1.
n. [Formation Evaluation]
The effective
resistivity of the
formation water or the
mud filtrate, as it affects the electrochemical potential. The electrochemical potential is the main source of the spontaneous potential log. The equivalent water resistivity, or equivalent resistivity, is defined as the reciprocal of the activity of a solution, so that the formula for the electrochemical potential, E
_{c}, can be written as:
E
_{c} = - K log
_{10} (R
_{mfe} / R
_{we})
where K is a coefficient, and R
_{mfe} and R
_{we} are the equivalent water resistivities. The importance of R
_{mfe} and R
_{we} is that they are equal to the actual water resistivities for NaCl solutions above about 0.1 ohm-m. In salty waters below 0.1ohm-m, the equivalent resistivity is lower by an amount that depends on temperature and salinity. In fresh waters, divalent ions such as Ca
^{++} and Mg
^{++} have a strong effect on E
_{c}. R
_{we} is then related to R
_{w} by an
empirical transform that assumes average concentrations of these ions. For fresh mud filtrates, it has been common practice to set R
_{mfe} = 0.85 * R
_{mf}.