Influence of wettability on water saturation modelling

In order to calculate water saturation using standard equations (e.g., Arch ie; Waxmann and Smits), the saturation exponent (n) and the cementation exp onent (m) must be determined. These exponents are normally derived from ele ctrical measurements made on cleaned core plugs [i.e., formation factor (FF ) and resistivity index (RI)]. However, numerous papers have shown that cor ing and cleaning processes may significantly alter the wettability of the c ores. In this case, the measurements of n and m are no longer representativ e of the in situ conditions and estimates of the water saturation may thus he seriously wrong. We will demonstrate that wettability will have a seriou s impact on log estimated water saturation, and that even a very rough esti mate of the wettability can significantly reduce the error in estimated wat er saturation. The cited field examples are taken from North sea sandstone reservoirs. (C) 1999 Elsevier Science B.V. All rights reserved.