Conventional resistivity tools run in horizontal wells give misleading readings in highly anisotropic formations. These tools are designed to read beyond the invaded zone to obtain true resistivity of the formation. While this design feature is an advantage in vertical or deviated wells, it leads to errors in anisotropic horizontal wells where the tools are affected by neighboring layers. Conventional shallow resistivity measurements would not give true formation resistivity because of mud filtrate invasion.

In horizontal injector wells with zones of actual water saturation exceeding 60%, logging with conventional resistivity tools resulted in calculated water saturation of less than 30%. Production testing of the formation and changing casing setting depths are parts of the consequences of the unexpected low water saturation. Four different resistivity tools were run in a well to investigate their relative responses.

The logging while drilling Geosteering Tool (GST*) was used for the first time in the Middle East to determine Rt of a horizontal well. The tool is shallow reading with the resistivity measurement located at the bit to minimize the effect of invasion. This resulted in a realistic water saturation of 70 – 80% compared to 20 - 30% obtained from conventional tools. Model simulation of the readings of both conventional tools and GST confirmed the field measurements.

For drill pipe conveyed application in horizontal wells, the Azimuthal Resistivity Imager (ARI*) provides 12 directional readings around the tool. Eliminating the adversely affected directional values allows proper Rt determination.

The paper will compare the results of the Azimuthal Resistivity Imager, Geosteering Tool, Resistivity at Bit and conventional tools. Modeling demonstrates the different response of electrical tools in the vertical versus horizontal plane. The paper will highlight the advantages and limitations of each resistivity tool particularly in horizontal anisotropic carbonates.

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