4 Oilfield Review

It is hard to believe that logging whiledrilling (LWD) has come such a long wayover the last decade. In the early 1980s,LWD measurements were restricted to sim-ple resistivity curves and gamma ray logs,used more for correlation than formationevaluation. Gradually, sophisticated resistiv-ity, density and neutron porosity tools havebeen added to the LWD arsenal.1 With theadvent of high-deviation, horizontal andnow slim multilateral wells, LWD measure-ments often provide the only means of eval-uating reservoirs. The quality and diversity ofLWD tools have continued to developquickly to meet this demand. Today, applica-tions include not only petrophysical analysis,but also geosteering and geological interpre-tation from LWD imaging (next page).2 Thisarticle focuses on the latest LWD resistivitytools—the RAB Resistivity-at-the-Bit tool andthe ARC5 Array Resistivity Compensatedtool—and the images they produce (see “AProfile of Invasion,” page 17).

Steve BonnerMark FredetteJohn LovellBernard MontaronRichard RosthalJacques TabanouPeter WuSugar Land, Texas, USA

Brian ClarkRidgefield, Connecticut, USA

Rodger MillsExxon USAThousand Oaks, California, USA

Russ WilliamsOXY USA Inc.Houston, Texas

Resistivity While Drilling—Images from the String

Resistivity measurements made while drilling are maturing to match

the quality and diversity of their wireline counterparts. Recent

advances include the development of multiple depth-of-investigation

resistivity tools for examining invasion profiles, and button electrode

tools capable of producing borehole images as the drillstring turns.

For help in preparation of this article, thanks to Saman-tha Duggan, Anadrill, Sugar Land, Texas; Tom Fett, GeoQuest, Houston, Texas and Mary Ellen Banks andMartin Lüling, Schlumberger-Doll Research, Ridgefield,Connecticut.

(continued on page 6)

nFormation evalua-tion made by com-bining data fromseveral LWD mea-surements. This loginterpretation wasmade using ELANElemental Log Anal-ysis software anddata from the RABResistivity-at-the-Bittool, CDR Compen-sated Dual Resistiv-ity and CDN Com-pensated DensityNeutron tools. Volumetric analysis(track 5) shows aquartz-rich zone ofrelatively highporosity. The brownshading indicatesthe movable gasvolume calculatedfrom CDR and RABdata run severaldays later. The RABresistivity image(track 4) shows thatthe sand body issplit into three mainlobes with shale per-meability barriers.

1:2400 ft

XX500

XX450

Net pay

Net sand

Gas effectDensity porosity

CDN60.0 p.u. 0Neutron porosity

CDN60.0 p.u. 0

Add. gas volume right after drilling CDR

Irreducible water

Moved water

Water

Gas volume 7 days after drilling RAB

Quartz

Bound water

Illite

Combined model0 p.u. 100

Perm to gas

Perm to water

Gamma ray

0 API 200

Perm to water

10000 md 0.1

Perm to gas

10000 md 0.1RAB image

0 deg 360

Ring res. RAB

0.2 ohm-m 20

Shallow res. RAB

0.2 ohm-m 20

Medium res. RAB

0.2 ohm-m 20

Deep res. RAB

0.2 ohm-m 20

Phase shift res.CDR

0.2 ohm-m 20Attenuation res.

CDR0.2 ohm-m 20

Diff. caliper-10 in. 10

5Spring 1996

AIT (Array Induction Imager Tool), ARC5 (Array Resistiv-ity Compensated tool), ARI (Azimuthal ResistivityImager), CDN (Compensated Density Neutron), CDR(Compensated Dual Resistivity tool), DIL (Dual InductionResistivity Log), DLL (Dual Laterolog Resistivity), DPT(Deep Propagation Tool), ELAN (Elemental Log Analysis),EPT (Electromagnetic Propagation Tool), FMI (FullboreFormation MicroImager), FracView (fracture synergy log),GeoFrame, INFORM (Integrated Forward Modeling),

1. Bonner S, Clark B, Holenka J, Voisin B, Dusang J,Hansen R, White J and Walsgrove T: “Logging WhileDrilling: A Three-Year Perspective,” Oilfield Review 4,no. 3 (July 1992): 4-21.

2. Bonner S, Clark B, Decker D, Orban J, Prevedel B,Lüling M and White J: “Measurements at the Bit: ANew Generation of MWD Tools,” Oilfield Review 5,no. 2/3 (April/July 1993): 44-54.

MicroSFL, Phasor (Phasor-Induction SFL tool), Power-Pulse (MWD telemetry tool), RAB (Resistivity-at-the-Bittool), SFL (Spherically Focused Resistivity), Slim 1 (slimand retrievable MWD system), StrucView (GeoFramestructural cross section software) and TLC (Tough Log-ging Conditions system) are marks of Schlumberger. FCR(Focused Current Resistivity tool) is a mark of ExplorationLogging. Dual Resistivity MWD tool is a mark ofGearhart Geodata Services Ltd. (now Halliburton).SCWR (Slim Compensated Wave Resistivity) is a mark of Halliburton. EWR (Electromagnetic Wave Resistivity),EWR-PHASE 4 and SLIM PHASE 4 are marks of Sperry-Sun Drilling Services.

This last feature allows the RAB tool to beused for geological interpretation.

Three 1-in. [2.54-cm] diameter buttonsare mounted along the axis on one side ofthe RAB tool. Each button monitors radialcurrent flow into the formation. As the drill-string turns, these buttons scan the boreholewall, producing 56 resistivity measurementsper rotation from each button. The data areprocessed and stored downhole for laterretrieval when the RAB tool is returned tothe surface during a bit change. Oncedownloaded to the wellsite workstation,images can be produced and interpretedusing standard geological applications like

Geology From the BitSimply stated, resistivity tools fall into twocategories: laterolog tools that are suitablefor logging in conductive muds, highly resis-tive formations and resistive invasion; andinduction tools which work best in highlyconductive formations and can operate inconductive or nonconductive muds.3 TheRAB tool falls into the first categoryalthough, strictly speaking, it is an electroderesistivity tool of which laterologs are onetype (see “From Short Normal to Axial Cur-rent,” page 9).4

6 Oilfield Review

The RAB tool has four main features:• toroidal transmitters that generate axial

current—a technique highly suited toLWD resistivity tools5

• cylindrical focusing that compensates forcharacteristic overshoots in resistivityreadings at bed boundaries, allowingaccurate true resistivity Rt determinationand excellent axial resolution

• bit resistivity that provides the earliestindication of reservoir penetration orarrival at a casing or coring point—alsoknown as geostopping

• azimuthal electrodes that produce aborehole image during rotary drilling.

