Gaseous slip flow simulation in a micro/nano pore-throat structure using the lattice Boltzmann model

In this study, the lattice Boltzmann method is utilized to simulate gas flow at micro/nano pores of tight and shale gas reservoirs. The flow regime domain for this work is in the Knudsen range of slip flow, i.e., 0.01 < Kn < 0.1. The Langmuir slip model incorporated in the lattice Boltzmann model is introduced using slip regime boundary condition, which is a combination of bounce-back and specular reflections. Appropriate parameters, such as reflection factor, relaxation time and Knudsen number, are considered to accurately predict the slip velocity on the pore walls. A pore-throat model is developed to observe changes in flow characteristics and slip velocity when gradual contraction happens along the flow channel (throat), followed by steady expansion to the original pore size. Although the literature shows various studies on the lattice Boltzmann simulation of gas flow under micro/nano-confinement, realistic pore-throat structures of shale have received limited attention to date. In fact, the shape, size and diameter of the pore-throat system and its effect on the gaseous flow are of crucial importance in micro/nano gas flow due to the dynamic behavior of the Knudsen number at different length scales. The throat to pore ratio is defined as the ratio of the diameter of pore throat, D2, to that of the pore body, D1. It is found that the gas flow behavior depended on the Knudsen number and the D2/D1 ratio. In addition, both the velocity profile and slip velocity are investigated at different Knudsen numbers (within the slip flow range) and D2/D1 ratios. At Kn numbers of 0.01, 0.05 and 0.1, and decreasing D2/D1 ratios from 0.4 to 0.1, the slip velocity adjacent to the wall increased by 57, 46 and 31%, respectively. It is also revealed that decreases in throat size and increases in Kn value enhance the slip effect within the throat. When the D2/D1 ratio decreases from 0.4 to 0.1 and Kn rises from 0.01 to 0.1, the ratio of maximum velocity to slip velocity alters from 8.11 to 1.2. This shows that at the highest Kn value and lowest throat to pore ratio, the slip effect is at the most significant level.