Several different types of tools have been used for cement investigation in wells. The original "cement top locator" was the unfocused gamma ray density log (a/k/a the 4p omni directional density log), which log has been more recently resurrected as the gravel pack log. Cement tops can be located with a
temperature log if run while the heat of hydration created during the curing process is still present. Radioactive tagging has also been used to identify the top of cement. Even a neutron type log has been proposed for cement evaluation behind pipe.
All modern commercial bond logging tools are acoustical instruments. The concept of an acoustical bond logging tool first appeared in the literature in 1961, and by 1962, a working tool was in use in the field. The original tool had a single transmitter and a single receiver, and produced only an amplitude curve; the limitations of this tool were soon recognized and thus began the evolution that continues to this day. Experimentation with the spacing between the receiver and transmitter led to the development of the single transmitter-dual receiver tool, usually referred to as the 3'-5' bond tool. Three foot spacing is optimal for cement to casing investigation, and five foot spacing is optimal for cement to formation investigation (actually the three and five foot spacings were never formally standardized, and a few companies even used a single receiver tool with four foot spacing as a compromise distance to avoid the cost of deploying the dual receiver tools). The 3'- 5' tool was in use by the middle sixties and remained the standard for the next twenty years. All manner of "high tech" acoustic bond tools have been developed in recent years, including the multi-transmitter compensated tool, the multi receiver or radial tool, the contact or segmented tool, the incremental pulse-echo tool, the continuous pulse-echo tool, and on and on. These newer tools are often referred to as cement evaluation tools, as opposed to the older single and dual receiver cement bond tools.
The single transmitter-single receiver tool, and the 3'-5' tool remain in daily use by independent loggers. The balance of this effort is dedicated to these old workhorses. If you are trying to identify the top of cement, the single transmitter-single receiver tool will work just fine. Such "top of cement" logs have been accepted by US EPA as a substitute for records of cementing in the underground injection control (UIC) program (see
Cement Bond Log MIT?). When going beyond simply identifying the top of cement to actually attempting to quantify the quality of bond, there is one important thing to remember: Acoustic cement bond logs do not directly measure hydraulic seal. If wells are condemned to workover solely on the strength of a cement bond log, a significant fraction will be perforated only to find cement cannot be squeezed, thus creating a serious problem where none existed to begin with! The cement bond log really measures the loss of acoustic energy as it propagates through casing; this loss can be theoretically related to the fraction of the casing perimeter covered with cement (this theoretical fraction of cement annular fill is called the "Bond Index"). A metaphor is sometimes used where the casing is compared to a bell; said bell rings loudly when in free space, but coat it with a layer of cement and you get a dull thud (nicely cemented pipe rings at less than 5% of the amplitude of free pipe).
Most single transmitter-single receiver, and most 3'-5' tools in current use, send a 20 kHz signal (a somewhat higher frequency is common in the smaller diameter tools due to the physics of the necessarily smaller transducer) from the transmitter to the receiver(s) at a repetition rate of 20 times a second or 20 Hertz. The sonic amplitude measurement is made with an adjustable fixed gate; the delay and width controls are used to set said gate. The travel time measurement is made with a floating gate (see
T2...The Easy Way for a suggested procedure to set said floating gate). The travel time curve serves as a quality control check on tool centralization, and can yield other useful information if correctly set up. Good centralization of cement bond tools is critical, 1/2 inch of decentralization will reduce the amplitude by 35% (some of the literature actually indicates over 50% reduction in amplitude for a mere 1/4 inch decentralization).
With the 3'-5' tool, the three foot spaced receiver is used to record the sonic amplitude (casing attenuation) curve, while the five foot spaced receiver is used to record the complete wavetrain. The full wavetrain is recorded from the five foot receiver to avoid interference with formation signals by the fluid wave (which arrives at the three foot receiver at about 600 microseconds, but at the five foot receiver at about 1,000 microseconds), and to a lesser extent by the actual casing signal (which arrives at the three foot receiver about 114 microseconds before the five foot receiver). It can thus be seen that the practice of recording the full wavetrain from the three foot receiver is of questionable value, and is only infrequently now seen. Three types of full wavetrain displays are available: the more common continuous seismic style a/k/a variable density log (VDL); the signature a/k/a "X-Y" display which actually consists of recorded oscilloscope traces as they would appear every one, two, or four feet; and the seldom seen "X-Y-Z" display which combines the two into a hybrid presentation (see illustration below for examples of the first two).