Gravimetric monitoring.

The idea behind offshore gravimetric monitoring is to place concrete benchmarks on the seabed above the reservoir, and a few stations outside the reservoir which serve as reference points. The station spacing is typically 2-3 km. At 2-3 year intervals, we measure gravity by placing a mobile gravimeter on top of the benchmarks. The mobile instruments are carried by ROV, measuring ~20 minutes on each site. At the same time the depth of the benchmark is measured.

As gas is produced in the reservoir, the seafloor gravity will change both due to gas extraction and potentially water influx. In addition, the pressure drop causes reservoir compaction, which in turn leads to a measurable seafloor subsidence. The method has been proven to have a precision of 3 uGal for time-lapse changes in gravity, and 5mm for time-lapse changes in seafloor depth, and in some cases the results are even better than this.

The measurements can be directly compared to the reservoir simulation model by forward-modelling the simulated reservoir density change into predicted changes in seafloor gravity. This makes the method well suited for use in history matching.

We expect the number of applications to increase in the years to come, and also to expand to deeper, smaller and more complex fields as precision improves.