Sequestration of anthropogenic CO2 into underground brine-saturated reservoirs is an immediate option for Australia to reduce CO2 emissions into the atmosphere. Many sites for CO2 storage have been defined within many Australian sedimentary basins. It is anticipated that seismic technology will form the foundation for monitoring CO2 storage within the subsurface, although it is recognised that several other technologies will also be used in support of seismic or in situations where seismic recording is not suitable. The success of seismic monitoring will be determined by the magnitude of the change in the elastic properties of the reservoir during the lifecycle of CO2 storage. In the short-term, there will be a strong contrast in density and compressibility between free CO2 and brine. The contrast between these fluids is greater at shallower depth and higher temperature where CO2 resembles a vapour. The significant change in the elastic moduli of the reservoir will enable time-lapse seismic methods to readily monitor structural or hydrodynamic trapping of CO2 below an impermeable seal. Because the acoustic contrast between brine saturated with CO2 and brine containing no dissolved CO2 is very slight, however, dissolved CO2 is unlikely to be detected by any seismic technology, including high-resolution borehole seismic. The detection of increases in porosity, associated with dissolution of susceptible minerals within the reservoir may provide a means for qualitative monitoring of CO2 dissolution. Conversion of aqueous CO2 into carbonate minerals should cause a detectable rise in the elastic moduli of the rock frame, especially the shear moduli. The magnitude of this rise increases with depth and demonstrates the potential contribution that can be made from repeated shear-wave and multi-component seismic measurements. Forward modelling suggests that the optimal reservoir depth for seismic monitoring of CO2 storage within an unconsolidated reservoir is between 1,000 and 2,500 m. Higher reservoir temperature is also preferred so that free CO2 will resemble a vapour.

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SUMMARYWe use full wave forward and inversion modelling to estimate the elastic properties of rock samples from ultrasonic waveforms. The finite element algorithm (ABAQUS modelling software) is used to model a forward wave propagation within a homogeneous medium. For 19 mm diameter P-wave transducers, the result of the displacement waveform for a uniform source signal is obtained using both a linear and radial (about 2 mm) receiver arrays. Also, the use of a non-uniform source amplitude such as Gaussian distribution improves the displacement waveforms by few percent. The results accuracy is increased with increasing values of Gaussian standard deviation. However, for a nominal frequency of 1MHz, the same error increases with the decreasing frequencies. Additionally, our inversion algorithm (written in Python) searches for the best Young modulus (E) and Poison ratio (ν) of the medium iteratively. Finally, without prior knowledge of any threshold, the elastic parameters are estimated, and the results are consistent with the experimental measurements. These results provide a new modelling workflow to estimate the elastic parameters of the homogeneous and isotropic sample.

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