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Isolation ScannerAdvanced evaluation of wellbore integrity
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Cement placement is a critical component of a wells architecturefor ensuring mechanical support of the casing, providing protectio
from fluid corrosion, and, most importantly, isolating permeable zones
with different pressure regimes to prevent hydraulic communication.
Conventional cement bond log (CBL) and ultrasonic pulse-echo tech-
niques are sometimes used together to diagnose zonal isolation but
encounter difficulties when attempting to evaluate cements with low
acoustic impedance or cements contaminated with mud. Ambiguity can
result because these tools rely on a significant contrast in acoustic
impedance between the cement and displaced fluid to identify solids.
Isolation Scanner cement evaluation service provides more certainty bycombining the pulse-echo technique with a new ultrasonic technique th
induces a flexural wave in the casing with a transmitter and measures the
resulting signals at two receivers. The attenuation calculated between th
two receivers provides an independent response that is paired with the
pulse-echo measurement and compared with a laboratory-measured
database to produce an image of the material immediately behind the
casing. By measuring radially beyond traditional cement evaluation boundarie
Isolation Scanner service confirms zonal isolation, pinpoints any channels in
the cement, and ensures confident squeeze or no-squeeze decisions.
The signals following the casing arrivals arising from the interface
between the annulus and the borehole or outer casing can be detected
and measured. These third-interface echoes (TIEs) provide the position
f the casing within the borehole, and if the borehole size is known, the
velocity of the annulus material can be determined. This additional
information, available only through the flexural measurements, can
provide useful information for remedial applications and serve to confirm
or determine the correct interpretation for complex evaluations.
Isolation Scanner* cement evaluation service integrates the conventionapulse-echo technique with flexural wave propagation to fully characterize
the cased hole annular environment while evaluating casing condition
APPLICATIONS Differentiate high-
performance lightweight
cements from liquids
Map annulus material as
solid, liquid, or gas
Assess hydraulic isolation
Identify channels and defects
in annular isolating material
Determine casing internal
diameter and thickness
Assess annulus beyond the
casing/cement interface
Difficult to diagnose with
acoustic impedance or
CBL measurements alone
Increasingcontamination6
4
2
0
Neat
Light
8
Acoustic impedance,
Mrayl
GasLiquid
CementContaminated cement
Identifying and distinguishing various well fluids from cement with low acousticimpedance is difficult for CBL and ultrasonic pulse-echo techniques.
Isolation Scanner cement evaluationservice fully characterizes the casedhole environment, including casingposition in the borehole, borehole andouter string imaging, and materialidentification from velocity analysis.
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PITCH-CATCH PROPAGATION
The Isolation Scanner pulse-echo acoustic impedance measurement
is made with a rotating subassembly containing four transducers.
The normal-incidence transducer is oriented 180 from the other
three transducers. The three obliquely aligned transducers transmit
and receive high-frequency pulsed beams (on the order of 250 kHz)
to excite the casing in a flexural mode. Once excited in the casing,
the flexural wave propagates while radiating acoustic energy into
the annulus and back toward the receiving transducers, resulting
in a circumferential scan of the casing, annulus, cement, and near-
wellbore formation. The annulus-propagating energy is reflected at
interfaces that present an acoustic contrast, such as the cement/
formation interface, and propagates back through the casing predomi-
nantly as a flexural wave that reradiates energy into the casing fluid.
Annulus
Borehole
Formation
Casing
Time, us
80 100 110 120 130 140 150 160 170
R
T
Pulseecho
Flexural wave imaging
Pulse-echo tool
90
The Isolation Scanner sub implements thepulse-echo (normal-incidence) techniquewith four transducers: a transreceiver andthe flexural wave imager comprising onetransmitter and two receivers obliquelyaligned to excite the casing flexural mode.
Geometrical interpretation of signal propagation for the pulse-echo(blue paths) and flexural wave imaging (green paths) techniques showsthat the pitch-catch flexural wave signal separates into an early-arriving(or casing) signal and a later-arriving (TIE) signal in reference to the firstinterface encountered in the annulus (the inner and outer walls of the casingare the first and second interfaces, respectively). The attenuation of thecasing arrival amplitude complements the pulse-echo measurement todetermine whether the material behind the casing is a fluid or a solid. IfTIEs are present in the acquired data, they are used to further enhancethe characterization of the cased hole environment in terms of the state andacoustic properties (wave speed) of the material filling the annulus, the posi-tion of the casing within the hole, and the geometrical shape of the hole.
Flexural wave (TIE)
Flexural wave (casing arrival)
Ultrasonic wave
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INSIGHT THROUGH ATTENUATION
The rate of energy radiation into the annulus depends on the acoustic
properties of the annular fill. The attenuation is estimated by captur-
ing the reflected signals at two of the receivers, which are a known
distance apart, and calculating the decay rate of the received signal.
Attenuation is expressed in decibels per centimeter.
