Oilfield Anisotropy Its Origins and Electrical Characteristics

5/26/2018 Oilfield Anisotropy Its Origins and Electrical Characteristics

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Across the many disciplines of the oil field, anearly universal phenomenon is anisotropythe variation of a property with the directionin which it is measured. Where anisotropyarises, convenient assumptions fall. Seismicreflectors appear at the wrong depth. Seismiclines dont tie. Waterflood programs fail.Induction logs are misinterpreted and mis-take water for pay.

As producers seek a finer comb to draw

through measurements of the earth, they arechanging the status of anisotropy fromunwelcome guest to hard-working collabo-rator. Advances in theory and computing

48 Oilfield Revie

Getting a grip on anisotropy of the earth can mean the difference between success and failure in reservoir

evaluation and development. Accounting for the affects of anisotropy in measurements of the earth begins

with understanding the geologic foundations of anisotropyhow sediments are laid down, converted to roc

and deformed. Here is a summary of some geologic mechanisms for anisotropy, and some recent progress o

anisotropy of electrical properties of rock formations.

power now allow anisotropy to be factoredinto field development decisions. A fullmathematical description of anisotropyremains difficultfor example, representingflow in three dimensions requires six vari-ables.1 But accounting for the simplesteffects of anisotropy is coming within reach.

What is meant by anisotropy depends onwho is talking. Geophysicists generally focuson variation of seismic wavefront velocity or

on the polarization of shear waves. Petro-physicists may measure resistivity anisotropy.Drillers and geologists may think first ofanisotropy in rock strength or stiffness pro-duced by earth stresses. Stratigraphers mayconcentrate on anisotropy of magnetic prop-erties. And reservoir engineers need go togreat lengths to characterize permeabilityanisotropy to plan an optimal productionstrategy (see Case Study: Anisotropy forSteering Horizontal Wells, page 50).

In all geoscience disciplines, however,there are two difficulties in dealing withanisotropy. One is that the conceptual

underpinnings of anisotropy originate fromthe laboratory study of crystalspure,homogeneous materials under pristine, con-trolled conditions. Todays physical modelsof the earth draw much from this work,even though the earth is a composite, het-erogeneous material sampled under any-thing but pristine, controlled conditions.Applying the physics of pure materials tothe earth is like putting an economics pro-fessor to work on the floor of the Tokyo

stock exchange. A smart one will figure ohow to survive, but not without a struggle

Physicists first met this challenge earlythis century when they left the laboratorymake measurements of the earths subsface. They brought with them the mathmatics of materials assumed to isotropichaving properties with the savalue in all directionsand homogeneoFor the most part, the convenient assum

tions held remarkably well. Where tassumptions began to fail, there arose tsecond difficulty with anisotropyif ydont have the tools to deal with anisotrothe temptation is to ignore it or sweepunder the rug.

Assumptions about isotropy began crack as early as the 1930s, when measuments made with electrodes laid in differedirections on the earths surface were seengive different results when strata were dping than when strata were flat.2 In gephysics, the introduction of shear wasources in mid-1970s showed that she

wave anisotropy was often significant acould be analyzed quantitatively (nepage).3 A leap, however, took place in tmid-1980s, when sensors again reclinwith the expansion of horizontal drilling.vertical wells, electrical anisotropy was oft

For help in preparation of this article, thanks to DavidAllen, Schlumberger Wireline & Testing, Sugar Land,Texas, USA; Blent Baygn, Bob Burridge, Darwin Ellis,Charlie Flaum, David Johnson, Bob Kleinberg, StefanLuthi, Bill Murphy, TS Ramakrishnan and Lisa Stewart,Schlumberger-Doll Research, Ridgefield, Connecticut,USA; Stuart Crampin, University of Edinburgh, Edinburgh,Scotland; Gregor Eberli, University of Miami, Miami,Florida, USA; Stan Gianzero, Consultant, Austin, Texas;Roy Nurmi, Schlumberger Wireline & Testing, Dubai,UAE; Jon Roestenburg, Schlumberger Wireline & Testing,

Jakarta, Indonesia; Mike Schoenberg, SchlumbergerCambridge Research, Cambridge, England; Frank Shray,Schlumberger Wireline & Testing, Stavanger, Norway;Julian Singer, ONGC-Schlumberger Wireline ResearchCentre, New Delhi, India; Sven Treitel, Amoco ProductionCompany, Tulsa, Oklahoma, USA; Don Winterstein,Chevron Petroleum Technology Company, La Habra,California, USA; Mark Zoback, Stanford University,Stanford, California.

In this article, CDR (Compensated Dual Resistivity) andRST (Reservoir Satuation Tool) are marks of Schlumberger.

