Physical and Imaging Borehole Methods

Borehole geophysics generally involves the collection and interpretation of physical measurements that are indirectly used to deduce properties of the measured borehole-intersected formations and evaluate the borehole/casing integrity. In addition to these techniques, physical and imaging borehole methods are used to describe boreholes, casing, and/or borehole-intersected formations directly or near-directly. That is, borehole imaging and physical tools allow for the visualization of the structure and/or surface of the borehole.

Borehole images are typically displayed two-dimensionally with the abscissa (i.e., x-axis) showing cardinal direction and the ordinate (i.e., y-axis) displaying depth below a reference point. Borehole images are composed of pixels, and each displays a value-based color derived from a user-defined scale that can be adjusted to emphasize or subdue features of interest. Because three-dimensional boreholes are depicted on two-dimensional planes (i.e. as unrolled cylinders), intersecting planar features appear as sinusoids with amplitudes related to dip angles, borehole diameter, and borehole inclination.

Images of open boreholes can be used to qualitatively identify and/or quantitatively characterize lithological and structural features (e.g., contact strike and dip, fracture aperture). Images may provide a sufficient substitute for unrecovered core and, by calibration to lab-analyzed samples, allow for porosity, permeability, and pore structure evaluation. Ancillary applications include assessing the cement, casing, fluid entry, and/or blockages within cased holes, and the core orientation is determined by feature correlation with north-oriented images (Prensky, 1999).

Borehole imaging tools collect high-resolution images of an open or cased borehole wall by employing an optical, acoustic, or electrical sensor or sensor array. Optical imaging techniques (e.g., optical televiewer and television logging) directly depict the borehole interior via photography or videography. Electrical imaging techniques (e.g., formation microresistivity) use high-density grids of electrodes to measure and map electrical resistivity. Finally, acoustic imaging techniques (e.g., acoustic televiewer) collect sonic data to map acoustic reflectivity of the borehole wall.

Though the technique chosen to be employed at a site depends on the application of image data, it is more heavily determined by the borehole conditions. Optical imaging involves the use of light, so the collection of useable optical data may only be possible in air- and water-filled boreholes with sufficient clarity. Electrical- and acoustic imaging require water- or mud- filled boreholes, and selection between them may depend on the density or conductivity of the mud.

In certain applications, method selection is based on the need for determining specific petrophysical parameters. Electrical-imaging tools respond to variations in resistivity (or conductivity), whereas acoustic-imaging tools are more sensitive to the geometric variations of the borehole wall. Thus, the targeted features (e.g., fluid content, borehole enlargements, etc.) can dictate the method most appropriate for the borehole. Additionally, because they respond to different petrophysical parameters, electrical- and acoustic-imaging tools can be used together to support a single interpretation (Prensky, 1999).

As a standalone method, electrical imaging is preferred for distinguishing lithology and describing fractures; though, acoustic imaging may be beneficial if the enlargement is related to lithology. Additionally, reflectivity data acquired by acoustic-imaging methods have been related to lithology and, in rarer cases, porosity. Combined interpretation of electrical and acoustic data sets is especially useful in some cases (e.g., fractured rock aquifers) and can distinguish between fracture types (e.g., open/closed, deep/shallow, natural/induced) (Prensky, 1999).

Physical logging methods are useful additions to many geophysical logs, as the deviations and diameter of the borehole may significantly influence the data collected with other methods. Azimuth and inclination, which are measured with a deviation tool, are typically collected concurrently with or as a component of an image log. Deviation data are used to determine the true vertical depth to reference points within a well and properly orient image logs, which allows for correct structural analysis.

Though borehole diameter may directly correlate to borehole integrity or mudcake buildup, it often allows for inferences of properties such as lithology, fracture progression, and porosity.

Caliper tools typically measure borehole diameter using two or more mechanical arms. However, acoustic-imaging tools can provide high-resolution “caliper” data that may be a sufficient substitute for traditional caliper logs. Whether complimentary or interchangeable, the physical and imaging methods most relevant to environmental investigations are as follows:

References

Prensky, S.E., 1999, Advances in borehole imaging technology and applications, in Lovell, M.A., Williamson, G., and Harvey, P.K., eds., Borehole Imaging: applications and case histories: London, England, Geological Society of London Special Publications, v. 159, p. 1-43, doi:10.1144/GSL.SP.1999.159.01.01.

Physical and Imaging Borehole Methods

Environmental Geophysics