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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A high rate of attenuation is indicative of a good cement bonding to the casing and an absence of channels within the cement. When conducting a cement evaluation, information on the type of cement used is essential. However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate.
This has the effect of improving the signal strength of both the casing and interpretatuon arrivals, respectively. If the gate opening is too large, there may be interference between early and late-arriving signals. However, they are not reliable as indicators of hydraulic sealing by the cement, because they cannot detect small channels therein.
Table 1  . Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:. The signal is received, processed, and displayed as a microseismogram.
This condition is indicated by the lack of later-arriving formation energy. 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 intterpretation strong formation signal.
Cement bond logs –
By analyzing the entire waveform, an acoustic-impedance map of the cement can be constructed. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white.
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro. Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond.
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. The application of statistical variance processing to the conjunctive use of CBL and ultrasonic impedance data has led to an improved cement evaluation. Gates are time periods during which measurements are made—they can be either of the following:.
Just as filtrate invasion and formation alteration may produce changes in formation acoustic properties, and thus variation in acoustic logs over time,    so too, cement-bond logs may vary over time as the cement cures and its properties change. The received waveform is the sum of the reflected waveform from the original burst and the exponentially decaying waveform from the resonant energy that is trapped between the inner and outer edges of the casing.
Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. Modern acoustic cement-evaluation bond devices are comprised of monopole axisymmetric transmitters one or more and receivers two or more.
The casing-evaluation presentation includes casing ovality, eccentricity, hole deviation, and gamma ray in Track 1. A BI value of 1. The shorter spacing e. Track 2 contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond.
One caveat regarding the use of the amplitude curve for bond evaluation: Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read. The bonding between cement and formation is investigated intdrpretation a CBL-type receiver array for wave-train presentation Fig.
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. Dark zones are the most strongly bonded.
The cement-evaluation presentation includes casing ovality and tool eccentricity in Track 1. A value of less than 1. These are the bond between casing and cement and, to a lesser extent, that between cement and formation. New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions. The data relate to an interval that overlaps with the conventional CBL in Fig.
In the first scenario, free pipe, there is no cement bond between the casing and cement. Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected by highly attenuating muds.
For example, the presence of shear-wave amplitudes on the full-waveform display is an indication of good acoustic coupling to the formation.
Cement bond logs
The bond index BI is a qualitative measure of cement bond based on signal amplitude. Amplitude, measured directly or as an attenuation ratio, is the primary bond measurement and is used to provide:.
Whenever possible, amplitude data should be used in conjunction with the other measurements presented on the log e. Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond. Courtesy of Baker Atlas.
In this example, the partial bonding is sufficient to provide hydraulic isolation. At greater depths, the pipe is well bonded. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal.
The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters.