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ARI*Azimuthal
ResistivityImager
Schlumberger
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ARI* AzimuthalResistivityImager
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Schlumberger 1993
Schlumberger Wireline & Testing
P.O. Box 2175
Houston, Texas 77252-2175
All rights reserved. No part of this book may be
reproduced, stored in a retrieval system, or tran-scribed in any form or by any means, electronic or
mechanical, including photocopying and recording,
without prior written permission of the publisher.
SMP-9260
An asterisk (*) is used throughout this document to
denote a mark of Schlumberger.
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ContentsIntroduction. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 1
Background . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 2
Principles . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . 3
Dual laterolog resistivity measurements . . . . . . 3
Azimuthal resistivity measurements . . . . . . . . . . . 4
Auxiliary azimuthal measurements . . . . . . . . . . . . 5
Orientation measurements . . . . . . . . . . . . . . . . . . . . . . . 5
Specifications . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . 6
Operation . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . 7
Modes of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Stand-alone operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Environmental corrections . . . . . . . . . . . . . . . . . . . . . . . . 8
Combinability . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 11
Resistivity. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 11
Porosity and lithology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Auxiliary . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 11
Others. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 11
Applications. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 12
Borehole correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Deep invasion . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . 13
Thin-bed analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Fractured formations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Heterogeneous formations . . . . . . . . . . . . . . . . . . . . . . . 17
Dip estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Horizontal wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Borehole profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Groningen effect correction . . . . . . . . . . . . . . . . . . . . . 20
Features and benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Common ARI curve names . . . . . . . . . . . . . . . . . . . . . . . 23
References . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. 24
Recommended reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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The ARI Azimuthal Resistivity Imager, a new-
generation laterolog tool, makes directional deep
measurements around the borehole with a higher
vertical resolution than previously possible.
Using 12 azimuthal electrodes incorporated in a
dual laterolog array, the ARI tool provides a dozen
deep oriented resistivity measurements while
retaining the standard deep and shallow readings.
A very shallow auxiliary measurement is incorpo-
rated to fully correct the azimuthal resistivities for
borehole effect.
The formation around the borehole is displayed
as an azimuthal resistivity image. Although this
full-coverage image has much lower spatial reso-
lution than acoustic or microelectrical images
those coming from the UBI* Ultrasonic Borehole
Imager tool or the FMI* Fullbore Formation
MicroImagerit complements them well becauseof its sensitivity to features beyond the borehole
wall and its lower sensitivity to shallow features
(Fig. 1).
ARI Azimuthal Resistivity Imager 1
Introduction
ARI AzimuthalResistivity Imager
Figure 1. Combining deep ARI images with shallowborehole surface images from the FMI tool, or even acoustic
UBI images, helps to discriminate between deep natural
fractures and shallow drilling-induced fractures.
(Courtesy of UK Nirex Ltd)
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2 Background
The laterolog technique was introduced in 1951;
20 years later the DLL* Dual Laterolog Resistivity
tool was developed (Fig. 2). Together with induc-
tion tools, the DLL tool provided key input for
basic formation saturation evaluation.
Although anomalies such as the Delaware
and anti-Delaware effects have been overcome
by repositioning the measure and current return
electrodes, other reference electrode effects have
influenced deep laterolog measurements since their
early days. The Groningen effect, for example,
remains a particularly complex problem that
manifests itself as an increase in the deep laterolog
(LLd) reading in conductive beds overlain by
thick, highly resistive beds.
The vertical resolution of the deep and shallow
laterologs is around 2.5 ft, with a typical beam
width of approximately 28 in. With the contribu-
tion of thin beds becoming more important for
optimizing production, this vertical resolution is
increasingly recognized as insufficient for their
proper evaluation.
A need has existed for a quantitative, deep-
reading resistivity measurement combining better
vertical resolution with azimuthal resolution and
full coverage. This measurement, which is pro-
vided by the ARI tool, bridges the gap between
high-resolution microimaging instruments and
conventional low-resolution resistivity tools.
Background
Figure 2.Dual Laterolog sonde electrode distribution and current path shape.
LLd LLs
A2
M2M1
A1
A0
M'1M'2
A'1
A'2
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ARI Azimuthal Resistivity Imager 3
The ARI tool incorporates azimuthal electrodes
into the conventional DLL array. The electrodes
are placed at the center of the DLL tools A2
electrode (Fig. 3).