3. It should be remembered that laterolog and inductiontools both work well in many environments.

Laterolog tools need a complete electric circuit towork. Current passes from an emitting electrodethrough the borehole into the formation and back tothe tool via a surface electrode or a return electrodeon the tool. Resistivity is a function of voltage drop,between return electrode and source, and source cur-rent. Laterolog tools have to make electric contactwith the formation through either a conductive mudsystem or by direct physical contact. They are capableof logging highly resistive formations and are good atspotting thin resistive beds.

Induction tools do not need to make contact withthe formation. Instead they transmit electromagneticwaves that induce formation eddy currents. The eddycurrents are a function of resistivity—the higher theconductivity, the greater the induced formation signal.The induced signals are picked up by receiver coilsand transformed into resistivity measurements. Induc-tion tools work best in high-conductivity formationsand can operate in nonconductive mud.

4. Bonner S, Bagersh A, Clark B, Dajee G, Dennison M,Hall JS, Jundt J, Lovell J, Rosthal R and Allen D: “A New Generation of Electrode Resistivity Measure-ments for Formation Evaluation While Drilling,”Transactions of the SPWLA 35th Annual Logging Symposium, Tulsa, Oklahoma, USA, June 19-22,1994, paper OO.

5. Arps JJ: “Inductive Resistivity Guard Logging Appara-tus Including Toroidal Coils Mounted on a ConductiveStem,” US Patent No. 3,305,771, February 1967.

Top of hole

Bottom of hole

Top of hole

Top of hole

Bottom of hole

Top of hole

Bed Dipping Away from Kickoff Point (4)

Folded Bed (5)

Bormap in Horizontal Hole (3)

3D View

Unrollingthe cylinder

Bormap

Top ofhole

Horizontalbedding plane

Verticalfracture

Nearly verticalnatural fracture

Vertical induced fracture

Vertical Well (1)

Horizontal Well (2)

Beddingplane

Fracture

0° 90° 180° 270° 360°

North

Beddingplane

Fracture Bottom of hole

Top of hole

nInterpretation of images. Resistivity images show the surface of the borehole—cut along the northerly direction for a vertical well (1) orthe top of the hole for a horizontal well (2)—laid out flat. The image is artificially colored to show contrasts in resistivity—dark brown islow resistivity and light brown is high resistivity—that highlight bed boundaries, faults or fractures. Features crossing the borehole at anangle show characteristic sinusoidal patterns (3). They also are wider at the bottom and top of the hole. Images of beds dipping awayfrom the kickoff point in horizontal boreholes produce an arrow-head pointing in the direction of drilling (4). Images of folded beds pro-duce a characteristic eye shape (5).

Gianzero S, Chemali R, Lin Y, Su S and Foster M: “A New Resistivity Tool for Measurement-While-Drilling,” Transactions of the SPWLA 26th AnnualLogging Symposium, Dallas, Texas, USA, June 17-20,1985, paper A.Grupping TIF, Harrell JW and Dickinson RT: “Perfor-mance Update of a Dual-Resistivity MWD Tool WithSome Promising Results in Oil-Based Mud Applica-tions,” paper SPE 18115, presented at the 63rd SPEAnnual Technical Conference and Exhibition, Hous-ton, Texas, USA, October 2-5, 1988.

6. Coiled tubing, run into a borehole, forms a naturalhelix. At some stage the frictional forces betweenborehole and coiled tubing become greater than theforce pushing the tubing downhole causing the helixto expand and lock tight against the boreholewall—helical lockup.

StrucView GeoFrame structural cross sec-tion software (previous page).

Wellsite images allow geologists toquickly confirm the structural position of thewell during drilling, permitting any neces-sary directional changes. Fracture identifica-tion helps optimize well direction for maxi-mum production.

Finding the Cracks in Master’s CreekMurray A-1 is a dual-lateral well drilled byOXY USA Inc. in the Cretaceous AustinChalk formation, located in the Master’sCreek field, Rapides Parish, Louisiana, USA(top right). The Austin Chalk is a low-per-meability formation that produces hydro-carbons from fractures, when present. Indi-cations of fractures were seen from cuttingsand gas shows obtained by mud loggers ona previous well. The intention was to drillthis well perpendicular to the fractureplanes to intersect multiple fractures andmaximize production.

OXY wanted to record borehole images inthe reservoir section for fracture evaluation.Fracture orientation would show if the welltrajectory was optimal for intersecting themaximum number of fractures. Knowledgeof fracture frequency, size and locationalong the horizontal section could be usefulfor future completion design, reservoir engi-neering and remedial work.

Ideally, the wireline FMI Fullbore Forma-tion MicroImager tool would have been run,but practical considerations precluded thisoption. Wireline tools can be conveyeddownhole by drillpipe or by coiled tubing inhigh-deviation or horizontal wells, but pres-sure-control requirements prevented the useof drillpipe conveyance in this case andcoiled tubing was considered too costly.Also, calculations showed that helicalcoiled tubing lockup would occur beforereaching the end of the long horizontal sec-tion.6 So OXY decided to try the RAB tool.

The first lateral well was drilled due northto cut assumed fracture planes at rightangles. During drilling, images wererecorded over about 2000 ft [600 m] of the81/2-in. horizontal hole. After each bit runthe data were dumped to a surface worksta-tion and examined using FracView software.

Images clearly showed the characteristicsinusoids of contrasting colors, indicatingchanges in resistivity as the borehole crossesbed boundaries (right).

7Spring 1996

Austin Chalk Trend

Texas

Houston

Arkansas0 100

miles

Pearsall

Mexico

Giddings

Brookeland

Master’sCreek field

North Bayou Jack

Gulf of Mexico

Louisiana Mississippi

nLocation of Master’sCreek field in rela-tion to other fields ofthe Austin Chalktrend.

nCrossing bedding planes. As the borehole crosses an almost horizontal, low-resistivitybed, the RAB image shows a characteristic high-amplitude sinusoidal image (darkbrown). Interpreters have picked the bed boundaries (green) for structural interpretation.The notation TD:11/26—True Dip: dip magnitude/dip azimuth—indicates that this bed-ding plane is dipping at 11° to the NNE, north 26° east to be exact.

Crossing the borehole almost vertically at XX896 ft is a fracture (yellow). TD:87/359indicates that the fracture is dipping north at an azimuth of 359° and is nearly vertical,87° from the horizontal. The strike, or trend, of the fracture is perpendicular to the dipdirection—east/west.

