Squirt flow in a tight sandstone: an interlaboratory study

In the spirit of interlaboratory benchmarking of related techniques, we have re-examined the seismic-frequency mechanical properties of a low-porosity Wilkeson sandstone specimen tested in axial stress oscillation under water-saturated conditions by Pimienta et al. The same specimen has been newly tested at periods of 1–1000 s under dry and argon-, water- and glycerine-saturated conditions by torsional and flexural oscillation methods, allowing direct measurement of the shear modulus G and Young's modulus E and associated strain-energy dissipations. The results show a steady increase of G and E for the dry specimen with increasing pressure, indicative of progressive closure of the compliant intergranular contacts. Under argon- and water-saturated conditions, the measured moduli differ only marginally from those for dry conditions, without any significant stiffening or dispersion, suggesting that such measurements probe the saturated-isobaric regime. In marked contrast, glycerine saturation results in substantially higher and frequency-dependent moduli, along with frequency-dependent dissipation. We attribute this behaviour to the squirt flow transition with decreasing frequency from the saturated-isolated to the saturated-isobaric regime, modelled with a log-normal distribution of relaxation times (broader than the Debye peak of the standard anelastic solid) superimposed upon a monotonically frequency-dependent background. Although there are differences in detail, these findings corroborate those of Pimienta et al. for the same material tested in axial stress oscillation to higher frequencies under water-saturated conditions. Taken together, the two studies thus provide robust support for theoretical models of squirt flow dispersion and dissipation, occurring at frequencies between those of conventional ultrasonic wave-propagation laboratory methods and those of seismic exploration of the shallow crust.