Abstract

Both structural and stratigraphic features are clearly distinguished from the borehole electrical resistivity images. These detailed images allow evaluations and interpretations and have been proven to be invaluable to geoscientists since they were first introduced more than 15 years ago. By providing high resolution logs of the wellbore, imaging tools are able to resolve not only structural features, but fine scaled stratigraphic details. First and second generation electrical resistivity imaging tools operate only in conductive water-based muds. Tools that operate in non-conductive oil-based muds have been developed in recent years, but they do not have the spatial resolution of the earlier tools. This limitation has now been largely overcome with a novel adaptation of the second generation tools that replaces the micro-resistivity buttons with miniature spring loaded scratcher knives that make electrical contact with the formation by cutting through the nonconductive layer on the borehole wall.

Introduction

Electrical borehole imaging tools are the evolutionary descendents of traditional dipmeter tools, and exponentially increase the value of the information gathered. Instead of 4, 6, or 8 buttons measuring resistivities circumferentially around the borehole, 150 to 196 buttons measure micro-resistivities from which false-colour images of the borehole wall are generated. These higher resolution images can be used for traditional dip and strike information, but also allow for more detailed geological interpretations to be made, both in terms of fine sedimentological features, and in large scale structural trends. Prensky in Lovell et al (1999) traced the advances in borehole imaging technologies from the earliest optical techniques to current wireline and LWD imaging tools. At that time, electrical borehole imagers were limited to obtaining their measurements in wells filled with conductive wellbore fluids. In oil-based mud systems however, the major applications of high resolution imagers, namely reservoir (facies identification) and fracture characterization were dependent upon acoustic imagers. Unfortunately, acoustic imagers suffer from borehole wall artifacts and signal amplitude attenuation from a variety of factors, including eccentricity, solids and weighting materials in drilling muds. Since then, the latest advances in wireline imaging technologies have focussed on obtaining electrical borehole images in wellbores containing nonconductive drilling fluids. Cheung et al (2001) and Lofts et al (2002) presented the industry?s first wireline electrical images taken in a non-conductive wellbore fluid. These tools are physically based upon the respective water-based imaging tools discussed above, but make changes in the pad and electronics to enable them to generate electrical images in non-conductive borehole fluids. These tools measure the Rxo component of the near-wellbore environment by using focussing and measuring currents to generate microlateral conductivity images. This paper presents examples of images using a new tool design in non-conductive mud systems. The key element to obtain micro-resistivity measurements in non-conductive fluids hinges on obtaining electrical continuity between the electrode and the formation. Weatherford?s Oil Mud Imager (OMI*), uses a novel approach to obtain this continuity in the form of knives that cut through mudcake to make physical, and therefore electrical contact with the borehole wall. Field testing of the OMI began in May 2005.

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