resistivity imaging ii.pdf

8/13/2019 Resistivity Imaging II.pdf

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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).2This

article focuses on the latest LWD resistivitytoolsthe RAB Resistivity-at-the-Bit tool andthe ARC5 Array Resistivity Compensatedtooland the images they produce (see AProfile of Invasion,page 17).

Steve BonnerMark FredetteJohn LovellBernard MontaronRichard RosthalJacques TabanouPeter Wu

Sugar Land, Texas, USA

Brian ClarkRidgefield, Connecticut, USA

Rodger MillsExxon USAThousand Oaks, California, USA

Russ WilliamsOXY USA Inc.Houston, Texas

Re sistivity Wh ile Drillin g Im a g e s from the Strin g

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 Lling, Schlumberger-Doll Research, Ridgefield,Connecticut.

(continued on page 6)


2/16

n Forma t ion eva lua -t ion ma de by com -

bining data from

severa l LWD m ea -

surem en ts. This loginterpretation wa s

m a de u sing ELAN

Eleme nta l Log Ana l-

ysis softwa re a nd

da ta from the RAB

Resistivity-a t-the -Bit

tool, CDR Com pe n-

sa ted Dua l Resistiv-

i ty a nd CDN Com-

p en sa te d De nsity

Neu tron tools.

Volum etric a na lysis

(track 5) shows a

qu a rtz-rich zone of

relatively high

p oros ity . Th e b row nshading indicates

t he mova b le ga s

volum e ca lculated

from CDR a nd RAB

da t a run severa l

da ys la ter. The RAB

resistivity ima ge

(track 4) shows tha t

the sand b ody is

split into three m a in

lobe s with sha le per-

m ea bili ty ba rriers.

1:2400 ft

XX500

XX450

Net pay

Net sand

G as effect

Density porosityC DN

60.0 p.u. 0

Neutron porosityC DN

60.0 p.u. 0

Add. gas volume right after drillingC DR

Irreducible water

M oved water

Water

G as volume 7 days after drillingRAB

Q uartz

Bound water

Illite

C ombined model

0 p.u. 100

Perm to gas

P erm to water

G amma ray

0 AP I 200

P erm to water

10000 md 0.1

Perm to gas

10000 md 0.1

RA B image

0 deg 360

R ing res. RA B

0.2 ohm-m 20

Shallow res. R AB

0.2 ohm-m 20

M edium res. RAB

0.2 ohm-m 20

Deep res. RAB

0.2 ohm-m 20

P hase shift res.C DR

0.2 ohm-m 20

Attenuation res.C DR

0.2 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 Review4,no. 3 (July 1992): 4-21.

2. Bonner S, Clark B, Decker D, Orban J, Prevedel B,Lling M and White J: Measurements at the Bit: ANew Generation of MWD Tools, Oilfield Review5,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 markof Halliburton. EWR (Electromagnetic Wave Resistivity),EWR-PHASE 4 and SLIM PHASE 4 are marks ofSperry-Sun Drilling Services.


3/16


4/16

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

Wellsite images allow geologists toquickly confirm the structural position of the

well during drilling, permitting any neces-sary directional changes. Fracture identifica-tion helps optimize well direction for maxi-mum production.

Find ing the Crac ks in Masters Creek

Murray A-1 is a dual-lateral well drilled byOXY USA Inc. in the Cretaceous AustinChalk formation, located in the MastersCreek 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 fracture

planes 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 in

high-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 helical

coiled 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 north

to 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 characteristic

sinusoids of contrasting colors, indicatingchanges in resistivity as the borehole crossesbed boundaries (right).

7Spring 1996

Austin Chalk Trend

Texas

Houston

Arkansas0 100

miles

Pearsall

M exico

G iddings

Brookeland

M asters

C reek field

North Bayou Jack

G ulf of M exico

Louisiana M ississippi

n Loca tion of Ma stersCree k field in rela -

tion to other fields of

the Austin Cha lk

t rend.

n Crossing be dd ing p lane s. As the b orehole crosses a n a lmost horizonta l , low-resistivitybed , the RAB ima ge shows a cha racter is tic h igh-am pl itude s inusoida l ima ge (dark

brown). In terpreters have picked the b ed bounda r ies (green) for structura l interpre ta tion.

