Study On The Solid-fluid-thermal Coupling Mathematical Models And Permeability Characteristics Of Coal Containing Gas

The permeability of coal containing gas is a physical quantity that characterizes the difficulty of gas flow in coal seams and an important indicator for evaluating gas drainage efficiency.Its value is affected by the geophysical field?stress field,seepage field and temperature field?,and the geophysical field changes with the change of coal seam occurrence conditions.So the permeability of coal containing gas also changes dynamically.Therefore,the study on permeability of coal containing gas under different combinations and the migration of gas under solid-fluid-thermal coupling is always the focus of research in the field of coalbed methane mining and gas prevention.Based on the engineering background of the No.8 coal seam of Xishan Coal and Electricity Malan Mine,the theoretical analysis,physical test and numerical simulation method are used to study on the permeability and solid-fluid-thermal coupling of gas in coal seam under different conditions.It has certain theoretical significance and engineering application value for the prevention and control of gas accidents and the injection of coalbed methane.The specific research results are as follows:?1?Considering the compressibility of gas,the expansion stress generated by coal adsorption gas,the slippage effect of coal,the change of gas dynamic viscosity coefficient with temperature,the true gas state equation of gas,establish the solid-fliud-thermal coupling mathematical models of coal containing gas.It can describe the migration law of gas in coal and is important for the prevention and control of gas accidents.?2?Under the action of stress-thermal coupling,the permeability of coal containing gas is generally"L"type.The gradient of permeability versus temperature is slightly more significant than the gradient of permeability versus effective stress at the"upper wing"of the relational surface?the effective stress is less than 0.7 MPa?,and the permeability changes from low to high as the temperature increases.The gradient of the relative effective stress of the permeability is significantly larger than the gradient of the permeability relative to the temperature at the"lower wing"of the relational surface?the effective stress is 2.7-8.7 MPa?,and the permeability changes from high to low as the temperature increases.?3?Under the action of hydrothermal coupling,the permeability of coal containing gas shows a nonlinear decreasing law with the increase of temperature and water content.The gradient of permeability versus temperature is slightly more pronounced than the gradient of permeability versus moisture.The gradient of permeability relative to water content has a slowing trend with the increase of temperature,and the gradient of permeability relative to temperature also slows down with the increase of water content.?4?Under the action of gas pressure and volume stress coupling,the permeability of coal containing gas is generally"V"type.The permeability decreases with the increase of volume stress and tends to be stable.The surface relationship between the two wings is high and the middle is low with the increase of gas pressure.At the"depression"of the relational surface,the gradient of permeability versus gas pressure is roughly equivalent to the gradient of permeability versus volumetric stress.At the"two wings"of the relational surface,the gradient of permeability versus gas pressure is significantly greater than the gradient of permeability versus volumetric stress.?5?The permeability loss rate and the effective stress are in accordance with the Boltzmann function variation law.In the process of temperature change,the rate of permeability loss is related to the critical value of effective stress.When the effective stress is lower than the critical value,the permeability loss rate changes greatly.After reaching the critical value,the permeability loss rate decreases and tends to be stable.When the effective stress remains constant,the value of the temperature sensitivity coefficient gradually decreases as the temperature increases.The permeability of the coal sample is gradually weakened to temperature.The initial value is the maximum value and small.The magnitude is 10^-2.Therefore,the permeability of gas containing coal is less affected by temperature than the effective stress at lower temperatures?below 70 degrees Celsius?.?6?The effects of temperature increase on coal seam and gas physico-mechanical parameters and on gas adsorption and desorption characteristics are fully considered when using COMSOL Multiphysics software for numerical simulation research.The effect of temperature on permeability was studied by numerical simulation.It was found that the permeability increased with the increase of temperature when the effective stress was constant at 0.7 MPa.The permeability increased with temperature slowing shrieking when the effective stress was constant at2.7-8.7 MPa.The numerical simulation results are in good agreement with thephysical testresults,and thecorrectness ofthe solid-fluid-thermal coupling mathematical models of coal containing gas is established.At the same time,the effect of gas pressure on permeability is studied.It is found that there is obvious slippage effect between gas pressure and permeability which is parabolic-type change law.The inflection point of slippage effect is between 1.4-1.5 MPa.