Reading Rw from the SP

Part 3, Chapter 3: Gamma Ray and Spontaneous Potential

Learning objectives

Invert the static SP for the equivalent water resistivity
Apply the Rwe-to-Rw correction
Identify the conditions where an SP-derived Rw is reliable
Use the SP as an independent source of Rw

The SP Pays Off

The SP earns its keep here. We needed Rw for every saturation, and getting it wrong was the most common error in the whole evaluation. The SP hands it to us, almost for free, in any clean wet sand: just run the static-SP relation backward.

The Inversion

Start from $SSP = -K,\log_{10}(R_{mf}/R_w)$ . Rearranged, $R_{mf}/R_w = 10^{-SSP/K}$ , so the equivalent water resistivity is

R_{we} = \frac{R_{mf}}{10^{-SSP/K}}

Measure the deflection opposite a clean wet sand, take Rmf from the mud report and K from the temperature, and out comes Rwe. For saline waters Rwe is essentially Rw; for fresher waters a small, charted Rwe-to-Rw correction finishes the job.

When to Trust It

An Rw read this way, in a thick clean wet sand, is one of the most trusted numbers in the toolbox, an independent check on Rw from any other source. But respect its limits: shaly sands suppress the SP and bias Rw low; very fresh muds give a feeble, noisy deflection; hydrocarbon and thin beds reduce the kick. Read Rw from the SP where the conditions are clean, and carry it as your anchor through the saturation work to come.

References

Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
Bateman, R. (2012). Openhole Log Analysis and Formation Evaluation, 2nd ed. SPE.