Azimuthal Resistivity Imager (ARI)

ARI*AzimuthalResistivityImager

Schlumberger

ARI* AzimuthalResistivityImager

Schlumberger 1993

Schlumberger Wireline & TestingP.O. Box 2175Houston, Texas 77252-2175

All rights reserved. No part of this book may bereproduced, stored in a retrieval system, or tran-scribed in any form or by any means, electronic ormechanical, including photocopying and recording,without prior written permission of the publisher.

SMP-9260

An asterisk (*) is used throughout this document todenote a mark of Schlumberger.

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Dual laterolog resistivity measurements . . . . . . 3Azimuthal resistivity measurements . . . . . . . . . . . 4Auxiliary azimuthal measurements . . . . . . . . . . . . 5Orientation measurements . . . . . . . . . . . . . . . . . . . . . . . 5

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Modes of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Stand-alone operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Environmental corrections . . . . . . . . . . . . . . . . . . . . . . . . 8Combinability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Resistivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Porosity and lithology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Auxiliary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Others. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Borehole correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Deep invasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Thin-bed analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Fractured formations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Heterogeneous formations . . . . . . . . . . . . . . . . . . . . . . . 17Dip estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Horizontal wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Borehole profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Groningen effect correction . . . . . . . . . . . . . . . . . . . . . 20

Features and benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Common ARI curve names . . . . . . . . . . . . . . . . . . . . . . . 23References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Recommended reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

The ARI Azimuthal Resistivity Imager, a new-generation laterolog tool, makes directional deepmeasurements around the borehole with a highervertical resolution than previously possible.

Using 12 azimuthal electrodes incorporated in adual laterolog array, the ARI tool provides a dozendeep oriented resistivity measurements whileretaining the standard deep and shallow readings.A very shallow auxiliary measurement is incorpo-rated to fully correct the azimuthal resistivities forborehole effect.

The formation around the borehole is displayedas an azimuthal resistivity image. Although thisfull-coverage image has much lower spatial reso-lution than acoustic or microelectrical imagesthose coming from the UBI* Ultrasonic BoreholeImager tool or the FMI* Fullbore FormationMicroImagerit complements them well becauseof its sensitivity to features beyond the boreholewall and its lower sensitivity to shallow features(Fig. 1).

ARI Azimuthal Resistivity Imager 1

Introduction

ARI Azimuthal Resistivity Imager

Figure 1. Combining deep ARI images with shallowborehole surface images from the FMI tool, or even acousticUBI images, helps to discriminate between deep naturalfractures and shallow drilling-induced fractures. (Courtesy of UK Nirex Ltd)

2 Background

The laterolog technique was introduced in 1951;20 years later the DLL* Dual Laterolog Resistivitytool was developed (Fig. 2). Together with induc-tion tools, the DLL tool provided key input forbasic formation saturation evaluation.

Although anomalies such as the Delawareand anti-Delaware effects have been overcomeby repositioning the measure and current returnelectrodes, other reference electrode effects haveinfluenced deep laterolog measurements since theirearly days. The Groningen effect, for example,remains a particularly complex problem thatmanifests itself as an increase in the deep laterolog(LLd) reading in conductive beds overlain bythick, highly resistive beds.

The vertical resolution of the deep and shallowlaterologs is around 2.5 ft, with a typical beamwidth of approximately 28 in. With the contribu-tion of thin beds becoming more important foroptimizing production, this vertical resolution isincreasingly recognized as insufficient for theirproper evaluation.

A need has existed for a quantitative, deep-reading resistivity measurement combining bettervertical resolution with azimuthal resolution andfull coverage. This measurement, which is pro-vided by the ARI tool, bridges the gap betweenhigh-resolution microimaging instruments andconventional low-resolution resistivity tools.

Background

Figure 2. Dual Laterolog sonde electrode distribution and current path shape.

LLd LLs

A2

M2M1

A1

A0M'1M'2A'1

A'2


The ARI tool incorporates azimuthal electrodesinto the conventional DLL array. The electrodesare placed at the center of the DLL tools A2electrode (Fig. 3).

Dual laterolog resistivity measurementsCurrent from the A2 electrode focuses the LLdcurrent. The A2 electrode also serves as a returnelectrode for the shallow laterolog (LLs) current.The relatively small azimuthal array at the centerof the A2 electrode does not interfere with eitherthe LLd or the LLs measurements.

