Digital rock physics – estimating the impact of unresolved voxels on elasticity

Abstract

Digital Rock is an evolving computational technique that uses high-resolution 3D micro-CT gray images to compute petrophysical and geomechanical properties of reservoir rocks. Conventional Digital Rock workflows compute rock properties using gray images segmented into pores and minerals and are known to predict effective elastic moduli that are significantly stiffer than laboratory measurements (Andrä et al., 2013; Saxena et al., 2019). This behavior can be attributed to incorrect assignment of unresolved voxels that contain both pores and minerals as pure minerals. We propose a novel technique that augments image segmentation into pores, minerals, and multiple unresolved voxels that contain sub-micron size pores and minerals. While the actual effective elastic property of these unresolved voxels depends on the details of sub-micron structure contained within the voxels (e.g., intergranular pores, cracks, grain contacts) their range can be predicted by Hashin-Shtrikman or Voigt-Reuss bounds. Our workflow yields a range of effective elastic moduli of rocks that bound those inferred from measured ultrasonic velocities.