Viscoplastic effects of Newtonian fluids in nanopores: a molecular dynamics study

Abstract

Understanding the influence of fluid–solid interactions on low-velocity non-Darcy flow in ultra-low permeability reservoirs is essential for enhanced oil recovery. The Poiseuille flows of C₁₂H₂₆ and argon under different pressure gradients in a nanopore are simulated using the molecular dynamics method. The results demonstrate that the confined fluids behave as elastic solids under lower pressure gradient but as viscous fluids under higher pressure gradient in the simulation. With increase of the external pressure gradient, the generated shear stress exceeds the yield stress related to the fluid-solid interaction and leads to a transition. This phenomenon indicates that the confined fluids in the nanopore exhibit apparent viscoplasticity. According to the viscoplastic hypothesis, the confined fluid can be divided into elastic and viscous layers, depending on the competition between the shear and yield stresses. The simulation results also indicate the yield stress increases near the wall. Assuming that the yield stress is inversely proportional to the distance from the wall, a non-Darcy flow model is proposed to calculate the flow rate by pressure gradient. The predictions of the model are consistent with the simulations for both C₁₂H₂₆ and argon, indicating the similarity of viscoplastic effects on the seepage of both fluid-nanopore systems.

GRAPHICAL ABSTRACT

KEYWORDS:

• Molecular dynamics
• nanopore
• non-darcy flow
• seepage
• viscoplastic effects

Acknowledgements

The computations were carried out at the National Laboratory for Information Science and Technology of Tsinghua University in China. The authors would like to thank Liqiang Ai, China Academy of Launch Vehicle Technology, for helpful discussions on topics related to this work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 51776104].