ABSTRACT:

The assessment of the reservoir oil or gas in place depends significantly on the accuracy of resistivity data and on the reliability of their interpretation. However, the evaluation of the formation resistivity from Russian BKZ logs is very troublesome for western oil companies as the western interpretation approach, based on laterolog measurements, is not suitable for the Russian tool characteristics and measures. Resistivity log data recorded in three Russian wells, for which a pronounced inconsistency was found between the water saturation values obtained according to the western interpretation method and the nature and rates of the produced fluids during well testing, were also interpreted applying the Russian methodology and a software, named Rt-Mod, suitable for interpretation of Russian logs. The Russian method involves a manual comparison between experimental resistivity measurements and theoretical type curves to obtain formation resistivity, invaded zone resistivity and invasion diameter. The Rt-Mod software performs a numerical resistivity simulation based on an inversion method used to generate formation and invaded zone resistivity profiles. Resistivity values obtained from both methodologies appear to be reliable, and more consistent with well testing results than those obtained when the western interpretation was applied. In particular, the Russian approach appears to be very reliable when layers are thicker than two meters and allows the evaluation of the formation resistivity and invasion diameter also when deep mud invasion is present. However, the Russian approach is complex and very time consuming. Results obtained from the software application are fairly consistent for any layer thickness. The numerical simulation is very quick, simple, and only requires few data to run log interpretation. However, calculated mud resistivity values are not always consistent with the reported fluid property.

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