Deep azimuthal wave resistivity sensors yield a new type of electrical images. Although traditional micro-electrical images are limited to the surface of the wellbore, the new deep resistivity images span a volume of several cubic feet around the borehole. This new window on the geology helps to geosteer with more confidence than a series of discrete non-azimuthal curves.
Deep azimuthal images feature frowning and smiling patterns similar to their micro-images counterparts, except for two important distinctions. First, they make it possible to recognize approaching boundaries long before they intersect the well path. Because they emanate from several feet away from the wellbore, deep electrical images effectively enable proactive geosteering. They guide proper and timely evasive actions before the well exits the reservoir into shale or an underlying water-bearing interval. Second, the deep propagation images feature a characteristic bright spot that appears when drilling through a reservoir and approaching an exit into a less resistive formation. Various spacings and frequencies generate images with differing depths of field that provide a sense of 3-dimensional view away from the wellbore. This important information is unavailable from micro-images limited to the wellbore surface.
New interpretation methods specific to deep electrical images are illustrated on modeled data and on real logs where the primary objective is often to steer near a boundary, while staying clear of it. To quantify the distance to nearby beds, deep images are supplemented by Geosignal, a new real-time electromagnetic log that is very sensitive to the distance to the nearest boundary. By integrating multiple deep images with multiple depths of field, electrical Geosignals, and quantitative azimuthal resistivity data, a real-time inversion software helps the geosteering engineer to locate the well precisely with respect to the geological layers.
Introduction
When borehole electrical images were first introduced in the mid-1980s, they were greeted with enormous interest by the geologists and sedimentologists (Ekstrom et al., 1986). Because of their ability to identify small features and their sensitivity to small scale resistivity contrasts, borehole electrical images are also known as micro-electrical images or micro-resistivity images.
This paper introduces and characterizes a family of electrical images obtained from a new type of wave resistivity measurement sensors. These new images originate from deeper within the formation than their micro-resistivity counterparts. Although these images have a much lower resolution, they see further away from the wellbore. Consequently, they are useful in geosteering and in imaging important geological events a few feet away from the wellbore. All of the intuitive knowledge developed over the years for small scale wellbore image interpretation can be adapted to deep image interpretation, but with some important differences. A comparative study of micro-resistivity images and deep resistivity images is first carried out. Next, a series of modeled and actually acquired deep images are used to develop a method for interpreting the new type of images. Finally, additional measurements from the deep azimuthal resistivity sensor are combined with the deep images for improved geosteering. One of these measurements is the Geosignal (Bittar, 2002). Its main advantage is the ability to detect lateral events from up to 18 ft away under favorable conditions.
