There is much literature concerning the effect of fractures on acoustic wave propagation in porous and fractured rock. Unfortunately, much of it is theoretical and not always supported by field examples; often it is contradictory. Nevertheless, the sonic log is the best fracture finder in older wells because other, more modern, methods were unavailable at the time.

Today, dipmeter and formation micro-scanner images provide more information, but at higher cost, so sonic logs are still used extensively for fracture identification. The modern full wave or array sonic and dipole shear sonic tools provide much new information, including shear wave travel time and amplitude plus full wave-train digitization This allows the wave train to be further processed.

In theory, the normal compressional interval transit time is little affected by fractures so long as there is a free matrix path between transmitter and receivers, as would be expected for vertical fractures. In practice, large vertical and most sub-horizontal fractures, create cycle skipping on the compressional transit time curve on all sonic logs that rely on detection of the first energy arrival. This is due to reduction in amplitude of the sound pulse by reflection at the fracture face, and by destructive interference caused by other propagation modes generated at the fracture. In addition, refraction caused by near vertical fractures diverts energy from the receivers, again reducing amplitude.

Cycle skipping makes the sonic travel time too long. Thus simple theory is overwhelmed by the complexity of sound transmission in a heterogeneous medium.

Sonic log cycle skips may indicate fractures

On the array and dipole shear sonic logs, travel time is usually found by waveform correlation and not by first arrival detection (although both methods are available). Therefore, it is less likely to skip a cycle due to low amplitude. Amplitude curves are presented as a matter of routine, so fractures can be identified by low compressional and shear amplitudes. Sonic curves on the array or dipole sonic can disappear or be shown as straight lines where amplitude is too low to obtain a waveform correlation.

Cycle skipping is an excellent fracture indication in hard formations. Shallow resistivity crossover might help confirm fractures in a typical well with only an induction and sonic log.

Gas in the formation or in the mud, poor borehole conditions, and poor tool condition or recording parameters, especially on older logs, may also cause skipping. Tool centralization is also important; compressional amplitude can be reduced to less than 20% of normal with the tool only 1 inch off center. This can cause skipping. Note that most modern sonic logs are designed to avoid cycle-skipping so this identification technique may not be useful in many newer wells.

Compressional and shear sonic amplitude versus fracture orientation and tool centering

The cause of the skipping can be checked; if the skips occur only in a competent zone and not in the surrounding shales, gas in the formation or fractures are the only possibilities. Cycle skipping is more common on long spaced than on short spaced sonic logs in hard formations, because of lower sound amplitude on longer tools. The reverse is often true in softer sands and shales, due to rock alteration near the wellbore.

In contrast, shear energy is, theoretically, strongly reduced by both horizontal and vertical fractures, but not much by fractures between 35 and 75 degrees. In practice, fluids and fluid filled fractures do not conduct shear waves, and shear arrivals are strongly attenuated in fractured zones. On full wave or array sonic presentations, the absence of shear arrivals or straight line segments on shear travel time logs are sure signs of fractures. The Stoneley wave amplitude is also strongly reduced by fractures.

To differentiate between fractures and other causes of skipping, a number of different logging tool designs and presentations of sonic data have been developed. Special presentations include the sonic amplitude, sonic wavetrain, and variable intensity (variable density) displays, discussed below.

The sidewall acoustic instrument was introduced to improve the bed resolution and measurement of acoustic properties, but it was not widely available or used. It is an acoustic pad device containing one transmitter and two receivers designed to reduce attenuation in the borehole and through the rock. The distance from the transmitter to the first receiver is 9 inches, and the spacing between the receivers is 6 inches. These dimensions allow for better bed definition for porosity measurements and improved wave forms for fracture studies.

Fractures are more readily identifiable from this short spaced measurement than from devices which measure and average a longer distance. However, the measurement is affected by borehole rugosity and only surveys a small portion of the borehole circumference. It is best suited for thin bedded formations.

Sonic Waveform Logs
The elastic constants of rocks determine the velocity of sound waves. Compressional waves travel faster than shear waves and theoretically can be seen separately on a display of sonic waveforms. Waveforms are a record of sonic energy level versus arrival time. In practice the two waves, and others, interfere with each other to some extent.

The amplitude of both waves are affected by the rock type, porosity, borehole rugosity, tool centralization, formation fluid, and fracture size and orientation. The fractures may be only those induced near the borehole wall by drilling or may be in-situ. Closed fractures reduce the amplitude less than open fractures. Refracted waves traveling other than direct ray paths can also reduce amplitude and give false impressions of fracturing.

The usual way to record these amplitude values is to present the amplitude of the first energy arrival, which is from the compressional wave, in the form of a log curve, or to present the entire wavetrain, or both. On the newer array and dipole shear sonic, the shear amplitude is also displayed. On older logs, some attempts were made to measure shear travel time and amplitude by adjusting gate times and trigger levels on the instrument panel. These logs were not too reliable, so take care if trying to use them.

Sonic amplitude log may indicate fractures

Numerous versions of these logs have been developed over the years, with little standardization. Names such as Micro-seismogram, Fracture Finder, 3-D Velocity, Acoustic Parameter, Shear Sonic, Variable Density, and Frac Log were used by various suppliers. We will use the generic term sonic amplitude log to cover all of these.