The n otation TD:11/ 26True Dip: dip ma gnitude / dip a zim uthindica tes that this bed-

ding plan e i s d ipping a t 11 to the NNE, north 26e a st to be exa ct .

Crossing the borehole a lmost vert ica lly a t XX896 ft is a frac ture (yellow). TD:87/ 359

indica tes that the frac ture is dipping north at a n a zim uth of 359 a nd is nea rly vert ica l ,

87 from the horizonta l . The strike, or trend, of the fra cture is pe rpend icular to the d ipdirectionea st/ we st .

The c ylind rica l 3D ima ge (inset) shows the boreh ole ima ge s as if viewed from the right

of the h ole.

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


5/16

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 (top

right), providing enough evidence that thewell trajectory was nearly perpendicular tothe fracture trend.7

Based on this information the second lat-eral was drilled south 10east, again to inter-

sect as many fractures as possible at 90.

Ima ge s of Ca lifornia

Complex 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 Resistivity

tool 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, such

as 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.

n Fra cture c lusters. Severa l frac tures cut the borehole a round XX956 ft . The large st anomal y (black) is either a cluster of frac tures or a very large frac ture. The borehole isp a ss in g p a ra lle l to th e in te rfa ce b e tw e en tw o b ed s. The m ore re sis tiv e b e d (white) is onthe b ottom side of the hole. The c ylind rica l im a ge (inset) gives an a lterna tive 3D view ofthe borehole ima ge.

n RAB a nd FMI im a ge s of dipp ing b ed s. Both RAB a nd FMI im a ge s show large-sca leeven ts tha t are se vera l feet long. Howe ver, the resolution of the FMI ima ge is m uchbe tter. Bed s less tha n a bout 4 in. [10 cm ] thick a re not clea rly seen on the RAB ima ge .8 Oilfield Review

7. The size of fractures seen by the RAB tool dependson several factors. The physical diameter of the buttonis 1 in. [2.54 cm], which produces an electric fieldslightly larger1.5 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 around1-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 36th

Annual Logging Symposium, Paris, France, June 26-29,1995, paper TT.

Top Bottom Top

RAB Image

Depth10ft

Top Bottom Top

FMI Image


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 : 90/167

TD : 86/173

TD : 84/355

TD : 86/177


6/16

n Electrode 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 measured

between the two so that resistivity can be calculated.

An improvement on this is the laterolog technique (middle). Additional electrodes provide a bucking current

that 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 J J 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 collar

radially and at the bottom of the collar. The amount of radial current at any point depends on the formation

resistivity at that location. Two different methods of measuring radial current are used: (1) by the difference

between axial current measured at two receiver toroids or (2) by direct electrode current meters.

Potentialelectrode

R eturn

Insulation

C urrentelectrode

Focused CurrentResistivity Tool

M easurementcurrent

R eturn

Insulation

R eturn

Transmitter

R eceivers

16in.

Short Normal

Tool

LateralresistivityR Lat

B itresistivityR Bit

Dual R esistivityM WD Tool

G uardelectrodes

9Spring 1996

1. Evans HB, Brooks AG, Meisner JE and Squire RE: A

Focused Current Resistivity Logging System for MWD,

paper SPE 16757, presented at the 62nd SPE Annual

Technical Conference and Exhibition, Dallas, Texas, USA,

September 27-30, 1987.

2. Arps, reference 5 main text.

Laterologs have their roots in a tool called the

short normal, one of the earli est wireline l og-ging tools. Its principles were a dapted by many

measurements-while-drilling (MWD) companies

in the early 1 980s to provide a simple re sistivity

log for correlation (r ight). The idea is fairly

straightforward: force current from a source

electrode to a return electrode through the for-

mation; mea sure the current and voltage drop

between the el ectrodes and use Ohms law to

derive formation resistivity. However, for accu-

rate petrophysical analysis in complex forma-

tions, more sophisticated devices are nee ded to

measure true formation resistivity, Rt.An improvement on the short normal is the

laterolog technique commonly used in wire line

logging. Exploration Logging introduced a lat-

erolog LWD resistivity tool in 1 987 based on the

laterolog 3 wireline tool of the early 1950s. 1

This FCR Focused Current Resistivity tool had

two additional current electrodes on either side

of the measurement e lectrode. They provided

guard currents that forced the main current

deeper into the formation to measure Rt.

