Bond Log Theory and Interpretation. April 16th, Variable Density Log ( VDL). • Examples of good . Single CBL 3′ amplitude can not identify channeling. Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of Cement to Pipe Bond & Formation VDL. interpretation of this recently developed measurement and will detail case studies from Gulf of Mexico offshore wells comparing results from traditional. CBL /VDL.
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Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
Acoustic logs do not measure cement quality directly, rather, this value is inferred from the degree of acoustic coupling of the cement to the casing and to the formation. Interpfetation run and interpreted, cement-bond logs CBL provide highly reliable estimates of well integrity and zone isolation.
Just as filtrate invasion and formation alteration may produce changes in formation acoustic properties, and thus variation in acoustic inerpretation over time,    so too, cement-bond logs may vary over time as the cement cures and its properties change.
Modern acoustic cement-evaluation bond devices are comprised of monopole axisymmetric transmitters one or more and receivers two or more. They operate on the principle that acoustic amplitude is interpretatlon attenuated in good cement bond but not in partial bond or free pipe.
These cased-hole wireline tools measure:. Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation. Amplitude, measured directly or as an attenuation ratio, is the primary bond measurement and is used to provide:.
Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity. On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength. However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. In these situations, users are advised to consult with the logging service company regarding the appropriate calibrations.
Presentation of the full acoustic waveform assists in resolving bond ambiguities arising from use of an amplitude measurement alone and provides qualitative information about the cement-to-formation bond. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white.
Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude.
On a variable-density log, free pipe and fluid arrivals if present are easily identified as straight dark and light lines indicating homogenous acoustic properties at either side of the display Fig.
The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time. In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig.
The height of the collar disturbance is a function of measurement TR spacing amplitude, 3-ft interval; variable density, 5-ft interval courtesy of Baker Atlas. Early CBL designs s used a single transmitter and single receiver for an amplitude measurement. In an evolution similar to that of openhole acoustic logs, new designs were subsequently introduced that measured signal amplitude at a near receiver and a full waveform from a far receiver.
Eventually, borehole-compensated devices using dual transmitters and dual receivers were introduced in the s, and today most commercial devices use multiple transmitters and receivers in a variety of arrangements to provide compensated measurements. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.
In addition to specialized cement-bond devices, modern openhole array tools are designed to also interpretstion conventional cement-evaluation measurements in cased hole. The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with the important casing signals. TR spacing typically ranges from 3 to 5 ft.
The shorter spacing e. This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding. These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz.
As with openhole tools, cement-bond tools require centralization to ensure accurate measurements. Centering in the cased hole is more critical because vd higher-operating frequencies i.
Tool eccentering reduces signal amplitude and travel time Fig. Cement-bond logging tools use gated systems to measure the specific parts ccbl the acoustic waveform needed for the primary bond-amplitude measurement. Gates are time periods during which measurements are made—they can be either of the following:. Fixed-gate systems are commonly used for amplitude measurements and floating gates for travel-time measurements. Fixed gates are set generally at interpreration wellsite to open, remain open, and close at designated times; opening time for the gate is a function of the casing size and the borehole-fluid velocity.
If the gate opening is too large, there may be interference between early and late-arriving signals. Floating gates remain open, but recording is only triggered by an amplitude value clb than a designated threshold value. Chl the first scenario, free pipe, there is no cement bond between the casing and cement. Consequently, there is no acoustic coupling with the formation and most of the transmitted acoustic energy is confined to the casing and the borehole fluid.
As a result, a free-pipe acoustic signal is:. In the second scenario, good bond, cement is properly bonded to casing and to the formation. This provides good bcl coupling and most of the acoustic energy is transmitted to the formation, resulting in little weak to no casing signals and little amplitude until the arrival of the strong formation signal. The third scenario, bond to casing only, is a common condition in which cement is bonded to the casing but not to the formation. This can occur because the mudcake dries and shrinks away from cement, or because the cement did not bond with mudcake in poorly consolidated cbo.
