WELLOG CEMENT BOND LOG
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CEMENT BOND LOG:
Acoustic logging is also used for determination of cement bond in cased wells. This type of log is most often referred to as a Cement Bond Log (CBL).
Acoustic signals propagated in steel casing are observed to have large amplitude in free (un-cemented) casing. The reason is because much of the energy is retained in the casing. The opposite effect is found in casing that is in contact with a solid such as cement. The casing signal is much smaller in cemented casing because the energy is coupled into the surrounding cement and formation.
The thin plate velocity of sound in steel is approximately 5300 meters per second or 188 microseconds per meter.
In terms of feet per second; The velocity is 17,388 ft/second or 57.51 microseconds per foot.
A transmitter and receiver having 3 feet spacing will receive the casing signal (first acoustic arrival) at 172.53 microseconds plus a short additional period allowing for transit time thru the borehole fluid. If the borehole fluid is water (200 microseconds per foot) then it is possible given casing ID and tool OD to calculate a very close approximation of the expected arrival time.
An oscilloscope is often used for visual presentation of the first acoustic arrival. Placement of a time gate at the appropriate time provides amplitude measurement of the desired arrival at the receiver.
Calibration can be performed first in “Free pipe”.
The tool is moved to a section of un-cemented “free” pipe which represents “zero bond”. The signal amplitude is measured.
Because the casing signal will be the first arrival at the receiver in free casing, the amplitude of that signal is recorded. The free pipe signal represents the highest amplitude. A high signal amplitude indicates poor cement bond (free pipe). A low signal amplitude indicates good cement bond. Amplitude is normally presented on a scale of 0 to 100 percent bond. No cement bond “zero bond” or “free pipe” is represented by maximum amplitude. Due to the fact that well cemented pipe can never reduce the signal to “zero”, a good reference for 100 percent bond is the best cemented portion of the cemented cased hole.
Formation - Cement bond:
Using information obtained from a Variable Density (waveform) display referred to as a VDL display, it is possible to observe the entire receiver wave train. When cementation is complete from casing to cement to formation, it is possible to observe formation related waveform shifts delta- time in the later arrivals. The VDL waveform can be correlated to open-hole acoustic delta-t logs.
The measurement of attenuation measured in decibels (dB) is obtained from the amplitude as follows:
Attenuation = 20/D x Log10(A/Ao)
Attenuation is measured in decibels
Ao is the transmitter amplitude measured in millivolts.
A is the receiver amplitude measured in millivolts.
D is the distance from the transmitter to receiver (spacing).
Note: Attenuation refers to the reduction of amplitude. Attenuation is therefore measured in –dB (dB is the abbreviation for deci-Bell).