Quick basics of purposes of cement & scenarios bond logs run in Variable Density Log (VDL). • Examples of good Log (CBL). 1. 2. 3. 4. Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of Cement to Pipe Bond & Formation VDL. Geosciences/Interpretation The Cement Bond Log (CBL) service provides a detailed evaluation of the cement bond to casing and 3`Amplitude, gamma ray, casing collar locator (CCL), and the VDL or signature waveform presentation.
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Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density.
Cement bond logs –
Interpretaton BI value of 1. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.
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 interpretattion either side of the display Fig. Channeling is significant because it prevents a hydraulic seal.
Properly run and interpreted, cement-bond logs CBL provide highly reliable estimates of well integrity and zone isolation.
Track 4 provides the average, minimum, and maximum values of the pipe radius that is shown in Track 5. Energy is transmitted at one pad and is received at an adjacent pad. Use this section to list papers in OnePetro that a reader who wants interpretatjon learn more should definitely read. On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength.
Amplitude, measured directly or as an attenuation ratio, is the primary bond measurement and is used to provide:. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white. An interesting example of the complementary nature of these data has been presented by De Souza Padilha and Da Silva Araujo.
The signal is received, processed, and displayed as a microseismogram. 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 thickening.
Cement-bond logging tools use gated systems to measure the specific parts of the acoustic waveform needed for the primary bond-amplitude measurement. Table 2 lists the limitations of conventional cement-bond logs. TR spacing typically ranges from 3 to 5 ft. However, they ihterpretation not reliable as indicators of hydraulic sealing by the cement, because they cannot detect small channels therein. A similar response can be caused by the presence of formation gas in shallow, high-porosity zones.
Cement bond logs
Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially. In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole.
Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. Gates are time periods during which measurements are made—they can be either of the following:. This provides cb,-vdl 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.
If the initial log run was not under pressure and the log indicates poor bond, the presence of a microannulus can be evaluated by running a second bond log under pressure to lgo if there is a difference. The data relate to an interval that overlaps with the conventional CBL in Fig. Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude.
In the second scenario, good bond, cement is properly bonded interpretstion casing and to the formation. Channels as narrow as 1. In the last scenario, partial bond, a space exists within an otherwise well-bonded casing.
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. The third scenario, bond to casing only, is a common condition in which cement is bonded to the casing but not to the formation.
Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond. 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. When conducting a cement evaluation, information on the type of cement used is essential.
These are the bond between casing and cement and, to a lesser extent, that between cement and formation. At greater depths, the pipe is well bonded. Eventually, borehole-compensated devices using dual transmitters and dual receivers were introduced in the s, and today most commercial devices use multiple transmitters and receivers in a variety of arrangements to provide compensated measurements.
The result is that little energy is transferred to the annular fluid and virtually none is transferred to the formation. In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath. The second reflection may be at the outside of the casing. Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity.
The ultrasonic tools compute the acoustic impedance of the material beyond the casing. In the first scenario, free pipe, there is no cement bond between the casing and cement.
In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig. Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected by highly attenuating muds. In this case, the eccentricity comprises both tool and casing eccentricity resulting from formation movement salt flow.