Effects of CO2 on P-wave attenuation in porous media with micro-cracks: A synthetic modelling study

The presence of CO2 in hydrocarbon reservoirs can cause significant changes in seismic wave properties. In turn these properties can be used to map CO2 saturation in hydrocarbon reservoirs or aquifers - either from natural sources or by injection from the surface. We present the results of a synthetic modelling study of the effects of supercritical CO2 saturation on P-wave attenuation in a medium consisting of four horizontal layers, including a target aquifer. The target aquifer is modelled fully by an effective medium containing pores saturated with brine and/or CO2 and randomly-aligned microcracks at different densities. The other layers are modelled solely by their bulk seismic velocities and densities. We first compute synthetic seismograms for a reference case where the third layer is completely isotropic with no cracks, no pores and no fluid saturation. We then calculate synthetic seismograms for finite crack densities of 0.01, 0.02 and 0.03 at varying degrees of CO2 saturation in the third layer. The results of our analysis indicate that attenuation is sensitive both to CO2 saturation and the crack density. For a given crack density, attenuation increases gradually with decreasing percentage of CO2 saturation and reaches a maximum at around 10% saturation. The induced attenuation increases with crack density and with offset. These observations hold out the potential of using seismic attenuation as an additional diagnostic in the characterisation of rock formations for a variety of applications, including hydrocarbon exploration and production, subsurface storage of CO2 or geothermal energy extraction.