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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Modern acoustic cement-evaluation bond devices are comprised of monopole axisymmetric transmitters one or more and receivers two or more. The result is that little energy is transferred to the annular fluid and virtually none is transferred to the formation.
Cement bond logs –
On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength. If the gate opening is too large, there may be interference between early and late-arriving signals. Dark zones are the most strongly bonded. A high rate of attenuation is indicative of a good cement bonding to the casing and vbl-vdl absence of channels within the cement.
CEMENT BOND LOG INTERPRETATION
These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:. The ultrasonic tools compute the acoustic impedance of the material beyond the casing. The tool has to be centered. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read.
Acoustic logs do not measure cement quality directly, rather, this value interpretatin inferred from the degree of acoustic coupling of the cement to the casing and to the formation. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond.
Cement bond logs
Radial-cement-evaluation devices were developed to overcome some limitations of conventional cement-bond tools and to permit more accurate evaluation of cement distribution behind casing by providing the precise location of interpretaion bond and channeling.
The first reflection is at the casing itself. One caveat regarding the use of the amplitude curve for bond evaluation: Measurements from these devices are influenced by the same factors as openhole televiewer devices.
Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. When channeling occurs, it is generally localized and nonuniform; that is, it occurs loy relatively short intervals and can frequently be identified by variations in the amplitude response.
They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free pipe. When conducting a cement evaluation, information on the type of cement used is essential.
A common practice is to run cement-bond logs with the casing under pressure to expand the casing against the cement, thereby decreasing any microannulus that might exist. Track 4 contains the average impedance of the image in Track 5 and a cement-bond index CBI as a quick indication of the degree of bonding. Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation.
It is used with a conventional CBL tool. These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing. At greater depths, the pipe is well bonded. Gates are time periods during which measurements are made—they can be either of the following:.
Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white. This dimensionless quantity is the ratio of measured attenuation to maximum attenuation:.
Track 2 contains the acoustic attenuation logs for the six pads. This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal.
Data from these tools are presented as individual log curves or intrepretation azimuthal images “maps” of cement quality generated by interpolating between the individual azimuthal measurements Xbl-vdl.
These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz. However, if only channeling exists, pressuring the casing will not significantly change the log.
Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. There is poor cement condition between X and X depth units.
Tracks 4 and 5 imart clarity to the interpretation of Fig. The method allows localized zones of good hydraulic seal to be identified in a way that is independent of borehole-fluid type. The waveform and amplitude data allow two bonds to be investigated.
New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools interpdetation provide improved cement evaluation in these conditions. It deals with the problem of gas-contaminated cement, which has been a longstanding interpretation problem in the industry. The pad spacing is such that the first arrival is the wave that has passed through the casing.