Venezuelan Annual Meeting, 6–9 November, Caracas, Venezuela


Resistivities related to the drilling fluid, i.e., Rm, Rmf, and Rmc, are fundamental in quantitative analysis of electric logs. However, these values are usually assumed to be "known" and analysis carried on from there. It is the authors opinion, thought to be substantiated by the field data presented, that serious errors are being introduced through inaccurate determination of these resistivity values.

He concludes that the Rmf/Rm relationship usually applied in fresh-water clay-base muds is too low and that the ratio 0.87 is more realistic. Also, that the Rmc/Rm relationship usually applied to these same muds is too high and that the ratio 1.11 is more accurate. However, as shown by scattering on the graphs, use of these average values may still admit large errors.

The conclusion is reached that similar errors exist in the determinations made in oil-emulsion muds and that more nearly correct ratios would again be 0.87 and 1.11, respectively. Scattering is even greater here, which indicates that these properties should be measured for each individual well at the time of logging rather than placing dependence upon their relationship to Rm.

Laboratory measurements as currently made in oil-emulsion muds do not represent true mud conditions, possibly due to a difference in degree of emulsification and/or further chemical changes in the mud with time.

Comparison of mud-cell measurements with collapsed Micro Sonde measurements may indicate introduction of an error through use of the standard NaCl temperature-resistivity chart in the case of oil-emulsion muds.

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