A critical review on pore to continuum scale imaging techniques for enhanced shale gas recovery

Imaging and image analysis of shale provides insights into the pore and fracture networks, disposition of compositional elements, and signatures of diagenesis and fluid flow. Extraction of features and reconstruction to get highly accurate models of shales using image analysis involves various techniques and tools. The present paper reviews different 3D and 2D imaging methods, their advantages, and limitations, along with recommendations on making the most appropriate choice for visualization of desired features and their applications. While fractures and other elements of deformation can be well resolved using X-ray CT, the FIB-SEM is a better suited 3D imaging tool for resolving the nano-scale pore attributes of shale. Among 2D imaging methods, TEM can provide direct information on the micropores (<2 nm), which cannot be obtained using SEM imaging. The paper discusses guidelines on filtering, thresholding, and optimization, with a special emphasis on image segmentation and feature extraction that may be selected based on the problem definition. It was found that adopting multiple imaging methods can provide a plethora of complementary attributes; however, the scale of observation can vary up to three orders of magnitude, which may lead to inaccurate upscaling. Experimental methods such as lowpressure gas adsorption, NMR, and small-angle scattering can not only validate the results of imaging but also extend the range of identification of pore characteristics to deliver a highly comprehensive understanding of shales. Choice of imaging tools and their combinations, appropriate sample preparation protocols, application of machine learning and advanced numerical simulation, and validation with native sub-surface will remain key to the applicability of imaging in unlocking the full potential of shale as a reservoir.