Seismic characterization of thin beds containing patchy carbon dioxide-brine distributions: A study based on numerical simulations

We studied the seismic attenuation and velocity dispersion effects produced by wave-induced fluid flow in weakly consolidated sandstones containing patchy carbon dioxide (CO2)-brine distributions. The analysis also focuses on the velocity pushdown because of the presence of this gas, as well as on the role of the wave-induced fluid flow (mesoscopic) effects on the amplitude variation with angle (AVA) seismic response of thin layers containing CO2, such as those found at the Utsira Sand, Sleipner field, offshore Norway. We found that this loss mechanism may play a key role on conventional surface seismic data, suggesting that further data analysis may provide useful information on the characteristics of the fluid distributions in these environments. Numerical experiments let us observe that although mesoscopic effects can be very significant in this kind of media, the seismic response of a given isolated thin layer computed considering such effects is very similar to that of a homogeneous elastic thin layer with a compressional velocity equal to that of the original porous rock averaged in the effective data bandwidth. This suggests that the thin-bed prestack spectral inversion method published by the authors could be used to estimate representative compressional velocities and layer thicknesses in these environments. In effect, results using realistic synthetic prestack seismic data show that isolated CO2-bearing thin beds can be characterized in terms of their thicknesses and representative compressional velocities. This information can be qualitatively related to CO2 saturations and volumes; thus, the prestack spectral inversion method could find application in the monitoring of the evolution of CO2 plumes at injection sites similar to that at the Sleipner field.