The saturation exponent (n) is an essential parameter that is required to solve the Archie Equation and obtain saturation values. This equation gives correlations between resistivity, porosity and saturations. When core measurements of n are unavailable, its value is assumed constant (usually 2). However, under certain conditions, this assumption of a constant value could lead to large errors in computed saturation values.
This paper presents a technique whereby electrical logs are used to solve simultaneously for n and the water saturation values Sw and Sxo in the uninvaded and invaded zone respectively. This technique combines open hole log values which are sensitive to saturation, namely the deep reading resistivity Rt, the shallow resistivity Rxo, and the electromagnetic transit time tpl, together with the through tubing cased hole measurement of the formation capture cross-section (Σ). Essentially four equations are used to solve for Sw, Sxo, n and the porosity exponent (m) at every log sampling point.
A data-base containing log data from 22 wells was constructed from the Upper Jurassic Arab Formation. This is a carbonate reservoir with anhydrite. The porosity distribution in sections of this formation is known to be heterogeneous, with the presence of vugs and fractures. The results of this study showed that the Arab Formation is water wet, and the computed values of n are consistent with the range of values obtained from core measurements. The n values were found to increase with increasing dolomitization, and were shown to be independent of both the pore type and of the porosity exponent m. The results presented demonstrates satisfactorily that this technique, under particular conditions, can give accurate computed values for n at downhole conditions.