ABSTRACT

A new generation of Laterolog tools provides azimuthal resistivity measurements around the borehole. The response of these mandrel tools in dipping beds has been modeled in order to evaluate the viability of computing formation dip from the output azimuthal images. The modeling results demonstrate that dip information is present on the azimuthal resistivity curves. They also show that structural formation dips can be computed with good accuracy although the Azimuthal Laterolog has a much coarser vertical resolution than the standard dipmeter tool. They also show that a dip correction is necessary in order to compensate for the fact that the electrical currents emitted by the device penetrate the formation and make the bore hole diameter appear slightly larger than actual when it comes to computing the dip plane orientation, It is shown that, the classical incremental electrical diameter concept used for all traditional dipmeters is also applicable to the Azimuthal Laterolog although there is no direct link to the depth of investigation of the tool. This important parameter depends mainly on the pre-processing applied to the input curves and the type of algorithm used to compute dip; it is relatively insensitive to formation geometry and to bed resistivity contrast. As a by-product of the dip computation, a better estimation of the formation resistivity is obtained by stacking the azimuthal resistivities along the apparent dip plane. This technique greatly improves the evaluation of laminated reservoirs when apparent dips are greater than 30 degrees. A software package using cross-correlation techniques, or local event detection, has been developed to compute dips for any tool yielding azimuthal data, including the Azimuthal Laterolog. Field examples comparing Azimuthal Laterolog dips with standard Dipmeters confirm the potential of the Azimuthal Laterolog for dip determination.

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