The impact of saturation on seismic dispersion in shales — Laboratory measurements

Abstract

Previous studies found a significant increase of acoustic velocities between seismic and ultrasonic frequencies (seismic dispersion) for shales, which would have to be taken into account when comparing seismic or sonic field data with ultrasonic measurements in the laboratory. We have executed a series of experiments performed with a partially saturated Mancos shale and a Pierre shale I in which the influence of water saturation on acoustic velocities and seismic dispersion was investigated. The experiments were carried out in a triaxial setup allowing for combined measurements of quasistatic rock deformation, ultrasonic velocities, and dynamic elastic stiffness at seismic frequencies under deviatoric stresses. Prior to testing, the rock samples were preconditioned in desiccators at different relative humidities. For both shale types, we present and analyze the experimental results that demonstrate strong saturation and frequency dependence of dynamic Young’s moduli, Poisson’s ratios, and Thomsen’s anisotropy parameters, as well as P- and S-wave velocities at seismic and ultrasonic frequencies. The observed effects can be attributed to water adsorption and capillary pressure that are functions of several factors including water saturation. Water adsorption results in a reduction of surface energy and grain-contact stiffness. The capillary pressure affects the effective stress and possibly also the effective pore-fluid modulus, which may be approximated by Brie’s empirical model. Reasonable fits to the low-frequency seismic data are obtained by accounting for these two effects and applying the anisotropic Gassmann model. The strong increase in dispersion with increasing water saturation is attributed to local flow involving adsorbed (bound) water, but a quantitative description is yet to be provided.

Static and dynamic stiffness measurements with Opalinus Clay

“…It was recently suggested that in shales it might be related to a local flow of bound water at grain contacts (Szewczyk et al . ).…”

“…21). The dispersion mechanisms are not fully understood yet but it may be related to local flow of bound water at grain contacts (Szewczyk, Holt and Bauer 2018).…”

“…The mechanisms of these large dispersion effects are not understood yet. It was recently suggested that in shales it might be related to a local flow of bound water at grain contacts (Szewczyk et al 2018).…”

Water retention effects on elastic properties of Opalinus shale

“…() and Szewczyk et al . (). This seeming discrepancy in the results obtained on different samples implies that variations in the elastic properties of shales with changing hydration states are driven by multiple competing phenomena.…”

“…Vernik and Nur 1992). The elastic moduli of shales have been measured at different hydration states (Vales et al 2004;Wild et al 2015;Ghorbani, Zamora and Cosenza 2009;Szewczyk, Holt and Bauer 2018). Vales et al (2004) reported that the drying of Tournemire shale resulted in a decrease in ultrasonic P-wave velocity propagating perpendicular to the bedding plane, V P (0°), and had no effect on the velocity of a P-wave propagating along the bedding, V P (90°).…”