High data quality and improved environmental corrections of array induction logging tools have expanded their operating range into wells drilled with high-salinity water-based mud. Multi-laterolog tools are preferred for these borehole conditions when formation resistivity is high, but if the ratio of formation-to-mud resistivity is moderate, array induction tools can be used successfully.
A fairly large region of formation resistivity and formation-to-mud resistivity contrasts exists where you would expect either array induction or multi-laterolog devices to perform well. However, each responds differently to borehole effects, near-borehole effects, and invasion.
Data from both array induction and multi-laterolog devices was acquired in an offshore well from China with low-to-moderate formation resistivity drilled with high-salinity water-based mud. In some sections of the well the drilling assembly produced a "spiral" effect with small, periodic changes in borehole size. The majority of the well had no invasion but there was shallow conductive invasion in some permeable intervals.
The differences in sensitivity to shallow conductive invasion and the significant effect of a very conductive "spiral" borehole on the array induction responses is illustrated and explained by modeling. A filtering technique to eliminate the spiral effect and improve the responses is described and illustrated. The multi-laterolog tool has no sensitivity to the spiral borehole shape.
The determination as to whether an array induction or a multi-laterolog is the preferred resistivity device should be based on more than the value of formation resistivity or the contrast between mud and formation resistivity. In some conditions one has preference, while in other conditions it would be advisable to acquire both.