Klaus HelbigConsultantHannover, Germany

Barbara AndersonIan BryantMartin LlingBrian SpiesRidgefield, Connecticut, USA

Oilfield Anisotropy:

Its Origins and Electrical Characteristics

ANISOTROPY

ANISOTROPY

ANISOTROPY


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the vertical direction. This is called tranverse isotropy in the vertical direction aderives from the early days of logging, whanisotropy was observed in vertical wells90 (transverse) to uniform (isotropic) fllying beds.5 Resistivity, for example, woappear to be the same for any wellboazimuth, but be different from the valuethe vertical direction.

A growing usage today, especially amo

geophysicists, is to qualify isotropy wrespect to an axis of symmetry. Transveisotropy in a vertical well that crosses hozontal beds would be transverse isotrowith a vertical axis of symmetry, abbreviaTIV. Properties measured in a horizonwell that crosses a series of vertical fractu

October 1994

observed to be negligible and could beignored. With horizontal boreholes, acousticand electrical anisotropy became obviousand demanded consideration.4 For many,the simple, isotropic days were ending and anew way of thinking was required.

This article gives an overview of the geo-logic basis of anisotropy as it is understoodtoday in the oil field. It begins with a reviewof basic concepts and geologic mechanisms

that produce various types of anisotropy andthen focuses on recent advances in the mea-surement and interpretation of electricalanisotropies. Two crucial and well-charac-terized anisotropiesacoustic/seismic andpermeabilityare detailed in other articlesin this issue (see pages 24 and 36).

What is Anisotropy?

A material is anisotropic if the value of avector measurement of a rock propertyvaries with direction. Anisotropy is typicallyused to describe physical properties, which,for the purposes of geoscience, can be

thought of as parameters intrinsic to thebody of the rock at a given state. Thenotable exception is that anisotropy is oftenused to describe a state of stress, which isnot a property but a condition that results inanisotropy of intrinsic physical properties.

In the simplest form of earth anisotropy, avector measurement has constant magni-tude in any horizontal direction that is dif-ferent from the magnitude of the vector in

1. Each variable here is a scalar, a quantity that has mag-nitude but no direction. For an introduction to theanalysis of anisotropy: Lake LW: The Origins ofAnisotropy, paper SPE 17652,Journal of PetroleumTechnology40 (April 1988): 395-396.

2. Maillet R and Doll HG: Sur un Thorme Relatif auxMilieux Electriquement Anisotropes et ses Applica-tions la Prospection Electrique en Courant Continu,Ergnzungshefte fr angewandte Geophysik3 (1932):109-124.

For a review of resistivity anisotropy: FormationAnisotropy: Reckoning With its Effects,OilfieldReview2, no. 1 (January 1990): 16-23.

3. Crampin S, Evans R, er B, Doyle M, Davis JP,Yegorkina GV and Miller A: Observations of Dila-tancy-induced Polarization Anomalies and EarthquakePrediction, Nature286 (August 1980): 874-877.

A shear wave is like the wave that travels down ajump rope that is attached to a tree at one end and atthe loose end, given a shake either side to side or upand down. Particle motion in a shear wave is perpen-dicular to the direction of wave propagation.

Crampin S, Bush I, Naville C and Taylor DB: Estiming the Internal Structure of Reservoirs with Shear-Wave VSPs, Geophysics: The Leading Edge5, no. (1986): 35-39.

Alford RM: Shear Data in the Presence of AzimuthAnisotropy, Expanded Abstracts, 56th SEG AnnuaInternational Meeting and Exposition, Dallas, TexasUSA (November 2-6, 1986): 476-479.

4. For acoustic anisotropy due to layering in deviatedand horizontal wells: White J: Recent North SeaExperience in Formation Evaluation of HorizontalWells, paper SPE 23114, presented at the SPE Off-shore Europe Conference, Aberdeen, Scotland,September 3-6, 1991.

For a discussion of electrical anisotropy, see refer-ences 27-36.

5. Maillet and Doll, reference 2.

n

How fractures split shear waves.A notable feature of acoustic anisotropy isshear wave splitting, or polarization, typi-cally caused by fractures. If a shear waveof a polarization is not parallel to the strikeof a fracture set, the wave will be split intotwo components as it passes through thefractures. The first, faster component willhave particle motion aligned parallel tothe fracture strike. A second, slower com-

ponent will have a wavefront aligned per-pendicular to the fracture strike.

[From Lynn HB: Seismic Detection of OrientedFractures, Oil & Gas Journal 84, no. 31 (August 4,

1986): 54-55. See also Crampin S: Evaluation of

Anisotropy by Shear-Wave Splitting, Geophysics

50 (January 1985): 142-152; Crampin S:

Anisotropy and Transverse Isotropy, Geophysi-

cal Prospecting 34 (February 1986): 94-99.]

