Petrophysical Interpretation of a Multispacing 2-MHz MWD Resistivity Tool in Vertical and Horizontal Wells

Abstract

The multiple phase shift and attenuation resistivities from a 2-MHz array resistivity tool provide the log analyst with the measurements needed to resolve complex interpretation problems associated with high relative dip formations typical in horizontal and highly deviated wells. With the support of modeling, productive zones can be located that may otherwise be missed.In high relative dip formations, the interpretation of multispacing 2-MHz resistivity tools and the comparison with wireline logs are not straightforward. Therefore, it is imperative for the log analyst to understand the unique response characteristics of these measurements in both vertical and horizontal wells. In high relative dip formations, the primary factors that may cause the resistivity curves of different spacings to separate are borehole effects, invasion, bed boundary proximity and anisotropy. With the help of forward modeling codes and inversion algorithms, the log analyst can resolve most of the interpretation ambiguities.The responses of a 2-MHz array resistivity tool, with five measurement spacings, to the above factors are illustrated and discussed. Traditional rules of thumb based on vertical well must be modified significantly. Productive zones that would be missed by logs made in a vertical well can easily be detected by logs made in a high-angle or horizontal well through the same formation. A comparison of the resistivity responses of a 2-MHz tool in low and high relative dip formations helps explain the differences between logs made in low-angle and high-angle wells.