Optical 3D micro-scanning for the determination of the envelope volume of rock samples

Density and porosity are key parameters in the petrophysical characterization of rock samples. To calculate those properties, the determination of the envelope volume of a rock sample is of central importance. Frequently used methods for unregularly shaped sample pieces are immersion-weighting or mercury porosimetry. For soluble or swellable rock samples an envelope density analyzer is frequently used, which measures the displacement of fluid-like particles to determine the envelope volume. This study investigates a new approach to derive the envelope volume of sample pieces, by directly capturing the surface of a rock, using a 3D micro-scanner. The device uses the triangulation principle to recreate the rock sample, and thus the volume, out of a recorded point cloud in a three dimensional coordinate system. The suitability and reproducibility for different surface properties were evaluated testing various materials (sedimentary rocks, igneous rocks, metamorphic rocks) and by comparing the results to those using an envelope density analyzer. The results show that optical 3D micro-scanning provides a higher reproducibility than the standard envelope density analyzer. Particularly accurate data can be expected for samples with a low surface roughness, regardless of the color and brightness, while recessed angles or shiny surfaces increase inaccuracy, but still with a comparable high reproducibility. Overall, optical 3D micro-scanning provides a fast and robust method to determine the envelope volume of rock samples.