The Spontaneous Potential (SP) Log
The spontaneous potential tool measures natural electrical potentials that occur in boreholes. The "battery" mechanism is caused by the drilling of the borehole with drilling mud that has a different salinity from formation waters. The invasion of formations by mud filtrate results in the juxtaposition of two aqueous solutions with different ion concentrations. Ions diffuse from the more concentrated solution (typically formation water) to the more dilute. The ion flow is electrical current, with an associated potential measured in millivolts. The cumulative potential is caused by several effects: the electrochemical components of a liquid-junction potential and a shale membrane potential, together with a minor electrokinetic potential.