Streaming potential, electrofiltration potential. A component of the SP produced as a result of movement of the invasion fluid through the mud cake or permeable formation. The electrokinetic potential is the potential difference which arises across a capillary when a stream of invading fluid is passed through it. The ions in the diffuse layer are swept along by the invading fluid so that a displacement of charge occurs. Opposite charges are built up at opposite ends of the capillary.
An electrofiltration potential also exists at the interface between the borehole and shale beds, a circumstance which tends to nullify somewhat the effect of the mud-cake electrofiltration potential in producing variations on the curve. In rare instances, in the case of a very low-permeability formation, little mud cake may be formed and a large electrofiltration potential may be generated across the formation itself.