Cement Integrity Logs - Part 2

4/11/2015 Crain's Petrophysical Handbook - CEMENT INTEGRITY LOGS - PART 2

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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



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.



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.



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 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 Print

Cement Integrity Logs - Part 2

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