Micro-gravimetric monitoring of geological CO2 reservoirs

The surface gravity effect of injected CO2 into geological reservoirs has always been modeled as a negative anomaly and the effect of porosity variations on the gravity signal has not been considered. This theory is generally valid in normal reservoirs where the injected CO2 replaces the ambient fluid without causing excessive pressure, resulting in a negative subsurface density anomaly and a negative gravity signal at the ground surface along with negligible reservoir deformation. Our investigations support an alternative hypothesis in tight reservoirs in which the injected CO2 generates a pressure increase, which in turn causes a volumetric change in the reservoir. The injected CO2 in tight reservoirs fills the extra porosity, and the CO2 mass is added to the ambient fluid without replacement. The gravimetric and geodetic signatures of injected CO2 into a tight reservoir are characterized by a positive gravity signal as a result of the CO2 mass balance changes only, an upward ground surface deformation signal, and a negative gravity signal originated from the free-air gradient effect of the ground surface deformation.