Evaluation of True Formation Resistivity Derived from the Dual Laterolog-Rxo and the Induction-Spherically Focused Log

The problem of determining whether a Laterolog or an Induction is better suited to measure true formation resistivity (Rt) according to prevalent conditions is reviewed. The question of which deep resistivity device (Laterolog or Induction) should be run in a given well is investigated. To aid with this decision, examples are shown that cover a wide range of cases in both oil-bearing and water-bearing zones. True resistivity values derived separately from the Dual Laterolog-Rxo (DLL-Rxo) tool2 and from the Induction-Spherically Focused (ISF) Log3 are compared. The comparison helps determine the accuracy of the measurements made by each tool with respect to Rt. This also provides the capability to correct the resistivity values for the invasion to obtain Rt, leading to a more accurate evaluation of water saturation. Practical applications of both tool types, recorded over a limestone formation in three Middle East wells, are shown for a variety of situations in both fresh and salt saturated mud systems. The responses of the basic deep resistivity devices, deep Laterolog (LLd) and deep Induction log (6FF40), are shown for cases of shallow invasion, moderate invasion, deep invasion, and very deep invasion.