In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath.
At the same time, the annulus outside the cement sheath provides poor acoustic coupling. The result is that little energy is transferred to the annular fluid and virtually none is transferred to the formation.
This condition is indicated by the lack of later-arriving formation energy. A similar response can be caused by the presence of formation gas in shallow, high-porosity zones.
In the last scenario, partial bond, a clb exists within an otherwise interprrtation casing. This may occur with the presence of a microannulus or channels within the cement.
The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal. When channeling occurs, it is generally localized and nonuniform; that is, it occurs over relatively short intervals and can frequently be identified by variations in the amplitude response. Channeling is significant because it prevents a hydraulic seal.
In contrast, a microannulus a small gap between the casing and cement sheath may extend over long sections of casing but may not prevent a hydraulic seal. A common practice is to run cement-bond logs with the casing under pressure to expand the casing against the cement, thereby decreasing any microannulus that might exist.
If the initial interprehation run was not under pressure and the log indicates poor bond, the presence of a microannulus can be evaluated by running a second bond log under pressure to see if there is a difference. Pressuring the casing improves the interpreration coupling interpretatjon the formation and the casing signal will decrease and the formation signal will become more obvious Fig.
However, if only channeling exists, pressuring the casing will not significantly change the log. When conducting a cement evaluation, information on the type of cement used is essential. For example, foam cements, which intentionally create void spaces in the cured cement, can be misinterpreted as partial bond if normal cement is assumed.
Table 1  . One caveat regarding the use of the amplitude curve for bond evaluation: Whenever possible, amplitude data should be used in conjunction interprtation the other measurements presented on the log e. For example, the presence of shear-wave amplitudes on the full-waveform display is an indication of good acoustic coupling to the formation.
Table 2 lists the limitations of conventional cement-bond logs. The VDL should be examined to ensure that the formation arrival is impacting the amplitude curve. The bond index BI is a qualitative measure of cement bond based on signal amplitude.
This dimensionless quantity is the ratio of measured attenuation to maximum attenuation:. A BI value of 1. A value of less than 1. Radial-cement-evaluation devices were developed to overcome some limitations of conventional cement-bond tools and to permit interpertation accurate evaluation of cement distribution behind casing by providing the precise location of partial bond and channeling.
These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing.
Data from these tools are presented as individual log curves or as azimuthal images “maps” of cement quality generated by interpolating between the individual azimuthal measurements Fig. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond.
The ultrasonic tools compute the acoustic impedance of vdp material beyond the casing. To do this, repeated acoustic interpretationn are directed at the casing to make it resonate in its thickness mode and the energy level attenuation of the interpretahion reflected wave is measured. Good cement bond to casing produces a rapid damping knterpretation impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. Measurements from these devices are influenced by the same factors as openhole televiewer devices.
Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:. The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters. The two measurements are combined to form a result that compensates for:. Transmitting elements and the firing sequence are controlled to direct steer and interprefation the cvl output of both the pad transmitters and the VDL transmitter.
This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively. The use of new high-performance low-density, foam, and complex cements is increasing. However, the presence of gas in cement slurries, as an inert component or as contamination, may seriously affect ultrasonic-tool interpretation.
New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions. Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers.
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. The signal is received, processed, and displayed as a intdrpretation. The recorded waveforms are presented together with the travel time and a casing-amplitude curve, vd displays the amplitude of the acoustic signal that has traveled through the casing but not through the cement and formation.
The waveform and amplitude data allow two bonds to be investigated. These are the bond between casing and cement and, to a lesser extent, that between cement and formation. A “straight” waveform display is traditionally interpreted to mean no cement bonding. Variations in the acoustic display are interpreted as indicating the presence of bonded cement. However, they are not reliable as indicators of hydraulic bcl by the cement, because they cannot detect small channels therein.