Tidal response on the reservoir pressure at the Gullfaks oil field

This paper presents detailed analyses of a bottom-hole pressure record observed at the Gullfaks oil field in the northern North Sea. The series contains variability which is superimposed on the large-scale reservoir pressure. This variability can lead to a misinterpretation of the gradients coupled with short-term pressure tests. The presents analysis has revealed that the mechanism behind these fluctuations is mainly the loading from the sea tides. A number of tidal constituents have been recovered from the observations. These are in phase with the local sea tidal oscillations, however the amplitudes are reduced by a factor of 3–4. Earth tides cannot be detected. A model has been constructed to describe the tidal influence on the reservoir pressure at the specific location. The model is able to resolve the “reservoir tide” with satisfactory accuracy and may be used to correct for or remove the tidal influences during pressure tests. The effect of atmospherically induced sea-surface displacement on the reservoir pressure has also been studied.