1. n. [Formation Evaluation]

With reference to the spontaneous potential log, the coefficient, K, in the equation relating electrochemical potential to the chemical activity of the mud filtrate and formation water. E_c = - K log₁₀ (a_w / a_mf). The coefficient is equal to kT/e in which k is the Boltzman's constant, e is the electron charge and T is the absolute temperature. K is equal to 71 at 25^oC [77^oF], 12 from the liquid junction potential and 59 from the membrane potential for a perfect shale.

See: liquid-junction potential,  membrane potential,  mud filtrate,  static spontaneous potential