A Review of Current Techniques for Determination of Water Saturation From Logs

Abstract

The basic saturation and log response equations are reviewed. It is concluded that conventional saturation calculations account for lithology and rock-type changes, but that they are susceptible to uncertainties in water resistivity (Rw), true resistivity, porosity and cementation factor (m). It is shown that resistivity-apparent porosity plots are useful in wells with minimum petrophysical data. Knowledge of Rw, m, the slope of the sonic log-porosity relation or the slope and intercept of the neutron log-porosity relation are not necessary, provided they are constant. Advantages and limitations are illustrated with examples. It is concluded that Rwa -depth plots are useful where Rw is unknown and lithology varies, provided m is known for all lithologies involved. Ros -So relations may be useful for determining water saturation when only a few porous intervals of constant rock type are present, and when either Rw or formation factor (but not both) are unknown. Finally, an example is reviewed to illustrate that, often, no one of the above techniques by itself may be diagnostic, and also to emphasize the need to utilize all available data.