The Calculation of Water Resistivities From Chemical Analyses

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Abstract
A method of calculating formation water resistivities from chemical analysesis presented which is somewhat faster and more accurate than previouslydescribed methods. For 26 formation water samples taken from wells in the TexasGulf Coast, the average deviation of calculated from measured waterresistivities was 3.3 per cent, with an average bias of + 1.1 per cent in thecalculated values. This compares with average deviations of 4.6 per cent and7.8 per cent, with corresponding average biases of +3.2 per cent and -6.3 percent, for the two methods previously described in the literature. All threemethods are sufficiently accurate for most purposes; the main advantage of themethod presented is its speed and convenience. Discussion It is frequently desirable to calculate the formation water resistivity fromthe chemical analysis of the water, as for example, in electric loginterpretation. Two methods have previously been given in the literature foraccomplishing this. Both methods require that the chemical analysis be given inthe Palmer system, as primary and secondary alkalinity and salinity. Thisconversion of the values of the ppm of the various ions to primary andsecondary salinity and alkalinity is time consuming, and many laboratoriesreport only the concentrations of the various ions in parts per million.However, all laboratories report the individual ionic concentrations, whetheror not the Palmer system of reporting is also used. Hence it was thought that amethod that would permit resistivity calculations directly from these datawould be useful. In the method developed, the various ions reported in the chemical analysisare converted to an equivalent amount of NaCl. The sum of the variouscontributions to the NaCl (actual or equivalent) can then be used inconjunction with a plot of NaCl concentration versus resistivity to obtain theresistivity of the solution in question. T.N. 67

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