The Utsira formation is a deep saline aquifer located in the North Sea that is composed of high porosity sandstone up to 300 m thick and is overlain by a low permeability shale caprock. It serves as a permanent large-scale storage reservoir for by-product CO2 resulting from the production of hydrocarbons at the Sleipner gas field, which is operated by Statoil.  The by-product CO2 is separated from the hydrocarbons and injected into the Utsira formation at about 1000 m below the seafloor as an environmentally friendly alternative to releasing the CO2 into the atmosphere.  In order to fully understand the behavior of the injected CO2, and to insure that this technique is safe and reliable, monitoring the changes in the reservoir over time are essential.  Time-lapse 4-D reflection seismic surveys have proven to be invaluable for this aim.  However, uncertainties remain regarding the temperature of the injected CO2 and the density of the CO2 within the reservoir.  These uncertainties make verifying the mass of CO2 within the reservoir difficult from the time-lapse seismic data alone.

Within the Utsira formation, the injected CO2 pushes the aquifer fluid aside and replaces it within the rock pore space (although this scenario is complicated by varying CO2 saturations and dissolution of CO2 into the aquifer brine).  This replacement of water by CO2 causes a net mass decrease in the reservoir, since the density of CO2 is less than that of water.  The total mass decrease depends on the density of the injected CO2, which in turn depends upon the temperature of the reservoir.  A decrease in reservoir mass causes the local gravity over the CO2 bubble to decrease.  Therefore, observing gravity changes over time on the seafloor above the injected CO2 has the potential to constrain the CO2 density within the reservoir.