Analysis of Uncertainties in Applying the Waxman-Smits Equation to the Determination of Remaining Oil Saturation

Summary An analysis of the Waxman-Smits equation is presented that describes how uncertainties in the input log data and evaluation parameters affect computed water saturation, Sw, values, taking into account the interdependence of uncertainties that arises when intermediate parameters in the evaluation (water resistivity and excess clay conductivity) are derived from log data. The analysis is applied to the evaluation of remaining oil saturation from openhole logs taken in a number of reservoirs in the Baram delta, offshore Sarawak, Malaysia. When an original water zone is available for the determination of intermediate parameters in these formations, it is concluded that remaining oil saturation in the 0.10- to 0.30-PV range can be determined with a standard deviation of ≈0.04 PV from openhole logs. A comparison with core-measured Sor values indicates that laboratory measurements and log-derived results differ significantly in these reservoirs. In one field example, this difference is attributed to reservoir inhomogeneity and the averaging inherent in the logging-tool resolution. In a second example, there is evidence that the discrepancy is a result of continued oil drainage after water breakthrough, which may be attributed to a mixed-wettability condition in the reservoir.