For a fluid filling the annulus, the attenuation is approximately propor-
tional to the acoustic impedance. For cement bonded to the casing, the
attenuation exhibits a more complex behavior as a function of the
velocities at which the compressional and shear waves propagate in
the cement. As shown in the plot of flexural attenuation versus acoustic
impedance for a well-bonded cement, below a critical impedance (Zc)
of approximately 3.9 Mrayl, the attenuation increases linearly with the
impedance of the annular fill, whether the fill is liquid or solid. Beyond
3.9-Mrayl Zc, only the shear waves can propagate in the cement, and
the attenuation drops sharply to small values. A high-impedance cement,
such as Class G, has an attenuation similar to that of a liquid. This
ambiguity in identifying cement is resolved by determining the acoustic
impedance of the cement with the pulse-echo technique. However, the
distinct attenuation of low-impedance cements, such as lightweight or
contaminated cements, is used to differentiate them from fluids.
Zc
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Acoustic impedance, Mrayl
SolidLiquid
Water
Gas
Flexural attenuation,
dB/cm
0 1 2 3 4 5 6
Flexural wave attenuation of a well-bonded cement is plotted as a functionof acoustic impedance for various materials. The value of Z
ccorresponds to
the critical compressional wave speed of the cement.
Three clouds of points are generated in SLG mapping of the measurementplane for a Class G cement. Zusit is the impedance determined by the pulse-echo technique; the attenuation is for the flexural wave technique.
86420-2 1
Zusit, Mrayl
1.6
1.4
1.8
1
0.8
0.6
0.4
0.2
0
0.2
GasLiquidSolid
Nominal uncontaminatedClass G cement
1.2
Attenuation,dB/cm
SOLID-LIQUID-GAS MAPPING
The first goal of Isolation Scanner processing is to provide a robust
interpreted image of the material immediately behind the casing. The
inputs are cement impedance determined by the pulse-echo mea-
surement and flexural wave attenuation computed from the amplitude of
the casing arrivals on the near and far receivers. These two independent
measurements are linked to the properties of both the fluid insidethe casing and the outside medium through an invertible relation.
Combining them accounts for the effect of the inside fluid, which
eliminates the need for logging specific fluid-property measurements
The processing output is a solid-liquid-gas (SLG) map displaying the
most likely material behind the casing. The map is computed during
an initialization step before logging by using a priori knowledge of the
possible materials:
Gas is defined as a very low impedance material, independent of
any input.
Liquid is defined as a liquid with the expected acoustic impedance
of the mud displaced by the cement, with provisions for possible
deviations from this value.
Solid is defined through the expected type of cement. A laboratory-
measured database is used to convert the material selection into
acoustic properties, again with provisions made for some contami-
nation or incompletely set cement.
Areas corresponding to inconsistencies between the measure-
ments (for example, at collars) are shown in white.
The mapped states are obtained for each azimuth by locating the
pulse-echo and flexural attenuation measurements, corrected for theeffect of the inside fluid, on the map with the areas encompassed by
each state.
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CasingCollar
Locator
10 30
Amplitudeof Echo
Minus Max.(Rugosity)
InternalRadii Minus
Average,in
MaximumInternalRadius
ExternalRadius
Average
(ERAV)
MinimumInternalRadius
4.5 3.0in
4.5 3.0in
4.5 3.0in
MinimumInternalRadius
ExternalRadius
Average
(ERAV)
MaximumInternalRadius
3.0 4.5in
3.0 4.5in
3.0 4.5in
ThicknessMinus
Average,in
Impedance,Mrayl
FlexuralAttenuation,
dB/m SLG Map
HydraulicCommun-
icationMap
500.0000
6.0000
5.6000
5.2000
4.8000
4.4000
3.6000
3.2000
2.6000
2.4000
2.0000
1.6000
1.2000
0.6000
0.40000.5000
500.0000
0.0780
0.0680
0.0520
0.0440
0.0380
0.0280
0.0200
0.0120
0.0040
0.0120
0.0280
0.0360
0.0440
0.0520
0.0600
0.0880
0.0760
500.0000
0.0780
0.0680
0.0520
0.0440
0.0380
0.0280
0.0200
0.0120
0.0040
0.0040
0.0120
0.0200
0.0280
0.03600.0440
0.0520
0.0600
0.0680
0.0760
500.0000
0.03000
2.8000
2.9091
3.0182
3.1273
3.2364
3.3454
3.5638
3.6727
3.7818
3.8909
4.0000
0.0000
50.0000
57.0000
64.0000
71.0000
78.0000
85.0000
92.0000
99.0000
106.0000
113.0000
120.0000
127.0000
134.0000
141.0000155.0000
SolidLiquidGas
SealNo seal
Depth, m
x200
x250
x300
x350
In additional to pulse-echo information on the rugosity, radius, cross section, and thickness of the 7-in [17.8-cm] casing, Isolation Scanner service processed theacoustic impedance and flexural attenuation data to produce an SLG map. Although the cement is heavy Class G, the flexural attenuation map clearly displayslow-density material from X,320 to X,270 m, revealing that the cement is contaminated in that interval. Regardless of the density difference, the material is correctlyindicated as solid on the SLG map.