At about this time, another approach was

developed by Gearhart Geodata Services Ltd.

from an idea by JJ Arps. 2 The Gearhart Dual

Resistivity 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 the

collar. This method is ideal ly suited to LWD

because resistivity tools have to be built into

mechanically strong steel colla rs. Below the

transmitter, current leaves the tool radially from

the collar and axially from the drill bit. The

amount of current leaving the collar at any point

depends on the induced drive-voltage and the

local formation resistivity. Two resistivity mea-

surements are made: a focused lateral re sistiv-

ity measurement and a trend resistivity mea-

surement at the bit. Two receiver toroids, 6 in.

apart, e ach measure axial current flowing past

them down the collar. The difference in axial

current equals the radial current lea ving the

drill collar betwee n the two receivers and is

used to calculate lateral resistivity. Bit resistiv-

ity is derived from the axial current measured

by the lower re ceiver.

Schlumberger also uses the Arps principle of

generating and monitoring axial-current flow in

the RAB tool. However, radial-current flow is

measured directly, and multiple toroidal trans-

mitters and receivers are used in a unique

focusing technique described later.

From Short Norma l to Axia l Curren t


7/16

RAB Tool The Works

The RAB tool m easures five resistivity val-

ues bit, ring and three button resistivities as

well as gamma ra y, plus axial and transverse

shock. 3 Built on a 6.75-in. drill collar, the 10-ft

[3-m ] long tool can be configured as a near-bit

or in-line stabilizer, or as a slick drill collar

(r ight). When real-time data are required, the

RAB tool communicates with a PowerPulseMWD telemetry tool via wireless telemetry or a

standard downhole tool bus, a llowing total BHA

design flexibility . However, it m ust be config-

ured as a stabilizer for ima ging.

Bit ResistivityA 1500-H z alternating current

is driven through a toroidal-coil transmitter, 1 ft

[30 cm] from the bottom of the tool, that

induces a voltage in the collar bel ow. Current

flows through the collar, out through the bit and

into the formation, returning to the collar far up

the drill string (below right ) . Knowing the volt-

age and measuring the axial current through thebit determines re sistivity at the bit. Corrections

are ma de for tool geometry, which varies

according to the BHA.

The resolution of the bit me asurement

depends 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, the

resolution is about 2 ft [60 cm]. As the bit-resis-

tivity measurement is not actively focused, the

current patterns and volume of investigation

are affected by nearby beds of contrasting

resistivity. As wellbore i nclination increases,

the effective length of the bit e lectrode becomes

shorter and, in horizontal wel ls, e quals hole

diameter.

Bit resistivity relies on a good bit-to-forma-

tion electrical path. The path is al ways excel-

lent in water-base m ud and generally sufficient

in oil-base mud.

Applications for the bit-resistivity measure-

ment include geostopping to precisely stop at

casing or coring point picks. For e xample, in a

Gulf of Mexico well the objective was to drill

only a few inches into the reservoir before set-

ting casing. An induction gamma ray log from a

nearby well was avai lable for correlation.

Drilli ng was stopped when bit resistivity

increased to 4 ohm-m, i ndicating reservoir

penetration (next page, bottom ) . Subsequent

modeling showed that the bit had cut only 9 in.

[23 cm] into the reservoir.

Focused Multidepth ResistivityThe RAB

tool with button sleeve provides four multidepth

focused resistivity measurements. For an

81/2-in. bit, the ring electrode has a depth of

investigation 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 and

acquire resistivity profiles as the tool rotates in

the borehole. The sampling rate dictates that a

full profile i s acquired at rotational speeds

above 30 rpm generally not a limitation.

Data from the azimuthal scans are stored

downhole and dumped from the tool between bit

runs. In addition, the azimuthal data may be

averaged by quadrant and transmitted to sur-

face in real tim e along with the ring and bit

resistivity, and gamma ray measurements.

All four resistivities use the same m easure-

ment principle: current from the upper transmit-

ter flows down the collar and out into the

formation, lea ving the collar surface at 90

10 Oilfield Review

Uppertransmitter

Azimuthal

electrodes

Ring

electrode

Lower

transmitter

Axial current

Lower transmitter

R ing monitor toroid

Upper transmitter

n Bit 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 Ohms law once the upper transmitterdrive voltage and the current are known. Correctionsare made to compensate for tool geometry and

transmitter frequency.

n RABtool.

along its length. The return path is along the

collar above the transmitter. The amount of

current leaving the RAB tool at the ring and but-

ton electrodes is m easured by a low- impedance

circuit. Axial current flowing down the collar is

measured at the ring ele ctrode and at the lowe r

transmitter. These measurements are repeated

for the lower transmitter.

