Quantifying remaining oil saturation using time-lapse seismic amplitude changes at fluid contacts

In this work, we consider the evaluation of the oil saturation left behind in a hydrocarbon reservoir, after oil is displaced by water as a consequence of natural drive oil production, or production supported by water injection in the oil leg. Our study shows that time-lapse changes in the amplitude of the seismic reflection at an oil-water contact and/or produced oil-water contact can be used to directly estimate the displacement efficiency of water displacing oil, ED without the need of a rock and fluid physics model. From this value it is possible to determine the remaining oil saturation if required. An application is performed using a 4D seismic dataset from a producing field in the North Sea. The results suggest a displacement efficiency of between 21 and 65% with an accuracy of 3% due to data non-repeatability (with a NRMS of between 11 and 13%). A pre-requisite for use of the proposed oil-water contact approach is that a discrete contact be interpreted on either the 3D or 4D seismic datasets. Therefore successful application of this technique requires moderate to high quality seismic data and a fairly thick reservoir sequence without significant structural complexity.