The DLL tool operates simultaneously at twofrequencies: 35 Hz for the LLd and 280 Hz for theLLs. In both cases the survey current (I0) flowsfrom the A0 electrode and is controlled by theoutput of a feedback loop. This loop equalizes thepotentials 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 measurementflows from the A1 and A'1 electrodes, and bothsurvey and focusing currents return to the A2and A'2 electrodes. For the LLd measurement, anauxiliary monitor loop makes the tool effectivelyequipotential at 35 Hz; focusing current flowsfrom both the A1, A'1 and A2, A'2 electrode pairs.The LLd survey current is focused so that it flowsperpendicular to the tool, and all deep currentreturns to electrode B at the surface.

The tool is connected to the logging cable bythe bridle, a flexible insulating connector about80 ft long. The potential difference (V0) betweenthe monitor electrodes (M2 and M'2) and the cablearmor at the torpedo is recorded, as is the surveycurrent (I0) flowing from the A0 electrode. Theresistivity (R) is computed according to

where k is a geometric factor.

Principles

Figure 3. ARI azimuthal electrodes are incorporated in the Dual Laterolog A2 electrode.

LLdanddeep

azimuthalresistivity

LLsand

azimuthalelectricalstandoff

A2

M2M1

A1

A0M'1M'2A'1

A'2

R k VI= 0

0,

4 Principles

Azimuthal resistivity measurementsThe detailed view of the azimuthal array (Fig. 4)shows current paths for the deep and auxiliarymeasurements made with the array. The deepazimuthal measurement operates at 35 Hz, thesame frequency as the deep laterolog, and thecurrents flow from the 12 azimuthal current elec-trodes to the surface. They are focused from aboveby the current from the upper portion of the A2electrode; from below they are focused by currentsfrom the lower portion of the A2 electrode and bycurrents 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 fromits neighbors.

To overcome electrochemical effects acrossthe electrode/mud interface, the azimuthal arrayis 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 electrodecurrent. The monitor electrode is thus maintainedat the mean potential of the annular monitor elec-trodes that lie just inside the A2 guard electrodeon either side of the array (M3 and M4 in Fig. 4).

The mud in front of the azimuthal currentelectrodes is effectively equipotential. The 12azimuthal currents (Ii) and the mean potential ofthe M3 and M4 electrodes relative to the cablearmor (Vm) are measured. From these data 12azimuthal 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 replacingthe azimuthal electrodes by a single cylindricalelectrode of the same height.

M3

M4

dV = 0

Vm

Ii

M3

M4

dVi

Ic

High-resolution deep mode Auxiliary mode

A2A2

A2A2

R k VIim

i=

' ,

Figure 4. Azimuthalelectrode array andcurrent paths in bothmeasurement modes.


Auxiliary azimuthal measurementsThe azimuthal resistivity measurements aresensitive to tool eccentering in the borehole and toirregular borehole shape. To correct these effects, asimultaneous auxiliary measurement is made withthe array at a frequency of 71 kHz, which is suffi-ciently high to avoid interference with the 35-Hzmonitor loops.

In this operating mode, current is passedbetween each azimuthal electrode and the A2guard electrode (Fig. 4). The azimuthal andannular monitor electrodes, M3 and M4, serve asmeasure electrodes. The difference between thepotential of the azimuthal monitor electrode andthe mean potential of the annular monitor elec-trodes (dVi) is measured.

Each azimuthal electrode passes the samecurrent (Ic), and 12 resistivities (Rci) are computedas follows:

where c is a geometric factor chosen so that, in aninfinite uniform fluid, Rci gives the fluid resistivity.

The auxiliary measurement is very shallow,with a current path close to the tool and most ofthe current returning to the A2 electrode near theazimuthal array.

Because the borehole is generally more conduc-tive than the formation, the current tends to stay inthe mud and the measurement responds primarilyto the volume of mud in front of each azimuthalelectrode. 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 theazimuthal resistivity measurement for the effectsof borehole irregularities and tool eccentering. Asecondary objective is to derive an electrical stand-off from which borehole size and shape can beestimated if mud resistivity (Rm) is known or ismeasured independently.

Orientation measurementsThe orientation of the ARI tool is measuredwith a GPIT* General Purpose InclinometryTool, the device used to orient many dipmeterand imaging logs.

R c dVIci

ci =

,

6 Specifications

The ARI tool is evolving; therefore, somespecifications 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 modeUp to 5 ohm-m in passive mode


The lower sections of the ARI tool contain thedual laterolog A1, A0, A'1 and A'2 electrodes,which are essentially identical to those used in theDLL tool. The upper azimuthal section uses thetop and bottom parts of the dual laterolog A2electrode as its LL3 guard electrodes. Thissection can be operated independently from thelower sections in a stand-alone configuration.

