A Discovery In The Lwd Resistivity Tool Response In A High-Angle Well - A Case Study Of Spiraled Borehole Respons

Anomalous" LWD propagation resistivity log res- ponses were observed in a high angle well. The ano- malous character includes anomalous curve separa- tions, periodic variations with depth, and anticorrela- tion of the phase and attenuation apparent resistivity curves. In particular, the long-spacing curves show larger depth variations than the short spacing curves. It was also observed that the cyclic variations remain in either compensated or uncompensated logs. The section of the well was drilled with oil-based mud (OBM) in a shale formation. Several possible hypo- theses were tested to explain the log. Numerical modeling showed that resistivity anisotro- py, tool eccentering, thin beds or dielectric effects alone could not explain the curve separations and the anticorrelation of the attenuation and the phase dif- ference apparent resistivity log responses. To test the hypothesis of spiraled borehole effect, a detailed 3D numerical study was performed. Contrary to an earli- er anticipation, a spiraled borehole itself does not necessarily cause significant cyclic log variation. Spirals gouges of reasonable geometrical dimensions failed to reproduce the magnitudes of variations in depth as observed in either the attenuation or phase difference logs, particularly for long spaced arrays. It is discovered that only a combination of a spiraled borehole with resistivity anisotropy, and optionally including tool eccentering effect, can explain the ob- served anomalous log responses, consistent with the anisotropic shale lithology. The numerical study fur- ther revealed that the alternation of the maximum and minimum responses in the logs does not depend on the transmitter-to-receiver or receiver-to-receiver spacing, but rather on the spiral period, which ex- plains the long- and short-spaced logs having closely similar period. Simulations in geometrically same spiral models, with and without anisotropy indicate an "amplifica- tion" of the spiral affects with inclusion of anisotropy in the formation model. Formation anisotropy can "amplify" spiral effects, which are normally a minor perturbation for apparent resistivity responses. This opens the idea that a mixture of various resistivity affecting environments may be more the norm. The combination of these effects must be corrected for simultaneously, instead of correcting for a single en- vironmental effect, or another, as is now the usual practice in the industry.