During the past 10 years the number of wells drilled with oil base muds has increased substantially in various parts of the world and quite often offshore on production platforms. The reasons of using oil base muds are various:" Avoid the swelling of the clays in water sensitive" reservoirs" Ease the drilling of highly deviated holes" Make the wells less sticky The use of such non conductive muds naturally had an effect on logging programs, limiting the resistivity tools to the Induction device, generally the Dual Induction. First the assumption was made that the invasion of the formation by the drilling fluid, now oil, was negligible. However, over the years it was observed that the oil of the mud was invading the formation as any water filtrate will do, so deeply sometimes that water zones were unnecessarily tested. Estimated diameters of invasion were over 120" when very light oils were used. In order to correct the Dual Induction for this invasion an estimation of the resistivity of the invaded zone is necessary. The percentage of the invading oil is also required to correct the porosity tools for this "hydrocarbon" effect.

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