Experimental Study and Theoretical Interpretation of Partial Saturation Effects on P- and S-Wave Velocities and Anisotropy in Artificial Tight Sandstones with Controlled Aligned Fractures

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This study investigates the combined effects of partial saturation and aligned fractures on P- and S-wave velocities and anisotropy in tight sandstones. Ultrasonic measurements (0.5 MHz) were conducted on three synthetic samples with a matrix porosity of 11.7% ± 1.2% and controlled fracture densities (0%, 3.12%, and 6.24%) under a full range of water saturation rate (Sw), from dry to fully water-saturated. Experimental results reveal that for fractured samples, the P-wave anisotropy parameter ε increases sharply as Sw decreases from 100% to approximately 60%, followed by a gentler variation at lower saturation. In contrast, fracture-induced shear-wave splitting (SWS) is predominantly governed by fracture density and exhibits weak dependence on Sw. To interpret these observations, we developed a coupled rock physics framework by integrating the MJGW partial saturation model with the Galvin fracture model, introducing a distribution coefficient to account for the non-uniform water distribution between the matrix and fractures. The coupled model accurately explains the Vp and SWS trends, while the overestimation of Vs and ε is attributed to near-dry surface effects at grain contacts and mutual interaction between fractures. This work provides experimental data and modeling insights for seismic-based characterization of multiphase-fluid-saturated fractured reservoirs.

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