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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In the first scenario, free pipe, there is no cement bond between the casing and cement. This may occur with cbl-vdk presence of a microannulus or channels within the cement.
Cement bond logs –
The bond index BI is a qualitative measure of cement bond based on signal amplitude. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. In the last scenario, partial bond, a space exists within an otherwise well-bonded casing. Track 5 is a CBL-type display. The third scenario, bond to casing only, is a common condition in which cement is bonded to the casing but not to the formation.
The recorded waveforms are presented together with the travel time and a casing-amplitude curve, which displays the amplitude of the interptetation signal that has traveled through the casing but not through the cement and formation.
Track 2 shows a cross-sectional presentation of the pipe shape. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.
Table 2 lists the limitations of conventional cement-bond logs. Track 4 contains the average impedance of the image in Track 5 and a cement-bond index CBI as a cbp-vdl indication of the degree of bonding.
CEMENT BOND LOG INTERPRETATION
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. The data relate to an interval that overlaps with the conventional CBL in Fig.
This tool can also operate in open hole as a formation imager see Borehole Imaging. Tool eccentering reduces signal amplitude and travel time Fig. 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.
These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing.
Dark zones are the most strongly bonded. The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy. Variable density is a continuous-depth time display of full-waveform amplitude presented as shades of black and white.
Modern acoustic cement-evaluation bond devices are comprised of monopole axisymmetric transmitters one cbp-vdl more and receivers two or more. Measurements from these devices are influenced by the same factors as openhole televiewer devices.
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 innterpretation provide compensated measurements. Tracks 4 and 5 imart clarity to the interpretation of Fig. New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions.
A similar response can be caused by the presence of formation gas in shallow, high-porosity zones. These cased-hole wireline tools measure:. Centering in the cased hole is more critical because the higher-operating frequencies i. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance.
The application of statistical variance processing to the conjunctive use of CBL and ultrasonic impedance data has led to an improved cement evaluation.
Track 2 contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond.
For interpretatioh, foam cements, which intentionally create void spaces in the interpretatioon cement, can be misinterpreted as partial bond if normal cement is assumed.
These are the bond between casing and cement and, to a lesser extent, that between cement and formation. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers. 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 see if there is a difference.
Track 3 shows the average and minimum attenuation at each sampling level.
interpretatiln TR spacing typically ranges from 3 to 5 ft. 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.
The second reflection may be at the outside of the casing. If there is unset cement or water behind the casing, there will be a weak reflection. Tool response depends on the acoustic impedance of the cement, which, in turn is function of density and velocity.
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. The attenuation in each segment is measured in two directions using a pair of acoustic receivers interpretafion two transmitters.
The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time. Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond. In cases of poor bonding, casing-collar signals may also be identified as “w” patterns anomalies Fig.
Cement bond logs
The USI cannot distinguish between gas-filled cement and fluids, but it can quantify the acoustic impedance of the cement. Courtesy of Baker Atlas. The casing-evaluation presentation includes casing ovality, eccentricity, hole deviation, and gamma ray in Track 1. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read. Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond.
Intrepretation to rectify this problem will be impeded by the hydraulic isolation above and below this interval. In these situations, users are advised to consult with the logging service company regarding the appropriate calibrations.
It is used with a conventional CBL tool.