What is it about?
This research examines natural convection within porous media, a fundamental process for storing CO2 in deep saline aquifers. Traditionally, scientists have modeled fluid mass transfer in these environments using simplified Darcy-Oberbeck-Boussinesq (DOB) equations that ignore the microscopic properties of the porous rock. These traditional models assumed that natural convection is uniquely determined by a single variable known as the Rayleigh number. To test this, we performed direct numerical simulations (DNS) that fully resolve the fluid flow field within the individual pores of the medium. By capturing all scales of motion inside the microscopic gaps, we evaluated how the physical pore scale explicitly influences the macroscopic properties of convection, mass concentration, and velocity statistics.
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Why is it important?
Our findings bridge a critical gap between theoretical models and real-world experimental results regarding CO2 sequestration. Previous DOB simulations yielded a linear relationship between the Sherwood number and the Rayleigh number at high values, which contradicted laboratory experiments that demonstrated nonlinear scaling. We proved that the boundary layer thickness is actually determined by the pore size rather than the Rayleigh number, and that the size of convective plumes increases with this pore size. This means the traditional mathematical models are missing crucial variables—specifically porosity and pore-scale parameters like momentum dispersion—which must be included to accurately predict long-term CO2 storage.
Perspectives
Writing this paper was a rewarding validation of our team's intuition regarding the limitations of the traditional Darcy-Oberbeck-Boussinesq equations. As researchers, we often rely on volume-averaged equations for computational convenience, but seeing our direct numerical simulation results visually prove the impact of individual pore spaces on mega-plume formation was a genuine breakthrough moment. I am particularly excited about how this work pushes the boundaries of our field's computational models. Incorporating momentum dispersion and viscous diffusion terms into future macroscopic equations will require significant effort, but it is a necessary step to ensure our theoretical models for combating climate change through CO2 sequestration are firmly grounded in physical reality.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: Effects of pore scale on the macroscopic properties of natural convection in porous media, Journal of Fluid Mechanics, March 2020, Cambridge University Press,
DOI: 10.1017/jfm.2020.164.
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