Study On Coalbed Methane Numerical Coupling Of Diffusion And Seepage In Inhomogeneous Medium

The aperture structure and pore size of coal samples from different depths havebeen studied based on scanning electron microscopy， mercury injection， lowtemperature liquid nitrogen adsorption and diffusivity test. And use the fitting ofexperimental data，concluded that the diffusion coefficient，and analyzes the factorsthat affect coalbed methane diffusion. A few ways of dispersal pattern have beendistinguished according to the relationship between gas molecular mean free path andthe the pore size. The study found that the micro hole to the disadvantage of gasdiffusion，so the pores content in coal had negatively correlated to the diffusioncoefficient；Transition pore is the main space of gas diffusion，transition qualitydirectly affect the gas’s diffusion ability， so the transition pore content and gasdiffusion coefficient were positively correlated；Large and middle size hole is seepagearea for gas，so the relationship between large and middle size poro content and thediffusion coefficient is unconspicuous. And in order to put the diffusion theory intoapplication assume that the leading mode of gas diffusion obey the Fick’law. Theevolution law of permeability under different temperature and pressure have beenstudied based on the permeability test of gas coal sample collected from Yangquanmine. In the condition of changing temperature，gas coal permeability and effectivestress has significant negative logarithmic relationship. In constant crustal stress，higher reservoir fluid pressure is conducive to the permeability of coal seam. Gas coalpermeability and effective stress has significant negative logarithmic relationship andthe correlation coefficient is above0.8，so the gas coal sample’s heterogeneity isrelatively weak. The dynamic evolution model of porosity have been import based onthe seepage experiment， the coupling between the seepage system and diffusionsystem have two aspects：1.adsorption-expansion of coal matrix；2.source termexchange of mass between seepage system and diffusion system. Refer to previousresearch results the mathematic model of coal bed methane migration ininhomogeneous medium under multi-physics field have been established，and theparameters obtained from experiment is used in the model’s numerical simulation.The model established in the paper have been imported into COMSOLMultiphysics to process numerical simulation，and there is a good match between thenumerical calculation result and the laboratory experiment， so the numericalcalculation result is reliable. The numerical calculation result reveals that increase the permeability of coalbed and decrease the pressure gradient is an effective way toguarantee the safe production and keep the coal-bed methane output stable.