(333h) Slip Length of Methane Flow Under Shale Reservoir Conditions: Effect of Pore Size and Pressure

Gas recovery from shale reservoirs is drastically different from the conventional reservoirs due to the presence of extensive amount of nano-sized pores. Within nanopores, gas molecule distributions are inhomogeneous and gas slippage on the pore surface plays a crucial role in gas flow. The conventional hydrodynamic equations can no longer describe gas flow in nanopores. Slip length has been widely used to quantify the deviation from the traditional hydrodynamic equations, such as Hagen-Poiseuille (H-P) equation. Determining slip length of a specific fluid can better assess the flow behavior in nanopores as well as multiscale modeling.

Many studies were carried out to clarify the slip length in nanopores in shale, such as experiments, numerical analysis and simulations. Although some qualitative and suggestive conclusions on gas transport have been obtained from previous studies, to our best knowledge, a systematic study about the effects of pore size, temperature and pressure on the slip length of methane transport in nanopores at shale reservoir conditions is still lacking.

Herein, we report a molecular simulation study of methane flow in organic nanopores under shale reservoir conditions (temperature: 300 to 450 K, pressure: 10 to 60 MPa), with pore sizes ranging from 2 to 20 nm. We use the grand-canonical Monte Carlo simulations to determine methane content in nanopores and analyze the density distributions using equilibrium molecular dynamics. The surface adsorption effect is almost negligible under high pressure (60 MPa) conditions on average density, while it plays an important role under low-pressure conditions. Finally, we use nonequilibrium molecular dynamics simulation to study the transport behavior of nano-confined methane molecules. We find that the pore size has a significant effect on the slip length under low pressure (10 MPa) conditions. In contrast, the slip length is almost constant under high-pressure conditions. Under most conditions, the slip length decreases as pressure increases. Such a trend is more obvious in the smaller pores under high-temperature conditions. Our work should provide important insights into the quantification of slip length under shale reservoir conditions.