Abstract
Water saturation determination from wireline resistivity log data depends on measurement of the resistivity index on core samples. Resistivity index measurements carried out in the laboratory are the input to the second Archie equation: I=Rt.Ro-Sw-n
Errors in the evaluation of the saturation exponent (n) can give rise to serious errors in the estimation of hydrocarbon saturations. This equation assumes that the same value of water saturation should give the same value of resistivity whatever the fluids characteristics used, because (S_w) and (R_t) are the only variables in the equation. It is assumed that this equation is independent of the fluids characteristics.
The continuous injection technique is used increasingly in the oil industry. In this technique, one fluid phase (brine) is continuously displaced by another (oil) at a fixed viscosity (Brine/Oil) ratio. An experimental approach is proposed here in which repeated resistivity index measurements are carried out on the same core samples at different viscosity ratios. The effect of viscosity ratio on the saturation exponent for the core sample can then be investigated. Four carbonate samples with a wide range of porosity and permeability were tested to demonstrate resistivity index measurements at low and high viscosity ratios. The results showed that the fluid characteristics, such as viscosity ratio, have a crucial effect on the I versus (Sw) relationship. For some of the viscosity ratios used, the second Archie equation consists of two straight-line segments on a log-log plot; this may be due to a bimodal pore size distribution in the core samples used or due to the influence of other factors. Use of the continuous injection technique at different viscosity ratios has demonstrated that for water-wet samples, a different I versus Sw correlation can be obtained. It is, therefore, recommended that for resistivity index measurements, fluids characteristics representing the reservoir of interest should be used in order to obtain a reliable I versus Sw relation and, hence, an appropriate value of saturation exponent.