Slowshear,S45E

Fasts

hear,N4

5E

W E

Fractu

res,N

45E


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might have transverse isotropy with a hozontal axis of symmetry, called TIH (spage 37). The nomenclature clings isotropy as the frame of reference, sinceremains far easier to model than anisotroTransverse isotropy is often assumed for siplicity of calculation. Increasingly, somworkers are considering anisotropy with dferent values along all three axes.6

Anisotropy is sometimes confused wheterogeneity. There are two important dtinctions. Foremost, anisotropy is variatiin a vectorial value with direction at opoint, whereas heterogeneity is variationvectorial or scalar values between two more points (next page, top). The secodistinction between anisotropy and hetegeneity is that anisotropy tends to descrmainly physical properties, whereas hetegeneity is typically used to describe poito-point variations in compositions, geomtries or physical properties.

Anisotropy and heterogeneity may coe

ist. There are four possible conditionisotropic and homogeneous, isotropic aheterogeneous, anisotropic and homogneous, and isotropic and heterogeneo(left). Fairly common is the third casanisotropic and homogenous. An examwould be a bed with an acoustic velocvarying in direction at any given po(anisotropy) but the same variation w

50 Oilfield Revie

n

Four possible conditions for isotropy/anisotropy and homogeneity/heterogeneity.Note that what is apparent at one scale may not be apparent at another. For example,when viewed at a large scale, a sample may appear homogeneous and isotropic(lower left), yet at a small scale may be heterogeneous and isotropic (lower right). Here,heterogeneity is expressed as bed boundaries. They may represent differences in com-

position, such as sands and shales, or differences in grain size and packing.

Case Study: Anisotropy for Steering Horizontal Wells

Lake Maracaibo, in western Venezuela, having

produced for more than 35 years, presents the

challenges of a typical mature oil field. Maraven

S.A., the operator of Block IV of the Bachaquero

field, is planning to use horizontal drilling to pro-

duce bypassed oil and thereby increase recovery.

Part of the interval of interest is composed of

elongated sands with laterally discontinuous shale

layers. The lateral extent of these bodies is usu-

ally less than well spacing, so they cannot be

described confidently using well data alone.Instead, scientists from Maraven and Schlum-

berger-Doll Research used a geostatistical method

to derive a model of permeability anisotropy in the

Blent Baygn

Ian Bryant

David J. Rossi

Schlumbe rger- Dol l Resea rch

Ridgefi eld, Connecti cut, USA

Carolina Coll

Helena Gamero de Villarroel

Luis Rondon

Mara ven S. A.

Cara cas, Vene zuela

sands and shales. This provided a means to mo

accurately locate horizontal drainholes.

One example of the modeling uses clay weig

percent, determined from gamma ray, geochem

cal logging, RST Reservoir Saturation Tool me

surements and infrared determination of miner

ogy from cores. Clay weight percent is inverse

related to permeability. If clay weight percent i

assumed to be isotropic, the geological model

comprises bulls-eye patterns, in which red is

high clay volume and blue is low (next page, botom). Bulls-eyes result because the model

assumes clay is evenly (isotropically) distribut

around the wells.

Geostatistical analysis from 31 wells in the

area indicated that the clay weight percent, an

Homogeneous Heterogeneous

Anisotropic

Isotropic


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direction observed at all points (homogene-ity). If different variations were observed atdifferent points, the bed would be bothanisotropic and heterogeneous.

Fundamental to both anisotropy and het-erogeneity is the concept of scale. Whetheranisotropy and heterogeneity are perceiveddepends on sample size and sampling reso-lution. In fact, to say this rock is anisotropic

is almost meaningless unless scale is alsodefinedthat is, both the size of the sampleand the resolution of the sampling method.Anisotropy, for example, can be detectedonly when the observing wavelength islarger than the ordering of elements creatingthe anisotropy.7 A crystal, for example, maybe homogeneous above the molecular scalebut highly anisotropic to larger wavelengthelectromagnetic and sound propagation.Many crystals together may form a homoge-neous rock that is anisotropic if the crystalsare aligned, or isotropic if they are ran-

Anisotropy HeterogeneityVector Vector or scalar

Variation in a vectorialvalue with directionat one point

Variation in vectorial or scalar valuesbetween two or more points

Physical properties Physical properties

Density,dielectric constant,magnetic properties,natural gammaray activity, neutron-capture cross

section, permeability,porosity,resistivity, rock strength,saturation,wave velocity,wettability

