Effects of Rock and Fluid Properties on Seismic Dispersion and Attenuation in Sandstone

A better understanding of the relationship between dispersion/attenuation and rock/fluid properties is of great interest in improving hydrocarbon identification and reservoir characterization. Presently it is less investigated at seismic frequency range, limited by the reliable laboratory data measured under varying physical conditions. Additionally, the roles of fluid type and distribution in enhancing wave dispersion and attenuation are still poorly understood. With this concern, I perform three groups of laboratory measurements on wave dispersion and attenuation for typical porous sandstones at both seismic and ultrasonic frequencies and under vacuum-dry and fluid-saturated conditions. More specifically, sandstone samples are fully or partially saturated by a series of fluids: methane, butane, water, and glycerin, aiming to investigate effects of fluid viscosity and distribution on wave dispersion and attenuation.
The experimental data suggests that distinct dispersion and attenuation can be found even at vacuum-dry conditions, especially for sandstones with relatively high clay contents. This finding might contradict the previous knowledge of no dispersion and attenuation in dry rocks, but has been extensively certified through a series of laboratory data in this study. Nevertheless, the comparison with fluid saturated data indicates that pore fluid related mechanisms are still the dominant cause for the dispersion and attenuation in sandstones. Significant dispersion and attenuation occur in the presence of relatively small amounts of gas both for partial glycerin and partial water saturation, yet varying in their magnitudes and characteristic frequencies. Generally, the overall characteristic frequencies shift to a relatively lower frequency range with the decrease of rock permeability or the increase of fluid viscosity. A complete attenuation curve is firstly observed in glycerin-saturated conditions at measured seismic frequency. Based on porous modeling analysis, the mesoscopic fluid flow in response to a heterogeneous fluid distribution in the pore space, might be the dominant mechanism accounting for the observed dispersion and attenuation in partially fluid-saturated rocks. The associations among velocity dispersion and wave attenuation, rock permeability, and fluid properties in the laboratory provide a potential indicator for the presence of fizz gas or high-permeability zones in fields during seismic surveys.