Multimodal Functions as Flow Signatures in Complex Porous Media
This study refutes the premise that the distribution of flow speeds in complex porous media can be described by a simple function such as a normal or exponential variation. In many complex porous media, including those relevant for subsurface storage and recovery applications, a separation of scales exists between larger inter-granular pores and micro-porosity inside the grains, leading to different flow signatures that need to be described by multimodal functions with distinct flow field characteristics. We demonstrate this finding by devising a novel methodology to simulate fluid flow in carbonate rock based on a representation of the pore space obtained by differential X-ray imaging. The permeability assignment in the micro-porous space is estimated from the pore size inferred from mercury injection porosimetry. Model predictions considering micro-porosity agree well with experimentally measured porosities and permeabilities. The micro-porosity can contribute significantly to the overall permeability, particularly in the more heterogeneous media.