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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Radial-cement-evaluation devices were developed to overcome some limitations of conventional cement-bond tools and to permit more accurate evaluation of cement distribution behind casing by providing the precise location of partial bond and channeling.
Floating gates remain open, but recording is only triggered by an amplitude value interpretatoon than a designated threshold value.
A similar response can be caused by the presence of formation gas in shallow, high-porosity zones. Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density. The shorter spacing e.
Track 1 contains the gamma ray for correlation and acoustic travel time for quality control. Track 2 shows a cross-sectional presentation of the pipe shape. 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.
The two measurements are combined to form a result that compensates for:. Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. Energy is transmitted at one pad and is received at an adjacent pad.
This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus.
However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. Straight lines in the CBL waveform, along with high amplitude readings, indicate poor cement-to-casing bond. In this example, the partial bonding is sufficient to provide hydraulic isolation. 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.
As with openhole tools, cement-bond tools require centralization to ensure accurate measurements.
Variable density is unterpretation continuous-depth time display of full-waveform amplitude presented as shades of black and white. Courtesy of Baker Atlas. 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. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. It deals with the problem of gas-contaminated cement, which has been a longstanding interpretation problem in the industry.
Attempts to rectify this problem will be impeded by the hydraulic isolation above and below this interval. Transmitting elements and the firing sequence are controlled to direct steer and enhance the acoustic-energy output of both the pad transmitters and the VDL transmitter. Table 1  . Ultrasonic tools are superior to the acoustic CBLs, interpretatuon they remain adversely affected by highly attenuating muds. These are the bond between casing and cement and, to a lesser extent, that between cement and formation.
Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation. These tools use one or more azimuthally sensitive transducers to evaluate cement cgl around the circumference of the casing.
One caveat regarding the use of the vl curve for bond evaluation: 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 Integpretation. The third scenario, bond to casing only, is a common condition in which cement is bonded to the casing but not to the formation.
If there is unset cement or cbbl behind the casing, there will be a weak reflection. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal.
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. Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro.
The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time. Pressuring the casing improves the acoustic coupling to the formation and the casing signal will decrease and the formation signal will become more obvious Fig.
An interesting example of the complementary nature of these data has been presented by De Souza Padilha and Da Silva Araujo. Interprstation the same time, the annulus outside the cement sheath provides poor acoustic coupling. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read.