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. Statoil has been one of the pioneers in both 4D seismic and gravimetric monitoring. The Oseberg 4D seismic project was started in 1992, and the Gullfaks 4D research project started in 1993, with a pilot time lapse seismic in 1995, and the first full field time-lapse survey in 1996. Today, we have 4D seismic on around 3/4 of the fields we operate.

Four-dimensional seismic uses 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.

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. The precision of the measurements has increased significantly throughout twelve years of development and deployment of the technique.

Illustration
Gullfaks 1985 - base	Gullfaks 1999 - monitor
Seismic data was acquired over the oil filled (green) reservoir of the Gullfaks field in 1985, one year prior to production start. After 13 years of production, a new full-field seismic survey was acquired in 1996. At this point, parts of the oil has been produced and replaced by water (blue). The seismic response from an oil field reservoir is different from the response from a water filled reservoir at Gullfaks, and this makes it possible to map the undrained areas. This information can be used to plan and drill new infill wells. The 4D seismic at Gullfaks is one of the factors contributing to the high recovery rate at the Gullfaks field (initial recovery factor was 46 %, recovery to date is 56 %, the ambition is 70 %).