Dielectric constant,magnetic properties,permeability, resistivity,

rock strength,thermal conductivity,wave velocity

Composition

Mineralology

Geometry

Grain size, grain shape, sorting,packing, bedding, foliation, folding,

jointing, faultingn

A scheme for classifying anisotropy and heterogeneity in the oilfield. All parameters listed as physical properties can be thoughtof as intrinsic to the fluid-saturated rock, and varying, to onedegree or another, with composition, geometry and state (pressure,

temperature, saturating fluid composition and stress). Physicalproperties listed in bold can be both anisotropic and heteroge-neous. Whether a parameter is a physical property or a state candepend on the time frame in which it is considered. Wettabilitymay have an imposed direction, so when viewed at themicroscale, it could be anisotropic. For example, a flat, solid sur-face with unidirectional scratches has an apparent contact angle(quantification of wettability) dependent on direction. A smalldrop will be elongated, like a rugby ball, instead of spherical.

therefore the horizontal permeability, have a ho

zontal anisotropy of 3:1value in thex plane i

three times that of the y plane. Analysis of

azimuthal variations in permeability indicated

principal axis of the anisotropy is at 340. Fact

ing these data into the reservoir model convert

the bulls-eyes into ellipses, with the main axis

340. The results of this model were transferre

a fluid flow simulator and indicated favorable

locations for horizontal drainholes along the ax

of these elongated sands. Orientation of thesesand bars derived from this geostatistical meth

agrees with that derived from regional well dat

The proof will come from the drilling program,

scheduled to begin in early 1995.

n

A three-dimensional

representation of clay

weight percent, which is

closely related to per-

meability, for Block IV

of the Bachaquero field,

Lake Maracaibo. Red is

high clay volume and

blue is low. The

isotropic model (top)

assumes clay is evenly

distributed around the

well, and results in the

bulls-eye pattern. Withanisotropic interpolation

of the same data (bot-

tom), the bulls-eyes are

converted into more

realistic ellipses, follow-

ing the trend of the

elongated sands.

6. Chemali R, Gianzero S and Su SM: The Effect ofShale Anisotropy on Focused Resistivity Devices,Transactions of the SPWLA 28th Annual LoggingSymposium, London, England, June 29-July 2, 1987,paper H.

For a review that deals with electrical biaxialanisotropy: Nekut AG: Anisotropy Induction Log-ging, Geophysics59 (March 1994): 345-350.

7. Helbig K: Foundations of Anisotropy for ExplorationSeismics, Handbook of Geophysical Exploration Sec-tion I Seismic Exploration, Volume 22. Oxford, Eng-land: Elsevier Science Ltd, 1994.

5000 ft

5000

ft

1000f

t


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Where Does Anisotropy Come From?

Anisotropy in the earth develops durideposition and during processes that taplace after deposition. In clastic sedimenanisotropy can arise during and after depsition.8 In carbonates, anisotropy is cotrolled mostly by fractures and diageneprocesses, and so tends to arise after depotion.9 Anisotropy in carbonates may be p

determined during deposition, as evidencby layering on seismic sections of slodeposits (below). Layering is thought to induced by subtle changes in carbonamineralogy, produced by variation in cbonate balance in the atmosphere awater. Changes in carbonate mineraloresult in changes in both texture and diagnetic potential, and consequently porosand permeability.

For anisotropy to develop during depotion of clastics, there needs to be ordering of sedimentsin essence, somdegree of homogeneity, or uniformity fro

point to point. If a rock were heterogeneoin the five fundamental properties of grains composition, size, shape, oriention and packinganisotropy could ndevelop because there would be no diretionality to the material.10 Anisotropy at tbedding scale that arises during deposititherefore may have two causes. One is peodic layering, usually attributed to changin sediment type, typically producing be

domly packed (above). A series of differentkinds of rock layers, each of which is homo-geneous and isotropic, can be heteroge-neous and anisotropic when viewed as a

unit. This is typically the case for laminatedsand-shale formations, in which each layerhas a thickness of 1 cm [0.4 in] or less. Amore recent insight is that resistivityanisotropy can also come from changes ingrain size that affect saturation distribution,as well as from shaliness. This will be dis-cussed later.

Thus, anisotropy on the scale of metersmay escape detection in core measure-

ments. On the other hand, anisotropy on thescale of centimeters may escape detectionby wireline-conveyed formation testing,which has such a large wavelengthseveral

metersthat it averages out any observableanisotropy. From all measurements, reser-voir engineers are usually after somethingmuch biggernot the permeability of a coreor of a single layer in a well test, but of areservoir compartment, which can be on thescale of square miles. The ultimate goal is toquantify anisotropy at any scale as accu-rately as possible.

n

Scanning electron photomicrographs showing aligned (left) and randomly orientedgrains. In the left image, the alignment is apparent because the solution effect has dis-solved intergranular cement. In the right image, layers are aligned within each kaolin-ite booklet, but booklets are randomly oriented. These are samples of Bassien limestonefrom the Mukta field, offshore Bombay, India.

