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Do you really know Rt**True Formation Resistivity
Despite improvements in resistivity logging tool design over the last 40 years, it is well known in the oil and gas industry that in complex formations traversed by a borehole, the measured deep-resistivity or apparent resistivity Ra is not equal to the true formation resistivity Rt.

Environmental effects and tool limitations
Distortions in resistivity log measurements are caused by various complex environmental effects (borehole, drilling mud, shoulder beds, invasion, anisotropy, formation dip) and also tool limitations (vertical resolution, radial depth-of-investigation, nonlinear behavior). 
These make interpretation and evaluation of formation resistivity a hard job even for the most experienced log analyst.

Complexity of Resistivity Tool Measurement  Click image to enlarge

Limits of correction charts
Applying correction charts to resistivity logs may be a first step towards an improvement of Rt estimation in very simple formation models. But this leads commonly to inaccurate corrections for Rt and therefore causes underestimation of hydrocarbon reserves.

The key to recover Rt
The most efficient way to overcome simultaneously all the environmental effects and the tool limitations is resistivity inversion processing involving iterative forward modeling. This resistivity modeling technology is now becoming a key step in petrophysical log interpretation, since it allows efficient quantitative evaluation of properties in the most complex formations and borehole environments. For hydrocarbon exploration, it is definitely a turning point in complex borehole fluid invasion determination and accurate water saturation estimation. Using this technology contributes to significantly improve oil saturation evaluation, commonly up to 20% compared to conventional methods or approximated modeling techniques.

Resistivity Modeling Technology allows:
  • re-assessment of old resistivity logs,
  • recovery of by-passed hydrocarbons,
  • study of thin beds and fractures,
  • validation of hypothetical earth models,
  • analysis and extension of correction charts,
  • pre-computation of a data base of tool responses,
  • sensitivity studies of tool responses,
  • understanding of anomalies and artifacts in logging data,
  • understanding of logs in highly-deviated wells and dipping formations,
  • planning and steering of horizontal wells.
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