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CEMENT INTEGRITY LOGS - PART 2 -- CMT / CET
This Page Cement Mapping Tools (CET, CMT)
See Also Cement Bond (CBL) Ultrasonic (USI) Courses Site Map
CEMENT MAPPING TOOLS (CET, PET, CMT)Cement evaluation (CET) and
cement mapping (CMT) tools continue touse many of the features of
cement bond (CBL) logs, but add a newfeature - a circumferential
image representing the cement quality, orlack of it. The ultrasonic
imaging (USI) tool, an offshoot of theopenhole acoustic image log,
is the most recent version of this class.The CET uses conventional
sonic log principles, with measurementsmade parallel to the tool
axis, with 6 or 8 segmented transmitterreceiver sets spaced radial
around the tool. CMT tools use anultrasonic pulse echo system,
measuring radially, again with 6 or 8radially spaced transducers.
USI type tools use a rotating head, pulseecho concept.
Classical cement bond (CBL) logging tools measure the amplitude
or attenuation of 20 to 30 kHz acousticpulses 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 theCBL, 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 ofthese
tools are pad-type devices.
The second type, the cement evaluation tool (CET) uses
ultrasonic acoustic pulses and measures radiallyinstead of axially.
This tool is described later in this Section. A third type of
cement mapping tool, therotating-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, bondindex, and cement compressive
strength. A cement map is made from the amplitude of the
individualreceivers, to locate channels and voids in the
cement.
These logs are sometimes referred to generically as segmented
cement bond logs.
CBL-CMT log example
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The cement evaluation tool (CET) tool investigates the cement
radially instead of axially. Eight ultrasonictransducers, operating
as both transmitters and receivers, are positioned radially around
the CET sonde 45degrees apart. Each transducer emits a beam of
ultrasonic energy in a 300 to 600 kHz band, which coversthe
resonant frequency range of most oilfield casing thicknesses. These
tools are also called pulse echotools (PET).
CET log presentations look similar to the CMT, but casing
diameter and other information is obtained byprocessing 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 behindthe casing. The majority of the energy is reflected
back to the transducer where it is measured, and theremainder
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 andreaches the transducer. A
ninth transducer continually measures acoustic travel time of the
casing fluidcolumn so that the other eight transducer travel times
can be converted to distance measurements. Thisfluid 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 eachwaveform by comparing the
energy in an early-time window W1 and a later-time window W2, as
shownbelow.
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Reflection waveforms and measurement gates for CET
calculations
INTERPRETATION RULE 1: Low Amplitude = Good CementINTERPRETATION
RULE 2: High Attenuation = Good CementINTERPRETATION 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 attenuationson the 8 transducers. These are
displayed as continuous log curves. The cement map is created from
theenergy of the early arrivals of the acoustic waveform in the 8
radial directions. A gas flag is generated whenlate 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 ofcasing diameter, casing
roundness, and tool eccentering are derived from the arrival times
of the 8transducers. 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 usuallygood, too. When the
cement sheath is very thin, the CET tool responds to formation
arrivals. However, whenthe cement is thick the formation
reflections may be too small to measure. So, if good pipe bond but
badformation bond is suspected, the best interpretation can be made
by combining the Cement Evaluation logwith the Cement Bond/Variable
Density log.
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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.
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CET with CBL-VDL before and after squeeze. Note improved cement
map, higher compressive strength, and formation signal on VDL.
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Casing diameter, casing thickness, and ovality logs from CET
tool are use for casing inspection.
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Data from CBL-VDL and CET are combined in Schlumbergers 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.
Copyright E. R. (Ross) Crain, P.Eng. emailRead the Fine
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