52 Oilfield Revie

n

Prograding margins of the Great Bahama Banvisible in a seismic section and interpreted refletors. [From Bally AW (ed): Studies in Geology Series #2Atlas of Stratigraphy, Vol. 2. Tulsa, Oklahoma, USA:

AAPG, 1988.]

Courte

syJulianSinger.

20 m 20 m

Two-waytraveltime,sec

Straits of Florida Prograding margin Bimini Bank Straits of Andros Andros Bank

10 km

Florida

Cuba

Gre

at

Baham

a

Bank

WNW E0

0

1.0

1.0


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of varying material or grain size. Anotherresults from the ordering of grains inducedby the directionality of the transportingmedium.11 The cause of this ordering andthe ultimate architect of this deposition-related anisotropy is gravity.

Deposition of clastics always begins withmovement of grains under the influence ofgravity. Whether carried by water or by

wind, grains tend to align in the direction ofleast resistance to the movement of air orwater. At a gravel beach face, for example,repeated washing by waves may formoblong pebbles, and orient them with theirlong axes parallel to the wavefront. This kindof grain alignment can set up a preferentialrock stiffness in one direction and a weak-ness 90 to that direction.12 Eolian sands, forexample, may have a greater grain-to-grainstrength in the downwind direction.13 Thereis also evidence that postdepositional tec-tonic deformation can result in shortening inone direction, changing the azimuthal distri-

bution of grain assemblages.14Under the action of gravity and transport,grains will also undergo sortingseparationby shape, weight or size. On riverbeds, theheaviest minerals concentrate where currentvelocity slows, typically in the troughs ofriverbed dunes. In eolian deposits, differentparts of a migrating dune are associatedwith distinct types of grain alignment andpacking, each giving rise to different perme-ability anisotropies.15 Pryor, at the Univer-sity of Cincinnati, Ohio, USA, observed thatin dunes and beaches, permeabilityincreases with increasing grain size and

decreasing sorting, but in river bars, perme-ability increases with increases in both grainsize and sorting.16 This is thought to be dueto greater irregularity in packing in river barscompared to dunes and beaches.Anisotropy is therefore governed not onlyby variation in the type of material but alsoby variation in its arrangement and grainsize (right).

In all depositional settings, variation intransport energy produces variation in thedegrees of grain orientation, packing andsorting. Because topography varies laterally,

n

Three faces of permeability anisotropy, with three differentcauses. The Mesaverde sandstone has a ratio of horizontal-to-vertical permeability (Kv/Kh) of 170, caused mainly by lamina-tions of shale (light-colored layers) in a sandstone matrix. TheNavajo sample is a clean sandstone with a remarkably high

permeability anisotropy of 276 caused by variation not in com-position but in grain size and packing. Dark bands are tightlypacked fine grains stained by ferric oxide. Light red intervals aremore loosely packed larger grains. The Cutbank sample has a

permeability anisotropy of only 2.7. Here, alternating dark andlight layers are the result of changes in abundances of chert andother silicates. Anisotropy is caused by small but suddenchanges in grain size and therefore sudden changes in porosity.(From Auzerais et al, reference 17.)

8. Clastics are rocks composed of broken fragmentsof other rocks that have been transported somedistance.

9. Diagenesis is all the physical, chemical and bio-logical changes undergone by a sediment afterdeposition.

10. There are many schemes for classification of sedi-ments. This one is from Griffith JC: ScientificMethod in Analysis of Sediments. New York,New York, USA: McGraw-Hill, 1967.

11. Rajan VSV: Discussion of the Origins ofAnisotropy, paper SPE 18394,Journal of PetroleumTechnology40 (July 1988): 905.

12. Lynn HB: Field Measurements of AzimuthalAnisotropy: First 60 Meters, San Francisco Bay Area,CA, and Estimation of the Horizontal StressesRatio from Vs1/Vs2, Geophysics56 (June 1991):822-832.

13. Nelson R: Distribution and Character of Fractures inSedimentary Basins and Their Importance for Hydro-carbon Reservoirs, Expanded Abstracts, 57th SEGAnnual International Meeting and Exposition, Tulsa,Oklahoma, USA (1987): 854.

14. Giesel W: Elastische Anisotropie in TekonischVerformten Sedimentgesteinen, GeophysicalProspecting11 (1963): 423-458.

15. Goggin DJ, Chandler MA, Kocurek G and Lake LWPatterns of Permeability in Eolian Deposits: PageSandstone (Jurassic), Northeastern Arizona, SPEFormation Evaluation3 (June 1988): 297-306.

