Characterization Of Self-affine Cleat Networks And LBM Simulation Of Flow-diffusion Process In Coals

As the main channel of fluid transport,cleat network in coal reservoir affects transport and output of coalbed methane deeply.Therefore,the quantitative assessment of the transport properties of cleat networks is of great significance to predict reservoir productivity and exploit coalbed methane.A large amount of evidence indicates that natural fractures in coal reservoirs have fractal features.Using fractal theory to characterize complex geometries is an effective research method.Based on the above understanding,this paper focuses on the representation of the cleat model and the pore-scale simulation of the mass transfer process.The following four aspects are mainly studied in the characterization of cleat networks and the flow-diffusion process:(1)Quantitative characterization of single cleat self-affine geometry and development of new Weierstrass-Mandelbrot functionThe Weierstrass-Mandelbrot(W-M)random fractal function is often used to characterize the natural fracture surface geometry.But according to the new fractal topography theory,the relationship between H(Hurst exponent)andD_f(fractal dimension)is notD_f?d-H(d Victoria Euclid dimension)but_fD=d(1+H).Thus the W-M function and its structure function was fixed.(2)Construction of coal reservoir cleavage networkCombining the geometric self-affine properties of the natural fracture face and the configuration and distribution of the cleat space in coal reservoir,the characterization of self-affine networks was achieved by using the new W-M function and a method to construct the cleat network model was developed,which can realize the quantitative characterization of self-affine rough cleat networks.(3)LBM simulation and permeability analysis of fluid flow in cleat network with pore scaleThe Lattice Boltzmann Method(LBM)was used to simulate the fluid transport process at the pore scale and calculate its permeability.The comparative analysis showed that the reservoir permeability satisfies the triple-effect fracture-permeability model without butt cleat.The introduction of butt cleat will enhance the permeability of the reservoir.Hurst exponent,the aperture of face cleat and butt cleat can all cause the change of seepage characteristics of the whole cleat network.More details,the permeability will increase when the Hurst exponent and the aperture of butt cleats increase,vice versa,while the butt cleat had the characteristics of high influence weight in small aperture and small influence weight in large aperture.And there was a power-law relationship between the ratio of the face cleat aperture and the weight value.At the same time,after ascertaining the differential contributions between them,an empirical permeability model under the influence of coal cleat network were established.(4)Simulation of advection-diffusion process and analysis of transport properties in cleat networkBased on the Taylor-Airs equations,the prediction model of the dispersion coefficient in the advection-diffusion process in tortuous fractures was analyzed and established.The new LBM program was used to simulate and verify the effectiveness of the new dispersion equation.Then simulated the advection-diffusion process in the cleat network characterization model,and analyzed the influence of the aperture of butt cleat and face cleat,the tortuosity on the advection-diffusion behavior in the cleat network.It was found that the advection-diffusion behavior in the cleat network was mainly dominated by the face cleat which direction was same as macroscopic flow.This paper mainly studied the seepage characteristics and the advection-diffusion behavior of cleat networks so as to provide a theoretical basis for the future study on the migration of coal-bed methane or mass transport of complex medium such as cleat networks.