The cylindrical 3D image (inset) shows the borehole images as if viewed from the rightof the hole.

xx870.0

xx875.0

xx880.0

xx885.0

xx890.0

xx895.0

TD:11/26

TD:87/359

Display: straight

Top display: xx885.39 Ft

Bottom depth: xx897.59 Ft

Although the resolution of the RAB tool isnot high enough to see microfractures, sev-eral individual major fractures and clustersof smaller fractures were clearly seen (topright), providing enough evidence that thewell trajectory was nearly perpendicular tothe fracture trend.7

Based on this information the second lat-eral was drilled south 10° east, again to inter-sect as many fractures as possible at 90°.

Images of CaliforniaComplex tectonic activity in southern Cali-fornia, USA, has continued throughout theTertiary period to the present time. Thisactivity influences offshore Miocene reser-voirs where folding and tilting affect reser-voir structure. Production is from fractured,cherty, dolomitic and siliceous zonesthrough wellbores that are often drilled athigh angle.

Wireline logs are run for formation evalu-ation and fracture and structural analysis—although in some cases they have to be con-veyed downhole on the TLC Tough LoggingConditions system.

The CDR Compensated Dual Resistivitytool was used to record resistivity andgamma ray logs for correlation whiledrilling. The oil company wanted to evalu-ate using the RAB tool primarily for correla-tion, but also wanted to assess the quality ofimages produced. In fact, it was the imagesthat, in the end, generated the most interest.

Good-quality FMI logs were available,allowing direct comparison with RAB images(right).8 Both showed large-scale events, suchas folded beds, that were several feet long, aswell as regular bedding planes. However,beds less than a few inches thick were notseen clearly by RAB images.

nFracture clusters. Several fractures cut the borehole around XX956 ft. The largestanomaly (black) is either a cluster of fractures or a very large fracture. The borehole ispassing parallel to the interface between two beds. The more resistive bed (white) is onthe bottom side of the hole. The cylindrical image (inset) gives an alternative 3D view ofthe borehole image.

nRAB and FMI images of dipping beds. Both RAB and FMI images show large-scaleevents that are several feet long. However, the resolution of the FMI image is muchbetter. Beds less than about 4 in. [10 cm] thick are not clearly seen on the RAB image.

8 Oilfield Review

7. The size of fractures seen by the RAB tool depends on several factors. The physical diameter of the buttonis 1 in. [2.54 cm], which produces an electric fieldslightly larger—1.5 in. [3.81 cm] in diameter. Conduc-tive zones thinner than 1.5 in. can be detected, how-ever, resistive zones need to be larger than this to bedetected. Typically fractures with apertures around 1-in. can be detected if the borehole fluid is conductive.

8. Lovell JR, Young RA, Rosthal RA, Buffington L andArceneaux CL: “Structural Interpretation of Resistivity-At-the-Bit Images,” Transactions of the SPWLA 36thAnnual Logging Symposium, Paris, France, June 26-29,1995, paper TT.

Top Bottom TopRAB Image

Dep

th 1

0 ft

Top Bottom TopFMI Image

(continued on page 12)

xx944.0

xx946.0

xx948.0

xx950.0

xx952.0

xx954.0

Display: straight

Top display: xx952.93 Ft

Bottom depth: xx959.03 Ft

xx956.0

xx958.0

TD : 90/167TD : 86/173

TD : 84/355

TD : 86/177

nElectrode resistivity tools. The first LWD resistivity tools used the normal principle (left). Current is forcedinto the formation, returning to the tool at a second electrode far away. Current and voltage drop are measuredbetween the two so that resistivity can be calculated.

An improvement on this is the laterolog technique (middle). Additional electrodes provide a bucking currentthat forces the central measurement current deeper into the formation. This helps suppress distortion to thecurrent path if nearby conductive beds are present.

A method proposed by JJ Arps uses a toroidal-coil transmitter that generates an axial current in a conduc-tor (right). This technique is ideally suited to LWD electrode resistivity tools. Axial current leaves the drill collarradially and at the bottom of the collar. The amount of radial current at any point depends on the formationresistivity at that location. Two different methods of measuring radial current are used: (1) by the differencebetween axial current measured at two receiver toroids or (2) by direct electrode current meters.

Potentialelectrode

Return

Insulation

Currentelectrode

Focused CurrentResistivity Tool

Measurementcurrent

Return

Insulation

Return

Transmitter

Receivers

16 in

.

Short NormalTool

LateralresistivityR Lat

BitresistivityR Bit

Dual ResistivityMWD Tool

Guardelectrodes

9Spring 1996

1. Evans HB, Brooks AG, Meisner JE and Squire RE: “AFocused Current Resistivity Logging System for MWD,”paper SPE 16757, presented at the 62nd SPE AnnualTechnical Conference and Exhibition, Dallas, Texas, USA,September 27-30, 1987.

2. Arps, reference 5 main text.

Laterologs have their roots in a tool called theshort normal, one of the earliest wireline log-ging tools. Its principles were adapted by manymeasurements-while-drilling (MWD) companiesin the early 1980s to provide a simple resistivitylog for correlation (right). The idea is fairlystraightforward: force current from a sourceelectrode to a return electrode through the for-mation; measure the current and voltage dropbetween the electrodes and use Ohm’s law toderive formation resistivity. However, for accu-rate petrophysical analysis in complex forma-tions, more sophisticated devices are needed tomeasure true formation resistivity, R t.

An improvement on the short normal is thelaterolog technique commonly used in wirelinelogging. Exploration Logging introduced a lat-erolog LWD resistivity tool in 1987 based on thelaterolog 3 wireline tool of the early 1950s.1

This FCR Focused Current Resistivity tool hadtwo additional current electrodes on either sideof the measurement electrode. They providedguard currents that forced the main currentdeeper into the formation to measure Rt.

At about this time, another approach wasdeveloped by Gearhart Geodata Services Ltd.from an idea by JJ Arps.2 The Gearhart DualResistivity MWD tool used a toroidal-coil trans-mitter to generate a voltage gap in a drill collar,which causes an axial current to flow along thecollar. This method is ideally suited to LWDbecause resistivity tools have to be built intomechanically strong steel collars. Below thetransmitter, current leaves the tool radially fromthe collar and axially from the drill bit. Theamount of current leaving the collar at any point

depends on the induced drive-voltage and thelocal formation resistivity. Two resistivity mea-surements are made: a focused lateral resistiv-ity measurement and a trend resistivity mea-surement at the bit. Two receiver toroids, 6 in.apart, each measure axial current flowing pastthem down the collar. The difference in axialcurrent equals the radial current leaving thedrill collar between the two receivers and isused to calculate lateral resistivity. Bit resistiv-ity is derived from the axial current measuredby the lower receiver.

Schlumberger also uses the Arps principle ofgenerating and monitoring axial-current flow inthe RAB tool. However, radial-current flow ismeasured directly, and multiple toroidal trans-mitters and receivers are used in a uniquefocusing technique described later.

