SP and Water Resistivity LAMBDAs
Maximum spontaneous potential and the equivalent water resistivity inverted from it.
2 lambdas
PO.PP.Sp.Maximum
The static spontaneous potential, which is the maximum deflection a thick clean bed can develop. It is fixed by the equivalent resistivities of the mud filtrate and the formation water and by temperature. SSP = −K log10(Rmfe / Rwe), with K = 61 + 0.133 T and T in °F. A fresher filtrate opposite saltier water gives a negative SSP, in millivolts.
PO.PP.Rw.FromSsp
Equivalent formation-water resistivity from the static spontaneous potential, for water in which NaCl dominates. The SP equation is SSP = −K log10(Rmfe / Rwe), with K = 61 + 0.133 T and T in °F. Solving for the water resistivity gives Rwe = Rmfe · 10^(SSP / K). SSP is negative in the usual case of a fresher mud filtrate opposite a saltier formation water, and Rwe then comes out smaller than Rmfe. The result is the equivalent resistivity at formation temperature, which is what the NaCl form of the SP equation solves. The chart that turns Rwe into true Rw is not a closed form and is not in this lambda.