Development and production from unconventional resources require understanding of flow mechanisms and rock/fluid interactions, which are different from those in conventional reservoirs. Several flow regimes including slip and transition can be dominated in these reservoirs due to the presence of micro- and nano-pores. Recently the lattice Boltzmann method has been considered as a well-accepted method for simulation of slip flow in shale rocks. In this paper, the two-relaxation-time-based Lattice Boltzmann approach was adopted to simulate the slip flow in a single channel and a simplified 2D and 3D porous media. Different boundary conditions including diffusive reflective and bounce back-specular reflection were used to capture the gas slippage at the wall surface. The simulation results were compared with the experimental data measured on three shale rock samples. The results showed that the gas slippage in the shale samples under study cannot be modelled using the literature tangential momentum accommodation coefficient values. It was also shown that the permeability enhancement was overestimated for Kn > 0.1 when the gas flow was simulated in a single channel. Furthermore, from the fitted line to the simulation results (up to Kn = 0.3), the first- and second-order slip coefficients of the N-S boundary conditions were found as 2 and 1.54, respectively.