From Short Normal to Axial Current

RAB Tool—The WorksThe RAB tool measures five resistivity val-ues—bit, ring and three button resistivities—aswell as gamma ray, plus axial and transverseshock.3 Built on a 6.75-in. drill collar, the 10-ft[3-m] long tool can be configured as a near-bitor in-line stabilizer, or as a slick drill collar(right). When real-time data are required, theRAB tool communicates with a PowerPulseMWD telemetry tool via wireless telemetry or astandard downhole tool bus, allowing total BHAdesign flexibility . However, it must be config-ured as a stabilizer for imaging.

Bit Resistivity—A 1500-Hz alternating currentis driven through a toroidal-coil transmitter, 1 ft[30 cm] from the bottom of the tool, thatinduces a voltage in the collar below. Currentflows through the collar, out through the bit andinto the formation, returning to the collar far upthe drillstring (below right). Knowing the volt-age and measuring the axial current through thebit determines resistivity at the bit. Correctionsare made for tool geometry, which variesaccording to the BHA.

The resolution of the bit measurementdepends on the distance between the transmit-ter and the bit face—the bit electrode length.When the RAB tool is run on top of the bit, theresolution is about 2 ft [60 cm]. As the bit-resis-tivity measurement is not actively focused, thecurrent patterns and volume of investigation are affected by nearby beds of contrasting resistivity. As wellbore inclination increases,the effective length of the bit electrode becomesshorter and, in horizontal wells, equals holediameter.

Bit resistivity relies on a good bit-to-forma-tion electrical path. The path is always excel-lent in water-base mud and generally sufficientin oil-base mud.

Applications for the bit-resistivity measure-ment include geostopping to precisely stop atcasing or coring point picks. For example, in aGulf of Mexico well the objective was to drillonly a few inches into the reservoir before set-ting casing. An induction gamma ray log from anearby well was available for correlation.

Drilling was stopped when bit resistivityincreased to 4 ohm-m, indicating reservoir penetration (next page, bottom). Subsequentmodeling showed that the bit had cut only 9 in.[23 cm] into the reservoir.

Focused Multidepth Resistivity—The RABtool with button sleeve provides four multidepthfocused resistivity measurements. For an 81/2-in. bit, the ring electrode has a depth ofinvestigation of about 9 in., and the three 1-in.buttons have depths of investigation of about 1 in., 3 in. and 5 in. [2.5, 7.6 and 12.7 cm]from the borehole wall into the formation. But-ton resistivity measurements are azimuthal andacquire resistivity profiles as the tool rotates inthe borehole. The sampling rate dictates that afull profile is acquired at rotational speedsabove 30 rpm—generally not a limitation.

Data from the azimuthal scans are storeddownhole and dumped from the tool between bitruns. In addition, the azimuthal data may beaveraged by quadrant and transmitted to sur-face in real time along with the ring and bitresistivity, and gamma ray measurements.

All four resistivities use the same measure-ment principle: current from the upper transmit-ter flows down the collar and out into the formation, leaving the collar surface at 90°

10 Oilfield Review

Uppertransmitter

Azimuthalelectrodes

Ringelectrode

Lowertransmitter

Axial current

Lower transmitter

Ring monitor toroid

Upper transmitter

nBit resistivity measurement. The lower toroidaltransmitter generates axial current that flows downthe tool and out through the bit. The ring monitortoroid measures the axial current. Formation resistiv-ity is given by Ohm’s law once the upper transmitterdrive voltage and the current are known. Correctionsare made to compensate for tool geometry andtransmitter frequency.

nRAB tool.

along its length. The return path is along thecollar above the transmitter. The amount of current leaving the RAB tool at the ring and but-ton electrodes is measured by a low-impedancecircuit. Axial current flowing down the collar ismeasured at the ring electrode and at the lowertransmitter. These measurements are repeatedfor the lower transmitter.

Cylindrical Focusing—In a homogeneous for-mation, the equipotential surfaces near the but-ton and ring electrodes on the RAB tool arecylindrical. However, in layered formations, this is no longer the case. Current will besqueezed into conductive beds distorting theelectric field (next page, top). By contrast, resis-tive beds will have the opposite effect: the cur-rent avoids them and takes the more conductivepath. These artifacts are called squeeze andantisqueeze, respectively, and lead to charac-teristic measurement overshoots at bed bound-aries called horns.

11Spring 1996

150.0. 1:240ft

RAB GRAPI

0.02 SFL Offset wellohm-m

200

0.02 ILM Offset wellohm-m

200

2000

200

20000.2 RAB RING resistivityohm-m

0.2 RAB BIT resistivityohm-m

0.02 ILD Offset wellohm-m

100.0. Wireline, GRAPI

Nonfocused System Active Focusing

Conductive bed

Ring electrode

Single transmitterBSBSBMBMBDBD

12

12

12

R1,R2

T2

BD

BM

BS

RM0

M2

T1

M01 M02

M12

By reciprocityM12 = M21

Upper transmitter

Upper transmitter current

Ring electrodeMonitor toroid

Lower transmitter

Lower transmitter current

Lower monitor toroid

M21

n“Geostopping.” Oneadvantage of a correla-tion tool that measuresresistivity right at the bitis the ability to recog-nize marker beds almostas soon as the drill bitpenetrates. This allowsdrilling to stop preciselyat casing or coringpoints. In this example,the bit penetrated only 9in. into the reservoir.

nCylindrical focusing technique. A conductive bed below the ring electrode causes currents to distort in a nonfocused system (left). Withactive focusing, the current paths penetrate the formation radially at the ring electrode and almost radially at the three button electrodes(right). Radial currents are measured at the ring electrode, R, and at each button, BS, BM, BD, for each transmission. Also the axial currentis measured at the ring electrode by a monitor toroid, M0, and at the lower transmitter by a monitor toroid, M2. There is no monitor toroidat the upper transmitter, the axial current there, M1, is assumed equal to M2 by symmetry. Software translates these measurements intoadjustments of transmitter strength so that the axial currents at M0 cancel.