Cylindrical FocusingIn a homogeneous for-mation, the equipotential surfaces near the but-

ton and ring electrodes on the RAB tool are

cylindrical. However, in layered formations,

this is no longer the case. Current will be

squeezed into conductive beds distorting the

electric field (next page, top) . By contrast, resis-

tive beds will have the opposite effect: the cur-

rent avoids them and takes the more conductive

path. These artifacts are called squeeze and

antisqueeze, respectively, a nd lead to charac-

teristic measurement overshoots at bed bound-

aries called horns.


8/16

11Spring 1996

150.0. 1:240ft

RAB GR

AP I

0.02 SFL O ffset well

ohm-m200

0.02 ILM O ffset well

ohm-m200

2000

200

20000.2 R AB R ING resistivity

ohm-m

0.2R AB BIT resistivity

ohm-m

0.02 ILD O ffset well

ohm-m

100.0. Wireline, GR

AP I

Nonfocused System Active Focusing

C onductive bed

R ing electrode

Single transmitter

BSBS

BMBM

BDBD

12

12

12

R 1,R 2

T2

BD

BM

BS

RM 0

M 2

T1

M 01 M 02

M 12

By reciprocityM 12= M 21

Upper transmitter

Upper transmitter current

R ing electrodeM onitor toroid

Lower transmitter

Lower transmitter current

Lower monitor toroid

M 21

n Geostopping. Oneadvantage of a correla-

tion tool that measures

resistivity right at the bit

is the ability to recog-

nize marker beds almost

as soon as the drill bitpenetrates. This allows

drilling to stop precisely

at casing or coring

points. In this example,

the bit penetrated only 9

in. into the reservoir.

n Cylindrical 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 m easurement

electrodes. Focusing is achieved by combining

the current patterns generated by the upper and

lower transmitters in software to effectively

impose a zero-axial -flow condition at the ring

monitor electrode. This ensures that the ring

current is focused into the formation and that no

current flows along the borehole. 4

Wireless TelemetryData from the RAB tool

may be stored in nonvolatile m emory or trans-

mitted uphole via the PowerPulse MWD teleme-

try tool. Data are transferred to the PowerPulse

tool by a downhole telemetry bus connection or

a wireless electromagnetic link. In the latter

case, the RAB tool transmits data to a receiver

module connected to the PowerPulse tool up to

150 ft [45 m] away

3. Bonner et al, reference 4 main text.



9/16

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 button

electrodes that produce RAB images arelarge in comparison1in. in diameter. How-ever, even with low-resistivity contrast acrossthe fractures, the largest fractures or densestgroups of fractures that appear on the FMI

images 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. Many

have 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 companys experience with the

RAB 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 during

drilling for real-time structural interpretation.

12 Oilfield Review

n Fra ctures ima ge d b y RAB a nd FMI tools. Frac tures with large a pe rtures or close spa c-ing tha t app ea r on the FMI im a ge (right) a re seen on the RAB ima ge (left).

n Structura l interp reta tion. Worksta tion interp reta tion of RABdips shows tha t the w ell pe netra tes a sync lina l fold.

RAB Image

Depth4ft

Top Bottom Top

FMI Image

Top Bottom Top

Depth100ft


10/16

Rea l-Tim e Dip Com pu ta tion

Most conventional dip processing relies oncrosscorrelation of resistivity traces gener-ated as the dipmeter tool moves along the

borehole (right).9This type of processingworks best when apparent dip is less than70typical of most formations logged invertical wells. However, in horizontal orhigh-angle wells, apparent dip will most

likely 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 appear

between 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 calculatedone for eachbuttonand the three pairs of curves can

be used independently for the dip magni-tude computation.The direction of dipthe azimuthis cal-

culated from the borehole orientation with

13Spring 1996

C orrelationbetween traces

C orrelation left to right

Traces from RAB tool

D irectionof logging

C orrelationbetween 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.

n Dip p rocessing com pa rison. Conventiona l dipme ter tools pro-du ce resist ivity curves a s the tool is m oved a long the b orehole(top). Processing relies on c rosscorrelation of similar eve nts logge d

a t d i fferent depths a nd works wel l for app a rent d ip b elow a bout

70. RAB dip c omp uta tion uses the resist ivity cu rves gen era ted a s

the three a zim uthal but tons sca n the b orehole (right). Proce ssing is

more robust as the three t races a re recorded wi th the tool a t one

de pth. There is a fixed interva l be twee n the b uttons.



