Suction-Induced Volume Changes in Rocks (Anisotropy and Cementation) – Geotechnical and Geoenergy Implications

Unsaturated conditions take place in both near-surface sediments (air, water, and NAPLs) and deeper formations containing immiscible fluids (such as oil, gas, brine, CO2, and H2). While volumetric changes on shallow sediments are well-documented, the available data is very limited for rocks. We combine existing data with new experimental results to analyze the underlying processes. Our experiments involved different rock types, including igneous and sedimentary rocks to encompass a wide range of pore size distributions, capillary-saturation relationships, and small-strain stiffness characteristics. The results emphasize the role of the matrix stiffness and pore size dependent capillary-saturation trends on volume change. Hydro-mechanical coupling is particularly complex in inherently anisotropic shales, further complicated by diagenetic cementation. Free swelling tests and Nuclear Magnetic Resonance NMR relaxation under controlled relative humidities highlight the interaction between cementation and stiffness anisotropy on the swelling response of clayey shales. These findings have significant implications for reservoir performance and management, seal integrity, potential seal-healing treatments, wellbore instability analyses, well decommissioning and long-term nuclear waste storage.