ABSTRACT

New propagation resistivity tools make measurements using more than one array. The result is the measurement of several apparent resistivity curves rather just the one or two that were provided by early propagation resistivity tools. The primary purpose of multiple measurements is to be able to determine several parameters simultaneously. Effects that influence propagation resistivity measurements include anisotropy, eccentricity, anomalous dielectric effects, vertical resolution differences, borehole conductivity, and invasion. After correcting for all of these effects (except invasion), the data can be combined to produce apparent resistivities at several fixed depths of investigation. The result is a display much like the log of modem multi-array induction tools. This fixed depth presentation is an even more important feature in propagation resistivity logging than in induction logging because the standard propagation resistivity curves have radically different depths of investigation at different resistivities. The fixed depth processing alleviates this problem by calculating curves at specific depths of investigation (radial depths of 10, 20, 35, and 60 inches).

Several field examples are used to demonstrate the utility of this technique. Logs made while drilling show very little or no invasion while logs through the same section after drilling show significant invasion. The fixed depth curves give a quantitative estimate of the invasion depth. The propagation resistivity tools are also compared to multi-array induction tools which were run in the same well. When both logs were made after drilling, the results are reasonably consistent, though the propagation resistivity processing seems to be more robust.