GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.1 GNGTS 2024 Time-lapse Gravity Monitoring at surface and Excess Mass Estmaton of CO 2 Stored in Deep Saline Aquifers M. Milano 1 , M. Fedi 1 1 Department of Earth, Environmental and Resources Sciences, University of Naples Federico II, Napoli, Italy. This study regards the assessment of surface gravity surveying for CO 2 plume monitoring in a deep saline aquifer (Milano and Fedi, 2023). We simulated surface gravity monitoring of CO 2 storage for the injecton and post-injecton phases and using diferent injecton rates. We show that tme lapse gravity data can be used to successfully estmate the CO 2 stored mass by means of DEXP multscale analysis, even when the anomaly is incompletely defned, due to a not proper areal coverage of the survey. The DEXP method has proven to be very stable with respect to noise and to be an efcient technique for simultaneously determining the CO 2 plume depth, its geometrical features and stored mass. We used the available benchmark model of the Johansen reservoir to conduct the simulaton. We calculated the gravity response at surface from the estmated models of reservoir density and saturaton at diferent tme intervals and for diferent injecton rates. We used a new approach for monitoring the mass stored into the reservoir based on the DEXP method. DEXP allows an efectve reducton of interference efects from nearby sources and it can be applied to high-order vertcal or horizontal gradients of the feld, maintaining high stability with respect to high-wavenumber noise. Moreover, this technique does not require any a-priori informaton about the geometry and physical parameter (density) of the source. The results show that the baseline scenario of 15 kg/s of CO 2 injected for 25 years resulted in a weak gravity response. Conversely, for a rate of 60 kg/s we observe a maximum amplitude of about -16 µGal at the end of the injecton period, while preserving low botom hole pressure within the reservoir. We also show that we can track the migraton of the CO 2 plume and contemporary the migraton of the brine with respect to the well positon. We also show that, by using the DEXP method, we can easily estmate the mass changes associated with the stored CO 2 for each injecton rate, which we found to be very close to the true values. DEXP analysis has also proven to be very stable vs. noise. Compared to the results inferred from other methods, we showed that the DEXP analysis can simultaneously provide results about the depth to the source, the source geometry and, most importantly, an accurate estmate of the stored mass. This study clearly shows that the appropriate choice of the injecton rate strongly impacts on the ability to recover useful gravity signal at the surface, beyond the measurement error threshold. We provide an in-depth analysis of the efect of noise on the mass change estmates. Our approach