Abstract

Use of incorrect values of m, n, and a in Archie’s water saturation equation, can lead to overlooking producible zones or the completion of poor zones. Often assumptions are made to approximate m, n, and a; frequently, m and n are 2.0, while a may be 0.81 and 1.0 depending on lithology. When core is available, accepted laboratory practices exist to calculate m, n, a; however, these laboratory procedures are generally not considered routine analysis, and may not be conducted. Log analysis methods exist for estimating m using correlations as a function porosity assuming a is constant and known, typically 0.81 or 1.0.

Core analysis for m and a require saturating multiple plugs of varying porosity 100% with water of know resistivity. The resistivity of each water saturated core is graphed on a log-log plot with a porosity to yield m and a. To acquire n, a single core plug is used. Resistivity of the core is measured at various saturations; a plot of saturation and the resistivity yield n.

This work illustrates a method to calculated m, n, and a simultaneously when saturation, effective porosity, and resistivity of the sample or zone are available. The data may be available from advanced core analysis, and special logs that estimate saturation independent of Archie’s saturation equation such as the dielectric and NMR logs. The use of this method using well logs is particularly advantageous as it is an in-situ measurement of m, n, and a. The method is sensitive to random scatter in the data, especially when the ranges of saturation, porosity, and resistivity are narrow.