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

Abstract. The envelope volume of rock samples is a key parameter in petrophysical rock evaluation, e.g. to determine standard properties such as density and porosity. For irregularly shaped rock samples the envelope volume is typically determined by immersion weighing or, for soluble or swellable rock samples (i.e. evaporites or clay), by measuring the displacement of fluid-like granular particles, using an envelope density analyzer. In this study, we investigate the use of an optical 3D micro-scanner as a non-destructive method to determine the envelope volume of a variety of rock samples of sedimentary, igneous and metamorphic origin. An optical 3D micro-scanner records a point cloud of the rock sample’s surface and uses the triangulation principle to recreate the surface – and thus volume – in a three-dimensional coordinate system. The optical 3D micro-scanner was evaluated regarding its suitability and reproducibility for imaging various surface properties and compared against envelope volume measurements using an envelope density analyzer. The results show that optical 3D micro-scanning performs best for samples with a low surface roughness, regardless of the color and brightness, while recessed angles or shiny surfaces increase inaccuracy. Nevertheless, the optical 3D micro-scanner always yields a higher reproducibility than the standard envelope density analyzer. We conclude that using optical 3D micro-scanning is a fast, non-destructive and reliable alternative for measuring the envelope volume of most rock samples.