The sonic amplitude log is a curve representing the first arrival energy, measured in milli-volts. Energy varies with many factors, so absolute values mean little, but low amplitude often means fractures. All the things that cause cycle skipping, described above, cause low amplitude, so fractures are only one possibility. This log is usually combined with a gamma ray, caliper, and a wavetrain presentation, as shown.

The sonic wavetrain log is a display of the recorded energy presented as wiggly trace signatures, usually one for every 6 inches to 2 feet of borehole. The variable intensity display, sometimes called a variable density log, displays the same waveform information, but the amplitude of the positive peaks are shaded gray or black and negative peaks are white. When plotted continuously, dark and light bands representing peaks and valleys are displayed versus depth. Conventions have varied, and arrival time has been plotted increasing right to left or left to right, with the latter used today.

Sonic waveform presentation

Two waveform logs, with associated gamma ray logs are shown above. Compressional amplitudes are lower than shear in most cases, but two areas on the left hand log show reduced amplitude on both compressional and shear waves, indicating fractures. Notice that waveform arrival time increases from left to right.

Reflections from fractures cause changes in amplitude and travel time of the main signal, and some waves arrive at later times, out of phase, thus causing irregular interference patterns on the waveform. Usually chevron patterns spanning several feet can be seen, indicating reflections from near horizontal fractures. Chevrons are difficult to see on older VDL presentations, but are much more obvious on more modern logs, as seen on the example below. Chevron patterns are not necessary as diagnostic tools. Low amplitude is all that is needed. Other interfering effects, such as Stoneley waves and rough borehole cause jittery patterns. Vertical fractures create less disturbance.

Example of modern sonic log with Stoneley wave travel timea (left) and VDL display with clear chevron patterns in limestone, indicating fractures, and none in sandstone interval. Permeability (right) is from Stoneley travel time.

Normal presentation of older sonic amplitude log includes:
   1. 3 ft. spacing borehole compensated compressional travel time
   2. 3 ft. spacing compressional amplitude curve
   3. 5 ft. spacing variable density display of entire wave train amplitude
   4. 3 ft. spacing single receiver compressional travel time

On modern logs, the shear travel time and shear amplitude are recorded, along with complete waveform displays and other diagnostic curves. Stoneley wave travel time and attenuation are also shown. Colour images of the waveform correlation amplitude or colour versions of the waveform display are now common (see Chapter Three for tool details).

Fractures are indicated when:
   1. amplitude of compressional first arrivals is low
   2. single receiver travel time skips or does not track compressional travel time
   3. high frequency chevron patterns are seen on variable intensity display
   4. amplitude of shear wave arrivals is low

Note that single receiver travel time may vary, often indicating poor tool centralization.

Circumferential sonic log for fracture detection is a rare commodity

A circumferential sonic log has also been developed but was not widely available. Sound pulses travel around the borehole wall and are attenuated most by vertical fractures, due to reflection at the fracture surface. Few examples exist outside the well logging literature. Both the sidewall and circumferential sonic rely on waveform analysis for fracture identification. By alternating between the two transmitters, four separate wavetrain or variable intensity displays are created, one for each quadrant around the hole.

Evaluation of any acoustic measurement is still complicated because many factors other than a fracture system can cause attenuation or distortion of the wave. Washout zones should be identified before a fracture interpretation is made because they give similar responses. In some shales, the compressive amplitude is larger than the shear amplitude, which again looks like a fractured zone. A gamma ray or SP log should be used to identify such zones.

Dipole shear image log shows directional stress - the Fast Direction is centered on
90 degrees (east - west) which is also the maximum stress direction.

Dipole shear sonic logs offer many display options

On the full wave or array sonic log, we can measure travel time and attenuation of the compressional, shear, and Stoneley wave energies, instead of merely the compressional energy as on conventional sonic logs.

Shear attenuation may locate fractures or vuggy porosity

These attenuations result primarily from the large contrast in acoustic impedance between the rock matrix and the fluid in the fractures and in porosity. As compressional and shear waves traverse a fracture their energies are significantly attenuated with the greatest attenuation occurring to the shear wave.  Remember that high attenuation is equivalent to low amplitude. Attenuation is measured in decibels per foot or per meter (db/ft or db/m).

Another cause of energy reduction is poor acoustic coupling in zones with vuggy porosity. This attenuation is due to acoustic wave scattering as it is being transmitted through the vuggy porosity. Analysis of acoustic energies must be supported by porosity information to distinguish this situation. Acoustic energy is not severely attenuated by normal intercrystalline porosity.


Waveform correlation map shows attenuation in fractures

Suitable processing of the digitally recorded waveforms can enhance the visibility of fractures. One example is to plot the velocity cross correlations to observe the compressional, shear, and Stoneley energy on a time versus velocity crossplot. The peaks of the contoured correlation amplitudes show where the sonic energy is located. Illustrated above is a comparison of a fractured and un-fractured zone, showing the loss of shear energy as fracture intensity increases. Note also that the log curve disappears (see left hand track) because no energy is being received at the tool. The gap in the log can be drawn as a straight line. This loss of data is equivalent to cycle skipping on older logs.

Another method involves filtering the waveforms to enhance the chevron patterns caused by mode conversion interference. This is similar to F-K or velocity filtering on seismic data. The dipole array sonic sharpens the chevron patterns naturally, due to the different propagation path of the directional acoustic beam compared to the omni-directional pattern of the monopole array sonic

Stoneley reflection coefficients, computed from adjacent Stoneley velocities, also help to pinpoint fractures.

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