Managing offshore hydrocarbon fields involves costly investments like drilling infill wells or installing compression facilities. A good understanding of the dynamical behavior of the reservoir is required for derisking such decisions. Time-lapse seismic is a suitable monitoring technology, but it involves large operational costs. On the Norwegian continental shelf (NCS), another geophysical method has been in use for two decades: 4D gravity and subsidence surveys, with a cost typically 15% of that of seismic surveys. Field cases show that, in some cases, this technology provides information beyond the reach of seismic within the required timeliness.
Time-lapse gravity changes at the seafloor are sensitive to changes in mass within the reservoir. As an example, vertical movements of water-gas contacts smaller than a meter can be detected under some circumstances by studying this observable (Ruiz et al., 2015). That is possible because of the high accuracy of the time-lapse gravity measurements, which is at the level of a few μGal.
Seafloor subsidence monitoring uses water pressure measurements at the seafloor as a starting point. Once the required tide corrections are applied, the method reaches accuracies of down to 2 mm, depending on the field conditions. This is much better than the accuracy specified by pressure sensor manufacturers, which amounts to typically 10 cm of water for a sensor qualified for 1000 m of water (Ruiz et al., 2016b).