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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CEMENT BOND LOG INTERPRETATION
This can occur because the mudcake dries and shrinks away from cement, or because the cement did not bond with mudcake in poorly consolidated formations. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.
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. In the first scenario, free pipe, interpretaiton is no cement bond between the casing and cement.
Whenever possible, amplitude data should be used in conjunction with the other measurements presented on the log e.
A “straight” waveform display is traditionally interpreted to mean no cement bonding. 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.
In this example, the partial bonding is sufficient to provide hydraulic isolation. There is nearly free pipe above an apparent top of cement at a depth of approximately X80 depth units. Properly run and interpreted, cement-bond logs CBL provide highly reliable estimates of well integrity and zone isolation.
This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively. To support the casing shear bond To prevent fluid from leaking to the surface Interpreration isolating producing zones from interpretattion zones hydraulic bond Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
An interesting example of the complementary nature of these data has been presented by De Souza Padilha and Da Silva Araujo. If there is unset cement or water behind the casing, there will be a weak reflection. As with openhole tools, cement-bond tools require centralization to ensure accurate measurements. A high interprettion of attenuation is indicative of a good cement bonding to the casing and an absence of channels within the cement.
Data from these tools are presented as individual log curves or as azimuthal images “maps” of cement quality generated by interpolating between the individual azimuthal measurements Inrerpretation. This separation allows for easier analysis of the formation-signal strength interpretaiton is used to monitor cement-to-formation bonding.
In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath.
Cement bond logs –
The tool has to be centered. 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.
Loh BI value of 1. The shorter spacing e. Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond.
Track 2 shows a cross-sectional presentation of the pipe shape. This map can cb,-vdl the presence of channels and their orientations. The USI cannot distinguish between gas-filled cement and fluids, but it can quantify the acoustic impedance of the cement.
Cement bond logs
Table 1  . One caveat regarding the use of the amplitude curve for bond evaluation: 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.
Track 5 is a CBL-type display. The bond index BI is a qualitative measure of cement bond based on signal amplitude. Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation.
They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free pipe.
Track 4 presents a variable-attenuation log or cement map of the casing periphery vs. Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density. However, they are not reliable as indicators of hydraulic sealing by the cement, because they cannot detect small channels therein. Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially.
Tracks 4 and 5 imart clarity to the interpretation of Fig. Track 3 shows a cross section of the pipe wall. 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. Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity.
The data for circumferential inspection of the casing, as described above, and for the evaluation of cement bonding are obtained on the same logging pass. In this case, the eccentricity comprises both tool and casing eccentricity resulting from formation movement salt flow.