Advanced microscopy and image analysis techniques revealed that massive nanpores exist in shale gas formations. The process of the microscopic gas transportation from nanpore systems (of shale) and the macroscopic produciton from shale gas reservoirs needs to be better understood. Here, we developed a multiscale simulation scheme for studying gas flow in nanpore systems using molecular dynamics (MD) and lattice Boltzmann method (LBM). MD is a powerful technique by which we can get thermodynamic properties of the simulation system at desired temperature and pressure, and simulate the gas flow in molecular scale. By this computational method, we will investigate transportation characteristics of methane molecule in molecular scale and estimate the slip velocities fordifferent solids at different temperatures and pressures. LBM, on the other hand, is a computational method for continuous fluid dynamics. In enables us to upscale MD simulation results from molecular scale to pore-scale with complicated geometries, and estimates the permeability to taking consideration of the slip velocities.
First, we calculated the mean free path and slip velocity, and evaluated the slip velocity as a function of Knudsen number using MD simulations. The MD simulations were conducted for three different constituents of shale nanpore surfaces; quartz, clay and kerogen at around 12 MPa and temperatore from300 K to 400 K. The fully hydroxlated α-quartz (001) surface, which contains vicinal silanol, was employed as a model of quartz system. The uncharged pyrophyllite surface was employed to represent a model of clay. The shinn model (Type III) kerogen slab was employed as a model of organic rich shale.
Second, we implemented the slip velocity obtained from MD simulations in LBM simulations. As for the fluid-solid boundary, we adopted counter slip boundary condition, which enables us to configure the slip velocity at the wall so as to satisfy that obtained from MD simulations. Then, we simulated the gas flow in nanpores with different pore geometries. For a simple slit geometry, it was found that the slip flow induced high permeability, which is about 0.1–1.8 times of that for non-slip flow. For systems with complicated geometries, we demonstrated that permeability of slip flow can be estimated from the pore profile. The application of multiscale simulation scheme to a more realistic system is straightforward.