This can be done by direct observation of the production of oil, gas and water in the wells. However, to get information about how the fluids move in the reservoir between the wells other monitoring techniques can be useful.

In Statoil we use geophysical reservoir monitoring techniques to monitor pressure changes and fluid movement in most of our major fields on the Norwegian continental shelf (NCS). The two major techniques used are time-lapse seismic (4D seismic) and gravimetric monitoring.

We can measure changes in a reservoir by using four-dimensional seismic- repeated seismic acquisitions to observe and estimate how a reservoir is being drained. One seismic survey is acquired prior to production start and then monitoring seismic data is acquired at different times in the life of a field. The 4D in 4D seismic is referring to the three spatial dimensions (x,y,z), with the fourth dimension being time.

As hydrocarbons are replaced with water and the pressure in the reservoir changes, the seismic data will change. Then we have a volumetric image of the reservoir.

In offshore gravimetric monitoring, changes in the gravity field of the Earth are measured by using relative gravimeters at permanently deployed concrete benchmarks on the seafloor.

Statoil has been one of the pioneers in the use of 4D seismic and gravimetric monitoring. On the Norwegian continental shelf, Statoil has ongoing 4D projects in some 80 percent of fields in production (2013).