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.