Sonic measurements have advanced considerably from early compressional logging days to include not only compressional and refracted shear velocities, but also slower-than-fluid shear, near-vs. far-well bore responses and anisotropy analysis. Following an industry-wide interest in imaging technologies, new methods of acoustic well bore imaging have lately attracted keen interest in both wireline and logging while-drilling (LWD) environments. This is an area where LWD sonic logging actually has an advantage over wireline logging, in that by its very nature, LWD data can be acquired while the tool string is rotating, thus providing the opportunity to acquire single depth data at multiple azimuths without costing rig time. These data are azimuthally binned such that a high resolution sonic image around the well bore can be presented, showing the variations due to intrinsic or induced acoustic anisotropy as well as nearby beds. In addition, coupling this azimuthal technology with recent advances in radial profiling (variable depth of investigation), makes it ideal for geosteering, especially in regimes where resistivity contrast is low and steering on porosity contrasts is preferred.
Three-dimensional modelling is used to explore various aspects of acoustic azimuthal imaging, including practical azimuthal resolution for compressional and shear velocities. Anisotropic sensitivity of acoustic imaging versus more traditional crossed-dipole methods is explored. The focus is largely on LWD technology, as the real-time applications of azimuthal sonic data are of particular interest. Considerations of tool design are explored, revisiting familiar concerns such as transmitter-to-receiver separation as well as more novel concepts such as the practicality of acoustic image and anisotropy measurements in off-centred cases.
Borehole logging is progressing from the world of single depth-of-investigation, single azimuth curves to three dimensional images showing azimuthal variation and complex layering. Wireline technologies include ultrasonic imaging/calliper tools and electrical imaging tools, which both have quite shallow depths of investigation (less than 6 inches). LWD tools are wellsuited to azimuthal sensitivity by their very nature of rotating while acquiring data. Thus, in recent years, LWD azimuthal density, azimuthal resistivity, and azimuthal induction tools have been introduced and are routinely used for geosteering, fracture detection, and other imaging applications.2,7
Sonic tools, wireline and LWD, offer azimuthal sensitivity as well as variable depth of investigation. However, with the exception of wireline crossed-dipole anisotropy calculations and recent wireline radial profiling9 the imaging possibilities with sonic tools have been largely unexploited. In particular, LWD sonic data, which is acquired while the tool is rotating (and thus at multiple azimuths) can be used to create azimuthal images at multiple radii for geosteering and stress profiling while drilling.
Many aspects must be considered when optimising azimuthal sonic tools, including source frequency, source-receiver spacing, azimuthal sensitivity, and centralisation effects.
Before discussing modelling and field results in detail, it is worthwhile to briefly review some practical aspects of sonic logging in the wireline and LWD environments.
Wireline
Wireline sonic tools, unlike their short body-length ultrasonic scanner relatives, are not designed to be rotated while logging.