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Landro, M, Zumberge M.  2017.  Estimating saturation and density changes caused by CO2 injection at Sleipner - Using time-lapse seismic amplitude-variation-with-offset and time-lapse gravity. Interpretation-a Journal of Subsurface Characterization. 5:T243-T257.   10.1190/int-2016-0120.1   AbstractWebsite

We have developed a calibrated, simple time-lapse seismic method for estimating saturation changes from the CO2-storage project at Sleipner offshore Norway. This seismic method works well to map changes when CO2 is migrating laterally away from the injection point. However, it is challenging to detect changes occurring below CO2 layers that have already been charged by some CO2. Not only is this partly caused by the seismic shadow effects, but also by the fact that the velocity sensitivity for CO2 change in saturation from 0.3 to 1.0 is significantly less than saturation changes from zero to 0.3. To circumvent the seismic shadow zone problem, we combine the time-lapse seismic method with time-lapse gravity measurements. This is done by a simple forward modeling of gravity changes based on the seismically derived saturation changes, letting these saturation changes be scaled by an arbitrary constant and then by minimizing the least-squares error to obtain the best fit between the scaled saturation changes and the measured time-lapse gravity data. In this way, we are able to exploit the complementary properties of time-lapse seismic and gravity data.

Nooner, SL, Eiken O, Hermanrud C, Sasayawa GS, Stenvold T, Zumberge MA.  2007.  Constraints on the in situ density of CO2 within the Utsira formation from time-lapse seafloor gravity measurements. International Journal of Greenhouse Gas Control. 1:198-214.   10.1016/s1750-5836(07)00018-7   AbstractWebsite

At Sleipner, CO2 is being separated from natural gas and injected into an underground saline aquifer for environmental purposes. Uncertainty in the aquifer temperature leads to uncertainty in the in situ density of CO2. In this study, gravity measurements were made over the injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to constrain the in Situ CO2 density. The gravity measurements have a repeatability of 4.3 mu Gal for 2003 and 3.5 mu Gal for 2005. The resulting time-lapse uncertainty is S.3 mu Gal. Unexpected benchmark motions due to local sediment scouring contribute to the uncertainty. Forward gravity models are calculated based on both 3D seismic data and reservoir simulation models. The time-lapse gravity observations best fit a high temperature for-ward model based on the time-lapse 3D seismics, suggesting that the average in Situ CO2 density is about to 530 kg/m(3). Uncertainty in determining the average density is estimated to be 65 kg/m(3) (95% confidence), however, this does not include uncertainties in the modeling. Additional seismic surveys and future gravity measurements will put better constraints on the CO2 density and continue to map out the CO2 flow. (C) 2007 Elsevier Ltd. All rights reserved.