| Coal seam gas is the main hazard threatening safety production in the coal mine.Meanwhile, methane is also a kind of clean and high calorific energy. Single gas flow becomes complex water-gas two-phase flow because of the existence of water. The research of water-gas two-phase flow is of great significance for gas extraction and CBM exploration. Based on the pore-fracture dual properties of coal, this paper analyzed the structure characteristics of coal, existing state of gas and water and gas diffusion mechanism, which laid foundation for further research of water-gas two-phase flow theory.The coal samples in the experiment are taken from Wei-Ding coal mine, Jincheng Anthracite Mining Group. Based on the self-developed experiment system, the isothermal adsorption experiment and water-gas two-phase flow test were carried out.Main conclusions are as follows:(1) Gas adsorption amount of saturated water coal decreases obviously, so Langmuir volume reduces and Langmuir pressure increases.With the increase of equilibrium pressure, damage of the gas adsorption amount tends to decrease and stability eventually.(2) According to water-gas relative permeability curves, water relative permeability is greater than gas at the right side of isotonic point and water flow occupies the leading role. Gas relative permeability is greater than water at the left side of isotonic point and gas flow occupies the leading role. Moreover,the change shows nonlinear relationship.(3) In the state of residual water, when the gas relative permeability stabilizes, gas relative permeability is only between0.14~0.34, and gas permeability damage causing by water is serious.(4) The residual water saturation is between 43.25%~77.91%, the water-gas two-phase flow range is relatively narrow, which shows the characteristics of low porosity and permeability,strong hydrophilicity.In the process of water-gas two-phase flow, the change of pore pressure and saturation can lead to the dynamic change of porosity and relative permeability. On the basis of Warren-Root model, this paper put forward the dynamic model of porosity and relative permeability evolution model. The flow of liquid water and gas in the coalreservoir are subject to the general Darcy’s law. Adsorption gas obeys to the Langmuir equilibrium equation, and free gas to the gas state equation. Substituting these equations into continuous equation, water and gas flowing control equation are deduced. According to the elastic theory of porous media, based on the principle of Terzaghi effective stress, combining equilibrium equation, geometric equation and constitutive equations, control equation of deformation field considering adsorption effect is set up. Complete two-phase fluid-solid coupling mathematical model consists of above equations in the seepage field and deformation field.The mathematical model is solved by the finite element analysis software COMSOL Multiphysics. The change law of gas pressure, water saturation, porosity and relative permeability at different time are obtained. Research results show:(1) Raw coals need more time to reach equilibrium state than briquette coals.(2) Pore pressure changes obviously in the early time, and then tends to decrease until equilibrium state.(3) Through analyzing gas relative permeability at the equilibrium state, simulation results keep good consistency with experimental data in gas relative permeability at residual water state. |
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