StatoilHydro is now, as the only operator in the world, conducting gravity reservoir monitoring on six Norwegian gas fields offshore to track changes in the gas/water contacts.
Field-wide gravity monitoring offers a unique possibility to directly measure changes in mass as they take place in a producing reservoir .
Such measurements are a useful tool for identifying the movements of gas/fluid contacts, optimising production and estimating in-place reserves.
Four time-lapse gravity surveys have been acquired over the Troll field in a pioneering attempt to image and monitor the rise of the liquid contact during gas production. On the Sleipner field two surveys show the average change in density when CO2 is injected in water-filled sandstone.
The results have later been confirmed by well and seismic data, and are actively used for reservoir management purposes.
These surveys represent the first offshore application of gravimetric reservoir monitoring in the world, and new surveys are being planned as production continues.
Similar projects have recently been started over onshore gas fields.
Unique gravimeter
StatoilHydro's state-of-the-art seabed gravimeter – ROVDOG II (Remote Operated Vehicle Deep Ocean Gravimeter) – was developed for the purpose in association with Scripps (University of California, San Diego).
Comprising three gravity sensors and three pressure sensors, it can also be used to monitor seabed subsidence caused by the reservoir rocks undergoing compaction as the gas is removed.
On the Troll field a subsidence of 1-3 cm has been observed over the thickest part of the reservoir over a three-year period.
The height of the instrument package is about 60 cm.
The instrument can measure subsidence of less than one centimetre.
In addition, its gravitational accuracy is better than 3 x 10-9 of the earth's total gravity field.
This is an improvement of one to two orders of magnitude compared with measurements acquired from conventional shipboard instruments.