16. Pryor WA: Permeability-Porosity Patterns and Vation in Some Holocene Sand Bodies, AAPG Bul-letin57, no. 1 (January 1973): 162-189.

October 1994

Mesaverde 255

Navajo 212

Cutbank 192


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so does transport energy. This produces lat-eral gradients in sediment texture, composi-tion and geometry. Over time, stacking of lat-eral gradients produces a vertical gradient.17

Many causes of anisotropy induced afterdeposition are lumped under the heading ofdiagenesisthe physical, chemical or bio-logical alteration of sediment after deposi-tion and during and after lithification.18

Compaction by overburden pressure cancause rotation of grain axes into the hori-zontal plane.19 Compaction and dewateringof muds cause clay platelet alignment thatgives rise to the pronounced anisotropy ofshales (page 40, bottom). Realignment ofgrains may also result from their fracturingor plastic deformation. Grains can alsoundergo significant alteration from pressuresolutiondissolution of grains at their con-tact points, causing flattening of formerlypointy contacts. This rearrangement of grainmaterial results in reduction of pore spaceand welding of grains.20 Pressure solution in

carbonates can develop stylolitestight,usually horizontal sawtooth surfaces thatconsist of the insoluble residue of dissolvedmaterial. Stylolites can act as laterally exten-sive flow barriers and appear as highly con-ductive (dark) layers on resistivity imaginglogs. In sandstones, pressure solution fea-tures are not usually confined to narrowbands, but are dispersed over a larger vol-ume, typically increasing with depth.

The third type of diagenesis, induced byburrowing animals, can take place in eithercarbonates or sandstones, and eitherenhance or undo depositional anisotropy.

For example, burrows can perforate a claylayer, making a former flow barrier perme-able. Burrows may also be confined to analready permeable sand, increasing its per-meability and thereby amplifying the perme-ability contrast between a sand and a shale.

In the evaluation of anisotropy, diageneticchanges cant be ignored because they maysignificantly alter anisotropies establishedduring deposition. For example, the controlof grain orientation and packing over poregeometry, and thereby over permeability,may be destroyed by quartz overgrowth andclays that develop in place (right). This

means that a model of permeabilityanisotropy may be flawed if it is based onlyon the depositional environment. The

exception, however, may be laminatedbeds. Here, diagenetic plugging of poresand pore throats may reduce permeability,but may not alter permeability anisotropy.21

Moving up from the grain-pore scale, thenext level of feature is bedding. A bed is atypically defined as layer thicker than 1 cmthat is distinguishable from layers above andbelow by a break in lithology, a sharp physi-

cal break, or both. Of significant interest tothe reservoir engineer is the effect of beddingscale geometry, namely, a common phe-nomenon called crossbedding (next page).

A crossbed is a single layer or a singlesedimentation unit consisting of internallaminae inclined to the principal surface ofsedimentation.22 Crossbedding is caused bymigration of wave ripples on a sedimentsurface. Settings in which crossbeds developinclude lateral shifting of tidal channels onintertidal flats, channel fill, beach and barmigration and eolian dune migration.

Crossbedding is of interest chiefly because

when all crossbeds in a formation have oneorientation, they have a much greater influ-ence on fluid flow anisotropy than effects atthe pore scale. Permeability is significantlylower across crossbed boundaries thanalong them. This permeability anisotropydoes not arise from variation in material,since crossbedded formations are fairly uni-form lithologically. Instead, it arises fromlarge variation in grain size and thereforelayering of high and low permeability.

The next scale up from bedding is foldingand fracturing. Folding can reorient direc-tional permeability established during depo-

sition and initial diagenesis.23 At the reser-

voir scale, however, the influence of foldon anisotropy may be a second-order effeThe first-order effect on anisotropy is fracting, which can be associated with foldiFractures tend to concentrate near the apof folds, although they may concentraelsewhere, since their distribution is goerned by the strain distribution in the foldFractures are distributed somewhat by t

mechanical properties of the rock, tendto concentrate in low-porosity formatiowhich are more brittle.

