Sensitivity, Limitations, and Error Inherent in Resistivity-tool-response Modeling
Resistivity-tool response is affected by the borehole mud, washouts, shoulder beds, thin beds, invasion, and bed dip, and in addition, every type of resistivity tool also has its inherent physical limits in measurement accuracy and resolution. Such limited vertical resolution and accuracy in detecting Rt by a specific resistivity tool may only be resolved by tool-response modeling if the minimum changes in tool response are greater than the specified accuracy of that tool. Because of the limitations imposed by tool physics, the uncertainty in resistivity-tool-response inversion is an issue even with an ideal data set measured in perfect borehole conditions. A clear understanding of how the smallest change in input (for instance, bed thickness or Rt in 1-D modeling) will produce a detectable change in output (the computed resistivity-tool response) is critical in the evaluation of the inversion results and uncertainty analysis. Using results from benchmark modeling, this chapter illustrates sensitivities and limitations of resistivity-tool-response modeling and potential errors in hydrocarbon-saturation calculation.
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Understanding resistivity-tool response and resistivity-log interpretation for formation evaluation is vital for the matching of the reconstructed deep-reading resistivity logs with the field log curves. AAPG Archie 2 introduces the fundamental concepts required. Resistivity-logging-tool physics and measurement accuracy are reviewed, and forward- and inverse-modeling resistivity-tool responses are introduced. In the case studies presented, well-deviation, shoulder-bed, bed-thickness, borehole, mud-resistivity, and invasion effects on restivity-log responses are discussed. This volume has been written for geoscientists and engineers working with and interpreting resistivity logs, petrophysicists and reservoir engineers integrating resistivity-based and capillary-pressure-based quantitative calculation of formation water saturation, and formation evaluation specialists.