azimuthal resistivity imager

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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 k V

    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 c dV

    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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    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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    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