ACOUSTIC IMAGE LOG BASICS
Acoustic image logs are an economic replacement for resistivity image logs for assessing structure and stratigraphy of reservoir rocks and for Identification of fracture intensity and fracture orientation. Tool design has improved considerably since its introduction. Acoustic image logs are identified by numerous tradenames that might not easily be recognized, such as continuous borehole image log (CBIL) and ultrasonic borehole imaging log (UBI).

This tool is sometimes called a borehole televiewer but it should not be confused with borehole television systems that can take continuous video images of a wellbore filled with a clear liquid or air.

The acoustic image log provides formation images in liquid filled open holes. In cased hole, the same tool is used as a cement and pipe integrity tool, but it does not provide any reservoir property information.

The tool has roots back to the 1970's and uses a rotating acoustic transducer/receiver operating at ultrasonic frequencies. The travel time of the sound from the transducer to the wellbore wall and back to the receiver is measured. This provides a complete image of wellbore diameter (a caliper log). Black colour indicates large diameter, white is "near" bit size. Fractures and wellbore breakout are easily seen (black). The poor resolution on older tools makes these artifacts larger than life and were quite misleading as to the width of fractures.  Even modern logs exaggerate fracture width.

The log also records the amplitude of the sound signal reflected from the borehole wall. This is a measure of the acoustic impedance of the rock. Low values represent shale or soft sandstone (shaded black on the log) and high values represent competent rock such as cemented sandstones or carbonates (shaded white on the log). Fractures and breakout will appear black, as on the caliper image.

Colours or grey-scales can be stretched or squeezed to enhanced particular features. Dipping beds appear on the image as sine-wave patterns. The amplitude of the sine-wave indicates dip angle and the valley of the sine-wave indicates downdip direction.

Ultrasonic (acoustic) image log; travel time image (left) indicates borehole diameter, amplitude image (right) shows acoustic impedance, dips calculated from image show both bedding (green) and fracture (blue) dips. The dark areas on the travel time image show borehole elongation in the NW-SE direction, with maximum stress direction at right angles to this axis. On real logs, check heading carefully, as travel time and amplitude images can be interchanged in position, and North may be on the right or the middle of the track.

A resistivity image log has about 10 times the spatial resolution of an acoustic image log and 500 times the amplitude resolution, due to the difference in contrast between the resistivity and acoustic impedance ranges measured by the respective tools.

Comparison of resistivity image log (left) and ultrasonic image log in the same borehole. The higher spatial resolution and the higher dynamic range of the resistivity image is clear. Black colour represents low acoustic impedance on CBIL/ UBI, or low resistivity on FMI, in this case representing fractures (near vertical) or shale beds (near horizontal).

OLDER ACOUSTIC IMAGE LOGS
Older acoustic image logs have even poorer resolution than current versions. If you are working with older logs that exist in the well files, be cautious in using them. Fracture apertures are usually extremely over emphasized.

 Older acoustic image log (left) and interpretation image (right). The large apparent fractures seen on older logs led many people to believe that fracture apertures were very large (many millimeters), when they are really quite small (less than a millimeter).

The resolution of the tool allows us to determine events of about 10 mm in width. Fractures are often accentuated in the wellbore by the drilling process, which breaks out the fracture on both sides of the opening. If it were not for this breakout, most fractures would not be seen by the acoustic imaging log, as their width is commonly less than 1 mm.

Considerable research has being conducted to enhance the images, using both arrival time and amplitude of the sound waves, plus computer methods for image enhancement, especially edge enhancement to resolve fractures and bed boundaries. Modern acoustic image logs can be used effectively in more rugged boreholes than older versions because of the new processing techniques. Be aware of the age of the log before you start your analysis.

The example below shows an older acoustic image log over a portion of a hole with a vertical fracture intersecting the borehole. The image is displayed as a 360 degree unwrap with East at the center of the image, and as an equivalent core image, with South in the middle.

Acoustic image in vertical fracture in a  vertical hole

Notice the enlarged borehole in some of the thin shale beds. The fracture plane is far from smooth and it wanders from one side of the borehole to the other. A dipmeter or older FMS might miss this fracture, or indicate discontinuous vertical fractures. Light colors are higher acoustic impedance, probably dolomite versus darker colored limestone and limey shales. Shale beds are black and washed out.

Below is a drill pipe conveyed acoustic image log run over a 1500 foot horizontal stretch from the intermediate casing shoe. The zone is an upper Cretaceous chalk in which fractures play a vital role in productivity. Most vertical wells penetrate only one or two fractures and deplete quickly. A horizontal well can penetrate many fractures and production can be significantly enhanced.

Acoustic image log with vertical fracture in a horizontal hole

The acoustic image and uranium precipitation shown on the spectral gamma ray log indicate fractures clearly (top image in illustration above). This allows the operator to position completion hardware, such as centralizers and external inflatable casing packers correctly. In this example, the hole was designed to run close to the top of the chalk, and it penetrated the marly zone above in a few places, shown by the dark bands (lower image in illustration above).

LWD ALTERNATIVES TO ACOUSTIC IMAGE LOGS
Aside from open hole resistivity image logs, there are other alternatives to acoustic images.

Logging while drilling (LWD) offers many alternatives that can be displayed as an image log. The example at right is a density image log. Low density values are shaded dark and can be interpreted as porosity or shale. A gamma ray log run with the drill string helps distinguish between these alternatives. White colours represent low porosity or tight rocks.

Resistivity and acoustic images are currently in the field testing stage and may become commercial as LWD logging tools.