This article describes how Cement Bond Logs are used to interpret the quality of the cement bond. The Cement Bond Logs (CBL) gives a continuous measurement of the amplitude of the first casing arrival at a receiver spaced 3 ft from an acoustic transmitter. The amplitude is a maximum in unsupported pipe, and a minimum for well cemented casing.

The absolute reading is a function of casing size and thickness, cement bond quality, wellbore fluid, tool type and tool centering. Generally, most of these parameters will be constant so that cement bond quality can be directly related to the amplitude measured.

Usually, the CBL is correlated to the open hole logs. This is achieved by running a Gamma Ray (GR) and Casing Collar Locator (CCL) in combination with the CBL to provide a correlation for any subsequent operations.

Variable Density Log  (VDL)

The Variable Density Log (VDL) provides a graphical representation of the actual sonic waveform recorded at a 5 ft transmitter/receiver spacing. The extra spacing gives more time for transit time differences to be shown up. In addition, information from the sonic waveform beyond the first arrival is provided.

This trace may be used as a major input to ensuring good log quality. A poor cement bond is noted by strong parallel black and white vertical stripes. A good cement bond is generally noted by a dull grey featureless response where the casing signal is expected.

The VDL shall always be run in combination with the CBL.

Cement Evaluation Tool (CET)

In the cement evaluation tool (CET), transducers are placed circumferentially on the sonde to emit ultrasonic energy and then receive the reflected waveform back from the casing and beyond. The dampening of the resonance induced by the transmitted pulse depends on the cementation of the casing and this is measured to infer cement bond quality. Well cemented casing exhibits rapid decay of resonant signals whereas poor cement shows slow decay periods.

The CET generally provides better cement bond evaluation than CBL/VDL and is unaffected by micro-annuli. In addition to providing cement bond information, the CET can be used for determining corrosion by presenting calliper measurements and, in some cases, casing thickness plots.

Determination of the cement quality is somewhat vague using this tool and consequently, the tool is usually run in combination with the CBL to provide extra cement quality information. CET is generally run 100ft over the liner lap.

CBL / VDL Log Quality Control

Cement bond logs are usually presented in the form of three outputs as follows:

• Transit time which represents the time from emission of the pulse to detection (usually of the first arrival) at the receiver.
• Amplitude of first arrival either scaled in mV or presented as percentage free pipe.
• Variable Density Log representation of the complete sonic waveform as detected at the 5 ft receiver.

The Gamma Ray and CCL traces should be presented in track one for depth correlation purposes.

CBL’s are particularly prone to operator induced error. The following guidelines must be followed to ensure quality of the CBL / VDL log:

• The log should only be run in casing sizes of 9 5/8in or less.
• The tool must be well centralised using at least three centralisers. It is essential that the transmitter and receiver are within 1/8in of the pipe centre for results to be meaningful.

Transit time measurements can give an indication of tool eccentricity, since a tool lying against the casing will give lower transit times than one which is properly centralised. The following table lists the expected transit times for a 3 ft transmitter/receiver spacing in various casing sizes.

Variation from the listed values will, in most cases, indicate excentering of the tool, however, the values must not be taken as absolute since variation in mud parameters and tool type can result in quite large variation in transit time. Fast formations may also account for shorter transit times.

• In deviated wells, the tool should be centralised for each diameter of casing logged. For example after logging 7in liner, the tool should be pulled and re-centralised before logging of the 9.5/8in.
• If possible the tool should be calibrated downhole in un-cemented pipe or in water.

CET Log Quality Control

The following guidelines must be followed to ensure quality of the CET log:

• The CET tool must be calibrated downhole in free pipe to determine tool constants in the specific environment of the well to be logged. The calibration plot produced must be presented on the log.
• The tool eccentricity must be less than 1/4in.
• In general, the cement quality should agree with the CBL.
• The four callipers should read within 1/8in of each other, and the ovality should be less than 1/8in.
• There must be no more than one black flag (indicating unreliable results) per foot.
• Free pipe should show as white.

The CET tool should not be used in mud with gradients higher than shown in the following table:

This tool should be run only after at least 24 hours following completion of the cementation (cement in place).