Saturation exponent n in well log interpretation: Another look at the permissible range

Saturation exponent n in Archie's water saturation equation is the exponent value in water saturation that establishes the relationship between water saturation of the rock to the ratio of fluid filled rock resistivity to the actual rock resistivity. Most theoretical derivations show that there is no exponent or that n is equal to one. However, decades of results from core analyses and openhole log analyses show n greater than one and most often assumed to be two. Most of the literature is in agreement that the saturation exponent is affected by wettability and saturation history (drainage or imbibition). Furthermore, the saturation exponent depends on rock type, primarily the manner in which the conductive water is connected and configured. Several authors quote values of n ranging from as low as 1 to over 20 for strongly oil wet rocks. Too often an assumed value is accepted as correct. This paper reevaluates the relationship between water saturation and rock resistivities in order to determine the significance of n and make recommendations on improving an assumed value for n.