Comparison of Modeling Codes for Resistivity and MWD Instruments: Part 2, 1-D Thin Beds

摘要

Part 1 of this series of reports examined the codes that are used to compute the effects of the borehole and radial invasion on the readings of induction and MWD borehole tools. In this paper the second part of the comparison exercise is reported for instrument responses in formations containing many thin beds. In one case, the 6FF40 induction tool responses in the Oklahoma formation are computed with no relative deviation. In the other case, we compare the computed logs of a generic MWD tool in an 85-degree well in a multiple-layer formation. In both cases the formation resistivity profiles are assumed to be one-dimensional (1-D), that is, the resistivity is a function of depth only and the borehole and invasion are neglected. The objective of this survey is to compare different computer modeling codes that have been developed by various academic and industrial groups. Only the numerical results are compared. The computation speed, memory requirements, and other factors are not covered in this project. Five groups participated in this exercise for the MWD tools and four groups for induction tools. Agreements between results obtained from those codes are generally good. The maximum difference in computed apparent conductivity is less than 0.1 mS/m in induction codes. The differences in computed amplitude ratio and phase shift in MWD codes are limited to 0.02 dB and 0.1 degree, respectively. However, primarily due to different schemes used to convert the amplitude and phase data to R_a and R_p (amplitude-based and phase-based apparent resistivity) of the MWD logs, the differences in these logs may be as high as 9 ohm-m or 30 percent in zones with resistivity greater than 50 ohm-m.