Dual laterolog resistivity measurements
Current from the A2 electrode focuses the LLd
current. The A2 electrode also serves as a return
electrode for the shallow laterolog (LLs) current.
The relatively small azimuthal array at the center
of the A2 electrode does not interfere with either
the LLd or the LLs measurements.
The DLL tool operates simultaneously at two
frequencies: 35 Hz for the LLd and 280 Hz for the
LLs. In both cases the survey current (I0) flows
from the A0 electrode and is controlled by the
output of a feedback loop. This loop equalizes the
potentials across pairs of monitor electrodes (M1,
M2 and M'2, M'1), focusing the current from theA0 electrode into the formation.
Focusing current for the LLs measurement
flows from the A1 and A'1 electrodes, and both
survey and focusing currents return to the A2
and A'2 electrodes. For the LLd measurement, an
auxiliary monitor loop makes the tool effectively
equipotential at 35 Hz; focusing current flows
from both the A1, A'1 and A2, A'2 electrode pairs.
The LLd survey current is focused so that it flows
perpendicular to the tool, and all deep current
returns to electrode B at the surface.
The tool is connected to the logging cable by
the bridle, a flexible insulating connector about
80 ft long. The potential difference (V0) between
the monitor electrodes (M2 and M'2) and the cable
armor at the torpedo is recorded, as is the survey
current (I0) flowing from the A0 electrode. The
resistivity (R) is computed according to
where kis a geometric factor.
Principles
Figure 3.ARI azimuthal electrodes are incorporated in the Dual Laterolog A2 electrode.
LLd
anddeep
azimuthalresistivity
LLs
andazimuthal
electricalstandoff
A2
M2M1
A1
A0
M'1M'2
A'1
A'2
R k VI
=
0
0
,
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4 Principles
Azimuthal resistivity measurements
The detailed view of the azimuthal array (Fig. 4)
shows current paths for the deep and auxiliary
measurements made with the array. The deep
azimuthal measurement operates at 35 Hz, the
same frequency as the deep laterolog, and thecurrents flow from the 12 azimuthal current elec-
trodes to the surface. They are focused from above
by the current from the upper portion of the A2
electrode; from below they are focused by currents
from the lower portion of the A2 electrode and by
currents from the A1, A0, A'1 and A'2 electrodes.
In addition, the current from each azimuthal elec-
trode is focused passively by the currents from
its neighbors.
To overcome electrochemical effects across
the electrode/mud interface, the azimuthal array
is implemented in a monitored laterolog 3 (LL3)
configuration. These effects would degrade theresponse of a simpler equipotential LL3 imple-
mentation.
A monitor electrode is set in each current elec-
trode, and a feedback loop controls the electrode
current. The monitor electrode is thus maintained
at the mean potential of the annular monitor elec-
trodes that lie just inside the A2 guard electrode
on either side of the array (M3 and M4 in Fig. 4).
The mud in front of the azimuthal current
electrodes is effectively equipotential. The 12
azimuthal currents (Ii) and the mean potential of
the M3 and M4 electrodes relative to the cable
armor (Vm) are measured. From these data 12
azimuthal resistivities (Ri) are computed:
where k'is a geometric factor.
From the sum of 12 azimuthal currents, a
high-resolution resistivity measurement, LLhr, isderived. This technique is equivalent to replacing
the azimuthal electrodes by a single cylindrical
electrode of the same height.
M3
M4
dV= 0
Vm
Ii
M3
M4
dVi
Ic
High-resolution deep mode Auxiliary mode
A2A2
A2A2
R kV
Ii
m
i
=
' ,
Figure 4.Azimuthal
electrode array and
current paths in bothmeasurement modes.
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ARI Azimuthal Resistivity Imager 5
Auxiliary azimuthal measurements
The azimuthal resistivity measurements are
sensitive to tool eccentering in the borehole and to
irregular borehole shape. To correct these effects, a
simultaneous auxiliary measurement is made with
the array at a frequency of 71 kHz, which is suffi-ciently high to avoid interference with the 35-Hz
monitor loops.