The cylindrical focusing technique (CFT) measures and compensates for this distortion,restoring the cylindrical geometry of the equipo-tential surfaces in front of the measurementelectrodes. Focusing is achieved by combiningthe current patterns generated by the upper andlower transmitters in software to effectivelyimpose a zero-axial-flow condition at the ringmonitor electrode. This ensures that the ringcurrent is focused into the formation and that nocurrent flows along the borehole.4

Wireless Telemetry—Data from the RAB toolmay be stored in nonvolatile memory or trans-mitted uphole via the PowerPulse MWD teleme-try tool. Data are transferred to the PowerPulsetool by a downhole telemetry bus connection ora wireless electromagnetic link. In the lattercase, the RAB tool transmits data to a receivermodule connected to the PowerPulse tool up to150 ft [45 m] away

3. Bonner et al, reference 4 main text.

4. Bonner et al, reference 4 main text.

Analysis of cores indicated wide distribu-tion of fractures throughout the reservoir withapertures varying from less than 0.001 in.[0.025 mm] to 0.1 in. [2.5 mm]. The buttonelectrodes that produce RAB images arelarge in comparison—1in. in diameter. How-ever, even with low-resistivity contrast acrossthe fractures, the largest fractures or densestgroups of fractures that appear on the FMIimages were seen on the RAB images (left).The RAB tool could not replace FMI data.

What intrigued the oil company, however,was the possibility of calculating dips fromRAB images. If this were successful, then theRAB tool could help resolve structuralchanges, such as crossing a fault, duringdrilling. The suggestion was taken up byAnadrill. With commercial software, dipswere calculated from RAB images. Goodagreement was found between RAB andFMI dips.

Dip correlation during drilling proved use-ful on subsequent California wells. Manyhave complex structures, and the absence ofclear lithologic markers during drillingmeans that the structural position of wellsmay become uncertain. Currently, RABimage data are downloaded when drillpipeis pulled out of the hole for a new bit anddips are subsequently calculated. The dataare used to determine if the well is on coursefor the highly fractured target area (left).

The oil company’s experience with theRAB tool in these formations has shown that:• RAB resistivity data are better in these for-

mations than CDR data.• RAB images compare well with FMI

images, but cannot produce the fine detailrequired for fracture analysis.

• Dips can be calculated from RAB images,leading to structural interpretation.

• Dips calculated during drilling aid direc-tional well control in highly faulted, high-angle, structurally complex wells.

• Dips determine when fault blocks arecrossed and, hence, when to stop drilling.

The close cooperation between Anadrill,GeoQuest, Wireline & Testing and oil com-panies has led to the recent development ofsoftware to process RAB dips downhole.Dips may then be sent to surface duringdrilling for real-time structural interpretation.

12 Oilfield Review

nFractures imaged by RAB and FMI tools. Fractures with large apertures or close spac-ing that appear on the FMI image (right) are seen on the RAB image (left).

nStructural interpretation. Workstation interpretation of RABdips shows that the well penetrates a synclinal fold.

RAB ImageD

epth

4 ft

Top Bottom TopFMI Image

Top Bottom Top

Dep

th 1

00 ft

Real-Time Dip ComputationMost conventional dip processing relies oncrosscorrelation of resistivity traces gener-ated as the dipmeter tool moves along theborehole (right ).9 This type of processingworks best when apparent dip is less than70°—typical of most formations logged invertical wells. However, in horizontal orhigh-angle wells, apparent dip will mostlikely be greater than 70°. This is the terri-tory of LWD tools. Automatic dipcomputation in such situations is useful forgeosteering applications in horizontal wells,especially if this can be done while drilling.

The new method uses the azimuthal resis-tivity traces generated by the three buttonsof the RAB tool. Bedding planes crossing theborehole will normally appear twice oneach trace as the buttons scan past the beds,first on one side of the hole and then theother. Dip computation is a two-part pro-cess that looks at where the beds appear oneach trace and then where they appearbetween traces.

Where the bed appears depends on itsazimuth with respect to the top of the RABtool. The same bed will appear twice onthe second and third traces, but will be dis-placed according to the dip magnitude.Finding the azimuth is simply a matter ofcorrelating one half of each trace againstthe other half. Dip magnitude depends onthe amount of event displacement betweenpairs of traces. Confidence in the computa-tion is increased because three separateazimuths can be calculated—one for eachbutton—and the three pairs of curves canbe used independently for the dip magni-tude computation.

The direction of dip—the azimuth—is cal-culated from the borehole orientation with

13Spring 1996

Correlationbetween traces

Correlation left to right

Traces from RAB tool

Directionof logging

Correlationbetween traces

Directionof logging

Traces from dipmeter tool

9. Rosthal RA, Young RA, Lovell JR, Buffington L andArceneaux CL: “Formation Evaluation and GeologicalInterpretation from the Resistivity-at-the-Bit Tool,”paper SPE 30550, presented at the 76th SPE AnnualTechnical Conference and Exhibition, Dallas, Texas,USA, October 22-25, 1995.

nDip processing comparison. Conventional dipmeter tools pro-duce resistivity curves as the tool is moved along the borehole(top). Processing relies on crosscorrelation of similar events loggedat different depths and works well for apparent dip below about70°. RAB dip computation uses the resistivity curves generated asthe three azimuthal buttons scan the borehole (right). Processing ismore robust as the three traces are recorded with the tool at onedepth. There is a fixed interval between the buttons.

(continued on page 17)

Measurement point

EWR tool

Receiver 1

Receiver 2

Transmitter

CDR tool

Transmitter 1

Receiver 1

Receiver 2

Transmitter 2

EWR-PHASE 4 tool SCWR tool

35-in. spacingupper transmitter

Multiarray MBHCpropagation tool

ARC5 tool

0

34-in. transmitter

22-in. transmitter

10-in. transmitter

Receiver

Receiver

16-in. transmitter

28-in. transmitter

2-MHzpropagation tool

Borehole-compensatedpropagation tool

Multiarray BHCpropagation tool

Multiarraypropagation tool

Directionalsensor andpulserDrillstringdynamicssensor

Gamma ray

Receivers

Transmitters

Wear bands

15-in. spacinglower transmitter

15-in. spacingupper transmitter

ReceiverResistivitymeasurement pointReceiver

35-in. spacinglower transmitter

Wear band

1.5-ftcrossover sub

Wear bands

In 1983, NL Industries introduced the first LWD

tool to tackle induction-type environments.1 The

EWR Electromagnetic Wave Resistivity tool has a

2-MHz transmitter and two receivers (above). The

high frequency makes it an electromagnetic wave

propagation tool rather than an induction tool (see

“Why 2 MHz?,” page 16). Induction tools measure

the difference in magnetic field between the two

receivers that is caused by induced formation

14 Oilfield Review

Evolution of the 2-MHz LWD Tool:From EWR to ARC5

nPropagation tools. The first 2-MHz propagation tool, the EWR tool, was designed by NL Industries. The tool had one transmitter and two receivers. Measurementswere made by comparing the formation signal phase shift between the two receivers. Later, borehole-compensated (BHC) tools, such as the Anadrill CDR tool, weredeveloped. Borehole-compensated tools have two transmitters equally spaced on either side of the receiver pair. In the case of the CDR tool amplitude and phase-shiftresistivities are measured. Development of multiarray tools, like the EWR-PHASE 4 tool, allowed multiple depths of investigation and the possibility of invasion profil-ing. Later tools, such as the SCWR tool, were also borehole compensated. The Anadrill ARC5 tool has three transmitters above and two below the receiver array andmeasures five attenuation and five phase-shift resistivities. Borehole compensation is achieved by using a linear mix of three transmitter measurements for each read-ing. This not only eliminates five transmitters required for standard borehole compensation (BHC), but also makes the tool shorter and stronger.

eddy currents. Propagation tools, however, mea-

sure amplitude and phase differences between

the receivers. All measurements can be trans-

formed into resistivity readings. However, the

EWR tool uses only the phase shift.

