The Cement Bond Log (CBL) service provides a detailed evaluation of the casing collar locator (CCL), and the VDL or signature waveform presentation. The variable-density log is commonly used as an adjunct to the cement-bond log, and offers better insights into its interpretation; in most cases microannulus. A digital sonic tool that provides conventional openhole sonic measurements, standard CBL amplitude and Variable Density log (VDL), and attenuation.
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As a result, a free-pipe acoustic signal is:. The casing-evaluation presentation includes casing ovality, eccentricity, hole deviation, and gamma ray in Track 1. Measurements from these devices are influenced by the same factors as openhole televiewer devices. The bond index BI is a qualitative measure of cement bond based on signal amplitude. The zigzag, wavy, or chevron pattern between these two arrivals is the formation signal indicating varying acoustic transit time.
The attenuation in each segment is measured in two directions using a pair of acoustic receivers and two transmitters. They operate on the principle that acoustic amplitude is rapidly attenuated in good cement bond but not in partial bond or free pipe. A high rate of attenuation is indicative of a good cement bonding to the casing and an absence of channels within the cement.
These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing. 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.
Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance.
Tracks 4 and 5 imart clarity to the interpretation of Fig.
CEMENT BOND LOG INTERPRETATION
The bonding between cement and formation is investigated through a CBL-type receiver array for wave-train presentation Fig. Attempts to rectify this problem will be impeded by the hydraulic isolation above and below this interval. The resulting waveform is comprised of a casing signal and a formation signal; the casing signal arrives first, followed by the formation signal. Track 3 shows a cross section of the pipe wall. To support the casing shear bond To prevent fluid from leaking to the surface For isolating producing zones from water-bearing zones hydraulic bond Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond.
The two measurements are combined to form a result that compensates for:. December Learn how and when to remove this template message. The received waveform is the sum of the reflected waveform from the original burst and the exponentially decaying waveform from the resonant energy that is trapped between the inner and outer edges of the casing.
They are often grouped as “cement evaluation tools. Conventional CBL tools provide omnidirectional measurements, while the newer radial cement-evaluation tools provide azimuthally sensitive measurements for channel evaluation. As with openhole tools, cement-bond tools require centralization to ensure accurate measurements.
When conducting a cement evaluation, information on the type of cement used is essential. There are production reasons and legal reasons governed by a petroleum regulatory body in each individual state that dictate the well must have specific areas of isolation.
The most recent tools have a single rotating transducer that incorporates both the source and receiver of ultrasonic energy.
A BI value of 1. Track 4 presents a variable-attenuation log or cement map of the casing periphery vs. A value of less than 1. Articles needing additional references from December All articles needing additional references All articles with unsourced statements Articles with unsourced loy from December The waveform and amplitude data allow two bonds to be investigated.
At greater depths, the pipe is well bonded. Unsourced material may be challenged and removed. Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude. At the same time, the annulus outside the cement sheath provides poor vdll coupling. Track vdo 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.
Early CBL designs s used a single transmitter and single receiver for an amplitude measurement. Track 2 contains the acoustic attenuation logs for the six pads.
In the second scenario, good bond, cement is properly bonded to casing and to the formation. However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. In the first scenario, free pipe, there is no cement bond between the casing and cement.
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 first reflection is at the casing itself. In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond.
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. Just as filtrate invasion and formation alteration may produce changes in formation 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.
From Wikipedia, the free encyclopedia. Please help improve this article by adding citations to reliable sources. 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.