A new technique is presented for the modeling of initial satu- rations in water wet oil/water reservoirs with large transition zones that have been exposed to imbibition as a result of structural tilting, hydrodynamic effects, gas cap leakage or compaction, or water influx during production. The method quantifies the extent of imbibition that the reservoir has un- dergone and determines the locations of the existing and the original, paleo free water levels. This is done by matching calculated and log derived water saturations utilizing existing empirical correlations for drainage and imbibition capillary pressure curves. From this information, the saturation in reser- voir regions without well control may be computed. In addition, a method is proposed for the identification of imbibition conditions. It is based upon an estimation of the capillary pressure from logged saturations. If a plot of the capillary pressure vs. depth deviates from a linear trend it is an indication of imbibition. The new method has been tested by field data from the North Sea. It is shown that the method can capture a large residual oil zone and a free water level far above the oil-water contact and that it is able to establish a good approximation to the saturation distribution resulting from the imbibition. It is also shown that a considerable error in the initial saturation distribution, and hence in the oil-in-place estimate, may be introduced if imbibition is not taken into account.
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