Water saturation determination from wireline resistivity log data depends on measurements of the resistivity index on core samples using Archie’s equation.The only variables in Archie’s equation are water saturation and rock resistivity,and it is assumed that the relationship is independent of the flow displacement mechanism.This paper reports an experimental investigation of the influence of flow injection rate and the viscosity ratio between the displaced and the displacing phases on the Archie saturation exponent.Resistivity index measurements were carried out by the continuous injection technique.The results show that the saturation exponent is a function of the displacement mechanism and fluids characteristics,and that different resistivity-saturation relationships can be obtained for a given sample under different experimental conditions.Because errors in the evaluation of saturation exponent can give rise to serious errors in the estimation of hydrocarbon saturations,it is concluded that the determination of the saturation exponent requires particular care,and that fluids characteristics and the desaturation method should be carefully selected whenever resistivity index measurements are performed.
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