Monitoring gas production and CO2 injection at the Sleipner field using time-lapse gravimetry

Thirty seafloor gravity stations have been placed above the carbon dioxide (CO2) injection site and producing gas reservoir at the Sleipner Ost Ty field. Gravity and depth measurements from 2002 and 2005 reveal vertical changes of the permanently deployed benchmarks, probably caused by seafloor erosion and biologic activity (fish). The original gravity data have been reprocessed, resulting in slightly different gravity-change values compared with earlier published results. Observed gravity changes are caused by height variances, gas production and water influx in the Ty Formation, and CO2 injection in the Utsira Formation. Simultaneous matches to models for these effects have been made. The latest simulation model of the Ty Formation was fitted by permitting a scale factor, and the gravity contribution from the CO2 plume was determined by using the plume geometry as observed in 4D seismic data and varying the average density. The best-fit vertical gravity gradient is 1.80 mu Gal/cm, and the response from the Ty Formation suggests more water influx than expected in the presurvey simulation model. The best-fit average density of CO2 is 760 kg/m(3). Estimates of the reservoir temperature combined with the equation of state for CO2 indicate an upper bound on CO2 density of 770 kg/m(3). The gravity data suggest a lower bound of 640 kg/m(3) at 95% confidence.