1. n. [Formation Evaluation]

The electromagnetic force, in millivolts, generated by an electrolyte flowing through a permeable medium. This potential is an unwanted contribution to the spontaneous potential (SP) log. In principle, there is no flow into a permeable reservoir at the time of logging, since the mudcake has isolated the reservoir from the borehole. However, it is possible for mudcakes and shales to produce an electrokinetic potential at the time of logging. In normal conditions, any potential is small and equal along the borehole, so that the effect on the SP is negligible. A significant electrokinetic potential can be generated in particular conditions, for example, high differential pressure or poor mudcakes.

See related terms: bimetallism,  differential pressure,  shale baseline,  spontaneous potential,  static spontaneous potential