Saturation Height Functions
Saturation Height functions (SWH, Swht) are a means of assigning a saturation to a volume of rock e.g. a geocell which is not penetrated by a well by virtue of that cells porosity, permeability and capillary pressure. At initial conditions capillary pressure is proportional to height above free water level (Ht) and an Sw may be assigned without knowing resistivity or other logged values. This is necessary in all parts of a geomodel not penetrated by a logged well and also along the well track itself wherever the saturation related log measurements provide less certainty for calculated Sw, e.g. in facies where there is little contrast between logged pay and non pay such as LRLCP. The Swht equation, like all equations, has uncertainty associated with its inputs, the crucial ones being permeability, interfacial tension, wettability (oil) and occasionally capillary pressure or height itself.
Ideally the petrophysicist will generate the Swht equation, developing and testing it thoroughly within the finer scale, cross referenced petrophysical domain before providing it to the reservoir geologist and reservoir simulation engineer in a seamless fashion as part of the common use Reservoir Rock Type Equation Set for Sw and other parameters.