In this operating mode, current is passed
between each azimuthal electrode and the A2
guard electrode (Fig. 4). The azimuthal and
annular monitor electrodes, M3 and M4, serve as
measure electrodes. The difference between the
potential of the azimuthal monitor electrode and
the mean potential of the annular monitor elec-
trodes (dVi) is measured.
Each azimuthal electrode passes the same
current (Ic), and 12 resistivities (Rci) are computed
as follows:
where c is a geometric factor chosen so that, in an
infinite uniform fluid,Rci gives the fluid resistivity.
The auxiliary measurement is very shallow,
with a current path close to the tool and most of
the current returning to the A2 electrode near the
azimuthal array.
Because the borehole is generally more conduc-
tive than the formation, the current tends to stay in
the mud and the measurement responds primarily
to the volume of mud in front of each azimuthal
electrode. Therefore, the measurement is less sen-
sitive to borehole size and shape and to eccenter-
ing of the tool in the borehole.
The primary objective of the auxiliary measure-
ment is to provide information for correcting the
azimuthal resistivity measurement for the effects
of borehole irregularities and tool eccentering. A
secondary objective is to derive an electrical stand-
off from which borehole size and shape can be
estimated if mud resistivity (Rm) is known or is
measured independently.
Orientation measurements
The orientation of the ARI tool is measured
with a GPIT* General Purpose Inclinometry
Tool, the device used to orient many dipmeter
and imaging logs.
R cdV
Ic
i
ci
=
,
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6 Specifications
The ARI tool is evolving; therefore, some
specifications in Table 1 may change.
Specifications
Table 1. ARI tool specifications.
Length 33.3 ft [10.1 m]
Weight 578 lbm [263 kg]
Diameter (small sub) 3 58 in. [9.2 mm] (4 78 in. [12.3 mm] with standoff)
Diameter (medium sub) 6 in. [15.2 mm] (7 14 in. [18.4 mm] with standoff)
Vertical resolution 8 in. in a 6-in. hole
Azimuthal resolution 60 degrees azimuthal angle for 1-in. standoff
Formation resistivity range 0.2 to 100,000 ohm-m
Temperature rating 350F
Pressure rating 20,000 psi
Mud resistivity Up to 2 ohm-m in active mode
Up to 5 ohm-m in passive mode
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ARI Azimuthal Resistivity Imager 7
The lower sections of the ARI tool contain the
dual laterolog A1, A0, A'1 and A'2 electrodes,
which are essentially identical to those used in the
DLL tool. The upper azimuthal section uses the
top and bottom parts of the dual laterolog A2
electrode as its LL3 guard electrodes. This
section can be operated independently from the
lower sections in a stand-alone configuration.
The ARI tool can be logged at 3600 ft/hr; when
dip estimation is required, however, logging speed
is reduced to 1800 ft/hr and data channels are
sampled every 0.5 in. for greater accuracy.
Modes of operation
In the principal mode of operation, the active
mode, current is emitted by each of the current
electrodes, and 12 calibrated resistivities are
available in real time. In addition, the conventional
deep and shallow laterolog measurements (LLdand LLs) are available.
A backup, passive mode was conceived for
cases where mud resistivity is above 2 ohm-m or
in case one of the azimuthal electrode circuit loops
fails. If one of the 12 azimuthal loops fails while
the tool is operating in the active mode, the
remaining loops may not function properly. In the
passive mode, one faulty channel does not affect
the remaining channels.
LLhr measurements from active and passive
modes are identical; however, an estimate of mud
resistivity is required to obtain the individual cali-
brated azimuthal resistivities in passive mode.
The tool can be switched downhole from one
mode to the other by software command.
Stand-alone operation
When induction devices are preferred to laterologs
and a deep-formation resistivity image is required,
the azimuthal section can be run in combinationwith an induction tool (for example, the AIT*
Array Induction Imager Tool).
Operation
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8 Environmental corrections
Any laterolog-type measurement is subject to
a borehole correction that is a function of the
borehole diameter and of the ratio of formation
resistivity to mud resistivity. The LLhr log reading
can be corrected according to the chart in Fig. 5.
Figure 6 shows that the high-resolution LLhr
Environmental corrections
Figure 5.Borehole corrections applied to the LLhr log recorded in active mode.
1.2
0.5
1 10 100 1000 10,000 100,000
Ra/Rm
Rcor/Ra
1.3
1.1
1
0.9
0.8
0.7
0.6
Borehole Corrections
358-in. ARI tool, active mode, tool centered, thick beds
10 in.
