Surface Gravity Response of CO2 Storage in the Johansen Deep Reservoir

This study regards the assessment of surface gravity surveying for CO2 plume monitoring in the deep Johansen saline aquifer, a potential offshore site for CO2 geologic storage. We used the available benchmark model and geological information to simulate the injection and postinjection phases. We calculated the gravity response at the surface from the estimated models of reservoir density and saturation at different time intervals and for different injection rates. The forward calculation is achieved by assuming a tetrahedral mesh discretization such as to ensure an accurate and detailed reconstruction of the complex reservoir. The results show a gravity anomaly extending radially around the well position and reaching a peak of about
$-15\,\,\mu $
Gal for an injection rate of 60 kg/s. During the postinjection period, the gravity maps clearly show an increase in brine saturation around the injection well and the migration of the CO2 plume toward shallower portions of the reservoir. We also show that time-lapse gravity data can be used to successfully estimate the CO2 stored mass by means of depth from extreme point (DEXP) multiscale analysis, even when the anomaly is incompletely defined 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 efficient technique for simultaneously determining the CO2 plume depth, its geometrical features, and stored mass.