Application of Digital Volume Correlation to X-ray Computed Tomography Images of Shale

Abstract

We investigated the Young’s modulus and Poisson’s ratio of Green River oil shale at elevated temperatures under triaxial conditions using an X-ray computed tomography (CT) scanner for in situ imaging, fast iterative digital volume correlation (FIDVC) for visualization of displacement and volumetric strain patterns, and a conventional linear variable differential transducer (LVDT) to measure the displacement independently. FIDVC was applied to time-lapse CT images to visualize the distribution of axial normalized displacement (Uz) and volumetric strain (εV) within the samples at the resolution of the CT images. Importantly, the displacement obtained by FIDVC was calibrated and verified with LVDT measurements. Both axial normalized displacement (Uz) and volumetric strain (εV) profiles from the FIDVC method present the location of greater strains within samples including layered material of greater density and lesser density organic matter (kerogen and bitumen) rich zones. The Young’s modulus remained unchanged upon heating to around 200 °C, but decreased following kerogen maturation at 350 °C. Young’s modulus decreased at 350 °C due to rock softening and conversion (maturation) of kerogen to oil or gas. Accordingly, the fraction of kerogen in a sample plays a critical role on Young’s modulus. These observations agree with trends found in the literature (White et al., 2017; Burnham, 2018), but without in situ imaging. Similarly, Poisson’s ratio did not change after pyrolysis but increased post maturation.