Electrical Logging

Abstract

Electrical logs are perhaps the most important tools available to a petrophysicist. This is because they provide a method for calculating the water saturation, upon which calculations of STOOIP are based. They were also some of he first logs to be used, with Marcel and Conrad Schlumberger testing out an electrical log for the first time in 1927 in the Pechelbronn field, France. These first measurements were continuous recordings using 2 or 3 electrodes and a direct current. It was discovered that high quality recordings of apparent resistivity could be obtained under favourable conditions of small diameter boreholes, high mud resistivities and shallow invasion in thick reservoirs. These early tools are called electric logging tools. The development of electrical tools has henceforward been intense. There are now tools that can cope with extremely highly resistive muds (oil-based muds or gas as the borehole fluid), which rely upon electromagnetic coupling and an induced alternating current (induction logs). The induction log actually measures conductivity, and hence is sometimes called the conductivity log. The modern tool for measuring resistivity in high salinity (low resistivity) muds is the laterolog, which focuses its current into a thin sheet to improve vertical resolution and penetration depth. The laterologs measure resistivity in the conventional sense, and are usually referred to as resistivity tools. Both the induction logs and the laterologs come in different types, which are sensitive to different depths of penetration into the borehole. Hence resistivity determinations for the invaded, partly invaded and undisturbed rock zones can be measured. In addition, there is a range of smaller electrical devices (micro-resistivity tools), which are designed to measure the resistivity of mudcake. There are also Array Logs, which are state of the art tools, and electrical measurements are used at high resolutions (small scale) to image the interior of the borehole electrically.