Effect of Saturation and Image Resolution on Representative Elementary Volume and Topological Quantification: An Experimental Study on Bentheimer Sandstone Using Micro-CT

Abstract

The establishment of the representative elementary volume (REV) in studies of porous media is crucial for linking microscopic structure and pore-scale fluid configurations to macroscopic flow processes. We present analysis of the REV of porosity, specific interfacial area, and topological measures (Betti numbers and the Euler characteristic) for an air–water–Bentheimer sandstone system imaged using micro-CT at multiple wetting-phase saturation levels, acquired during primary imbibition (an unsteady-state displacement process). The original high-resolution tomographic data (1.66 μm voxel size) were downsampled to provide additional images at a range of voxel sizes, and REV analysis was also performed for these images. The results demonstrate that the REV is dependent on the image resolution: at large voxel size features are lost, resulting in unreliable parameter measurements. The wetting-phase saturation impacts the REV of the specific interfacial area, depending on the phase pair under consideration. Quantification of topological measures is especially sensitive to voxel size, as microporosity associated with clays in this sandstone sample significantly biases the results; treatment of microporosity is required to obtain connectivity results applicable to flow processes occurring within macropores. Additionally, the REV of the Euler characteristic is dependent on the heterogeneous structure of the air phase at intermediate saturations during the unsteady-state displacement processes. These results inform experimental design and 3D tomographic data acquisition parameter targets and provide insight into the microstructure of fluid phases during primary imbibition. Show more