11/16

M easurement point

EWR tool

Receiver 1

Receiver 2

Transmitter

CD R tool

Transmitter 1

R eceiver 1

R eceiver 2

Transmitter 2

EWR-PH ASE 4 tool SC WR tool

35-in. spacingupper transmitter

M ultiarray M BH Cpropagation tool

ARC5 tool

0

34-in. transmitter

22-in. transmitter

10-in. transmitter

Receiver

Receiver

16-in. transmitter

28-in. transmitter

2-M Hzpropagation tool

Borehole-compensatedpropagation tool

M ultiarray BH Cpropagation tool

M ultiarraypropagation tool

Directionalsensor andpulser

Drillstringdynamicssensor

G amma ray

Receivers

Transmitters

Wear bands

15-in. spacinglower transmitter

15-in. spacingupper transmitter

Receiver

Resistivitymeasurement point

Receiver

35-in. spacinglower transmitter

Wear band

1.5-ftcrossover sub

Wear bands

In 198 3, NL Industries introduced the first LWD

tool to ta ckle i nduction-type environments.1 The

EWR Electromagnetic Wave Resistivity tool has a

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

high frequency makes i t an el ectromagnetic 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

Evolut ion of th e 2-MHz LWD Tool:From EWR to ARC5

n Propagation 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, m ea-

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 198 8, Schlumberger introduced a borehole-compensated 2-MHz tool. 2 This CDR Compen-

sated Dual Re sistivity tool has two transmitters

symmetrically a rranged around two receivers

built into a drill collar. Each transmitter alter-

nately broadcasts the e lectromagnetic waves:

the phase shifts and attenuations are m easured

between the two receivers and averaged. The

phase shift is transformed i nto a shallow resis-

tivity measureme nt and the attenuation into a

deep resistivity measurement.

The EWR tool described earli er was developed

further by Sperry-Sun Drilli ng 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, w ere not borehole compensated.

A slim hole version SLIM PHASE 4 was intro-

duced in 1994. 4 Halli burton also offers a slim

4.75 -in. tool the SCWR Slim Compensated

Wave Resistivity tool. 5


12/16

Propagating the ARC5 Tool

The latest generati on 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 a bove and two below the rece ive rs

broadcast in sequence providing five raw phase-

shift and five raw attenuation measurements. In

addition, there are gamma ray and transverse

shock measureme nts.Borehole compensation (BHC) i s achieved by a

method unique to the ARC5 tool. Standard BHC

combines data from two transmitters placed sym-

metricall y around the receiver array for one com-

1Spring 1996

n ARC5 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 G R , transverse

shocks, electronics andSlim 1 connection

Tota

ltoollength21ft

6in.

43/4 in.

0.5T1+0.5T2

f(T5, T 4, T3)

f(T3, T 4, T5)

f(T2, T 3, T4)

f2(T1, T2, T 3)

f1(T1, T2, T 3)

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

0M easurement point

Total tool length = 21 ft

X(T R ) = phase shift or attenuation measuredfrom transmitter at spacing T R

TR = 10, -16, 22, -28, 34

T1 R1 R2 T2

0M easurement point

+x in. -x in.

T5 T3 T1 R 1 R2 T2 T4

1. Rodney PF, Wisler MM, Thompson LW and Meador RA:

The Electromagnetic Wave Resistivity MWD Tool, paper

SPE 12167, presented at the 58th SPE Annual Technical

Conference and Exhibition, San Francisco, California,

USA, October 5-8, 1983.

Various acquisitions and disposals by NL Industries has

lead to this technology being transferred to Sperry-Sun

Drilling Services, a Dresser Industries, Inc. company.

