Bond Log Theory and Interpretation. April 16th, Variable Density Log ( VDL). • Examples of good cement bond and Cement Bond. Log (CBL). 1. 2. 3. 4. 2 Sep Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of Cement to Pipe Bond & Formation VDL. 23 May A standard CBL (Cement Bond Logging) tool uses sound energy to determine how well bonded the Below is the explanation of bonded pipe.
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Ultrasonic tools are superior to interpreattion acoustic CBLs, although they remain adversely affected by highly attenuating muds. The bond index BI is a qualitative measure of cement bond based xbl signal amplitude.
The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig. Proper cement placement between the well casing and the formation is essential: Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation.
Cement bond logs –
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions. These are the bond between casing and cement and, to a lesser extent, that between cement and formation.
The pad spacing is such that the first arrival is the wave that has passed through the casing. The waveform and amplitude data allow two bonds to be investigated.
The height of the collar disturbance is a interpretxtion of inteprretation TR spacing amplitude, 3-ft interval; variable density, 5-ft interval courtesy of Baker Atlas. On a variable-density log, free pipe and fluid arrivals if present are easily identified as straight interoretation and light lines indicating homogenous acoustic properties at either side of the display Fig.
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro. A “straight” waveform display is traditionally interpreted to mean no cement bonding.
To do this, repeated acoustic pulses are directed at the casing to make it resonate in its thickness mode and the energy level attenuation of the decaying vel wave is measured. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. Track 4 presents a variable-attenuation log or cement map of the casing periphery vs. In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath.
If the gate opening is too large, there may be interference between early and late-arriving signals. Gates are time periods during which measurements are made—they can be either of the following:.
For example, foam cements, which intentionally create void spaces in the cured cement, can be misinterpreted as partial bond if normal cement is assumed. 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.
In this example, the partial bonding is sufficient to provide hydraulic isolation. Courtesy of Baker Atlas. This provides good acoustic 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.
These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:. At greater depths, the pipe is well bonded. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read. Tool response depends on the acoustic impedance of the cement, which, in turn is intepretation of density and velocity. Tool eccentering reduces signal amplitude and travel time Fig.
Channels as narrow as 1. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal. In the first scenario, free pipe, there is no cement bond between the casing and cement. 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.
Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. 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. This dimensionless quantity is the ratio of measured attenuation to maximum attenuation:.
Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. In the last scenario, partial bond, a space exists within an otherwise well-bonded casing. Early CBL designs s used a single transmitter and single receiver for an amplitude measurement. This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus.
The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters. Dark zones are the most strongly bonded.
Cement bond logs
TR spacing typically ranges from vdo to 5 ft. In an evolution similar to that of openhole acoustic logs, cdl designs were subsequently introduced that measured signal amplitude at a near receiver and a full waveform from a far receiver. There is poor cement condition between X and X depth units. Radial-cement-evaluation devices were developed to overcome some limitations of conventional cement-bond tools and to permit more vvl evaluation of cement distribution behind casing by providing the precise location of partial bond and channeling.
However, if only channeling exists, pressuring the casing will not significantly change the log. These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz. Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially.
However, they are not reliable as indicators of hydraulic sealing by the cement, because they cannot detect small channels therein. The tool has to be centered.
CEMENT BOND LOG INTERPRETATION
Track 3 shows the average and minimum attenuation at each sampling level. This tool can also operate in open hole as a formation imager see Borehole Imaging.
In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig.