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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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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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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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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


13/28


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


= 1 ohm-m

= 10 ohm-m

= 0.1 ohm-m


14/28

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


= 1 ohm-m

= 10 ohm-m

= 0.1 ohm-m


15/28


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


16/28


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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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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


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


20/28

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.


21/28


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.


22/28

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.


23/28


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.


24/28

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.


25/28


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.


26/28

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

Download - Azimuthal Resistivity Imager

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