Influence of wettability on water saturation modelling

In order to calculate water saturation using standard equations (e.g., Archie; Waxmann and Smits), the saturation exponent (n) and the cementation exponent (m) must be determined. These exponents are normally derived from electrical measurements made on cleaned core plugs [i.e., formation factor (FF) and resistivity index (RI)]. However, numerous papers have shown that coring and cleaning processes may significantly alter the wettability of the cores. In this case, the measurements of n and m are no longer representative of the in situ conditions and estimates of the water saturation may thus be seriously wrong. We will demonstrate that wettability will have a serious impact on log estimated water saturation, and that even a very rough estimate of the wettability can significantly reduce the error in estimated water saturation. The cited field examples are taken from North sea sandstone reservoirs.