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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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This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding. Well logging Petroleum engineering. The waveform and amplitude data allow two bonds to be investigated. Floating gates remain open, but recording is only triggered by an amplitude value greater than a designated kog value. Pressuring the chl improves the acoustic coupling to the formation and the casing signal will decrease and the formation signal will become more obvious Fig.

Retrieved from ” https: They are often grouped as “cement evaluation tools.

Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers. However, the presence of gas in cement slurries, vdk an inert component or as contamination, may seriously affect ultrasonic-tool interpretation.

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These are the bond between lpg and cement and, to a lesser extent, that between cement and formation. However, if only channeling exists, pressuring the casing will not significantly change the log.

variable-density log

In addition, each lof design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. An interesting example of the complementary nature of these data has been presented by De Souza Padilha and Da Silva Araujo. The USI cannot distinguish between gas-filled cement and fluids, but it can quantify the acoustic impedance of the cement.


This can occur because the mudcake dries and shrinks away from cement, or cl the cement did not bond with mudcake in poorly consolidated formations.

variable-density log – Schlumberger Oilfield Glossary

The shorter spacing e. A value of less than 1. Proper cement placement between the well casing and the formation is essential: The first reflection is at the casing itself. The method allows localized zones of good hydraulic seal to be identified in a way that is independent of borehole-fluid type. December Cdl how and when to remove this template message.

This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively.

Cement bond logs –

It deals with the problem of gas-contaminated cement, which has been a longstanding interpretation problem in the industry. Essentially, the CBL reads low-amplitude values in gas-contaminated cements. This condition is indicated by the lack of later-arriving formation energy. Track 2 shows a cross-sectional presentation of the pipe shape.

This article needs additional citations for verification. Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:. In the second scenario, good bond, cement is properly bonded to casing and to the formation. The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with the important casing signals.

In these situations, users are advised to consult with the logging service company regarding the appropriate calibrations. Track 2 contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond.

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. Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density. The VDL should be examined to ensure that the formation arrival is impacting the amplitude curve.


The bond index BI is a qualitative measure of cement bond based on signal amplitude. Articles needing additional references from December All articles needing additional references All articles with unsourced statements Articles with unsourced statements from December The acoustic signal travels through borehole fluid, casing, cement, and the formation itself.

As a result, a free-pipe acoustic signal is:. On the basis of empirical data, the log can be calibrated directly in terms of cement compressive vld. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance.

This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its cl.

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. If the gate opening is too large, there may be interference between early and late-arriving signals. Just as filtrate invasion and formation alteration may produce changes in formation acoustic properties, and thus variation in vbl logs over time, [6] [7] [8] so too, cement-bond logs may vary over time as the cement cures and its properties change.

In an evolution similar to that of openhole acoustic logs, ccbl designs were subsequently introduced that measured signal amplitude at a near receiver and a full waveform from a far receiver. Tool eccentering reduces signal amplitude and travel cvl Fig. As with openhole tools, cement-bond tools require centralization to ensure accurate measurements.