Reading True Rt

The Deep Still Lies a Little

The deep tool gets closest to Rt, but invasion still biases it. In an oil zone with deep invasion the deep reading falls below the true Rt; with a fresh-mud-over-water profile it can sit above. Take the deep reading alone and the saturation inherits that error. The fix uses the fact that we have not one reading but three, and three readings carry enough information to solve for both unknowns at once, the invasion depth and the true Rt.

Three Readings, Two Unknowns

The tornado chart (also called the butterfly chart) is that solution drawn out. Form two resistivity ratios from the three tools, the deep-over-shallow and the medium-over-deep, and plot the point. Behind the chart is a pre-computed mesh: one family of curves for constant $R_t/R_{xo}$ and another for constant invasion depth $d_i$. Where the point lands names both. All the curves spring from the lower-left corner, where there is no invasion and the three tools agree.

Off the Chart and Into Archie

Reading off the mesh gives the true $R_t$, corrected for the invasion the deep tool could not fully see past, and the invasion depth as a bonus. That corrected $R_t$, not the raw deep reading, is the number carried into Archie's equation. With the formation factor, the porosity, and now a trustworthy $R_t$ all in hand, the resistivity chapter has assembled everything the saturation needs. Putting it on the Ogbon-1 well is the close of the chapter.

References

• Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
• Schlumberger (2009). Log Interpretation Charts. Schlumberger.