The ARI tool can be logged at 3600 ft/hr; whendip estimation is required, however, logging speedis reduced to 1800 ft/hr and data channels aresampled every 0.5 in. for greater accuracy.

Modes of operationIn the principal mode of operation, the activemode, current is emitted by each of the currentelectrodes, and 12 calibrated resistivities areavailable in real time. In addition, the conventionaldeep and shallow laterolog measurements (LLdand LLs) are available.

A backup, passive mode was conceived forcases where mud resistivity is above 2 ohm-m orin case one of the azimuthal electrode circuit loopsfails. If one of the 12 azimuthal loops fails whilethe tool is operating in the active mode, theremaining loops may not function properly. In thepassive mode, one faulty channel does not affectthe remaining channels.

LLhr measurements from active and passivemodes are identical; however, an estimate of mudresistivity is required to obtain the individual cali-brated azimuthal resistivities in passive mode.

The tool can be switched downhole from onemode to the other by software command.

Stand-alone operationWhen induction devices are preferred to laterologsand 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

8 Environmental corrections

Any laterolog-type measurement is subject toa borehole correction that is a function of theborehole diameter and of the ratio of formation

resistivity to mud resistivity. The LLhr log readingcan 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

0.51 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 Corrections358-in. ARI tool, active mode, tool centered, thick beds

10 in.8 in.6 in.

12 in.

Hole diameter


curve reads almost as deep into the formation as adeep laterolog LLd curve, particularly when Rt isless than Rxo. An LLhr log can therefore replace an

LLd log for interpretation, especially when itsexcellent vertical resolution is an advantage.Individually selected azimuthal resistivities can

Figure 6. Depth of investigation of the LLhr curvecompared with the LLd and LLs curves in two differentresistivity environments.

LLhr

LLdLLs

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

0.10

0.4

0.3

0.6

0.7

0.5

0 10 20 30 40 50 Invasion radius (in.)

60 70 80 90 100

Rt RaRt Rxo

Rt RaRt Rxo

RtRxoRmHole diameter = 8 in.

LLhr

LLdLLs

= 50 ohm-m= 10 ohm-m= 0.1 ohm-m

RtRxoRmHole diameter = 8 in.

= 1 ohm-m= 10 ohm-m= 0.1 ohm-m

10 Environmental corrections

be used in the same way when the logged intervalis 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 boundarywith a resistivity step from 1 to 10 ohm-m. Theresponses of the LLd and LLs curves are shownacross the same boundary for comparison.

Figure 7. LLhr log response compared with LLd and LLs logs across aresistivity step boundary. The significant improvement in vertical reso-lution is apparent.

20

10

1

0.530 24 18 12 6 0

Distance to boundary (in.)

Ra (ohm-m)

6 12 18 24 30

LLhrLLdLLs

Rt1Rt2RmHole diameter = 6 in.

= 1 ohm-m= 10 ohm-m= 0.1 ohm-m


The ARI tool is combinable with a wide variety ofother 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

12 Applications

New applications are being developed and discov-ered as experience with the ARI service grows in avariety of environments. We discuss here the moreimportant applications known and proven withexamples at this time.

Borehole correctionThe electrical standoff measurements can be usedto correct the azimuthal resistivities for tool eccen-tering and variations in borehole shape and size.The correction to be applied is a function of theelectrical standoff measurements, mud resistivityand formation resistivity. Correction algorithmshave been derived from tool modeling.

Figure 8 shows two ARI log passes over thesame intervalone with the tool centered and onewith it eccentered. The 12 electrical standoffmeasurements of each pass on the left of the logdisplay show that the tool is not perfectly centered,even in the centered pass, and that the toolrotates during logging. On the right, the 12 uncor-rected azimuthal resistivity measurements of eachpass are shown with the corrected measurementsof the eccentered pass. It is obvious that the stand-off measurements and corrections are good sincethe corrected curves are much more coherent thanthe uncorrected curves, even of the centered pass.

Applications

Figure 8. Electrical diameters and uncorrected azimuthal resistivities with the ARI tool centeredand eccentered, and borehole-corrected azimuthal resistivities.


Deep invasionFigure 9 shows ARI and MicroSFL logs over adeeply invaded zone. Conductive-invasion separa-tion between the MSFL, LLs and LLd curves isapparent. The LLhr curve, while showing moredetail, generally follows the LLd curve quite

closely, and its fine-detail variations reflectmovement in the MSFL curve.