Fracturing is a leading contributor anisotropy induced postdepositionalespecially in carbonates. Either open filled with porous breccia, fractures fozones with physical properties sharply dferent from those of the surrounding rocFractures by definition also have a wedefined directionality. These two featutogethercontrasting properties and diretionalitymake them potent generatorsanisotropy at a scale as large as whole res

voirs, or as small as a core plug.Healed, or mineralized, fractures have tsame well-defined directionality as opfractures, but may not contrast as sharpwith properties of the surrounding rock, aso may not generate anisotropy that is pronounced or as detectable. For exampif the mineralizing material has an acousimpedance or resistivity close to that of surrounding rock, the fracture may not pduce detectable anisotropy of acoustic wavelocity or resistivity. Most of the interestanisotropy induced by open fractures ceters on their effect on seismic energy and

fluid flow.25

A Closer Look at Electrical Anisotropy

Of all the investigations into anisotropythe oil field, some of the most intriguirecent work has been in the oldest arenaanisotropyresistivity measurements. Tearliest observations of anisotropy wenoted by discrepancy between surface msurements in different directions. Later, dcrepancies were noted between surface aborehole measurements, and betweinduction and laterolog measurementsSeveral papers have reviewed advances

interpreting anisotropy from wireline restivity measurements.27 Now, much attentis focused on harnessing anisotropy intpreted from logging-while-drilling (LWmeasurements. Here is a brief descriptionelectrical anisotropy and a historical pspective on some recent developments.

In electrical anisotropy, resistivity depenon the direction of current flow in the ro

54 Oilfield Revie

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How diagenesis can affect permeabilityanisotropy. Pore-filling clay, such as thiskaolinite, can destroy the permeability ofthis Bassien limestone.

CourtesyJulianSinger

100 m


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27. See Oilfield Review, reference 2.

Moran JH and Gianzero S: Effects of FormationAnisotropy on Resistivity Logging Measurements,Geophysics44 (July 1979): 1266-1286.

28. To avoid the ambiguity of using the horizon as aframe of reference, vertical resistivity is sometimecalled transverse resistivity and horizontal resistivis called longitudinal resistivity.

29. Polarization horns are large peaks in the resistivitylog that take place at a steeply dipping bed boundary. They are described fully in:Anderson B, Minerbo G, Oristaglio M, Barber T,Freedman B and Shray F: Modeling Electromag-netic Tool Response, Oilfield Review4, no. 3(July 1992): 22-32.

Bonner S, Clark B, Holenka J, Voisin B, Dusang J,Hansen R, White J and Walsgrove T: Logging WDrilling: A Three-Year Perspective, Oilfield Revi4, no. 3 (July 1992): 4-21.

30. Anderson B, Bonner S, Lling MG and Rosthal R:Response of 2-MHz LWD Resistivity and WirelinInduction Tools in Dipping Beds and Laminated Fmations, Transactions of the SPWLA 31st AnnuaLogging Symposium, Lafayette, Louisiana, USA,June 24-27, 1990, paper A.

22. Reineck H-E and Singh IB: Depositional Sedimen-tary EnvironmentsWith References to TerrigenousClastics. New York, New York, USA: Springer-Ver-lag, 1992.

23. Alexander WG: Geologic Controls on Producibilityat Clear Creek Field, Uinta County, Wyoming,paper SPE 15528, presented at the 61st SPE AnnualTechnical Conference and Exhibition, NewOrleans, Louisiana, USA, October 5-8, 1986.

24. Hobbs BE, Means WD and Williams PF: An Outlineof Structural Geology. New York, New York, USA:John Wiley & Sons, Inc., 1978.

25. Sayers CM: Inversion of Ultrasonic Wave VelocityMeasurements to Obtain the Microcrack Orienta-tion Distribution Function in Rocks, Ultrasonics26(1988): 73-77.

Sayers CM: Stress-Induced Ultrasonic Wave Veloc-ity Anisotropy in Fractured Rock, Ultrasonics26(1988): 311-317.

Gibson RL and Toksz MN, Permeability Estima-tion from Velocity Anisotropy in Fractured Rock,Journal of Geophysical Research95, no. B-10(September 1990): 15643-15655.

26. Chemali et al, reference 6.

The effect of anisotropy on resistivity logsdepends on the angle between the boreholeand formation. In a vertical well crossingflat-lying beds, an induction tool measureshorizontal resistivity, the current flowingparallel to bedding. Vertical resistivity, thecurrent flowing normal to bedding, isalways at least as much as, if not more than,horizontal resistivity.28 Therefore the notion

of a single number for true resistivity, Rt,becomes less useful as electrical anisotropyincreases. A more descriptive approach is tothink of two resistivitiesRt, vertical andRt, horizontal. With increasing well devia-tion, the contribution from the vertical com-ponent becomes stronger.

In a horizontal well through flat-lyingbeds, resistivity logs read higher because thecontribution of vertical resistivity reaches amaximum. This, combined with polariza-tion horns, results in correlation difficultiesand overestimation of hydrocarbon satura-tion.29 As long as the resistivity anisotropy

remains incompletely quantified, the loganalyst is working with a resistivity some-where between the horizontal and verticalvalues, so hydrocarbon saturation cannot beprecisely determined. The goal of account-ing for resistivity anisotropy is to obtain acorrect estimate of hydrocarbon saturation.

