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Recent experiences in offshore Indonesia using an MWD multiple depth of investigation resistivity sensor provided information about shaly sands which differed with the wireline dual laterolog run over the same intervals several days later. The difference in response between the two tools were substantial enough to cause differing results when utilising a log analysis program designed to determine prospective net pay. Analysis using the MWD log indicated that a marginal zone had low enough water saturation to be considered pay, in contrast to the wireline log analysis, which was more pessimistic. The cost of testing was debated but undertaken on the strength of the MWD evaluation. Good test results verified the MWD analysis, making a discovery out of a marginal zone. Substantial differences in calculated water saturation over several other reservoir sections were also the direct result of MWD-wireline resistivity discrepancies.
A comparison of the two technologies shows that the differences between the log responses can be explained by a combination of borehole and invasion corrections. After applying these corrections, the MWD response shows negligible changes to apparent resistivity (Ra), indicating that the raw response is very close to true resistivity (Rt). In contrast, the dual laterolog required substantial corrections which led to errors in water saturation (Sw) of over 10 per cent.
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