“Comparison of modeling codes for resistivity and MWD instruments: part 2 1-D thin beds,” Petrophysics, Vol. 43, No.1, pp. 20-25, 2002.
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 111 formations containing many thin beds. In one case, the 6FF-40 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 8s-degree well in a multiple-layer formation. In both cases the formation resistively profiles arc assumed to be one-dimensional ( 1 -D), that is, the resistively 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 amplitud and phase data to R,, and R,, (amplitude-based and phase-based apparent resistively) of the MWD logs, the differences in these logs may be as high as 9 ohm-in or 30% in zones with resistively greater than 50 ohI1i-l11.