A new approach of monitoring CO2 storage in deep saline aquifers from time-lapse gravity data

Here we assessed surface gravity monitoring as tool for detecting the CO₂ plume in deep saline aquifers during the injection and post-injection phases. We used the available benchmark model of the Johansen reservoir to conduct the simulation of CO₂ storage at about 3 km of depth for 70 years and using different injection rates. We calculated the gravity response at surface from the estimated models of reservoir density and saturation at different time intervals. The forward calculation is achieved by assuming a tetrahedral mesh discretization, such as to ensure an accurate and detailed reconstruction of the complex reservoir.

We proposed a new approach for monitoring the mass stored into the reservoir based on the DEXP method, which allows an effective reduction of interference effects from nearby sources and provide accurate results even when the anomaly is incompletely defined, due to a not proper areal coverage of the survey.

This study clearly shows that the appropriate choice of the injection rate strongly impacts on the ability to recover useful gravity signal at the surface, beyond the measurement error threshold. We also provide an in-depth analysis of the effect of noise on the mass change estimates.

Our approach could be a valid tool for conducting real time monitoring of the CO₂ as it could accurately determine the effective mass stored in the reservoir. This is particularly important as it does not require information about the source and could make surface gravity surveying as an independent monitoring strategy.