WELLOG                 Acoustic Log Interpretation



REVISED 10-15-2009

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Part III, page 5





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 casing because much of the energy is retained in the casing. Whereas the opposite effect is found in casing that is in contact with a solid such as cement. The casing signal is much smaller 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 (188 microseconds per meter).



                        1 meter = 39 inches (approx) and 3 ft = 36 inches therefore .92 * 188 = 174 microseconds.


A receiver having 3 feet spacing will receive the casing signal (first arrival) at 174 microseconds plus an additional period allowing for transit time thru the borehole fluid.


Consideration for fluid travel time at approximately 200 microseconds per foot shows that for example in a 12 inch casing, 6 inches from tool center to casing and 6 inches from casing to tool will add 1 foot = 200 microseconds to total travel time.


Variation in total travel time due to fluid type will cause the casing signal to move or change position in relative time.


A receiver signal “time gate” is set at the time of the expected casing signal. The casing signal will be the first arrival at the receiver in free casing. The signal amplitude is recorded.


Signal amplitude is affected by the acoustic coupling of cement in the annulus (space between the casing and formation). Acoustic energy is absorbed when cement is in contact with the casing and formation.   





In general terms,


A high signal amplitude indicates poor cement bond. A low signal amplitude indicates good cement bond. Amplitude is normally presented on a scale of 0 to 100 percent amplitude. No cement bond is represented by 100 percent amplitude. Due to the fact that well cemented pipe can never reduce the signal to “zero”, a good reference for zero signal is the best cemented portion of the cased hole. 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 (good bond) from casing to cement to formation, it is possible to observe waveform shift in delta- time in the later arrivals that can be correlated to open-hole acoustic delta-time logs. This is referred to as “formation” signal on the CBL.




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 abbreviated dB.


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. Therefore, attenuation is measured in  dB.




Commercially available CBL services offer a log containing the measurement of amplitude and VDL.





The variable density display is a Z axis representation of the receiver waveform. The display is usually monochrome (black and white) however in some presentations, the display is in color.




A variation in application of the VDL is in an open-hole environment that is used for the purpose of defining fractures in surrounding formations. The service is referred to as a fracture finder or micro-seismogram log.




Acoustic bond tools have been developed that have segmented receivers. A typical eight segment array is arranged radially around the circumference of the tool. Each segment has a view of 45 degrees of the total 360 degree circumference. The advantage of this configuration is that eight discrete receivers that are able to define vertical channels in cement.




Ultrasonic waveforms having frequencies that range from 200 KHz to 700 KHz are used. A rotating acoustic sensor is employed to measure acoustic energy through a 360 rotation. The Ultrasonic Imager Tool (Schlumberger USIT) is an example. It performs the functions of cement evaluation, casing inspection, corrosion detection and monitoring, detection of internal and external damage or deformation and casing thickness analysis for collapse and bust pressure calculations.




The acoustic borehole imager (ABI) televiewer is a tool that acoustically scans a borehole – usually in open-hole. A very small acoustic receiver aperture is used to increase the acoustic resolution. A stepper motor rotates the acoustic aperture and at each step records the acoustic signature of the borehole. Logging speeds are slow. The log is an acoustic image of the borehole that defines structural details in a relatively high resolution format.


                        CONTINUE:                                FORMATION EVALUATION