Study Of Coal-bed Methane Seepage Characteristic In Micro-cleat And Its Network Topological Space

Extensive macro-fractures,micro-fractures and pores exist in coal reservoir,which dominate the migration of coal-bed methane.Especially,micro-fractures,connecting both the macro-fractures and pores,are the main seepage field of coal-bed methane in coal reservoir.Coal-bed methane,reserved in confined state under the original stress,keeps a dynamic balance,which can be regarded as stationary.The previous equilibrium state will be changed by human activities such as mining,removal of water and reducing hydrostatic pressure on coal-bed reservoir,which will trigger the corresponding physicochemical process of water and coal-bed methane in coal reservoirs.The migration of coal-bed methane in reservoir can be divided into three interrelated processes:(1)gas desorb from internal surface of coal matrix pore with the underpressure of coal reservoir;(2)under the concentration difference and pressure drop,gas diffuse to micro-fracture through matrix and pore;(3)gas in the fracture and cleat network migrate in a Darcy flow under the pressure difference.Proceeding from fractal theory and spatial topological relations,self-affine,rough fractures and cleat networks are generated by Weierstrass-Mandelbrot(W-M)randomized function.Based on the digital models and Lattice Boltzmann Method,fluid flow in fractures and networks were simulated with the lattice model of D2Q9,what's more,the simulation results are verified by Finite Element Method.Quantitatively measure hydraulic tortuosity ? in arbitrary porous media is a crucial work to the estimation of permeability.However,it is hard to experimentally or analytically derive the hydraulic tortuosity ? of porous media flow because of their complex microstructures and the ambiguous definition of ?.In this work,we propose a kinematical measurement method for ? by introducing the concept of local tortuosity.Based on the simulated fluid fields in inclined,U-shape and Cos-shape channel,the measurement of ? is verified,furthermore,the hydraulic tortuosity measure method of arbitrary porous media is tested by the porous media randomly filled with identical disks without overlap.Surface roughness effects were divided into surface tortuosity,hydraulic tortuosity and local stationary surface roughness factor by their different actions to fluid flow.We propose a triple-effect permeability model by comprehensively considering the physical meaning of each parameter and their mechanism to flow.We obtain the surface tortuosity and residual rough surface by iterative approach.Combing the geometry of residual surface and the simulation results;the local roughness factors are derived.By comparing the simulation results of LBM and the analytical results,the triple-effect permeability model is verified.Finally,by considering the scaling equation and the size effect of self-affine objects,we reexamine the existence of three flow regimes in fracture and establish different permeability models for them.Since there are great differences between the properties of face and butt cleat in coal reservoir,such as connectivity,development degree,surface roughness and so on,the permeability of coal reservoir exhibits obvious anisotropy in different direction.Generally,face cleat has smoother surfaces,better connectivity and development degree than butt cleat,thus,permeability reaches summit when the maximum pressure parallel to face cleat.Then,proceeding form the geometrical morphology of cleat and its statistical behavior,we generate cleat network using W-M randomized function by considering the topological relations between butt and face cleat.Simulate the fluid flow in different pressure directions to study the permeability anisotropy of coal reservoir,thus,provide reference to the estimation of coal reservoir productivity and the work of pattern arrangement.