Constraining the density of CO_2 within the Utsira formation using time-lapse gravity measurements

摘要

At Sleipner, CO_2 is being separated from natural gas and injected into an underground saline aquifer, known as the Utsira formation, for environmental purposes. In this study, gravity measurements were made over the Sleipner CO_2 injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to study the behavior and physical properties of the injected CO_2. The gravity measurements show a repeatability of 4.3 μGal for 2003 and 3.5 μGal for 2005. The formal time-lapse uncertainty is 5.3 μGal, however vertical benchmark motions that are likely due to local sediment scouring, obscure the sought after signal. Forward models of the gravity change are calculated based on both 3-D seismic data and reservoir simulation models from other studies. These forward models indicate that the magnitude of maximum gravity change is primarily related to CO_2 density rather than flow geometry. The time-lapse gravity observations best fit a high temperature forward model based on the seismically determined CO_2 geometry, suggesting that the 3-D reflection seismics are imaging the geometry of the injected CO_2, and that the in situ CO_2 density is around 530 kg/m~3. Uncertainty in determining the average density using this technique is estimated to be ±65 kg/m~3 (95% confidence), however, additional seismic surveys are needed before firm conclusions can be drawn. Future gravity measurements will put better constraints on the CO_2 density and continue to map out the CO_2 flow.