This example demonstrates that the LLhr curvehas a depth of investigation close to that of theLLd measurement and a vertical resolutionapproaching that of the MSFL curve.

Figure 9. Deep conductive invasion example showing that the LLhr curve has adepth of investigation similar to that of the LLd curve and a vertical resolutionapproaching that of the MSFL curve.

14 Applications

Thin-bed analysisThe deep, high-resolution resistivity measurements(vertical resolution less than 1 ft) can be used toimprove the quantitative evaluation of laminatedformations. In such formations the resistivityimage helps ensure that potential hydrocarbonzones are not missed and guides the selection ofsubsequent logs.

Figure 10 is a log recorded across a series ofthin beds. The LLd and LLs curves between X662and X677 ft have little character, while the LLhrcurve and the azimuthal measurements show thinbedding with an average bed thickness of less than1 ft. The conductivity image shows other detailssuch as azimuthal heterogeneity (X650 to X652 ft,and X660 to X662 ft) and dipping features (X658to X660 ft).

Figure 10. 1-ft beds barely visible on the LLd and LLs curves areclearly seen by the azimuthal resistivity curves. Dipping beds andazimuthal heterogeneities can also be seen on the ARI image.


Fractured formationsAs with any resistivity device, the ARI responseis strongly affected by fractures filled with con-ductive fluids. Fig. 11 shows a simulated log ofthe 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 fracturedrops sharply. The signal departs from the baseline(the matrix resistivity reading) for an intervalshorter than 1 ft. The fracture signal can becharacterized by measuring the area of addedconductivity1,2 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 isenhanced by dynamic normalization to improvethe visibility of features by locally increasing theimage contrast. The log presents several highlydipping, darker (conductive) events (at X945,X947, X953 and X967 m), which are interpretedas open fractures. The log also shows a verticalfracture from X975 to X985 m. The large separa-tion between the LLs and LLd curves over thiszone is characteristic of vertical fractures.3

Figure 11. LLhr log response in front of a 1-mm horizontal fracture.

ERmRbHole 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

16 Applications

A dynamically normalized image does not havea calibrated image scale because the conductivityassociated with a particular color or shade variesalong the image.

Figure 1 compares ARI, FMI and UBI imagesin a fractured formation. Although the ARI imagesdo not have the definition and resolution of detailof the FMI images, open fractures are clearlyidentified. Some vertical fracturing seen on the

FMI image does not appear as clearly on theARI image. This vertical fracturing is probablydrilling-induced fracturing and cracks that aretoo shallow to be detected by the deeper-readingARI measurement. ARI images, therefore, com-plement FMI borehole images by helping todiscriminate between deep natural and shallowdrilling-induced fractures.

Figure 12. Highly dipping fractures can be identified on the ARI imagesat the depth of each sharp resistivity trough. Separation between LLs andLLd curves confirms a vertical fracture below X975 m.


Heterogeneous formationsResistivity readings of the LLd and LLhr logs canbe strongly affected by azimuthal heterogeneities.In such cases the azimuthal image can greatlyimprove the resistivity log interpretation. Aselected 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 formationwith dipping beds and surfaces, and with someazimuthal heterogeneities. It is interesting to

compare the low-resistivity readings at X91.4 andX92.2 m. The deeper low reading is due to hetero-geneity, with a very low-resistivity localizedfeature, and the shallower is an azimuthally con-tinuous event. The deeper event would certainlybe misinterpreted using a standard azimuthallyaveraged resistivity log reading.

A more coherent answer can be obtained if toolorientation information is recorded with the den-sity log. The formation resistivity in the sameazimuthal direction can be selected from the ARIlog data for saturation computation.

Figure 13. ARI and FMI images in a heterogeneous formation. Compare the low-resistivitydepths (X91.4 and X92.2); one is a heterogeneity, and the other is an azimuthally continuousevent.

18 Applications

Dip estimationAn estimate of formation dip can be derived fromthe azimuthal resistivity image. Generally, dipscomputed 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 ofexpected features. Figure 14 shows the agreementbetween sedimentary dips derived from ARIimages and dips from the SHDT* StratigraphicHigh-Resolution Dipmeter Tool.

Horizontal wellsThe responses of azimuthally averaged measure-mentsLLd, LLs and induction logs, for exam-pleare influenced by beds lying parallel andnear the borehole. This situation often arises inhorizontal wells, particularly when the well issteered to closely follow the top of the reservoir.The quantitative azimuthal image of the ARI toolhelps to detect and identify these nearby beds sothe most representative reading can be selectedfrom the quantitative azimuthal deep resistivitymeasurements.

