Cement Mapping Tools (CET, PET, CMT)
Cement evaluation (CET) and cement mapping (CMT) tools continue to use many of the features of cement bond (CBL) logs, but add a new feature - a circumferential image representing the cement quality, or lack of it. The ultrasonic imaging (USI) tool, an offshoot of the openhole acoustic image log, is the most recent  version of this class. The CET uses conventional sonic log principles, with measurements made parallel to the tool axis, with 6 or 8 segmented transmitter receiver sets spaced radial around the tool. CMT tools use an ultrasonic pulse echo system, measuring radially, again with 6 or 8 radially spaced transducers. USI type tools use a rotating head, pulse echo concept.

Classical cement bond (CBL) logging tools measure the amplitude or attenuation of 20 to 30 kHz acoustic pulses propagating axially along the casing between a single transmitter and a single receiver.

There are three types of cement mapping tools. The CMT operates with the same acoustic principles as the CBL, but uses oriented acoustic receivers to recover amplitude data from 6, 8 , or ten radial directions (depending on tool design). They may use a single transmitter or one transmitter for each receiver. Some of these tools are pad-type devices.

The second type, the cement evaluation tool (CET) uses ultrasonic acoustic pulses and measures radially instead of axially. This tool is described later in this Section. A third type of cement mapping tool, the rotating-head bond log (RBT) or ultrasonic imaging log (USI) is described in the next Section.

On a CMT, the average amplitude curve is used in the same manner as a CBL to obtain attenuation, bond index, and cement compressive strength. A cement map is made from the amplitude of the individual receivers, to locate channels and voids in the cement.

These logs are sometimes referred to generically as segmented cement bond logs.


CBL-CMT log example

The cement evaluation tool (CET) tool investigates the cement radially instead of axially. Eight ultrasonic transducers, operating as both transmitters and receivers, are positioned radially around the CET sonde 45 degrees apart. Each transducer emits a beam of ultrasonic energy in a 300 to 600 kHz band, which covers the resonant frequency range of most oilfield casing thicknesses. These tools are also called pulse echo tools (PET).

CET log presentations look similar to the CMT, but casing diameter and other information is obtained by processing the echo signal. The pulse echo concept is illustrated below.


Sound paths for pulse echo (CET) type cement bond log

The energy pulse causes the casing to ring or resonate in its thickness dimension, as shown above, perpendicular to the casing axis. The vibrations die out quickly or slowly, depending on the material behind the casing. The majority of the energy is reflected back to the transducer where it is measured, and the remainder passes into the casing wall and echoes back and forth until it is totally attenuated.

Each time the pulse is reflected off the inner casing wall, some energy passes through the interface and reaches the transducer. A ninth transducer continually measures acoustic travel time of the casing fluid column so that the other eight transducer travel times can be converted to distance measurements. This fluid travel time can be presented on the log, if desired, to indicate the type of casing fluid.

CET logs record attenuation of the acoustic signal directly by computing the decay of energy on each waveform by comparing the energy in an early-time window W1 and a later-time window W2, as shown below.


Reflection waveforms and measurement gates for CET calculations

INTERPRETATION RULE 1: Low Amplitude = Good Cement
INTERPRETATION RULE 2: High Attenuation = Good Cement
INTERPRETATION RULE 3: High Bond Index = Good Cement
INTERPRETATION RULE 4: High Compressive Strength = Good Cement

Minimum and maximum compressive strength are computed from the minimum and maximum attenuations on the 8 transducers. These are displayed as continuous log curves. The cement map is created from the energy of the early arrivals of the acoustic waveform in the 8 radial directions. A gas flag is generated when late arrivals are very low energy and a fast formation flag is generated when late arrivals are high energy.

The tool can be oriented to the low side of the borehole or to true north. In addition, measurements of casing diameter, casing roundness, and tool eccentering are derived from the arrival times of the 8 transducers. These caliper curves show casing wear, corrosion, or collapse.

Experience has shown that when there is good cement around the pipe, the bond to the formation is usually good, too. When the cement sheath is very thin, the CET tool responds to formation arrivals. However, when the cement is thick the formation reflections may be too small to measure. So, if good pipe bond but bad formation bond is suspected, the best interpretation can be made by combining the Cement Evaluation log with the Cement Bond/Variable Density log.

eXAMPLES


Layout of CET log. The important curves are the attenuation and compressive strength values
 in Track 2 and the cement map in Track 3. Other data is annotated on the side of the plot.


CET with CBL-VDL before and after squeeze. Note improved cement map, higher
compressive strength, and formation signal on VDL.


Casing diameter, casing thickness, and ovality logs from CET tool are use for casing inspection.


Data from CBL-VDL and CET are combined in Schlumberger’s Cement Advisor program to
 generate a more informative cement map and zone isolation flags (Track 2) in both neat and
foam cement. A section of well with neat cement is shown at left and a section of foam cement
 on the right. Track 1 contains colour coded cement volume map and Track 3 shows a
 computed cement impedance map. The colour code is dark for good cement shading to
yellow for gas (nitrogen in the case of foam cement). Colour code thresholds vary from job to job.
 

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