Continuous well logging during drilling

Continuous well logging during drilling is carried out by (a) subjecting the drilling fluid to be injected into the well to a "unipolar electric treatment" while simultaneously measuring its redox potential and maintaining it at a constant value (V1); (b) continuously measuring the redox potential (V2) of the fluid withdrawn from the well; and (c) using the difference between V2 and V1 to determine the mineralogical compsn. of the formation rock being penetrated by comparing it with the normal redox potentials of candidate minerals. The drilling fluid is "stabilised" in an electrolysis cell in which the electrodes (2,3) are supplied with a direct current from a suitable source (4). By varying the voltage across the electrodes and the current, the redox potential is adjusted to a value corresp. to oxidn.-redn. equilibrium (generally 1.6-1.8 V for drilling muds). The drilling fluid is treated in the catholyte chamber if the chemical reactions in the fluid are reductive in character, or in the anolyte chamber if the reactions are oxidative in character.