Effects of Water-Rock Interaction on Elastic Behaviors of Fully Water-Saturated Lacustrine Clay-Rich Shales: Experimental Investigation and Theoretical Modeling

Summary Unlike marine shales, lacustrine Gulong shales deposited in semi-deep-to-deep lakes typically exhibit complex fluid-phase behavior and high clay mineral content. To study the saturation effects (e.g, frame stiffness, clay-water weakening) of in-situ shale, ultrasonic investigations into lacustrine shales from the Qingshankou (QSK) Formation were conducted under 100% water-saturated and confining (e.g, 5 -25 MPa) conditions. The data indicate that water saturation increases the P-wave velocity propagating perpendicular to the bedding plane (P0) by ~14%, while exerting a negligible effect on the P-wave velocity parallel to the bedding plane (P90); S-wave velocities consistently decrease (~10% for SH-wave and ~6% for SV-wave) from dry to fully saturated states. Furthermore, the experimental observations indicate a saturation-induced frame-weakening response in clay-rich shales, leading to a ~18% decrease in shear modulus at 100% water saturation. In addition, to qualitatively capturing the frame-weakening effect in 100% water-saturated shales in the velocity estimation, we proposed a Porosity-Clay modified Gurevich Squirt Flow (PCGS) model, which was introduced as a data-constrained phenomenological extension that accounts for effective saturation-induced frame weakening through empirical fitting of modulus variations. Compared with traditional fluid-substitution models, the PCGS model exhibits substantially improved accuracy (~10% for bulk modulus and ~15% for shear modulus).