ABSTRACT
Pore-scale modeling of flow in porous and fractured media is a first key step to provide a firm knowledge basis for effective aquifer management practices in environmental applications or to improved reservoir performance in the oil and gas field. We present a performance comparison of two numerical approaches for the pore-scale simulation of fully saturated flow in natural porous media. Real samples of rocks at the millimeter scale whose complex pore space geometry is reconstructed through X-ray computer tomography are considered. The governing flow equation in the system are solved by employing a Body-Fitted (BF) grid and an Immersed Boundary (IB) method, as respectively embedded in FLUENT and EULAG (Prusa et al., 2008) software environments. The BF technique allows solving the flow problem within the real pore space and is widely adopted in computational fluid dynamics since it allows to fit complex geometries while preserving accuracy, up to a certain level of detail. The method is associated with some limitations in the presence of extremely complex geometrical setting of the kind encountered in natural or reconstructed micro scale porous systems, mainly due to the time-demanding mesh generation stage. On the other hand, IB methods can take full advantage of uniform Cartesian grid implementations describing both the fluid and solid regions in the domain. Grain size distributions are modeled by considering an additional forcing term in the governing flow equation. This term inhibits Sow through the solid matrix and hence acts to enforce the no-slip condition at solid walls. The effectiveness of such term, i.e., its ability to prevent mass flow rate within the solid domain, is controlled by a parameter, a, which is directly related to the characteristic time scale of the forcing action. First, the performance of each model is analyzed separately on the same rock samples. Grid dependence is tested for the BF method upon comparing simulations at three different levels of resolution. A detailed sensitivity analysis is performed to assess the role of the parameter α in the IB method simulations. Finally, the two models are compared in terms of resulting flow fields, mass-conservation capability, hydraulic conductivity estimates and computational resources requirement.
