J. P. Wa1lace
Mobil Exploration and Producing Services, Inc.
F. F. Osborn, L. C. Shen
University of Houston, Department of Electrical Engineering
W. D. Lyle Jr. nbsp;
Mobil Research and Development Corporation, Dallas Research Laboratory
ABSTRACT
A major problem in the interpretation of induction log data is the correction of the recorded resistivity to account for bedding effects. In the conventional approach the analyst first determines the bed boundaries and then uses charts to correct the resistivity values for bed thickness effects. These charts assume ideal conditions, such as equal resistivities of adjacent shoulder beds or infinitely thick shoulder beds. The validity of these assumptions is often in doubt In addition to the idealized assumptions used to construct the correction charts, there are two further drawbacks to the chart-based interpretation process. First, in determining corrected resistivity values, only the peaks and valleys of the resistivity logs are normally taken into account, with the shape of the log at other points usually ignored. Second, when the estimated resistivities are obtained using the conventional process, there is no means to validate the interpretation. Prior to the development of induction log computer models, the conventional approach having the limitations described above was all that was available. These models do not eliminate all of the interpretation problems, but they do make possible an interpretation that is considerably more accurate than one which uses the conventional method.
In the computer-aided induction log interpretation approach considered here, it is assumed that invasion effects are negligible. This assumption is not required to implement the technique; however, the accuracy of the results will depend on its validity. For the cases considered here, this assumption appears to be valid. Two-dimensional modeling codes have been developed that will allow for the inclusion of invasion, but the computational requirements are substantially increased relative to the no-invasion situation. Borehole corrections are made prior to implementation of the modeling codes.
The first step in the computer-aided approach is to determine the bed boundaries as in the conventional approach. Next, an initial estimate of formation resistivity for each bed is made. Subsequently, the induction log simulation codes are used to compute the induction log in the formation based on the estimated formation resistivity profile. The analyst compares the computed log with the field log, adjusts the profile resistivity values and/or the bed boundaries, and repeats the process until the computed log agrees with the field log. Examples of interpretation of actual field logs using this procedure are presented.