Gravity measurements are most useful for monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells in saline aquifers. The CO2Carbon dioxide will push away the brine and change the mass (locally) since the density of the CO2Carbon dioxide is significantly lower (i.e. 600 kg/m3 at a depth of 800 m, and 700 kg/m3 at 1.5 km depth; IPCCIntergovernmental Panel on Climate Change, 2005) in comparison to brine. For example, Bickle et al. (2007)2007 - M. Bickle, A. Chadwick, H. E. Huppert, M. Hallworth and S. LyleModelling carbon dioxide accumulation at Sleipner: Implications for underground carbon storagesee more assume a brine density of 1020 kg/m3 for the Sleipner field. Provided that CO2Carbon dioxide replaces the brine, the injectionThe process of using pressure to force fluids down wells will cause a negative gravity anomaly. Note that the gravity response is linearly dependent on the mass of injected CO2Carbon dioxide.
Time-lapse gravity measurements have been used at the Sleipner field to monitor the CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells in the Utsira formationA body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it and the gas production in the deeper Ty formationA body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it (Alnes et al., 2008;Alnes et al., 20112011 - H. Alnes, O. Eiken, S. Nooner, G. Sasagawa, T. Stenvold and M. ZumbergeResults from Sleipner gravity monitoring: Updated density and temperature distribution of the CO2 plumesee more). Using a remotely operated underwater vehicle, the gravimeters were placed on a number of fixed benchmarks on the seafloor. The measurements show a negative anomaly due to the CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells, as expected.
In addition, the density of the CO2Carbon dioxide in the reservoirA subsurface body of rock with sufficient porosityMeasure for the amount of pore spaceSpace between rock or sediment grains that can contain fluids in a rock and permeabilityAbility to flow or transmit fluids through a porous solid such as rock to store and transmit fluids can be estimated based on the gravity measurements. There have also been plans to test gravity measurements on land in connection with the CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells at In Salah, Algeria (Mathieson et al., 20102010 - Allan Mathieson, John Midgley, Kevin Dodds, Iain Wright, Philip Ringrose, Nabil SaoulCO2 sequestration monitoring and verification technologies applied at Krechba, Algeriasee more).
Based on the experiences at Sleipner and the noise conditions at this site it has been suggested that the detection limit for a time-lapse gravity anomaly is on the order of 5 µGal (Alnes et al., 2008). This would correspond to an injected mass of about 470 000 tons CO2Carbon dioxide, assuming that the plume can be approximated as a point mass and using a density of 650 kg/m3 for the CO2Carbon dioxide and 1020 kg/m3 the brine. At other locations detection limits may be different.