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A study has been made to try to improve the determination of water saturation of a given zone by use of the SP log. A method has been derived which appears to be an improvement in previous methods.
Previous studies have shown that the formation water resistivity, a needed factor in the Archie equation for determining water saturation, is related to the magnitude of the SP deflection and to the mad filtrate resistivity by a coefficient which is a function of formation temperature. Evidence has indicated that this coefficient also varies with geographical location.
An attempt was made to find a variable which would express this apparent variation in geographical location. The formation resistivity factor was selected and found to be satisfactory. The resulting empirical correlation relates the SP coefficient, formation temperature, and the formation resistivity factor. For the data used, this method produced results which were better than those of previous methods. An experimental study has been made which tends to confirm the trends and influences discovered in the empirical work.
In order to calculate the formation water saturation using the familiar Archie equation,
................(1)
the formation resistivity factor, F, the formation water resistivity, Rw, the true formation resistivity, Rt, and the saturation exponent, n, must all be determined or estimated. Values for Rw and Rt may be estimated by use of logs and correction and correlation charts. Values for F and n are usually determined from core analyses, or they may be estimated from logs and/or correlation charts. The accuracy of the estimated values of Rt, F, and n is usually 30%, but the approximated value of Rw is often in error by more than 100 percent. An incorrect assumption of Rw may lead to a very large error in Sw.
Many times water resistivities can be measured from DST samples from the well in question or from the same zone in nearby wells.
