Swelling Anisotropy of Shales

Abstract Hydrocarbon extraction from unconventional formations requires pressurized introduction of a substantial volume of water to create permeable pathways in situ, thereby accessing optimal reservoir zones. Consequently, the interaction between shale and injected water leads to shale hydration, causing swelling and softening, which presents risks related to the stability of wellbores and the potential diminishment of induced permeability. Despite extensive investigations into the causes of shale expansion, a comprehensive evaluation of anisotropic swelling has been overlooked. Therefore, this study aims to investigate the extent of unconfined anisotropic expansion in shales with significant variations in mineralogical content, providing a comprehensive overview of the anisotropic swelling intensity and its underlying causes. Digital Image Correlation (DIC) was employed for quantitative spatial deformation analysis in well-preserved and intact samples of the Tuscaloosa Marine Shale and Eagle Ford Shale. The results indicate that unconfined swelling anisotropy can reach values exceeding 10 in highly reactive and laminated shales. While the vertical expansion of shale is primarily dependent on clay content and the presence of laminations, it was found that all structural constituents, including clay, quartz, and carbonates, highly influence the degree of horizontal swelling. Furthermore, the unconfined swelling anisotropy was discovered to be almost equally dependent on all primary minerals present. The shale samples with limited carbonate content exhibit significant expansion and are vulnerable to washout. The bidirectional expansion anisotropy was predicted for 503 shale samples of Eagle Ford and Tuscaloosa Marin Shale formations. Overall, this work sheds light on previously unexplored aspects of shale behavior, with potential implications for drilling and production optimization, as well as safe operations in unconventional reservoirs.