Study Of Microscopic Transport Mechanisms Of Shale Gas Using The Lattice Boltzmann Method

The reserves of shale gas are huge in the world. It is reported that the preliminarily verified reserves of shale gas are close to the total reserves of the tight sandstone gas and coalbed methane worldwide. Moreover, the huge reserves of shale gas in China have a considerable exploration potential. The nano-pores of shale determine the occurrence and flow characteristics of shale gas in reservoirs, and then determine the development characteristics of the shale gas reservoirs. Recently, there are a number of scientific and technical problems in the development of the shale gas reservoirs. Owing to the complexity of the gas flow in shale, the traditionally macroscopic seepage theory has limited capabilities to describe the shale gas flow and to reveal the flow laws. Introducing new microscopic or mesoscopic methods to study the flow mechanisms of shale gas at different scales is becoming an important trend in this field.Up to now, the main strategies for studying the flow mechanisms of shale gas can be classified into three categories, i.e., macro approach based on the continuum assumption, molecular dynamics approach, and mesoscopic approach such as the direct simulation Monte Carlo (DSMC) and the lattice Boltzmann method (LBM). By contrast, the lattice Boltzmann method, which is not based on the continuum assumption and has higher computational efficiency and lower statistical noises, is considered as a promising method for simulating multi-scale, multi-physics-field complex flowsIn this paper, the lattice Boltzmann method is employed to investigate the micro-flow mechanisms of shale gas. The flow laws of shale gas are studied at the pore scale and representative-elementary-volume (REV) scale, respectively. The main contents and conclusions are as follows.(1) The extensive investigation on the domestic and foreign literature is conducted, the mesoscopic characteristics of the lattice Boltzmann method are introduced, and the research methods and research status are summarized.(2) The principle of the lattice Boltzmann method is systematically discussed based on the Boltzmann equation, the common boundary schemes and program structures of the lattice Boltzmann method are introduced.(3) The rectangular lattice Boltzmann model is studied. Through the Chapman-Enskog multi-scale expansion, the macroscopic equation from the rectangular lattice Boltzmann model includes an error term. Then the rectangular lattice Boltzmann model for gaseous microscale flows is established, and the microscale boundary conditions of the rectangular lattice Boltzmann model are analyzed in detail. Through adjusting the combination coefficient r (or the discrete accommodation coefficient ?) and the relaxation parameter s4, a reasonable approach to overcome the numerical discrete effects is presented. In addition, the computational efficiency and relative difference between the rectangular lattice Boltzmann model and the square lattice Boltzmann model are analyzed in detail.(4) A pore-scale lattice Boltzmann model for shale gas is established under the ideal and nonideal gas conditions, respectively. Owing to the high pressure in the shale gas reservoirs, the transport mechanisms of shale gas in kerogen pores under the shale-gas-reservoir conditions are studied based on the pore-scale lattice Boltzmann model for nonideal gas. Some physical laws for gas flow in kerogen pores are summarized based on the present model.(5) The combined bounce-back/diffuse-reflection boundary condition for curved boundary is studied under the nonideal gas condition, and a method to determine the combination coefficient in the boundary condition is presented. The pore-scale lattice Boltzmann model is used to simulate the shale gas flow in the artificial porous media. The effect of the shale gas transport mechanisms in kerogen on the apparent permeability is analyzed.(6) A REV-scale lattice Boltzmann model for shale gas is established under the nonideal gas condition. The gas-flow laws in shale with natural fractures are investigated at the REV scale. The effect of the natural fractures on the shale gas flow is studied.(7) Owing to the presence of the surface diffusion in nanoscale kerogen pores, especially the presence of the natural fractures in shale, the seepage capacity of shale increases significantly, which makes the economical development of the shale gas reservoirs come true.