The Dual Laterolog-Rxo Tool

Abstract

Introduction The Laterolog*, developed by H.G. Doll, has answered the need for a tool capable of producing useful resistivity measurements even when R /R is very high, as in the case of carbonates and evaporites drilled with salty mud. In invaded formations where R is greater than R it is better adapted for R determination than the induction log. The laterolog has also provided improved thin-bed resolution. Field laterologs have taken two different forms: Laterolog 7 using seven small electrodes, and Laterolog 3 with a central electrode and two long guard electrodes. As shown in Figs. 1a and 1b, in each tool a measure current i, from the central electrode is focused into the formations by bucking currents, i, of the same polarity. These are fed from symmetrical, short-circuited electrodes above and below the central electrode. The intensity of this focusing current (i a) is automatically and continuously adjusted so that under most conditions the measure current (i) flows outward from the sonde into a horizontal slice of formation of approximately constant thickness.