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. 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.
One caveat regarding the use of the amplitude curve for bond evaluation: Properly run and interpreted, cement-bond logs CBL provide highly reliable estimates of well integrity and zone isolation. Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially.
Track 1 contains the gamma ray and two quality curves for pad contact with the borehole wall and for centralization, both of which are of high quality in this example.
This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus. If cement is bonded to the casing, there will be a strong reflection. Table 2 lists the limitations of conventional cement-bond logs. When conducting a cement evaluation, information on the type of cement used is essential. The acoustic signal travels through borehole fluid, casing, cement, and the formation itself.
Attempts to rectify this problem will be impeded by the hydraulic isolation above and below this interval. TR spacing typically ranges from 3 to 5 ft. Modern acoustic cement-evaluation bond devices are comprised of monopole axisymmetric transmitters one or more and receivers two or more.
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. Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by:.
At the same time, the interpretatoon outside the cement sheath provides poor acoustic coupling. They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free pipe. These interpretatoin wireline tools measure:. The cement-evaluation presentation includes casing ovality and tool eccentricity in Track 1.
In an evolution similar to that of openhole acoustic logs, new designs were subsequently introduced that measured signal amplitude at a near receiver and a full waveform from a far receiver. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond.
Gates are time periods during which measurements are made—they can be either of the following:.
A BI value of 1. For example, the presence of shear-wave amplitudes on the full-waveform display is an inteerpretation of good acoustic coupling to the formation. Acoustic logs do not measure cement quality directly, rather, this value is inferred from the degree of acoustic coupling of the cement to the casing and to the formation.
Cement bond logs
In cases of poor bonding, casing-collar cbo-vdl 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. Variations in the acoustic display are interpreted as indicating the presence of bonded cement. Just as filtrate invasion and formation alteration may produce changes in lig acoustic properties, and thus variation in acoustic logs over time,    so too, cement-bond logs may vary over time as the cement cures and its properties change.
Straight lines interpretaion the CBL waveform, along with high amplitude readings, indicate poor cement-to-casing bond. When channeling occurs, it is generally localized and nonuniform; that is, it occurs over relatively short intervals and can frequently be identified by variations in the amplitude response.
A value of less than 1. Amplitude, measured directly or as an attenuation ratio, is the primary bond measurement and is used to provide:. The pad spacing is such that the first arrival is the wave that has passed through inrerpretation casing.
Cement bond logs –
Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, interpretatioj the casing-cement bond. This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding.
This dimensionless quantity is the ratio of measured attenuation to maximum attenuation:. Measurements from these devices are influenced by the same factors as openhole televiewer devices. The height of the collar disturbance is intrepretation function of measurement TR spacing amplitude, 3-ft interval; variable density, 5-ft interval courtesy of Baker Atlas.
CEMENT BOND LOG INTERPRETATION
As a result, a free-pipe acoustic signal is:. Tool eccentering reduces signal amplitude and travel time Fig. Track 3 shows the average and minimum attenuation at each sampling level. Ultrasonic tools can be operated to address two objectives concurrently: There is poor cement condition between X and X depth units. For example, foam cements, which intentionally create void spaces in the cured cement, can be misinterpreted as partial bond if normal cement is assumed.
A similar response can be caused by the presence of formation gas in shallow, high-porosity zones.
The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with interpretatlon important casing signals.
Track interpretatioj contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond. Essentially, the CBL reads low-amplitude values in gas-contaminated cements. Fixed gates are set generally at the wellsite to open, remain open, and close at designated times; opening time for the gate is a function of the casing size and the borehole-fluid velocity.