The Effect of Partial CO2 Saturation on the Elastic Properties of Castlegate Sandstone Measured at Seismic and Ultrasonic Frequencies
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The Oil & Gas industry started to tackle the problem of CO2 emissions through subsurface storage in depleted reservoir formations. Such are the cases of the Sleipner West Field in the North Sea (Eiken, Brevik et al. 2000), and the Otway Basin in South East Australia (Lebedev, Pervukhina et al. 2013). Monitoring of CO2 injection is crucial to make sure that it remains in place through very long time (geological time). Seismic methods are used to achieve this purpose since wave propagation depends strongly on the mechanical properties of the medium (Lebedev, Iglauer et al. 2014). The study conducted in this research aimed to determine through experimentation if the elastic properties of a CO2-saturated reservoir-like rock are frequency dependent, i.e. dispersive. To tackle this, experimental measurements were performed when CO2-saturated water was injected to the pore space of a sample of Castlegate sandstone, where later pore pressure was reduced and gaseous (free) CO2 came out of solution in a homogenous way. Young s modulus and Poisson s ratio were measured at seismic frequencies (1Hz 155Hz), and P-wave velocity was measured in the ultrasonic frequency range (@500kHz). It was observed that the measured magnitudes decreased with increasing free CO2 saturation, as expected. But also, it was found that elastic properties such as P-wave velocity and P-wave moduli had little dispersion within the seismic frequency range. Nevertheless, it increased considerable compared to the measurements performed at 500kHz. Biot-Gassmann model replicated well the measured data up to when CO2 saturation was 2%, but it did not give a good description of the data obtained at higher saturation values. The data was fitted to a Cole-Cole relaxation model. An increase in attenuation, as well as a shift of the relaxation frequency towards higher values was observed as the saturation of CO2 increased. Even though the Cole-Cole model is based on a single fluid relaxation mechanism, it cannot be ruled out that two (or more) dispersion mechanisms are superimposed. Finally, a Rock Physics Template was built based on the measured data and two trends were proposed: frequency and CO2 saturation.