1. n. [Formation Evaluation]
An equation for calculating the water saturation from resistivity and other logs. There are many saturation equations. Practical equations are all extensions of the Archie equation, which is valid for a rock with very little clay, or very high salinity water, and with a regular pore structure. The majority deal with the problem of shaly sands, and can be divided into two main groups?those that treat the shale as a volume of conductive material (Vsh models), and those that analyze the effect of clay counter-ions. Vsh models take many forms. Typical examples are the Simandoux, laminated sand and Indonesian equations. The other group includes the Waxman-Smits, Dual Water and SGS equations. Most nonshaly sand equations deal with the problem of mixed pore types, for example combinations of fractures, isolated pores and intergranular pores.
See: isolated porosity