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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The USI interpretattion distinguish between gas-filled cement and fluids, but it can quantify the acoustic impedance of the cement. Tool eccentering reduces signal amplitude and travel time Fig.
Cement bond logs –
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. Courtesy of Baker Atlas.
Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond. A “straight” waveform display is traditionally interpreted to mean no cement bonding.
In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole. The ultrasonic tools compute the acoustic impedance of the material beyond the casing. In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig. When conducting a cement evaluation, information on the type of cement used is essential.
Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:. These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing. Whenever possible, amplitude data should be used in conjunction with the other measurements presented on the log e.
Table 2 lists the limitations of conventional cement-bond logs. In the second scenario, good bond, cement is properly bonded to casing and to the formation. Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read.
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. The cement-evaluation presentation includes casing ovality and tool eccentricity in Track 1. Track 3 shows the average and minimum attenuation at each sampling level. The data for circumferential inspection of the casing, as described above, and for the evaluation of cement bonding are obtained on the same logging pass.
The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig. Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity. Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
However, if only channeling exists, pressuring the casing will not significantly change the log. If there is unset bdl or water behind the casing, there will be a weak reflection. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white.
They operate on the principle that acoustic amplitude is rapidly attenuated in onterpretation cement bond but not in partial bond or free pipe. Track 5 is a CBL-type display. Track 2 contains the acoustic attenuation logs for the six pads.
Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal. Ultrasonic tools can be operated to address two objectives concurrently: Amplitude, measured directly or as an attenuation ratio, is the primary cl measurement and is used to provide:.
A high rate of attenuation is indicative of a good cement bonding to the casing and an absence of channels within the cement. 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.
As a result, a free-pipe acoustic signal is:. In this case, the eccentricity comprises both tool and ingerpretation eccentricity resulting from formation movement salt flow. The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters.
CEMENT BOND LOG INTERPRETATION
There is nearly free pipe above an apparent top of cement at a depth of approximately X80 interpretatiom units. These are the bond between casing and cement and, to a lesser extent, that between cement and formation.
As with openhole tools, cement-bond tools require centralization to ensure accurate measurements. By analyzing the entire waveform, an acoustic-impedance map of the cement can be constructed.
The use of new high-performance low-density, foam, and complex cements is increasing. These cased-hole wireline tools measure:.
Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected interrpetation highly attenuating muds.
To support the casing shear bond To prevent fluid from leaking to the surface For isolating producing zones from water-bearing zones hydraulic bond Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
Floating gates remain open, but recording is only triggered by an amplitude value greater than a designated threshold value. Centering in the cased hole is more critical because the higher-operating frequencies i.
Cement bond logs
The signal is received, processed, and displayed as a microseismogram. They are often grouped as “cement evaluation tools. This can occur because the mudcake dries and shrinks away from cement, or because the cement did not bond with mudcake in poorly consolidated formations. 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. Conventional CBL tools provide vrl measurements, while the newer radial cement-evaluation tools provide interpreyation sensitive measurements for channel evaluation.