PetrophysicsResistivity Logs
Resistivity Index - Archie's Law Formula
Resistivity Index - Archie's Law calculates resistivity index for resistivity logs workflows in petrophysics.
How engineers use this formula
Use this formula when the listed inputs (R_t, R_o) are known and the assumptions behind the cited resistivity logs relationship match the engineering case being checked.
Assumptions
- Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
- The declared units match the field-unit constants used in the formula.
- The cited formula applies to the selected petroleum engineering workflow.
Limitations
- The calculation does not replace a full engineering model or operating procedure.
- Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.
Common mistakes
- Mixing unit systems without converting the inputs.
- Using default example values as field recommendations.
- Applying the formula outside the source assumptions.
Default example
Using the default inputs, RI equals 4 dimensionless.
R_tohm m
20
R_oohm m
5
Inputs
R_t
ohm mTrue Resistivity
R_o
ohm mResistivity of Water-Filled Rock
Outputs
RI
dimensionless
Resistivity Index
R_t
ohm m
True Resistivity
R_o
ohm m
Resistivity of Water-Filled Rock
Source and review
reviewedCore Laboratories. 2005. Formation Evaluation and Petrophysics, Page 42.
Source