Experimental Study On Permeability Changes And Adsorption-induced Deformation In Coal

In the process of coal mining and gas extraction, there exists a close interaction between coal and gas. This interaction is complicated by the coupled relationships between gas desorption, gas flow , deformation and infiltration. Therefore, it seems to be of great importance to investigate the gas flow in the process of coal deformation and the coal infiltration by considering time and space factors. Meanwhile, due to people's limited knowledge of the occurrence mechanism of coal and gas outburst, the study of coal of adsorption and desorption law in coupled effect will be significant for the prevention of disasters and guidance of gas extraction.This paper is an exploration of the coal infiltration law and desorption-adsorption law. Based on rock mechanics, percolation mechanics, experimental mechanics and mining engineering theory, an experiment is carried out to investigate the coal infiltration law by taking into account the possible affecting factors such as absorption, stress loading paths, time effect and size effect. Another experiment focuses on infiltration law considering the coal rock under the process all stress and strain and cyclic loading. The third experiment probes into the characteristics of coal in absorbing and desorbing different gas and the law of different types of coal in methane adsorption and desorption. The major findings are as follows:(1) Maximum deformation varies when coal adsorb different gas,ε_CO2 >ε(CH₄) >ε_N2>ε(Ar). The deformation after adsorbing carbon dioxide is twice that after adsorbing methane, 4 times that of adsorption of nitrogen, but coal does not adsorb helium.(2) The capacity of adsorption deformation of different types of coals in methane is different, and the time needed for different types of coal to reach maximum deformation varies. The outburst coal adsorption can reach its maximum deformation instantly while the non-outburst coal needs a few hours or dozens of hours to reach maximum deformation.(3) Considering the adsorption effect of coal and gas, coal permeability reduces; the permeability of coal is related to stress direction, and different applied load direction has conspicuous effect on permeability.(4) The permeability of complete stress and strain process of coal takes on two forms. one form of infiltration is characterized by decreased- increased- decreased process; Another one is that with the increase of stress, the permeability diminishes until the yielding permeability suddenly rises. The permeability of coal increases regularly in the process of cyclic loading whereas the permeability of cyclic loading before and after yield manifests different change patterns. The permeability change law of axial and radial cyclic loading presents different patterns. The permeability anticlockwise goes up during the process axial cyclic loading while clockwise declines during the process radial cyclic loading. The permeability-strain relationship of the three stages during the coal deformation and damage is established by the permeability testing experiment of complete stress and strain process.(5) The size effect of coal infiltration is obvious when considering the time, that is, permeability takes the form of negative exponent attenuation; permeability change law of two space scales, which are laboratory scale and engineering scale, is studied. As to small scale coal sample, permeability of coal increases along with coal infiltration area and come to stabilize after reaching a certain value. Coal of fracture, especially effective connected fracture has significant impact on gas permeability. Multiscale computing method of permeability is put forward. The permeability of coal reservoir is computed based on the discharge and mining data of coalbed well, and it's three orders of magnitude higher than the test result in laboratory.