Relation between REV of Porosity and Tortuosity in Unconsolidated Sand Using SMT

The Representative Elementary Volume (REV) for geometrical tortuosity in unconsolidated sand reservoirs, is important in simulating and upscaling models for flow and geomechanical applications. In order to accurately reflect the pore connectivity and morphology in predictive models of sandy porous media, both porosity and tortuosity should not vary over the sample volume. Determining tortuosity REV of a geomaterial specimen requires a resource-intensive process while measuring the minimum REV porosity is considerably easier. Hence if the REV of tortuosity can be related to that of porosity for unconsolidated sand reservoirs, substantial saving can be achieved in both lab and software analysis. Twenty-six specimens of unconsolidated sand with varying sand type, diameter, and gradation, were repacked in columns and scanned using X-ray Synchrotron Micro-computed Tomography (SMT) at the Argonne National Laboratory. The reconstructed images from the 3D scans were used to label sand and void space. Porosity and geometrical tortuosity were measured from the labeled 3D images. The REV was determined by measuring twenty incremental subvolumes within the sample until the property was constant with the increase in subvolume over a 5-point moving average. The sand specimens were also characterized for the distributions of grain diameter, roundness, sphericity, pore and throat radiuses. Findings reveal that all the unconsolidated sand specimens had minimum REV of porosity that is higher than the minimum REV for tortuosity. This discovery indicates that for many unconsolidated sand samples there is no need to carry a separate REV analysis for tortuosity. Also, a weak negative correlation between REV and sand grains diameter was found while the study found no significant correlation between REV and roundness nor sphericity of the grains. The results can boost laboratory and software analysis protocols for unconsolidated sand cores by reducing time and cost in selecting the sample test volume. Benefiting scantiest and engineers, working on water reinjection and recovery from aquifers; production of hydrocarbons and carbon geological storage in reservoirs.