Temperature drift Calibration In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole. The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with the important casing signals. TR spacing typically ranges from 3 to 5 ft. The shorter spacing e. This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding. These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz.
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Temperature drift Calibration In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole. The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with the important casing signals.
TR spacing typically ranges from 3 to 5 ft. The shorter spacing e. This separation allows for easier analysis of the formation-signal strength and is used to monitor cement-to-formation bonding. These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz. As with openhole tools, cement-bond tools require centralization to ensure accurate measurements. Centering in the cased hole is more critical because the higher-operating frequencies i.
Tool eccentering reduces signal amplitude and travel time Fig. Cement-bond logging tools use gated systems to measure the specific parts of the acoustic waveform needed for the primary bond-amplitude measurement. Gates are time periods during which measurements are made—they can be either of the following: Fixed Sliding floating Fixed-gate systems are commonly used for amplitude measurements and floating gates for travel-time measurements.
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.
If the gate opening is too large, there may be interference between early and late-arriving signals. Floating gates remain open, but recording is only triggered by an amplitude value greater than a designated threshold value. A casing cement job can result in one or more of the following situations: Free pipe Bond to casing only Partial bond 1. In the first scenario, free pipe, there is no cement bond between the casing and cement.
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. As a result, a free-pipe acoustic signal is: Long-lived High-amplitude Of uniform frequency 2. In the second scenario, good bond, cement is properly bonded to casing and to the formation.
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 third scenario, bond to casing only, is a common condition in which cement is bonded to the casing but not to the formation.
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.
In this situation, energy traveling through the casing is attenuated drastically because of the highly attenuating cement sheath. At the same time, the annulus outside the cement sheath provides poor acoustic coupling. The result is that little energy is transferred to the annular fluid and virtually none is transferred to the formation. This condition is indicated by the lack of later-arriving formation energy.
A similar response can be caused by the presence of formation gas in shallow, high-porosity zones. In the last scenario, partial bond, a space exists within an otherwise well-bonded casing. This may occur with the presence of a microannulus or channels within the cement. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal.
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. Channeling is significant because it prevents a hydraulic seal. In contrast, a microannulus a small gap between the casing and cement sheath may extend over long sections of casing but may not prevent a hydraulic seal.
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. 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.
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. However, if only channeling exists, pressuring the casing will not significantly change the log. When conducting a cement evaluation, information on the type of cement used is essential.
For example, foam cements, which intentionally create void spaces in the cured cement, can be misinterpreted as partial bond if normal cement is assumed.
Table 1   One caveat regarding the use of the amplitude curve for bond evaluation: pipe amplitude represents the quality of the bond of pipe to cement but provides no indication of the quality of the bond between the cement and the formation. Whenever possible, amplitude data should be used in conjunction with the other measurements presented on the log e. For example, the presence of shear-wave amplitudes on the full-waveform display is an indication of good acoustic coupling to the formation.
Table 2 lists the limitations of conventional cement-bond logs. The VDL should be examined to ensure that the formation arrival is impacting the amplitude curve. The bond index BI is a qualitative measure of cement bond based on signal amplitude. This dimensionless quantity is the ratio of measured attenuation to maximum attenuation A value of less than 1.
Radial-cement evaluation 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 partial bond and channeling. These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing.
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 Fig. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond.
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. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. Measurements from these devices are influenced by the same factors as openhole televiewer devices.
The pad device makes multiple measurements that are: Short-spaced Compensated Azimuthal-attenuation Because the pads are in direct contact with the casing—in contrast to ultrasonic measurements—measurements are unaffected by: Gas in the borehole Heavy-mud conditions Minor tool eccentricity The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters.
The two measurements are combined to form a result that compensates for: Surface roughness The effects of minor residual cement on the inside of the casing Transmitting elements and the firing sequence are controlled to direct steer and enhance the acoustic-energy output of both the pad transmitters and the VDL transmitter.
This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively. The use of new high-performance low-density, foam, and complex cements is increasing.
However, the presence of gas in cement slurries, as an inert component or as contamination, may seriously affect ultrasonic-tool interpretation. New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions.
Interpretation techniques combining these different measurements provide improved evaluation in lightweight cements, especially in the annulus, beyond the casing-cement bond. Table 3 Cement-evaluation logs Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers.
The acoustic signal travels through borehole fluid, casing, cement, and the formation itself. The signal is received, processed, and displayed as a microseismogram. The recorded waveforms are presented together with the travel time and a casing-amplitude curve, which displays the amplitude of the acoustic signal that has traveled through the casing but not through the cement and formation. The waveform and amplitude data allow two bonds to be investigated.
These are the bond between casing and cement and, to a lesser extent, that between cement and formation. A "straight" waveform display is traditionally interpreted to mean no cement bonding. Variations in the acoustic display are interpreted as indicating the presence of bonded cement. 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. Track 1 contains the gamma ray for correlation and acoustic travel time for quality control.
Track 2 contains the amplitude curve and amplified amplitude, which indicates cement-to-casing bond. Track 3 contains the CBL waveform, which indicates cement-to-casing bond as well as cement-to-formation bond.
Straight lines in the CBL waveform, along with high amplitude readings, indicate poor cement-to-casing bond. There is nearly free pipe above an apparent top of cement at a depth of approximately X80 depth units.
At greater depths, the pipe is well bonded. Courtesy of Halliburton. Energy is transmitted at one pad and is received at an adjacent pad. The pad spacing is such that the first arrival is the wave that has passed through the casing. A high rate of attenuation is indicative of a good cement bonding to the casing and an absence of channels within the cement.
The method allows localized zones of good hydraulic seal to be identified in a way that is independent of borehole-fluid type. The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig. 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.
Track 2 contains the acoustic attenuation logs for the six pads. Track 3 shows the average and minimum attenuation at each sampling level. Track 4 presents a variable-attenuation log or cement map of the casing periphery vs. Dark zones are the most strongly bonded. Track 5 is a CBL-type display. In this example, the partial bonding is sufficient to provide hydraulic isolation.
There is poor cement condition between X and X depth units. Attempts to rectify this problem will be impeded by the hydraulic isolation above and below this interval.
Courtesy of Baker Atlas. Ultrasonic tools are superior to the acoustic CBLs, although they remain adversely affected by highly attenuating muds. They are often grouped as "cement evaluation tools. This Schlumberger tool comprised an array of eight ultrasonic transducers that allowed a limited radial inspection of the casing and its annulus. The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy.
The tool has to be centered. 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.
Cbl Vdl Interpretation
The term of CBL is particularly refer to amplitude measurement of acoustic wave. CBL logging tool is run on electric wire line cable after cementing job is done. The measured sound wave is then analyzed. The attenuation is due to the dissipation of acoustic-wave energy by cement sheath behind the casing.
Cement bond logs