In 1988, Schlumberger introduced a borehole-

compensated 2-MHz tool.2 This CDR Compen-

sated Dual Resistivity tool has two transmitters

symmetrically arranged around two receivers

built into a drill collar. Each transmitter alter-

nately broadcasts the electromagnetic waves:

the phase shifts and attenuations are measured

between the two receivers and averaged. The

phase shift is transformed into a shallow resis-

tivity measurement and the attenuation into a

deep resistivity measurement.

The EWR tool described earlier was developed

further by Sperry-Sun Drilling Services into a

multispacing tool.3 This EWR-PHASE 4 tool con-

sisted of four transmitters and two receivers pro-

viding four phase-shift resistivity measurements

which, however, were not borehole compensated.

A slimhole version—SLIM PHASE 4—was intro-

duced in 1994.4 Halliburton also offers a slim

4.75-in. tool—the SCWR Slim Compensated

Wave Resistivity tool.5

Propagating the ARC5 Tool

The latest generation LWD propagation tool is the

4.75-in. ARC5 Array Resistivity Compensated

tool, a self-contained 2-MHz multiarray borehole-

compensated resistivity tool developed to log the

increasing number of slim holes being drilled

(above left). 6 The array of five transmitters

—three above and two below the receivers—

broadcast in sequence providing five raw phase-

shift and five raw attenuation measurements. In

addition, there are gamma ray and transverse

shock measurements.

Borehole compensation (BHC) is achieved by a

method unique to the ARC5 tool. Standard BHC

combines data from two transmitters placed sym-

metrically around the receiver array for one com-

15Spring 1996

nARC5 tool.

Wear band

34-in. transmitter

22-in. transmitter

Wear band

10-in. transmitter

Receiver

Receiver

Wear band

16-in. transmitter

28-in. transmitter

Wear band

To GR, transverseshocks, electronics andSlim 1 connection

Tota

l too

l len

gth

21 ft

6 in

.

43/4 in.

0.5T1+0.5T2

f(T5, T4, T3)

f(T3, T4, T5)

f(T2, T3, T4)

f2(T1, T2, T3)

f1(T1, T2, T3)

34 in. 22 in. 10 in. 3 in. -3 in. -16 in. -28 in.

0Measurement point

Total tool length = 21 ft

X(TR) = phase shift or attenuation measuredfrom transmitter at spacing TRTR = 10, -16, 22, -28, 34

T1 R1 R2 T2

0Measurement point

+x in. -x in.

T5 T3 T1 R1 R2 T2 T4

1. Rodney PF, Wisler MM, Thompson LW and Meador RA:“The Electromagnetic Wave Resistivity MWD Tool,” paperSPE 12167, presented at the 58th SPE Annual TechnicalConference and Exhibition, San Francisco, California,USA, October 5-8, 1983.

Various acquisitions and disposals by NL Industries haslead to this technology being transferred to Sperry-SunDrilling Services, a Dresser Industries, Inc. company.

2. Clark B, Allen DF, Best D, Bonner SD, Jundt J, Lüling MGand Ross MO: “Electromagnetic Propagation LoggingWhile Drilling: Theory and Experiment,” paper SPE18117, presented at the 63rd SPE Annual Technical Con-ference and Exhibition, Houston, Texas, USA, October 2-5, 1988.

3. Bittar MS, Rodney PF, Mack SG and Bartel RP: “A TrueMultiple Depth of Investigation Electromagnetic WaveResistivity Sensor: Theory, Experiment and PrototypeField Test Results,” paper SPE 22705, presented at the66th SPE Annual Technical Conference and Exhibition,Dallas, Texas, USA, October 6-8, 1991.

4. Maranuk CA: “Development of the Industry’s First MWDSlimhole Resistivity Tool,” paper SPE 28427, presented atthe 69th SPE Annual Technical Conference and Exhibition,New Orleans, Louisiana, USA, September 25-28, 1988.

5. Heysse DR, Jackson CE, Merchant GA, Jerabek A, Beste Rand Mumby E: “Field Tests of a New 2 MHz ResistivityTool for Slimhole Formation-Evaluation While Drilling,”paper SPE 30548, presented at the 76th SPE AnnualTechnical Conference and Exhibition, Dallas, Texas, USA,October 22-25, 1995.

6. Bonner et al, reference 10 main text.

nCompensating forborehole effects. Stan-dard borehole compen-sation uses a symmetri-cal arrangement oftransmitters around thereceiver pair (top).Resistivity measure-ments from each areaveraged to compensatefor effects such as holerugosity or drifts inreceiver electronics. TheARC5 tool uses mixedborehole compensation(MBHC) to achieve thesame effect, but withoutthe need to duplicatetransmitters (bottom). By placing transmittersasymmetrically aroundthe receiver pair, variouscombinations of mea-surements may be used.For example, to achieveMBHC for the 22-in.spacing, a combinationof 22-in., 16-in. and 28-in. resistivity measure-ments is used.

pensated measurement (above). The ARC5 tool

dispenses with the second transmitter, relying

instead on linear combinations of three sequen-

tially spaced transmitters to provide what is

called mixed borehole compensation (MBHC),

The advantage of this system is that tool costs

and length are reduced by eliminating five trans-

mitters. Five MBHC phase shifts and attenuations

are then transformed into five calibrated phase-

shift and five calibrated attenuation resistivities

(next page, top).

Oilfield Review

nARC5 logs before andafter MBHC. The spikyappearance of the logwithout MBHC (top) iscaused by overshoots—horns—in resistivitymeasurements atwashouts. These arti-facts are canceled outby MBHC (bottom).

nOperating frequenciesof Schlumberger resis-tivity tools.

Since the depth of investigation increases as

the transmitter spacing increases, the five phase-

shift resistivities represent five different depths

of investigation with nearly identical axial resolu-

tion. Similarly, the five attenuation resistivities

represent five deeper reading measurements.

