Pore-scale Multiphase Flow Simulation Based On Lattice Boltzmann Method

Core provides basic datas for the reservoir evaluation.The skeleton properties,pore structure and porous flow properties of rock can be acquired by core analysis.Digital rock is a powerful tool for understanding the process of pore-scale fluid flow and determining the physical properties of rock.Digital rock physics makes use of image processing technology to acquire the grain size distribution,porosity and pore size distribution of rock.Meanwhile,computational fluid dynamics method are adopted to solve the problem of pore-scale single phase and two phase fluid flow,and to calculate absolute permeability,capillary force and relative permeability.Digital rock physics can intuitively understand the pore-scale flow characteristic,compared with core analysis in the laboratory.The lattice Boltzmann equation is derived from the Boltzmann equation based on the theory of fluid dynamics and gas kinetics.And then the macroscopic equation is recovered from the lattice Boltzmann equation by Chapman-Enskog analysis.Image processing technology is applied to discretize the body-centered cubic and face-centered cubic,and to calculate the grain size distribution,porosity,pore size distribution and pore connectivity.The basic algorithms and program implementation of single-relaxation-time model and two-relaxation-time model are given.Meanwhile,the absolute permeability of body-centered cubic,face-centered cubic and berea sandstone sample are calculated with the optimized algorithm.The berea sandstone is selected as the research object whose relative permeability under different capillary number,viscosity ratio and wetting conditions are calculated by the color gradient model.When lattice Boltzmann method is used to simulate incompressible Navier-Stokes equation,Ma should be as small as possible.Berea sandstone is a fine sandstone with even and small grain size.The pore size distribution of berea sandstone sample is relatively uniform,and pore radius ranges mostly between 10 ?m and 35 ?m,about 74.40% of the pore volume.The average pore radius of berea sandstone subsample is 25.252 ?m.The permeability is related to the size of the digital rock.The larger the digital rock is,the closer the permeability is to the actual value.The relaxation time of the single-relaxation-time model obviously affects the permeability,and the relaxation time is related to the fluid kinematic viscosity,so the fluid motion viscosity will affect the permeability.The magic parameters of the two-relaxation-time model control the stability and accuracy of the model,and the fluid kinematic viscosity can be adjusted independently.The permeability of non-wetting phase and wetting phase increases with the increase of capillary number,but the permeability of non-wetting phase increases more obviously than that of wetting phase.When the viscosity ratio is relatively large,the relative permeability of the wetting phase fluid will decrease because there are more wetting phase fluids adsorbed on the rock surface.If the rock surface changes from strong wetting to intermediate wetting,the relative permeability of the non-wetting phase decreases obviously,while that of the wetting phase increases to a certain extent.