High Resolution From a New Laterolog With Azimuthal Imaging

Abstract

A new resistivity logging tool is described that improves the efficiency of drilling and logging operations by halving the length of traditional Dual Laterolog arrays to 16 ft. The reduced length and weight of the monolithic sonde lead to faster rig-up and can save several meters of rat-hole. The array, based on the configuration of a Dual Laterolog and Complemented by an azimuthal electrode array in the center, was designed for high vertical resolution and optimized for invasion response and borehole effect. The tool's operation is based on a number of independent and simultaneous measurement modes that are combined by software to yield a series of resistivity measurements with different depths of investigation and resolutions. The standard deep and shallow laterolog curves are output with improved resolution, but the flexible computed focusing also allows the substitution of different focusing conditions, which simultaneously yield deep and shallow resistivity curves with still higher resolutions of around 8 in. The passive azimuthal array at the center of the tool provides resistivity imaging of the formation in deep and shallow mode and an auxiliary ultra-shallow measurement sensitive to the radial distance from the borehole wall. An additional in-situ determination of the mud resistivity permits accurate real- time borehole corrections to be made. Examples from recent field tests show the invasion and thin- bed response of the tool together with applications of the resistivity images. Introduction The laterolog has become the standard formation device in salt drilling muds and in hard rock environments. Doll (1951) first presented the principles of the measurement, and two decades later the first simultaneous Dual Laterolog followed, described by Suau et al. (1972). The Dual Laterolog not only permitted the two measurements to be made simultaneously, but also was combinable with a microresistivity tool for a complete resistivity measurement suite. Since the introduction of the Dual Laterolog, few notable advances in laterolog devices have been made until Davies et al. (1994) presented the ARI* (Azimuthal Resistivity Imager tool), that introduced azimuthal resistivity measurements in addition to the standard deep and shallow resistivities. The azimuthal array further allowed a deep resistivity measurement to be made with increased resolution. In practice, resistivity tools are seldom run alone for complete formation evaluation. Reference has already been made above to the combination of laterologs with microresistivity tools and, in fact, operators seek to combine these tools with porosity measurements into the so-called "triple-combo." More complex and complete combinations result in tool strings of considerable length - often over 90 ft. While these combinations improve efficiency by reducing the number of logging runs, they also pose specific problems such as lengthy rig-up/rig-down, reduced logging speed and the need for more rat-hole. A new laterolog tool, the HALS* (High Resolution Azimuthal Laterolog Sonde), is presented here which was designed to overcome these problems. It is only 16 ft long, half the length of the Dual Laterolog and monolithic in construction. Like the ARI, it has an azimuthal resistivity array that gives measurements having manifold applications. Directional measurements aid understanding and interpretation in horizontal wells, while formation images provide visual presentation of fractured zones or other heterogeneities. P. 563

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