designing the mining sequence for the coal-seam group with high interaction potential, mining the neighbouring coal seams of the targeted seam first could be an effective method to increase its permeability and gas drainage efficiency.