Pore network characterization of shale reservoirs through state-of-the-art X-ray computed tomography: A review

Shale microstructures control the capacity of the formation to store hydrocarbons and its fluid flow. However, microstructural complexity and multiscale heterogeneity is a challenge in a comprehensive understanding of shale plays. As a non-destructive visualization technology, X-ray computed tomography (X-ray CT) scanning can be used to characterize multiscale shale pore structures. Herein, we review recent advancements in X-ray CT scanning for multifaceted characterization of organic rich shale plays. We first introduce the basic principle of X-ray CT scanning technology with a general workflow of image processing, followed by a sequential overview of its current applications in shale pore structure analysis, permeability evaluation, and reservoir in-situ conversion. We understood that a multiscale imaging framework involving micro-CT, nano-CT, focused ion beam scanning electron microscopy (FIB-SEM), and other imaging techniques can offer a powerful approach for assessing shale microstructures. Meanwhile, various contrast agent injections can provide a new entry point for investigating shale pore space. Moreover, digital rock physics (DRP) which is based on 3D visualization of natural samples to reconstruct digital specimens can be used in flow modeling to predict transport properties from molecular to microscale. In addition, microstructures can evolve significantly during geological processes or hydrocarbon exploitation (e.g., pyrolysis, fracturing deformation and chemical reactions), which can be observed with CT scanning. Collectively, this review primarily highlights the applications of X-ray CT scanning in characterization of shale pore structures and their evolution and proposes directions for future research in this realm where a knowledge gap exists.