At present, the ARC5 tool communicates to the

surface using the Slim 1 slim and retrievable

MWD system. This is essentially a tool that

latches onto the ARC5 tool. After connection to

the ARC5 tool, data are transferred by an induc-

tive coupling to the Slim 1 system and then con-

tinuously transmitted to the surface acquisition

system by a mud-pulse link.

Why 2 MHz?

A wireline induction tool generates an oscillating

magnetic field—typically 10 to 40 kHz—that

induces eddy currents in a conductive formation.

These, in turn, generate a much weaker, sec-

ondary magnetic field that can be measured by a

receiver coil set. Measuring the secondary mag-

netic field gives a direct measurement of conduc-

tivity—the higher the conductivity, the stronger

the eddy currents, and the larger the secondary

magnetic field.

Induction tools use a trick to cancel the pri-

mary magnetic field’s flux through the receiving

coil set and allow measurement of the secondary

magnetic field only. This is accomplished by

arranging the exact number of turns and precise

positions of the coils such that the total flux

through them is zero in an insulating medium

such as air. In a conductive formation, the flux

from the secondary magnetic field doesn’t exactly

cancel, so the induction tool becomes sensitive

to the eddy currents only. If the same trick were

tried on a drill collar, then similar precision for

coil placement and dimensional stability would

be required. In the harsh conditions of drilling, a

drill collar striking the borehole wall can easily

produce 100 g shocks—more than enough to ruin

any precise coil positioning.

At 2 MHz, precise coil placement doesn’t mat-

ter, because the phase shift and attenuation are

measurable with a simple pair of coils—both

quantities increase rapidly with frequency. While

the two receivers may be slightly affected by

pressure, temperature and shock, borehole com-

pensation completely cancels any such effects.

Increasing the frequency further reduces the

depth of investigation and leads to dielectric

interpretation issues (left).

16

100

101

102

103

Rps

, ohm

-m

100

101

102

103

Rps

, ohm

-mWithout MBHC

With MBHC

PH10PH22PH34

2 GHz

200 MHz

20 MHz

2 MHz

200 kHz

20 kHz

2 kHz

200 Hz

Propagation dielectricEPT Electromagnetic Propagation Tool 1.1 GHz

Propagation dielectric resistivityDPT Deep Propagation Tool 25 MHz

Propagation resistivityCDR Compensated Dual Resistivity tool 2 MHzARC5 Array Resistivity Compensated tool 2 MHz

Induction resistivityAIT Array Induction Imager Tool 25,50,100 kHzPhasor Phasor-induction SFL tool 20 and 40 kHzDIL Dual Induction Resistivity Log 20 kHz

Conduction resistivityRAB Resistivity-at-the-Bit tool 1.5 kHzSFL Spherically Focused Resistivity tool 1 kHzDLL Dual Laterolog Resistivity toolARI Azimuthal Resistivity ImagerLLS Laterolog shallow 280 HzLLD Laterolog deep 35 Hz

algorithm that can be implemented in thetool microprocessor, allowing real-time trans-mission of structural dips (above).

A Profile of InvasionThe ARC5 tool is a 4.75-in. slimhole, multi-spacing, 2-MHz, propagation LWD tooldesigned, in record time, to operate in5.75- to 6.75-in. holes (see “Evolution ofthe 2-MHz LWD Tool: From EWR toARC5,” page 14 ).10 Propagation LWDdevices are similar in principle to wirelineinduction logging tools. They transmit elec-tromagnetic waves that induce circulareddy currents in the formation and pair ofreceivers monitors the formation signal. Atthis stage, however, the physics of measure-ment similarities stops.

LWD propagation tools operate at 2MHz, much higher than the 10- to 100-kHzfrequencies of induction tools (see “Why 2MHz?,” previous page). They are built onsturdy drill collars and are capable of takingthe violent shocks imposed by drilling.Wireline induction tools are essentiallybuilt on well-insulated fiberglass mandrelsthat cannot tolerate such heavy handling.

However, they both perform best in similarenvironments, such as conductive and non-conductive muds and low-to-medium resis-tivity formations.

The ARC5 tool was designed to exploitinterpretation methods developed for thewireline AIT Array Induction Imager Tool. Tothis end, both tools provide resistivity mea-surements at five different depths of investi-gation allowing radial resistivity imaging.

The ARC5 has other advantages over pre-vious LWD propagation technologiesincluding:• improved estimation of Rt• improved estimation of permeability

index• better evaluation of thin beds through

improved resolution• inversion of complex radial invasion

profiles• better interpretation of complex

problems, such as invasion, resistivityanisotropy and dip occurring simultaneously

• reservoir characterization based on time-lapse logging.

Unique to the ARC5 tool is mixed-boreholecompensation (MBHC). This method pro-vides five MBHC attenuation and fiveMBHC phase resistivity measurements pro-cessed from only five transmitters. Standardborehole-compensation (BHC) requires 10transmitters (see “Propagating the ARC5Tool,” page 15).

17Spring 1996

300

320

340

360

380

400

420

Dep

th, f

t

70 80 90 100 110Dip, degrees

0 360 Azimuth, degrees

nReal-time dip com-putation. Dip can becomputed from theresistivity image (left)using a real-timealgorithm (right).Results indicate highapparent dips, near90°. Shown on theresistivity image isthe computed dipazimuth, which runsalong the direction ofthe borehole.

respect to north plus the orientation of thebedding plane with respect to the borehole.For example, if on a trace, a bed appears tocut the borehole at 10° and 70°, then theorientation of the bed is 40° with respect tothe top of the borehole. The second tracemay see the same bed at 0° and 80° and thethird trace, at 350° and 90°. Both give theorientation as 40° providing additional con-fidence in the calculation.

To determine the apparent dip, correlationis made between the three traces. In theabove example, the bed appears on oneside of the hole at 10°, 0° and 350° on eachtrace, respectively. As the distance betweenRAB buttons is fixed, simple geometry canbe used to calculate apparent dip betweenany pair of traces. Knowing the boreholetrajectory leads to true dip.

This method does not rely on data col-lected at different depths and is effective inhorizontal wells. Also, the two-step approachof first calculating the dip azimuth and thendip magnitude provides a robust and fast

10. Bonner SD, Tabanou JR, Wu PT, Seydoux JP, Mori-arty KA, Seal BK, Kwok EY and Kuchenbecker MW:“New 2-MHz Multiarray Borehole-CompensatedResistivity Tool Developed for MWD in Slim Holes,”paper SPE 30547, presented at the 76th SPE AnnualTechnical Conference and Exhibition, Dallas, Texas,USA, October 22-25, 1995.