8 in.
6 in.
12 in.
Hole diameter
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ARI Azimuthal Resistivity Imager 9
curve reads almost as deep into the formation as a
deep laterolog LLd curve, particularly whenRtis
less thanRxo. An LLhr log can therefore replace an
LLd log for interpretation, especially when its
excellent vertical resolution is an advantage.
Individually selected azimuthal resistivities can
Figure 6.Depth of investigation of the LLhr curve
compared with the LLd and LLs curves in two different
resistivity environments.
LLhr
LLd
LLs
0.9
1
0.8
0.2
0.1
0
0.4
0.3
0.6
0.7
0.5
0 10 20 30 40 50
Invasion radius (in.)
60 70 80 90 100
0.9
1
0.8
0.2
0.1
0
0.4
0.3
0.6
0.7
0.5
0 10 20 30 40 50Invasion radius (in.)
60 70 80 90 100
RtRa
RtRxo
RtRa
RtRxo
Rt
Rxo
Rm
Hole diameter = 8 in.
LLhr
LLd
LLs
= 50 ohm-m
= 10 ohm-m
= 0.1 ohm-m
Rt
Rxo
Rm
Hole diameter = 8 in.
= 1 ohm-m
= 10 ohm-m
= 0.1 ohm-m
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10 Environmental corrections
be used in the same way when the logged interval
is azimuthally anisotropic or includes highly dip-
ping thin beds.
The fine vertical resolution of the LLhr curve
is shown in Fig. 7 across a formation boundary
with a resistivity step from 1 to 10 ohm-m. The
responses of the LLd and LLs curves are shown
across the same boundary for comparison.
Figure 7.LLhr log response compared with LLd and LLs logs across a
resistivity step boundary. The significant improvement in vertical reso-
lution is apparent.
20
10
1
0.5
30 24 18 12 6 0
Distance to boundary (in.)
Ra
(ohm-m)
6 12 18 24 30
LLhr
LLd
LLs
Rt1
Rt2
Rm
Hole diameter = 6 in.
= 1 ohm-m
= 10 ohm-m
= 0.1 ohm-m
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ARI Azimuthal Resistivity Imager 11
The ARI tool is combinable with a wide variety of
other tools including the following:
Resistivity
AIT Array Induction Imager Tool
DIL* Dual Induction Resistivity Log
MicroSFL* tool
Porosity and lithology
Gamma ray tool
CNL* Compensated Neutron Log tool
Litho-Density* tool
NGS* Natural Gamma Ray Spectrometry tool
Auxiliary
EMS* Environmental Measurement Sonde
Auxiliary Measurement Sonde
GPIT inclinometry tool
Others
DSI* Dipole Shear Sonic Imager
FMI Fullbore Formation MicroImager
ADEPT* Adaptable Electromagnetic
Propagation Tool
RFT* Repeat Formation Tester
Combinability
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12 Applications
New applications are being developed and discov-
ered as experience with the ARI service grows in a
variety of environments. We discuss here the more
important applications known and proven with
examples at this time.
Borehole correction
The electrical standoff measurements can be used
to correct the azimuthal resistivities for tool eccen-
tering and variations in borehole shape and size.
The correction to be applied is a function of the
electrical standoff measurements, mud resistivity
and formation resistivity. Correction algorithms
have been derived from tool modeling.
Figure 8 shows two ARI log passes over the
same intervalone with the tool centered and one
with it eccentered. The 12 electrical standoff
measurements of each pass on the left of the log
display show that the tool is not perfectly centered,
even in the centered pass, and that the tool
rotates during logging. On the right, the 12 uncor-
rected azimuthal resistivity measurements of each
pass are shown with the corrected measurements
of the eccentered pass. It is obvious that the stand-
off measurements and corrections are good since
the corrected curves are much more coherent than
the uncorrected curves, even of the centered pass.
Applications
Figure 8.Electrical diameters and uncorrected azimuthal resistivities with the ARI tool centered
and eccentered, and borehole-corrected azimuthal resistivities.
