Assessing the monitorability of CO saturation in subsurface saline aquifers

We propose a method to assess the geophysical monitorability of petrophysical parameters (in this case CO saturation) in subsurface geological reservoirs. The approach is based on measuring and inverting up to six geophysical parameters: P- and S-wave impedances and quality factors (1/attenuation), density and electrical resistivity. We use Shannon's information measure to quantify the information obtained from the inversion of different combinations of geophysical parameters. We thus develop a generic approach to assess the joint monitorability of supercritical CO saturation when CO is stored in subsurface reservoirs. Uncertainty analysis shows that expected information is in general nonlinearly related to the level of a priori or measurement uncertainty in the geophysical and petrophysical parameters, to the true CO saturation, and also to the measurement frequency in the case of seismic measurements. Prior uncertainties in petrophysical parameters such as porosity have a significant effect on the monitorability, highlighting the need for accurate benchmark (pre-injection) measurements and reservoir characterization in the case of time-lapse (4D) monitoring. We show that estimates of P-wave seismic attenuation contribute most to the overall information obtained, and that in principle the joint application of different geophysical methods can significantly improve monitorability. We applied the approach to assess the monitorability of the CO saturation after 10 years of simulated injection into a saline aquifer reservoir in the near-shore UK North Sea. Application of the approach to design optimal data combinations for hydrocarbon saturation assessment and monitoring is straightforward.