18 Oilfield Review

XX50 X1000 X1500 X2000 X2500

ARC5 Phase Resistivities R

ps, o

hm-m

ARI Resistivities

10 0

10 1

10 2

10 0

10 1

10 2

LLD

, LLS

, Mic

roS

FL, o

hm-m

XX500 X1000 X1500 X2000 X2500

PH10PH28PH34

nARC5 phase-shift resistivity comparison. Deep ARC5 phase-shift resistivity curves fromthe 34-in. and 28-in. spacing, PH34 and PH28 (orange and black curves, top log), correlatewith deep laterolog readings, LLD, recorded by the ARI tool (orange curve, bottom log)several days after drilling. The shallowest reading ARC5 curve, PH10 (green curve, toplog), correlates with the shallow laterolog, LLS (purple curve, bottom log), but reads higherthan the MicroSFL curve (green curve, bottom log). This implies that there was little inva-sion at the time of drilling.

nInvasion profile. The radial resistivity image generated from the ARC5 resistivity curvesshows little invasion. Light brown is high resistivity and dark brown, low resistivity. AtXX500 ft, XX550 ft and X2080 ft are possible sources of seawater influx from nearbyinjection wells.

XX500 X1000 X1500 X2000 X2500

-15

-10

-5

0

5

10Rad

ial r

esis

tivity

, in.

Depth, ft

Raiders of the ARC5A slim horizontal sidetrack in an offshoreMiddle East well provided a good field testfor the ARC5 tool.11 Oil company objectiveswere to gain experience with horizontaldrilling and to understand why more waterthan expected was being produced. Thecarbonate reservoir has major faults andseveral fractured zones, and is being pro-duced under seawater injection.

The 6-in. sidetrack was drilled with theARC5 tool run in record mode above thedownhole motor in a steerable bottomholeassembly (BHA) and an interval of morethan 2000 ft was logged from the kickoffpoint. Later, drillpipe-conveyed wirelinelogs were recorded over the same interval.

Comparisons were made between ARC5phase resistivity readings and deep laterolog(LLD), shallow laterolog (LLS) and MicroSFLmeasurements recorded by the ARIAzimuthal Resistivity Imager and MicroSFLtools (left). Deep ARC5 phase resistivitycurves, PH34 and PH28, agree well withLLD readings implying that applications forLWD propagation tools and laterolog toolsoverlap. The shallowest ARC5 curve, PH10,correlates with the LLS curve and readsmuch higher than the MicroSFL curve. Laterprocessing suggests that there was littleinvasion at the time of drilling.

Wireline log interpretation indicateshydrocarbons throughout most of the inter-val. Water saturation is at a minimum fromX1150 ft to X1250 ft, where ARC5 resistivi-ties read higher than 100 ohm-m.

An invasion profile image produced fromARC5 data clearly shows the effects ofdrilling history, as well as formation perme-ability (left ).12 For example, the intervalfrom X2000 to X2050 ft shows increasedinvasion, because it was logged 24 hours

Imagine the FutureThe ARC5 and RAB tools are part of a newgeneration of LWD resistivity tools capableof producing quality resistivity data for awide variety of applications. Both introducemeasurement techniques unique to LWDand wireline logging. For example, MBHCis a cost-effective alternative to doubling upon transmitters for borehole compensationand cylindrical focusing is a more stablealternative to traditional laterolog focusing(see “Cylindrical Focusing,” page 10).

With the development of INFORM Inte-grated Forward Modeling software, interpre-tation in horizontal wells will be greatlyimproved.13 Couple this with downhole dipprocessing and real-time imaging, and thearguments for resistivity-while-drilling mea-surements become powerful.

The value of LWD data will be furtherincreased by close collaboration with Wire-line & Testing and GeoQuest. For example,the concept of invasion-profile measure-ments leads to exciting possibilities. It offersa chance to look at the invasion process indetail. Resistive invasion infers water-filledporosity, whereas conductive invasion infersoil-filled porosity. In the near future, itshould be possible to predict water cut anddraw some conclusions about permeabilitydirectly from LWD fluid invasion-profile log-ging and resistivity anisotropy processing.

What is the next step in development?Although future possibilities are exciting forresistivity while drilling, the next step willbe more evolutionary than revolutionary.With the development of a family of differ-ent sized ARC5 and RAB tools, measure-ments described in this article can beapplied to more borehole sizes. —AM

X1950

-15

-10

10

-5

5

0

Depth,ft

Rad

ial R

esis

tivity

, in.

X2000 X2050 X2100 X2150 X2200

ARC5 Radial Resistivity Image and Diameter of Invasion

ARC5 Phase-Shift Resistivity

Drilling Summary

0

20

40

60

80

100

RO

P, m

in/f

t, G

R, g

api

gamma ray

time atbottom

ROP

TAB

, hr

10

0

Pha

se re

sist

ivity

, ohm

-m

100

101

102

X1950 X2000 X2050 X2100 X2150 X2200

X1950 X2000 X2050 X2100 X2150 X2200

BA C

19Spring 1996

11. Bonner et al, reference 10.12. Howard AQ: “A New Invasion Model for Resitivity

Log Interpretation,” The Log Analyst 33, no. 2(March–April 1992): 96-110.

13. INFORM software allows an analyst to construct adetailed model of the geometry and petrophysicalproperties of the formation layers along a well path.Simulated tool responses along the well are thencompared to acquired log data allowing the modelto be adjusted until they match. For a more detaileddescription:Allen D, Dennis B, Edwards J, Franklin S, KirkwoodA, Lehtonen L, Livingston J, Lyon B, Prilliman J,Simms G and White J: “Modeling Logs for Horizon-tal Well Planning and Evaluation,” Oilfield Review7, no. 4 (Winter 1995): 47-63.

nDrilling summary (top), ARC5 phase resistivities (middle) and resistivity image (bottom)shown in detail for the interval X1950 ft to X2200 ft. ARC5 data (middle) recorded 24 hrafter a bit change show increased invasion (interval A) compared to the previous interval,which was logged only a few hours after being drilled. Little invasion occurs across a low-permeability streak (interval B). All resistivity curves converge (interval C) indicatingwater breakthrough.

after a bit change (above). Other intervalswere logged within a few hours of drillingand show less invasion. Invasion is deeperwhere drilling is slow and also in high-per-meability streaks. The latter coincide withthe position of fractures and faults that areshown on FMI data.

Two intervals were of special interest tothe oil company—around XX550 ft and

X2080 ft. Formation resistivity approaching1 ohm-m in both intervals indicated thatseawater injection had broken through thesezones. Increased pore pressure in theseintervals resulted in dramatic increases inthe rate of drilling. Several days later, theARI tool showed that invasion had pro-gressed to about 35 in. [89 cm].

The well was completed with a slottedliner and produced 4000 BOPD and 600BWPD compared to 1000 BOPD in theoriginal well. The interval at X2080 ft is themost likely contributor to water production.