Monitoring CO2 injection in a carbonate reservoir with Controlled source electromagnetic sounding and Gravity: a modeling study

Abstract Monitoring and verification of CO2 injected in carbon sequestration projects is an important task which will in general combine surface geophysical and downhole measurements. The most commonly applied geophysical method is seismic, which detects changes in the acoustic and elastic properties of the sub-surface. However in some settings, seismic monitoring can be challenging or expensive (or both). Non-seismic methods can be considered as a complement in such cases. In this paper we consider the application of two such methods to CO2 storage in a depleted carbonate gas reservoir: 4D gravity surveying, which is sensitive to changes in sub-surface density, and 4D Controlled Source ElectroMagnetic (CSEM) surveying, which is sensitive to changes in resistivity. Three dimensional models of density and resistivity are first built from dynamic simulations of the reservoir, and 4D synthetic data generated from these. A combination of forward modeling and inversion is then used to demonstrate that both gravity and CSEM methods have good sensitivity to 4D changes caused by the injection of CO2. This is an important first step in understanding the contribution that non-seismic methods can make in optimising the overall CO2 monitoring and verification plan.