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ARI Azimuthal Resistivity Imager 13
Deep invasion
Figure 9 shows ARI and MicroSFL logs over a
deeply invaded zone. Conductive-invasion separa-
tion between the MSFL, LLs and LLd curves is
apparent. The LLhr curve, while showing more
detail, generally follows the LLd curve quite
closely, and its fine-detail variations reflect
movement in the MSFL curve.
This example demonstrates that the LLhr curve
has a depth of investigation close to that of the
LLd measurement and a vertical resolution
approaching that of the MSFL curve.
Figure 9.Deep conductive invasion example showing that the LLhr curve has a
depth of investigation similar to that of the LLd curve and a vertical resolution
approaching that of the MSFL curve.
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14 Applications
Thin-bed analysis
The deep, high-resolution resistivity measurements
(vertical resolution less than 1 ft) can be used to
improve the quantitative evaluation of laminated
formations. In such formations the resistivity
image helps ensure that potential hydrocarbonzones are not missed and guides the selection of
subsequent logs.
Figure 10 is a log recorded across a series of
thin beds. The LLd and LLs curves between X662
and X677 ft have little character, while the LLhr
curve and the azimuthal measurements show thin
bedding with an average bed thickness of less than
1 ft. The conductivity image shows other details
such as azimuthal heterogeneity (X650 to X652 ft,
and X660 to X662 ft) and dipping features (X658
to X660 ft).
Figure 10. 1-ft beds barely visible on the LLd and LLs curves are
clearly seen by the azimuthal resistivity curves. Dipping beds and
azimuthal heterogeneities can also be seen on the ARI image.
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ARI Azimuthal Resistivity Imager 15
Fractured formations
As with any resistivity device, the ARI response
is strongly affected by fractures filled with con-
ductive fluids. Fig. 11 shows a simulated log of
the ARI tool as it passes in front of a horizontal
(perpendicular to the wellbore) fracture of infiniteextension filled with conductive fluid.
The resistivity reading in front of the fracture
drops sharply. The signal departs from the baseline
(the matrix resistivity reading) for an interval
shorter than 1 ft. The fracture signal can be
characterized by measuring the area of added
conductivity1,2 in front of the fracture.
Figure 12 shows a fractured formation.
The azimuthal image on the left has a fixed con-
ductivity scale, while the image on the right is
enhanced by dynamic normalization to improve
the visibility of features by locally increasing the
image contrast. The log presents several highly
dipping, darker (conductive) events (at X945,
X947, X953 and X967 m), which are interpreted
as open fractures. The log also shows a vertical
fracture from X975 to X985 m. The large separa-
tion between the LLs and LLd curves over this
zone is characteristic of vertical fractures.3
Figure 11.LLhr log response in front of a 1-mm horizontal fracture.
ERm
Rb
Hole diameter = 6 in.
200
100
10
Distance from fracture (in.)
LLhr
(ohm-m)
24 21 18 15 12 9 6 3 0 3 6 9 12 15 18 21 24
= 1 ohm-m
= 0.1 ohm-m
= 100 ohm-m
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16 Applications
A dynamically normalized image does not have
a calibrated image scale because the conductivity
associated with a particular color or shade varies
along the image.
Figure 1 compares ARI, FMI and UBI images
in a fractured formation. Although the ARI images
do not have the definition and resolution of detail
of the FMI images, open fractures are clearly
identified. Some vertical fracturing seen on the
FMI image does not appear as clearly on the
ARI image. This vertical fracturing is probably
drilling-induced fracturing and cracks that are
too shallow to be detected by the deeper-reading
ARI measurement. ARI images, therefore, com-
plement FMI borehole images by helping to
discriminate between deep natural and shallow
drilling-induced fractures.
Figure 12.Highly dipping fractures can be identified on the ARI images
at the depth of each sharp resistivity trough. Separation between LLs and
LLd curves confirms a vertical fracture below X975 m.
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ARI Azimuthal Resistivity Imager 17
Heterogeneous formations
Resistivity readings of the LLd and LLhr logs can
be strongly affected by azimuthal heterogeneities.
In such cases the azimuthal image can greatly
improve the resistivity log interpretation. A
selected azimuthal resistivity can be used forquantitative evaluation of the formation.
