Correlative core- to pore-scale imaging of shales

Abstract

Abstract Unconventional reservoirs (e.g., shales) remain poorly understood, compared to conventional reservoirs, due to their complex compositional and structural anisotropy. These heterogeneities profoundly influence petrophysical and geomechanical properties of shales. Current advances in correlative multi-scale and multi-modal 2D/3D imaging provide a tremendous opportunity to image and characterize shales across multiple length scales – from core-to pore-scale. In this study, a Mancos Shale rock sample was characterized across multiple length scales – from a few centimeters to a few nanometers via digital rock analysis using correlative micro 3D X-ray computed tomography (micro-CT), micro 3D X-ray microscopy (micro-XRM), light microscopy (LM), scanning electron microscopy (SEM), and focused ion beam (FIB) – SEM (FIB-SEM) image datasets. These multi-scale/-modal 2D/3D image datasets were then correlated with each other and used to reconstruct digital rock 2D/3D models from which petrophysical properties (porosity and mineralogy) were quantified. Additionally, the SEM/FIB-SEM imaged porosity was compared with bulk porosity measured with the traditional laboratory technique of helium porosimetry. The micro-CT, LM, and (low-resolution) SEM indicated that the investigated Mancos Shale rock sample consisted of interlaminated silt- and mud-rich laminae. The silt-rich laminae were characterized further using micro-XRM, whereas mud-rich laminae were characterized in great detail using high-resolution SEM and FIB-SEM. The SEM and FIB-SEM showed the presence of various fine-grained minerals (clay) and micrometer- and nanometer-sized pores within the mud-rich laminae, whereas micro-XRM showed coarse-grained minerals (quartz) cemented with the mud-rich nanoporous matrix within the silt-rich laminae. Furthermore, the results indicated that micro-fractures significantly contributed to the porosity of the investigated core-plug rock sample.