| As a kind of efficient and clean energy,coal bed gas has become an important part of energy in China.Unlike conventional natural gas reservoirs,coal reservoirs as a typical double porosity medium owns a low porosity,low permeability and a strong heterogeneity.The pore structure of coal is complex,pore size is multi-scale,and there are many kinds of flow channels,such as nano matrix pores / micro cracks,micron cleats and exogenic fractures.In this paper,using coal samples from eastern Yunnan and western Guizhou,China,parameter tests,such as the pore type,pore radius distribution and stress sensitivity are carried out.Based on the experiments,the digital rock modeling method of the matrix pore and the fracture network are developed.Lattice Boltzmann method is used to simulate the gas flow characteristics in coal matrix and fracture,and the matrix apparent permeability and fracture network stress-depended permeability are discussed respectively.Finally,based on the grey lattice Boltzmann method,the interlayer interference in the multi seams in process of CBM straight well is studied.What follows are the main achievements obtained from this research.(1)An improved kinetic equation considering the volume diffusion effect for nonideal gases is presented based on Klimontovich's kinetic equation and Enskog equation-based lattice Boltzmann Bhatnagar-Gross-Krook(LBGK)model.Then,with the modified effective viscosity and the second-order slip boundary condition,a series of numerical simulations of gas flows with different mean Knudsen numbers were carried out based on the proposed model.Compared with other solutions,we find that the present model can be valid up to a wider Knudsen number.Based on the pore size distribution data obtained from liquid nitrogen adsorption experiments,QSGS method is used to reconstruct the coal matrix pore,using the micro scale lattice Boltzmann model simulations are carried out.Then the apparent permeability coefficient is analysised under different average Knudsen number.(2)Considering the fracture aperture and roughness,we present a 3D geometry model to reconstruct coal fracture networks on the basis of the Voronoi tessellations.The lattice Boltzmann method(LBM)is applied to simulate fracture flows and to predict the associated permeability.Based on the stress-strain constitutive equation,the 3D fracture network model of coal under stress conditions is established.Lattice Boltzmann method is applied to simulate the 3D fracture network model of coal and rock under the condition of stress.The influence rule of fracture density,roughness,aperture and stress on permeability of coal is obtained.(3)The local rate of mechanical dissipation is applied to determine an appropriate REV.The simulation results of REV with different sizes indicate that: The size of REV determined in this paper is reasonable.To solve the problem of interlayer interference in multi-layer commingled production CBM vertical wells,for different permeability,heterogeneity and pressure system of coal,numerical simulations were carried out using the grey lattice Boltzmann method.The pressure and velocity distribution under different condition of multi-layer commingled production were obtained.We analyzed the influence of different permeability,heterogeneity and pressure difference between layers on multi-layer commingled production.The research has provided a set of system simulation of lattice Boltzmann method for coal coalbed gas migration in porous media.The effects of stress,gas slippage and non Darcy flow are analysised.It reveals the mechanism of micro flow in coal seam gas reservoir.The research also provides a basis for revealing the interlayer interference mechanism of multi-layer commingled production,production dynamic analysis and production system optimization in multi seam coal seam. |
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