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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Cement-bond logging tools use gated systems to measure the specific parts of the acoustic waveform needed for the primary bond-amplitude measurement.
Track 2 contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond. Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:.
The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig. On a variable-density log, free pipe and fluid arrivals if present are easily identified as straight dark and light lines indicating homogenous acoustic properties at either side of the display Fig. Eventually, borehole-compensated devices using dual transmitters and dual receivers were introduced in knterpretation s, and today most commercial devices use multiple transmitters and receivers in a variety of arrangements to provide compensated measurements.
For example, the presence of shear-wave amplitudes on the full-waveform display is an indication of good acoustic coupling to the formation. Channeling is significant because it prevents a hydraulic seal. Centering in the cased hole is more critical because the higher-operating frequencies i. This Interpretatioj tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus.
The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy. The use of new high-performance low-density, foam, and complex cements is increasing.
The received interpretatio 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. In this example, the partial bonding is sufficient to provide hydraulic isolation. This dimensionless quantity is the ratio of measured attenuation to maximum attenuation:. Gates are time periods during which measurements are made—they can be either of the following:.
In the first scenario, free pipe, there is no cement bond between the casing and cement. On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength.
Cement bond logs
Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation. In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig.
Measurements from these devices are influenced by the same factors as openhole televiewer devices. Presentation of the full acoustic waveform assists in resolving bond ambiguities arising from use of an amplitude measurement alone and provides qualitative intrrpretation about the cement-to-formation bond.
Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity. Track 4 provides the average, minimum, and maximum values of the pipe radius that is shown in Track 5.
Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of vdo velocity and density. Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected by highly attenuating muds.
Data from imterpretation tools are presented as individual log curves or as azimuthal images “maps” of cement quality generated by interpolating between the individual azimuthal measurements Fig. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:. The casing-evaluation presentation includes casing ovality, eccentricity, hole deviation, and gamma ray in Track 1. Attempts to rectify this problem will be impeded by the inherpretation isolation above and below this interval.
In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath. Track 6 provides the average, minimum, and maximum values of the pipe thickness that is the image shown in Track 7, where red indicates pipe thinning and blue indicates pipe bdl. In this case, the eccentricity comprises both tool and casing eccentricity resulting from formation movement salt flow.
CEMENT BOND LOG INTERPRETATION
The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read. Tracks 4 and 5 imart clarity to the interpretation of Fig. When conducting a cement evaluation, information on the type of cement used is essential.
Cement bond logs –
This tool can also operate in open hole as a formation imager see Borehole Imaging. Channels as narrow as 1. 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 reflected wave is measured. However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. It deals with the problem of gas-contaminated cement, which has been a longstanding interpretation problem in the industry.
Track 1 contains the gamma ray for correlation and acoustic travel time for quality control. Track 2 contains the acoustic attenuation logs for the six pads. The USI cannot distinguish between gas-filled cement and fluids, but it can quantify the acoustic impedance of the cement. Straight lines in the CBL waveform, along with high amplitude readings, indicate poor cement-to-casing bond. A BI value of 1. A value of less than 1. Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude.
As with openhole tools, cement-bond tools require centralization to ensure accurate measurements. Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
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. It is used with a conventional CBL tool. Table 1  . Inter;retation the same time, the annulus outside the cement sheath provides poor acoustic coupling.