2. Clark B, Allen DF, Best D, Bonner SD, J undt J , Lling MG

and Ross MO: Electromagnetic Propagation Logging

While Drilling: Theory and Experiment, paper SPE

18117, 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 True

Multiple Depth of Investigation Electromagnetic Wave

Resistivity Sensor: Theory, Experiment and Prototype

Field Test Results, paper SPE 22705, presented at the

66th SPE Annual Technical Conference and Exhibition,

Dallas, Texas, USA, October 6-8, 1991.

4. Maranuk CA: Development of the Industrys First MWD

Slimhole Resistivity Tool, paper SPE 28427, presented at

the 69th SPE Annual Technical Conference and Exhibition,

New Orleans, Louisiana, USA, September 25-28, 1988.

5. Heysse DR, J ackson CE, Merchant GA, J erabek A, Beste R

and Mumby E: Field Tests of a New 2 MHz Resistivity

Tool for Slimhole Formation-Evaluation While Drilling,

paper SPE 30548, presented at the 76th SPE Annual

Technical Conference and Exhibition, Dallas, Texas, USA,

October 22-25, 1995.


n Compensating forborehole effects. Stan-

dard borehole compen-sation uses a symmetri-

cal arrangement of

transmitters around the

receiver pair (top).

Resistivity measure-

ments from each are

averaged to compensate

for effects such as hole

rugosity or drifts in

receiver electronics. The

ARC5 tool uses mixed

borehole compensation

(MBHC) to achieve the

same effect, but without

the need to duplicatetransmitters (bottom) .

By placing transmitters

asymmetrically around

the receiver pair, various

combinations of mea-

surements may be used.

For example, to achieve

MBHC for the 22-in.

spacing, a combination

of 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 linea r combinations of three sequen-

tiall y 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 M BHC phase shifts and attenuations

are then transformed into five cali brated phase-

shift and five calibrated a ttenuation resistivities

(next page, top).


13/16

Oilfield Review

n ARC5 logs before andafter MBHC. The spiky

appearance of the log

without MBHC (top) is

caused by overshoots

hornsin resistivity

measurements at

washouts. These arti-

facts are canceled out

by MBHC (bottom) .

n Operating frequenciesof Schlumberger resis-tivity tools.

Since the depth of i nvestigation 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 mea surements.

At present, the ARC5 tool communicates to the

surface using the Slim 1 slim and retrievable

MWD system. This is essentially a tool thatlatches 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 oscilla ting

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 areceiver coil set. Measuring the secondary mag-

netic field gi ves a direct me asurement of conduc-

tivity the higher the conductivity, the stronger

the eddy currents, and the la rger the secondary

magnetic field.

Induction tools use a trick to cancel the pri-

mary m agnetic fields flux through the receiving

coil set and allow measurement of the secondary

magnetic fiel d 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 medi um

such as ai r. In a conductive formation, the fl ux

from the secondary ma gnetic field doesnt exactly

cancel, so the induction tool becomes sensitive

to the eddy currents only. If the sam e 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 e nough to ruin

any precise coil positioning.

At 2 MHz, precise coil placement doesnt 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 a ny such effects.

Increasing the frequency further reduces the

depth of investigation and lea ds to dielectric

interpretation i ssues (left) .

16

100

101

102

103

Rps,ohm-m

100

101

102

103

Rps,ohm-m

Without MBHC

With MBHC

PH10

PH22

PH34

2 GH z

200 M Hz

20 MH z

2 M Hz

200 kHz

20 kH z

2 kHz

200 Hz

Propagation dielectric

EP T Electromagnetic Propagation Tool 1.1 GH z

Propagation dielectric resistivity

DP T Deep Propagation Tool 25 M Hz

Propagation resistivity

C DR C ompensated Dual Resistivity tool 2 M Hz

ARC 5 Array Resistivity C ompensated tool 2 M Hz

Induction resistivity

AIT Array Induction Imager Tool 25,50,100 kHz

P hasor Phasor-induction SFL tool 20 and 40 kHz

DIL D ual Induction Resistivity Log 20 kH z

Conduction resistivity

R AB Resistivity-at-the-Bit tool 1.5 kH z

SFL Spherically Focused Resistivity tool 1 kH z

DLL Dual Laterolog Resistivity tool

AR I Azimuthal Resistivity Imager

LL S Laterolog shallow 280 Hz

LLD Laterolog deep 35 Hz


14/16

algorithm that can be implemented in thetool microprocessor, allowing real-time trans-

mission of structural dips (above).

