In Egypt's Western Desert, water saturation evaluation of the Upper Bahariya reservoirs is complicated by significant uncertainties in the resistivity-based saturation equation inputs, mainly the formation water salinity, but also the cementation factor and the clay cation exchange capacity (CEC).
A novel dielectric multifrequency measurement was able to clearly identify zones in which oil was present, regardless of these inputs. It provided a residual oil saturation and clear differentiation of oil-bearing zones from water-saturated zones. The measurement also permitted us to apply constraints to the conventional saturation interpretation from shallow and deep resistivities. This process highlighted the variability of formation water salinity across the well and the mixing of filtrate and formation waters in the zone investigated by microresistivity tools.
The dielectric tool also measures dielectric dispersion, from which a parameter (MN) related to the tortuosity of the conduction path can be extracted. This MN parameter was used to refine the water saturation computation from deep-resistivity logs. Variation of the MN parameter between 1.8 and 2 was observed in the reservoir zones. In the zone of interest, a water saturation difference of 7-8 % was observed in the cleanest zone between water saturation computed using the variable MN parameter and that computed using a fixed cementation factor of 1.9. The water saturation recomputed using the data from the dielectric log matches well the irreducible water computed from the nuclear magnetic resonance (NMR) data.
Dielectric dispersion was also used to derive the CEC, which was then input as a direct measure of shaliness in the deep-resistivity saturation equation.
The dielectric measurement increased the accuracy of the water saturation computation in this challenging environment and provided data that are not directly available with conventional logging.