Evaluation Of Water Saturation In Shaly Formations

In order to determine the water saturation of a shaly formation, use is made of a formula expressing the relationship between the true resistivity (Rt) and the formation parameters affecting this resistivity. These include water resistivity (Rw), clay resistivity (Rclay), porosity (phi), clay content (Vclay), and water saturation (Sw). Satisfactory results are usually obtained with a formula of the type:1/Rt = [(Vclay)^c * Sw/Rclay] + [(phi^m * Sw^n) / (a*Rw)]where the exponent "c" is usually taken equal to 1, "but sometimes larger values are used, up to 2. In some Indonesian wells, however, water saturations calculated with this formula were found to be too high, especially for clay contents in the range of 30 to 70%. A study of computer-made cross-plots has made it possible to establish an empirical relationship giving much better results in these cases: 1/sqrt(Rt) = [{Vclay^d / sqrt(Rt)} + {phi^(m/2) / sqrt(a*Rw)}] * Sw^(n/2)For the value of exponent "d", both d = [1- (Vclay/2)] and d = 1 have been found satisfactory; however, at high clay content the Sw value may tend to be slightly overestimated when d = 1 is used. This type of relationship, between Rt and Sw, expresses that the conductivity of the formation is made up of three terms, two of these involving the conductivities of the clay and formation-water networks. The third term represents the additional conductivity resulting from the cross-linkage of these two networks. EDITORS NOTE: sqrt = square root