The following favourable conditions are required to use the spontaneous potential to determine water resistivity (Rw): the interval is shale free (clean) and water bearing; the drilling mud must have a reasonable resistivity; formation water must be saline; the interval must have appreciable permeability; interval thickness must be adequate; and, hole size should be less than 25 cm (10 in).
The spontaneous potential log should first be baseline-shifted to a vale of zero. Input the static spontaneous potential value (SSP), along with mud filtrate resistivity (Rmf) and the associated reference temperature for the Rmf value. Also enter the reservoir temperature. Outputs include Rw at 25 deg. C (77 deg. F) and Rw at reservoir temperature.
Inputs are highlighted in yellow and outputs are in green. Both reference and reservoir temperatures must be entered in degrees Celsius, degC=5/9*(degF-32). The point associated with inputs is plotted on the cross plot.
Bates, R.M, and C.E. Konen, 1977, SP Algorithms For Rw Determination For HP-67 Calculators: The Log Analyst, v.9, p.38-52.