Analysis of surface gravity and ground deformation responses of geological CO₂ reservoirs to variations in CO₂ mass and density and reservoir depth and size

The strength and direction of gravity and ground deformation anomaly vectors over normal and tight reservoirs after injection of CO₂ depend on the reservoir and CO₂ properties. Some of these properties, such as porosity, permeability, and size, define the reservoir type and therefore determine the existence or sign of the anomalies, indirectly. However, some other properties of reservoirs, such as reservoir depth, horizontal extension of CO₂ plume, and CO₂ mass amount and density, affect the strength of the measurements directly. Gravimetric and geodetic modelling of synthetic CO₂ reservoirs can quantify each of the direct effects that represent the expected signals over different geological settings. We present gravity signal as the gravity increase due to CO₂ mass attraction, and its combination with the gravity decrease caused by ground uplift. Our results indicate that the reservoir depth and horizontal extension, along with the injected mass, have significant influences on both ground deformation and gravity signals. The results also demonstrate that the horizontal extension of the CO₂ distribution decreases the dependency on the depth. In addition, the observed gravity signal over CO₂ reservoirs is dominated by the free-air effect from large ground deformation. Finally, the gravity effect over both normal and tight reservoirs and the ground surface deformation over tight reservoirs are highly dependent on the density change of the injected CO₂ inside the reservoir as a result of depth dependant changes of pressure and temperature.