Figure 14. Excellent agreement between sedimentary dips derived from ARIimages and dipmeter data.


Borehole profileFigure 15 shows the 12 auxiliary-mode azimuthalborehole curves, recorded in conductivity units.The spread of the curves indicates some tooleccentering 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 ARIcalipers are more sensitive to sharp variations,particularly small washouts.

In this case the FMI caliper arms were partiallyclosed to log a sticky section of the hole. Caliperinformation was recovered from the ARI log.

Figure 15. Borehole profile from ARI caliper measurements compared with measurements madewith FMI calipers. Agreement is good except where the FMI caliper arms have not been fullyopened below X770 ft.

20 Applications

Groningen effect correctionThe Groningen effect on the deep laterolog mea-surement is encountered in conductive formationsoverlain by thick, highly resistive beds.

The LLd measurement voltage reference, takenat the torpedo connector between the logging cableand the top of the insulated bridle, normally repre-sents infinity. The reference becomes negativeas the torpedo enters the resistive bed, and theGroningen effect occurs.

In cases without Groningen effect, the out-of-phase (quadrature) voltagewith reference to thetotal currentis normally zero. When the effectoccurs, 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 responsiblefor the Groningen effect and V90 represents thequadrature voltage. The coefficient g depends onthe mud resistivity, the formation/mud resistivitycontrast and the borehole diameter. This coefficientis determined from charts obtained by modeling.

dV g V0 90= ( ),

The value of the ratio V90/V0 is used to indicatethe presence of a Groningen effect. Figures 16and 17 show the application of the detection andcorrection schemes in a well with the casing stringset well above the resistive bed.

When casing is set in the resistive bed, thiscorrection method no longer applies; the onset ofthe effect, however, is still detected by an increasein the out-of-phase voltage. The Groningen effectis stronger and the effect extends deeper in thewell, occurring even when the torpedo is wellbelow the resistive bed.

A second pass is made with an enlarged A2electrode. The mass-isolation sub on top of theA2 electrode is short-circuited by a software com-mand, extending the electrode. This techniquealters the tools geometrical factor and the ratioof the total to measured current. These two passesexhibit Groningen effects of different magnitudefrom which a Groningen-free LLd reading canbe computed. The second pass is only needed overa short section below the casing.

The Groningen effect correction is appliedautomatically if the well and casing configurationpermit the single-pass correction.


Figure 16. The appearance of a Groningen effect canbe flagged.

Figure 17. Correction for Groningen effect is confirmed bythe LLs and IDPH curves.

22 Features and benefits

The ARI tool brings such an innovative approachto deep resistivity logging, opening new opportu-nities for interpretation and applications, that it is

useful to summarize here its principal features andbenefits.

Features and benefits

Features Benefits

Improved vertical resolution with narrow Better Rt estimation in thin bedsbeam 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 characterizationdeeper than microelectrical image

Differentiates between natural and drilling-induced fractures

Adjacent (nonintersecting) bed distanceDynamic normalization for enhanced Detection of heterogeneous formationsimage with improved contrast

Structural 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 holesinterchangeable half-shell design

Backup passive mode Images possible in high-resistivity muds

Stand-alone mode Short tool string (for example, in combinationwith induction tools)

Combinable with resistivity, Significant rig time savingsporosity and lithology, andother borehole imaging tools


The following curve names may appear on ARIand 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)

24 References and recommended reading

1. Luthi SM and Souhait P: Fracture Aperturefrom Electrical Borehole Scans, Geophysics(1990), 55, No. 7, 821833.

2. Faivre O: Fracture Evaluation fromQuantitative Azimuthal Resistivities, paperSPE 26434, presented at the 68th SPE AnnualTechnical Conference and Exhibition,Houston, Texas, October 36, 1993.

3. Sibbit AM and Faivre O: The Dual LaterologResponse in Fractured Rocks, presented atthe SPWLA Twenty-Sixth Annual LoggingSymposium, June 1985.

Davies DH, Faivre O, Gounot M-T, SeemanB, Trouiller J-C, Benimeli D, Ferreira AE,Pittman DJ, Smits J-W and Randrianavony M:Azimuthal Resistivity Imaging: A NewGeneration Laterolog, paper SPE 24676,presented at the 67th SPE Annual TechnicalConference and Exhibition, Washington, DC,October 47, 1992.

References

Recommended reading

Documents

Azimuthal Resistivity Imager (ARI)