The analysis of borehole sonic data in high-angle and horizontal wells requires special consideration as a result of the difficulty in centralizing multipole logging tools and, in the LWD case, attributable to the influence of the drill collar on the slower-than-fluid shear measurements. Furthermore, in high-angle wells, the measured acoustic "anisotropy" sometimes leads to misinterpretations because it is frequently a function of the relative dip (the non-perpendicular angle between the bedding plane and the borehole), rather than to real (intrinsic) anisotropy in the rocks or to drilling-induced anisotropy. This paper reviews the fundamentals of acoustic-logging measurements in vertical and high-angle boreholes. In particular, we consider the effect of tool design and compensation techniques, as well as best practices and data interpretation methods in deviated wells. We present data for ideal cases and difficult datasets obtained using multipole tools. We also suggest quality control methods that can be applied to acoustic wireline and LWD data obtained in high-angle wells.
Logging high angle wells presents opportunities to explore new applications, but it also presents particular challenges to acquiring good log data. This is definitely true with sonic tools. Anisotropy, imaging, fracture characterization, and geosteering are all particularly applicable in high angle holes. Centralization, borehole compensation, compressional anisotropy, and logging-while-drilling (LWD) shear anisotropy all present challenges to data acquisition and interpretation, and should be handled carefully during both the data acquisition and processing steps.
Before beginning a detailed discussion of logging issues and resolutions, it may be helpful to review modern wireline and LWD sonic tool designs and operation. The equipment of individual service providers may differ in detail, but will operate using the following common acoustic mechanisms: Wireline. Modern acoustic sondes rely on two or more arrays of receivers located along the length of the sonde. There are usually four to eight receivers in each array, with four or more azimuthally spaced arrays to a sonde. Receivers at each depth are identified by their position relative to an azimuthal reference point. Typically, the arrays are designated A, B, C, and D,. During the firing of the monopole source, the four receivers at each depth station are summed to compute the monopole response. During dipole firing, opposing pairs of receivers are summed to provide the XX, XY, YX and YY waveforms. Modern cross-dipole wireline tools consist of a monopole transmitter and two dipole transmitters that are oriented perpendicular to each other.
They also have (at least) four azimuthal receivers spaced 10+ ft from the transmitters. The monopole source is omni-directional and is used at high frequencies (6+ kHz) to excite compressional and refracted shear arrivals (the most common basic service for sonic tools) and at low frequencies (<5 kHz) to excite Stoneley waves (not always used). When the source is fired, the compressional waves from the transmitter reach the borehole wall and are critically refracted, splitting into compressional and shear waves.