A Profile of Inv a sion

The 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 2

MHz, much higher than the 10- to 100-kHz

frequencies 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 exploit

interpretation methods developed for the

wireline 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 ofRt 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

Depth,ft

70 80 90 100 110

Dip, degrees

0 360

Azimuth, degrees

n Rea l-t im e dip c om-p u ta tion . Dip c a n b ecomp uted f rom theresist ivity ima ge (left)

using a rea l-t im e

algorithm (right).

Results ind icate high

ap pa ren t d ips , near

90. Show n on the

resistivity ima ge is

the compu ted dip

a zimu th, which runsa long the d irection of

the borehole.

respect to north plus the orientation of thebedding plane with respect to the borehole.

For example, if on a trace, a bed appears to

cut 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 0and 80and thethird trace, at 350 and 90. Both give theorientation as 40 providing additional con-fidence in the calculation.To determine the apparent dip, correlation

is made between the three traces. In theabove example, the bed appears on oneside of the hole at 10, 0and 350on eachtrace, respectively. As the distance betweenRAB buttons is fixed, simple geometry canbe used to calculate apparent dip between

any 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 then

dip 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-Compensated

Resistivity 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.


15/16

18 Oilfield Review

XX50 X1000 X1500 X2000 X2500

ARC5 Phase Resistivities

Rps,ohm-m

ARI Resistivities

100

101

102

100

101

102

LLD,LLS,MicroSFL,ohm-m

XX500 X1000 X1500 X2000 X2500

PH10

PH28

PH34

n ARC5 p ha se-shift resistivity com pa rison. Deep ARC5 ph a se-shift resistivity curve s fromthe 34-in. a nd 28-in. spa cing, PH34 a nd PH28 (orange and black curves, top log), correla tewith de ep late rolog rea ding s, LLD, record ed by the ARI tool (orange curve, bottom log)severa l da ys a fter dri ll ing . The sha llowest rea ding ARC5 curve, PH10 (green curve, toplog), correla tes w ith the sh a llow la terolog, LLS (purple curve, bottom log), but rea ds higher tha n the MicroSFL cu rve (green curve, bottom log). This im plies tha t there w a s l it t le inva -sion a t the time of drilling.n Inva sion profile. The ra dia l resistivity ima ge g en era ted from the ARC5 resistivity curve sshow s little inva sion. Light b rown is high re sistivity a nd da rk brown , low resistivity. At

XX500 ft, XX550 ft a nd X2080 ft a re possible sou rces of sea wa ter influx from ne a rby

injec tion w ells.

XX500 X1000 X1500 X2000 X2500

-15

-10

-5

0

5

10Radialre

sistivity,in.

Depth, ft

Ra ide rs of the ARC5

A slim horizontal sidetrack in an offshoreMiddle East well provided a good field testfor the ARC5 tool.11 Oil company objectives

were 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 resistivity

curves, 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 from

ARC5 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


16/16

Ima gine the Future

The ARC5 and RAB tools are part of a newgeneration of LWD resistivity tools capableof producing quality resistivity data for a

wide variety of applications. Both introducemeasurement techniques unique to LWDand wireline logging. For example, MBHCis a cost-effective alternative to doubling upon transmitters for borehole compensation

and 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 further

increased 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

RadialResistivity,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

ROP,min/ft,GR,gapi

gamma ray

time atbottom

R O P

TAB,hr

10

0

Phaseresistivity,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 ResitivityLog Interpretation, The Log Analyst33, no. 2(MarchApril 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 detailed

description: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.

n Drill ing sum m a ry (top), ARC5 p ha se re sistivities (middle) a nd resistivity im a ge (bottom)show n in d eta il for the interva l X1950 ft to X2200 ft. ARC5 da ta (middle) recorded 24 hr

a fter a bit cha nge show increa sed inva sion (interva l A) com pa red to the p revious interva l ,

which w a s logge d only a few hours a fter being d ril led . Litt le inva sion occu rs across a low-

p e rm e a b ility st re a k (in te rva l B). All re sis tiv ity cu rve s con ve rg e (in te rva l C) in d ic a tin g

water 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 to

the oil companyaround 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.

resistivity imaging ii.pdf

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