Analysis of a Darcy-Stokes system modeling flow through vuggy porous media
Our goal is to accurately model flow through subsurface systems composed of vuggy porous media. A vug is a small cavity in a porous medium which is large relative to the intergranular pore size. A vuggy porous medium is a porous medium with vugs scattered throughout it. While the vugs are often small, they can have a tremendous effect on the flow of fluid through the medium. We first introduce our microscale mathematical model for flow of an incompressible, viscous fluid in vuggy porous media. Our next step is to obtain a homogenized macroscale model. In order to do so, we assume periodicity of the medium. We obtain necessary existence and uniqueness results, error estimates, and slight generalizations of two-scale convergence results for bi-modal media. First using formal homogenization and then the rigorous two-scale convergence method, we show that our microscale model homogenizes to give a much simpler modified Darcy’s law macroscale model. In this homogenized model, the permeability tensor is modified to capture the effects of the vugs on the flow through the medium. In order to compute the homogenized permeability tensor, we essentially compute our microscale system on a (much smaller) representative cell. Toward this end, we introduce two numerical methods for the microscale model. We combine a discontinuous Galerkin method with a low order RaviartThomas element and obtain suboptimal convergence rates for the first method. The second method differs only slightly from the first, but yields optimal convergence rates. Unfortunately, it is less efficient in practical implementations.