R. Chemali, S.M. Su and J.F. Goetz
Halliburton Logging Services
R. E. Maute and F. F. Osborn
Mobil Research and Development Corporation
ABSTRACT
The effective electrical diameter for the six arm dipmeter is characterized and quantified through computer modeling. This parameter is critical for accurate dip computation, especially in highly deviated wells or in steeply dipping formations. From the dipmeter analyst’s point of view, the electrical diameter accounts for the finite depth of investigation of the dipmeter focused resistivity pads. Due to this depth of investigation, highly dipping features are seen above or below their intersection with the borehole wall, as if the pads were deployed within a hole with an electrical diameter larger than the actual borehole diameter.
In the first part of the study, simulated logs for water-based muds are produced for several dipping interfaces and thin dipping beds through extensive computations on a Cray supercomputer. Using the simulated logs through specified formation dips as test beds, a series of values for the effective electrical diameter are derived and tabulated for typical cases.
To a surprisingly large extent, the electrical diameter is found to depend on the particular correlation method used to determine the shift between curves. For instance, within a given correlation function, use of the logarithm of the resistivity rather than the resistivity itself yields a smaller and less variable electrical diameter. A new and robust correlation method which was developed during the course of the study reduces the variability of the electrical diameter. Additionally, excluding the slowly varying components from the resistivity curves brings the electrical diameter considerably closer to the borehole mechanical diameter, thus minimizing the associated uncertainty in dip computation. Advantages and drawbacks of the various
correlation methods are illustrated on real and simulated dipmeter logs.