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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New interpretation methods integrate cbl-vd and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions. 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. The recorded waveforms are presented together with the travel time and llg casing-amplitude curve, which displays the amplitude of the acoustic signal that has traveled through the casing but not through the cement and formation.
A value of less than 1. Floating gates remain open, but recording is only triggered by an amplitude value greater than a designated threshold value.
The application of statistical variance processing to the conjunctive use of CBL and ultrasonic impedance data has led to an improved cement evaluation. As a result, a free-pipe acoustic signal is:. Courtesy of Baker Atlas.
Variations in the acoustic display are cbl-vld as indicating the presence of bonded cement. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers.
Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond. Track 3 shows a cross section of the pipe wall. Table 1 inferpretation . To do this, repeated acoustic pulses are directed at the interprdtation to make it resonate in its thickness mode and the energy level attenuation of the decaying reflected wave is measured.
Cement bond logs
The waveform and amplitude data allow two bonds to be investigated. Pressuring the casing improves the acoustic coupling to the formation and the casing signal will decrease and the formation signal will become more obvious Fig.
Proper cement placement between the well casing and the loh is essential: 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 interpretatoin indicates pipe thickening.
The two measurements are combined to form a result that compensates for:.
The pad spacing is such that the first arrival is the wave that has passed through the casing. In this example, the partial bonding is sufficient to provide hydraulic isolation. If cement is bonded to the casing, there will be a strong reflection. In contrast, a lnterpretation a small gap between the casing and cement sheath may extend over long sections of casing but may not prevent a hydraulic seal.
The signal is received, processed, and displayed as a microseismogram. If the gate opening is too large, there may be interference between early and late-arriving signals. A “straight” waveform display is traditionally interpreted to mean no cement bonding. This tool can also operate in open hole as a formation imager see Borehole Imaging. Centering in the cased hole is more critical because the higher-operating frequencies i.
CEMENT BOND LOG INTERPRETATION
By analyzing the entire waveform, an acoustic-impedance map of the cement can be constructed. An interesting example of the complementary nature of these data has been presented by De Souza Padilha and Da Silva Araujo.
Consequently, there is no acoustic coupling with the formation and most of the transmitted acoustic energy is confined to the casing and the borehole fluid. 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. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.
Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude. Dark zones are the most strongly bonded.
Cement bond logs –
This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus. In the second scenario, good bond, cement is properly bonded to casing and to the formation.
These are the bond interprteation casing and cement and, to a lesser extent, that between cement and formation. Energy is transmitted at one pad and is received at an adjacent pad. However, if only channeling exists, pressuring the casing will not significantly change the log.
For example, the presence of shear-wave amplitudes on the full-waveform display is an indication of good acoustic coupling to the formation. At the same time, the annulus outside the cement sheath provides poor acoustic coupling. The VDL should be examined to ensure that the formation arrival is impacting the amplitude curve.
When conducting a cement evaluation, information on the type of cement used is essential. 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 cl-vdl under pressure to see if there is a difference.
There is poor cement condition between X and X depth units. The first reflection is at the casing itself.
They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free interpretatjon. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance.
It is used with a conventional CBL tool.