Advances in borehole imaging technology and applications

Abstract

Abstract Borehole imaging devices are currently available for both wireline and logging-while-drilling (LWD) environments for use in both openhole and cased wells. Most wireline, and all LWD devices provide indirect images that are derived from a high-density grid of electrical or ultrasonic acoustic measurements. Optical techniques, such as television and photography, provide direct images of the borehole. Imaging devices consist of (1) a rotating sensor (acoustic or electrical) that scans the borehole with each revolution, (2) multiple, azimuthally placed sensors, or (3) a downhole television or photographic camera. Advances in digital acquisition and processing permit real-time display and evaluation of images of the open borehole or the inside of casing. The resolution and borehole coverage of microelectrode devices has increased with newer designs that allow additional measurement sensors per pad, additional pad segments on a four-arm tool, and the introduction of six-arm imaging tools. In nonconductive borehole fluids, a four-arm microinduction device can provide crude images. Macroelectrode imaging tools (laterolog-type; wireline) provide lower-resolution electrical images from either an azimuthal arrangement of sensors around a mandrel (wireline), or from azimuthally sensitive sensors (logging-while-drilling), that scan the borehole during rotation of the bottomhole assembly. In acoustic imaging, recent devices incorporate design changes that provide improved image resolution and operate in a wider range of borehole conditions. Ultrasonic transducers now use lower transmitting frequencies and spherical focusing and these tools also offer surface-selectable acquisition parameters. Acoustic and microresistivity imaging techniques measure different physical properties and these data are complementary. One recently introduced wireline service combines microresistivity and acoustic imaging devices in a single tool for simultaneous, single-pass acquisition. Recent advances in cable and sonde design permit downhole video in a wider range of petroleum environments, in flowing wells, and in tubing. Openhole image interpretation involves qualitative (visual) identification and quantitative characterization of strike and dip on planar features, lithology, bedding, fractures, and vugs. When core is available, features identified on log-derived images can be correlated or calibrated to it, or when core is absent, images may serve as a substitute. Borehole images are useful in formation evaluation (when calibrated), sedimentology, stratigraphy, and structural analysis. Current applications include core orientation, ‘reconstructing’ missing core, identification and characterization of fractures and depositional features (e.g. bedforms), reservoir architecture, bedding, grain size, porosity, permeability, net sand and net pay counts, faults, folds, and fractures (hydrocarbon, water, geothermal wells, and in rock engineering), evaluation of borehole stress (borehole stability), directional placement of wellbore for optimal drainage (fractured reservoirs), evaluation of fracture stimulations, and perforation control. Cased hole applications include cement evaluation, casing inspection, detection of fluid entry, identifying production blockages, and assisting in fishing operations.