An experimental study of acoustic responses on the injection of supercritical CO2 into sandstones from the Otway Basin

Abstract

Quantitative knowledge of the acoustic response of rock from an injection site on supercritical [Formula: see text] ([Formula: see text]) saturation is crucial for understanding the feasibility of time-lapse seismic monitoring of [Formula: see text] plume migration. A suite of shaley sandstones from the injection interval of the CRC-2 well, Otway Basin, Australia is tested to reveal the effects of supercritical [Formula: see text] injection on acoustic responses. [Formula: see text] is first injected into dry samples, flushed out with brine and then injected again into brine-saturated samples. Such a suite of experiments allows us to obtain acoustic velocities of the samples for a wide range of [Formula: see text]/brine saturations from 0% to 100%. On injection of [Formula: see text] into brine-saturated samples, the rocks exhibit a decrease of compressional velocities by about 7% with the increase of [Formula: see text] saturation from 0% to a maximum of about 50%. Anisotropy of the shaley sandstones from the Otway Basin must be taken into account as the difference in the velocities normal and parallel to bedding is comparable with the perturbation due to [Formula: see text] injection and the samples of different orientations exhibit transition from Gassmann-Hill to Gassmann-Wood bound at different [Formula: see text] saturations. Changes of the dry samples before and after the [Formula: see text] injection (if any) are not traceable by acoustic methods.