Investigation of multiphase fluid imbibition in shale through synchrotron-based dynamic micro-CT imaging

While substantial advances were made in pore system characterization in shale in the past decade, understanding of fundamentals of multiphase fluid flow in shale remains limited. The complexity of multiphase fluid flow in shale is related to many factors including nanoscale to submicron-scale pore systems, differences in mineralogy and associated surface wettability, heterogeneity of these properties at different scales, and possible existence of microfractures. Previous work related to fluid flow, such as permeability measurement, spontaneous imbibition, and wettability analysis, includes many simplified assumptions/hypotheses with limited consideration of small-scale details. In addition, these studies were mostly focused on single-phase phenomena. Direct observation of multiphase fluid flow in shale showing small-scale details is highly desired. In this study, dynamic imaging using synchrotron-based micro-CT (computed microtomography) was conducted on two shale samples to investigate multiphase (oil and water) spontaneous imbibition. The results revealed many details of multiphase flow in shale, including pore connectivity, porosity and permeability layered distribution, and wettability of microfractures and pores. At the same time, results illustrate the complexity of multiphase flow in shale and gaps in understanding that need further investigation. Plain Language Summary Shale oil and gas are important energy supply worldwide. While understanding on how the pore system looks like in shale is much advanced in the recent decade, how fluid flows in such type of unconventional reservoir rock remains a mystery. This is mainly caused by the small pores (nanoscale) and complicated structure and surface property in shale. In this study, how oil and water flow in two shale samples was directly visualized using micro-CT scan for the first time, as to our knowledge. Many details were revealed from the dynamic imaging of the flow processes, such as where the fluid goes, how far they go in shale matrix, and the process of immiscible water/oil displacement in microfractures and shale matrix. At the same time, the results also suggest many unknowns and gaps in understanding that requires further investigation.