Evolution of mechanical behavior and its influence on coal permeability during dual unloading

In order to reveal the response of mechanical behaviors and its impact on gas flow in reservoir during insitu coal mine gas recovery,the changes of mechanical parameters and permeability during dual unloading (unloading confining stress during reducing gas pressure) was studied based on the experiments and theoretical analyses.First,the changes of coal permeability under different unloading paths were studied.Then,the evolutions of ultrasonic wave velocity and the mechanical parameters were monitored synchronously and analyzed.Finally,the coal stability and its failure mechanism during dual unloading were discussed.The main conclusions are shown as follows:① when the stress reduction gradient dσ h/dp<1,the coal permeability decreases with a reduction of the gas pressure.Due to the Klinkenberg effect,the permeability increases slightly with the decrease of gas pressure when dσ h/dp=1.When dσ h/dp>1,the permeability increases slowly at the initial stage because of the opening of the primary fractures,and it rises rapidly at the later stage due to the formation of new fractures.② There is a significant corresponding relationship among the ultrasonic wave velocity,mechanical parameters and permeability of coal during dual unloading.The change of the ultrasonic wave velocity reflects the alteration of the fracture structure in coal.The decrease of ultrasonic wave velocity indicates the opening of primary fracture or the formation of new fracture in coal,and correspondingly,the dynamic elastic modulus and shear modulus of coal decrease,the Poisson’s ratio increases and the permeability of coal increases.③ BP(=3\\*GB3BP) The model of horizontal stress developed based on poroelasticity under uniaxial strain condition deviates from the experimental results,especially at the later stage of unloading.The main reason is that the theoretical model ignores the reduction of elastic modulus caused by coal damage.④ BP(=4\\*GB3BP) During dual unloading,the coal under high dσ h/dp has a higher failure probability than that under low dσ h/dp.This is because that the deviation between the vertical and horizontal effective stresses of coal under high dσ h/dp increases rapidly,and the coal strength weakens rapidly,which makes the coal more prone to failure.