Figure 13 shows ARI and FMI images dis-
played with ARI resistivity curves in a formation
with dipping beds and surfaces, and with some
azimuthal heterogeneities. It is interesting to
compare the low-resistivity readings at X91.4 and
X92.2 m. The deeper low reading is due to hetero-
geneity, with a very low-resistivity localized
feature, and the shallower is an azimuthally con-
tinuous event. The deeper event would certainly
be misinterpreted using a standard azimuthally
averaged resistivity log reading.
A more coherent answer can be obtained if tool
orientation information is recorded with the den-
sity log. The formation resistivity in the same
azimuthal direction can be selected from the ARI
log data for saturation computation.
Figure 13.ARI and FMI images in a heterogeneous formation. Compare the low-resistivity
depths (X91.4 and X92.2); one is a heterogeneity, and the other is an azimuthally continuous
event.
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18 Applications
Dip estimation
An estimate of formation dip can be derived from
the azimuthal resistivity image. Generally, dips
computed from ARI images do not have the accu-
racy of those computed by a dipmeter. They can,
however, give a good estimate of the structuraldip, detect unexpected structural features (uncon-
formities and faults) and confirm the presence of
expected features. Figure 14 shows the agreement
between sedimentary dips derived from ARI
images and dips from the SHDT* Stratigraphic
High-Resolution Dipmeter Tool.
Horizontal wells
The responses of azimuthally averaged measure-
mentsLLd, LLs and induction logs, for exam-
pleare influenced by beds lying parallel and
near the borehole. This situation often arises in
horizontal wells, particularly when the well issteered to closely follow the top of the reservoir.
The quantitative azimuthal image of the ARI tool
helps to detect and identify these nearby beds so
the most representative reading can be selected
from the quantitative azimuthal deep resistivity
measurements.
Figure 14. Excellent agreement between sedimentary dips derived from ARI
images and dipmeter data.
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ARI Azimuthal Resistivity Imager 19
Borehole profile
Figure 15 shows the 12 auxiliary-mode azimuthal
borehole curves, recorded in conductivity units.
The spread of the curves indicates some tool
eccentering or borehole irregularity such as oval-
ity. Tracks 2 and 3 show FMI calipers recordedwith orthogonal pairs of caliper arms and an
orthogonal presentation of ARI electrical calipers.
Although agreement is generally good, the ARI
calipers are more sensitive to sharp variations,
particularly small washouts.
In this case the FMI caliper arms were partially
closed to log a sticky section of the hole. Caliper
information was recovered from the ARI log.
Figure 15.Borehole profile from ARI caliper measurements compared with measurements made
with FMI calipers. Agreement is good except where the FMI caliper arms have not been fully
opened below X770 ft.
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20 Applications
Groningen effect correction
The Groningen effect on the deep laterolog mea-
surement is encountered in conductive formations
overlain by thick, highly resistive beds.
The LLd measurement voltage reference, taken
at the torpedo connector between the logging cableand the top of the insulated bridle, normally repre-
sents infinity. The reference becomes negative
as the torpedo enters the resistive bed, and the
Groningen effect occurs.
In cases without Groningen effect, the out-of-
phase (quadrature) voltagewith reference to the
total currentis normally zero. When the effect
occurs, the quadrature voltage becomes significant.
This phenomenon can be used to identify and,
under favorable conditions, correct for the effect.
The correction is based on the formula
where dV0 represents the voltage shift responsible
for the Groningen effect and V90 represents the
quadrature voltage. The coefficient g depends on
the mud resistivity, the formation/mud resistivity
contrast and the borehole diameter. This coefficient
is determined from charts obtained by modeling.
dV g V 0 90= ( ),
The value of the ratio V90/V0 is used to indicate
the presence of a Groningen effect. Figures 16
and 17 show the application of the detection and
correction schemes in a well with the casing string
set well above the resistive bed.
When casing is set in the resistive bed, this
correction method no longer applies; the onset of
the effect, however, is still detected by an increase
in the out-of-phase voltage. The Groningen effect
is stronger and the effect extends deeper in the
well, occurring even when the torpedo is well
below the resistive bed.
A second pass is made with an enlarged A2
electrode. The mass-isolation sub on top of the
A2 electrode is short-circuited by a software com-
mand, extending the electrode. This technique
alters the tools geometrical factor and the ratio
of the total to measured current. These two passes
exhibit Groningen effects of different magnitudefrom which a Groningen-free LLd reading can
be computed. The second pass is only needed over
a short section below the casing.
