X-ray tomography imaging of shale microstructures: A review in the context of multiscale correlative imaging

Shale microstructure determines hydrocarbon reserves and productivity. However, despite its high economic importance, the shale microstructure remains poorly understood. This is largely due to the complex pore space configuration in shale, and associated heterogeneity at multiple length scales. Here we review imaging techniques with which shale microstructure can be observed, and we highlight recent advancements in this area. Moreover, the limitations pertinent to x-ray tomography (μCT) methods and the outlook of research in this area are outlined. The presented datasets include recent investigations on shale microstructure characterization. We conclude that μCT can resolve shale microfractures, shale anisotropy, and microscale pore structure (at μm resolution). However, the relatively low μCT image resolution cannot precisely capture nanosized pores and organic matter. Indeed, it was observed that no single imaging technique is capable to fully characterize shale microstructure, instead a multiscale correlative imaging framework that spans multiple length scales is required (e.g. a combination of μCT, SEM, FIB-SEM). Thus, multiscale correlative imaging offers a powerful tool for shale microstructure quantification. This review article, therefore, provides an entry point for new researchers in this area, gives an overview of the current state of the art, and aids in further developments in this area, to improve hydrocarbon production from shale reservoirs.