Experiences on Seafloor Gravimetric and Subsidence Monitoring Above Producing Reservoirs

During the last 12 years, Statoil has collaborated with Scripps institution of Oceanography to develop a method for precision seafloor measurements of time-lapse gravity and depth changes. The precision is now at a level where effects of pressure depletion and hydrocarbon production are visible, and the data can be used in history matching the reservoir simulation model. This paper shows some results from two ongoing monitoring programs offshore Norway. At Troll, a subsidence of about 1 cm/year is observed, and both gravity decrease due to hydrocarbon production and gravity increase due to edge water influx are seen. At Sleipner, the gravity data yields an estimate on water influx to the producing gas reservoir. In addition, a clear signal from the CO2 injected into a nearby aquifer is seen. Finally, the potential of the method in various geological and production settings is discussed.