The Groningen effect correction is applied
automatically if the well and casing configuration
permit the single-pass correction.
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ARI Azimuthal Resistivity Imager 21
Figure 16. The appearance of a Groningen effect can
be flagged.
Figure 17. Correction for Groningen effect is confirmed by
the LLs and IDPH curves.
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22 Features and benefits
The ARI tool brings such an innovative approach
to deep resistivity logging, opening new opportu-
nities for interpretation and applications, that it is
useful to summarize here its principal features and
benefits.
Features and benefits
Features Benefits
Improved vertical resolution with narrow BetterRtestimation in thin beds
beam width (compared to the DLL tool)
12 deep azimuthal resistivities, Improved evaluation of deviated and
comparable with the LLd curve horizontal wells
Deep azimuthal image, much Fracture detection and characterization
deeper than microelectrical imageDifferentiates between natural and
drilling-induced fractures
Adjacent (nonintersecting) bed distance
Dynamic normalization for enhanced Detection of heterogeneous formations
image with improved contrastStructural dip
Quadrature signal processing Groningen-corrected resistivity
(no casing present)
Log quality control
Software-controlled Groningen-corrected resistivity
extendable electrode (casing present)
Electrical standoff measurement Better deep resistivity measurement
to correct azimuthal resistivities in irregular holesfor individual standoff
Borehole profile
Measurement not degraded by eccentering
Flexible system architecture with Resolution maintained in large holes
interchangeable half-shell design
Backup passive mode Images possible in high-resistivity muds
Stand-alone mode Short tool string (for example, in combination
with induction tools)
Combinable with resistivity, Significant rig time savings
porosity and lithology, andother borehole imaging tools
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ARI Azimuthal Resistivity Imager 23
The following curve names may appear on ARI
and other log presentations.
Common ARI curve names
Curve name Sample Descriptionrate
AC01 to AC12 0.5 in. Corrected azimuthal conductivity curves 1 to 12 (mmho/m)
AR01 to AR12 0.5 in. Corrected azimuthal resistivity curves 1 to 12 (ohm-m)
CALE 0.5 in. Borehole diameter from electrical standoff (in.)
CC01 to CC12 0.5 in. Electrical standoff conductivity curves 1 to 12 (mmho/m)
CLLD 6 in. Deep laterolog conductivity (mmho/m)
LDCG 6 in. Casing Groningen-corrected deep resistivity (ohm-m)
LHCG 6 in. Casing Groningen-corrected high-resolution resistivity (ohm-m)
LLD 6 in. Deep laterolog resistivity (ohm-m)
LLDG 6 in. Groningen phase-corrected deep resistivity (ohm-m)
LLG 6 in. Standard deep Groningen-referenced resistivity (ohm-m)
LLHC 0.5 in. High-resolution conductivity (mmho/m)
LLHG 0.5 in. Groningen phase-corrected high-resolution resistivity (ohm-m)
LLHR 0.5 in. High-resolution deep resistivity (ohm-m)
LLS 6 in. Shallow laterolog resistivity (ohm-m)
RC01 to RC12 0.5 in. Azimuthal deep conductivity curves 1 to 12 (mmho/m)
RR01 to RR12 0.5 in. Azimuthal deep resistivity curves 1 to 12 (ohm-m)
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1. Luthi SM and Souhait P: Fracture Aperture
from Electrical Borehole Scans, Geophysics
(1990), 55, No. 7, 821833.
2. Faivre O: Fracture Evaluation from
Quantitative Azimuthal Resistivities, paper
SPE 26434, presented at the 68th SPE AnnualTechnical Conference and Exhibition,
Houston, Texas, October 36, 1993.
3. Sibbit AM and Faivre O: The Dual Laterolog
Response in Fractured Rocks, presented at
the SPWLA Twenty-Sixth Annual Logging
Symposium, June 1985.
Davies DH, Faivre O, Gounot M-T, Seeman
B, Trouiller J-C, Benimeli D, Ferreira AE,Pittman DJ, Smits J-W and Randrianavony M:
Azimuthal Resistivity Imaging: A New
Generation Laterolog, paper SPE 24676,
presented at the 67th SPE Annual Technical
Conference and Exhibition, Washington, DC,
October 47, 1992.
References
Recommended reading
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