By measuring radially and beyond traditional cement evaluation boundaries,
Isolation Scanner service confirms zonal isolation, pinpoints any channels
in the cement, and ensures confident squeeze or no-squeeze decisions.
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NEW MEASUREMENTS FROM FORMATION-WALL ECHOES
In addition to the SLG map identifying the annular fill immediately
behind the casing, a further Isolation Scanner objective is to extract
relevant information from the annulus/formation reflection echo or
echoes for quantifying the state of the annulus between the casing
and formation. First, the echoes following the casing arrival are detected
and their time of arrival and amplitude measured. From the time differ-ences between the reflection echoes and the casing arrivalprovided
sufficient echo azimuthal presence is available in the datathe casing
centering within the borehole can be determined. This is conveniently
presented as a percentage, with 100% representing perfect centering
and 0% for fully eccentered casing, in contact with the formation wall.
If the borehole diameter is known, the time-difference processing can
be further converted into material wave velocity and is displayed as
an annulus velocity map.
Other new measurements possible with the Isolation Scanner TIE
reflected from the cement/formation interface are
estimated wave velocity, which can be used to confirm the
SLG map and better understand cement placement
imaged borehole shape
imaged outer string to reveal corrosion and damage.
Isolation Scanner imaging of the formation wall through casing and cement revealshole enlargement (caving) in the reflection echo from the cement/formation interface attwo opposite azimuths in the intervals X,673X,675 m and X,679X,682 m. The left-sideimage, displaying the raw data at all azimuths, shows that the formation-wall echo ispresent at nearly all azimuths. Echo moveout appears sinusoidal because of casingeccentering. Each cycle represents a tool azimuthal scan.
Holeenlargement
Hole
enlargement
Casingarrival
Echo fromformation
wall
X,673
X,684
X,674
X,675
X,676
X,677
X,678
X,679
X,680
X,681
X,682
X,683
Depth, m
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Isolation Scanner Tool
Output Solid-liquid-gas map of annulus material, hydrauliccommunication map, acoustic impedance, flexuralattenuation, rugosity image, casing thickness image,internal radius image
Max. logging speed Standard resolution (6 in, 10 sampling): 823 m/h
[2,700 ft/h]High resolution (0.6 in, 5 sampling): 172 m/h[563 ft/h]
Range of measurement Min. casing thickness: 0.38 cm [0.15 in]Max. casing thickness: 2.01 cm [0.79 in]
Vertical resolution High resolution: 1.52 cm [0.6 in]High speed: 15.24 cm [6 in]
Acoustic impedance Range: 0 to 10 MraylResolution: 0.2 MraylAccuracy: 0 to 3.3 Mrayl = 0.5 Mrayl,>3.3 Mrayl = 15%
Flexural attenuation Range: 0 to 2 dB/cmResolution: 0.05 dB/cmAccuracy: 0.01 dB/cm
Min. quantifiable channel width 30 mm [1.2 in]Depth of investigation Casing and annulus up to 7.62 cm [3 in]
Mud type or weight limitation Conditions simulated before logging
Combinability Bottom only, combinable with most wireline toolsTelemetry: fast transfer bus (FTB) or enhanced FTB(EFTB)
Special applications H2S service Investigation of annulus width depends on the presence of third-interface echoes. Analysis and processing
beyond cement evaluation can yield additional answers through additional outputs, including the VariableDensity log (VDL) of the annulus waveform and polar movies in AVI format.
Differentiation of materials by acoustic impedance alone requires a minimum gap of 0.5 MRayl betweenthe fluid behind the casing and a solid.
For 8-mm [0.3-in] casing thickness.
Max. mud weight depends on the mud formulation, sub used, and casing size and weight, which aresimulated before logging.
Measurement Specifications
Isolation Scanner Tool
Max. temperature 177 degC [350 degF]
Pressure range 1 to 138 MPa [145 to 20,000 psi]
Casing sizemin. 412in (min. pass-through restriction: 4 in)
Casing sizemax. 958in
Outside diameter IBCS-A: 8.57 cm [3.375 in]IBCS-B: 11.36 cm [4.472 in]IBCS-C: 16.91 cm [6.657 in]
Length without sub 6.01 m [19.73 ft]
Weight without sub 151 kg [333 lbm]
Sub length, weight IBCS-A: 61.22 cm [24.10 in], 7.59 kg [16.75 lbm]IBCS-B: 60.32 cm [23.75 in], 9.36 kg [20.64 lbm]IBCS-C: 60.32 cm [23.75 in], 10.73 kg [23.66 lbm]
Sub max. tension 10,000 N [2,250 lbf]
Sub max. compression 50,000 N [12,250 lbf] Limits for casing size depend on the sub used. Data can be acquired in casing larger than 9 58in with
low-attenuation mud (e.g., water, brine).
Mechanical Specifications
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www.slb.com/scanner
*Mark of Schlumberger
Copyright 2011 Schlumberger. All rights reserved. 11-PR-0028
Isolation Scanner