The Cement Bond Log (CBL) service provides a detailed evaluation of the casing collar locator (CCL), and the VDL or signature waveform presentation. The variable-density log is commonly used as an adjunct to the cement-bond log, and offers better insights into its interpretation; in most cases microannulus. A digital sonic tool that provides conventional openhole sonic measurements, standard CBL amplitude and Variable Density log (VDL), and attenuation.
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Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white. Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond.
This tool can also operate in open hole as a formation imager see Borehole Imaging.
However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. However, if only channeling exists, pressuring the casing will not significantly change the log. By analyzing the cbbl waveform, an acoustic-impedance map of the cement can be constructed.
CEMENT BOND LOG INTERPRETATION
The second reflection may be at the outside of the casing. They are often grouped as “cement evaluation tools. The ultrasonic tools compute the acoustic impedance of the material beyond the casing. 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.
Positive waveform amplitudes are shown lkg dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude. For example, foam cements, which intentionally create void spaces in the cured cement, can be misinterpreted as partial bond if normal cement is assumed. Straight lines in the CBL waveform, along with high amplitude readings, indicate poor dbl bond.
New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions. Fixed-gate systems are commonly used for amplitude vd and floating gates for travel-time measurements. Attempts to rectify this problem will be impeded by the hydraulic isolation above and below this interval.
This condition is indicated by the lack of later-arriving formation energy. One caveat regarding the use of the amplitude curve for bond evaluation: Track 1 contains the gamma ray and two quality curves for vsl contact with the borehole wall and vd centralization, both of which are of high quality in this example.
A common practice is to run cement-bond logs with the casing under pressure to expand the casing against the cement, thereby decreasing any chl that might exist. If cement is bonded to the casing, there will be a strong reflection.
Whenever possible, amplitude data should be used in conjunction with the other measurements presented on the log vdk. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers. The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy. TR spacing typically ranges from 3 to 5 ft. Track 3 shows the average and minimum attenuation at each sampling level.
In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation lg. These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing.
Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond.
Cement bond log
Essentially, the CBL reads low-amplitude values in gas-contaminated cements. This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively.
Transmitting elements and the firing sequence lo controlled to direct steer and enhance the acoustic-energy output of both the pad transmitters and the VDL transmitter. The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time.
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variable-density log – Schlumberger Oilfield Glossary
This article needs additional citations for verification. The waveform and amplitude data allow two bonds to be investigated. There are production reasons and legal reasons governed by a petroleum regulatory body in each individual state that dictate the well must have specific areas of isolation. Track 4 provides lig average, minimum, and maximum values of the pipe radius that is shown in Track 5.