In one of the earliest papers on LWDresistivity measurements, in 1990, Andersonet al at Schlumberger compared the logresponse of wireline induction and 2-MHzresistivity LWD tools in dipping forma-tions.30 The work focused on developmentof a modeling code to calculate the degree

of LWD signal distortion in thin, dippingbeds. The code modeled the likely CDRCompensated Dual Resistivity response forany dip angle. The modeling code provideda good first approximation, and worked for

October 1994

17. Auzerais FM, Ellis DV, Luthi SM, Dussan V EB andPinoteau BJ: Laboratory Characterization ofAnisotropic Rocks, paper SPE 20602, presented atthe 65th SPE Annual Technical Conference andExhibition, New Orleans, Louisiana, USA, Septem-ber 23-26, 1990.

18. For a current review of diagenesis: Wolf KH andChiligarian GV (eds): Diagensis, IV(Developmentsin Sedimentology 51). New York, New York, USA:Elsevier Science, Inc., 1993.

19. Manrique JF, Kasap E and Georgi DT: Effect of Het-erogeneity and Anisotropy on Probe PermeameterMeasurements, Transactions of the SPWLA 35thAnnual Logging Symposium, Tulsa, Oklahoma,USA, June 19-22, 1994, paper R.

Helbig, reference 7.

20. Taylor JM: Pore-Space Reduction in Sandstones,AAPG Bulletin34, no. 4 (April 1950): 701-716.

21. Evans RC: An Investigation into the Influence ofCommon Sedimentary Structures and Diagenesis onPermeability Heterogeneity and Anisotropy inSelected Sands and Sandstones, unsolicited paperSPE 17130, 1987.

n

Crossbedding ia Devonian fluviasandstone bar, frthe Catskill deltaNew York, USA.

StefanLuthiphoto

TIV isotropic resistivity. The authors showedthat in anisotropic beds logged at dips

above 30, the shallow and deep resistivitymeasurements become a combination ofvertical and horizontal resistivities. At thispoint, LWD resistivities behave differentlyfrom wireline induction resistivity in dipping

laminated or shaly formations: the deep ashallow LWD resistivity measurements ea

respond differently to anisotropy and thefore separate.

This year, a new method was publishthat derives both a vertical and horizonresistivity from the two LWD resistivities a


9/9

the known dip angle, iteratively using thesolution from Moran and Gianzero, thenboth at Schlumberger.31 This gives analyststheir first opportunity to actually calculatethe horizontal and vertical resistivitiesdirectly from log measurementssome-times with surprising results (right). Largeanisotropies (10:1 or greater) have beenobserved, and not just in shales and lami-

nated sand-shale sequences. Some of thelargest anisotropies are showing up in whathave in the past been taken to be clean,homogeneous sands. In these formations,the anisotropy is thought to be due to varia-tion in grain size and the close cousin ofgrain size, irreducible water saturation.

Without this processing, the original LWDresistivities are higher than the vertical wellinduction resistivity. The derived horizontalLWD resistivity, however, reconciles thedirectional well log with the vertical welllog. Because it is the horizontal resistivitythat corresponds to Rt in offset vertical

wells, this horizontal resistivity is used forcorrelation and to keep the borehole trajec-tory in anisotropic pay zones.32 A similarapproach to correct for resistivity anisotropyhas been developed.33

Considering the case of laminated sand-shale resistivity, Hagiwara, now at Hal-liburton, has proposed another similarmethod using the horizontal and verticalresistivities of laminated sand-shalesequences to determine sand resistivity andnet-to-gross ratio, making the simplifyingassumption that the shale laminations areelectrically anisotropic.34

In a parallel development, James Klein atARCO Exploration and Production Technol-ogy has been studying the petrophysicsunderlying electrically anisotropic reser-voirs. Building on work by Chemali and col-leagues at Halliburton, he proposed amethod to disentangle the resistivity of sandlaminates (taken as being isotropic) from theanisotropic contribution of shales in a lami-nated sand-shale sequence. Input parame-ters are the volume of shale laminations, dipangle, and both the horizontal and verticalresistivity of the shale laminations.35

Kleins on-going work, however, extends

the understanding of electrical anisotropybeyond shales and sand-shale sequences.36

He has been investigating the origin of elec-trical anisotropy in clean, seemingly homo-geneous sandstones as observed by Leake,

formerly of Oxy USA, and Shray and Lling,both of Schlumberger. The key insight con-

cerns what appears to be homogeneoussandstone consisting of layers with more orless constant porosity but varying grain sizesand capillarity.37 While this sandstone isisotropic when 100% water saturated, itbecomes anisotropic when desaturated toreservoir conditions.

Klein and collaborator David Allen ofSchlumberger Wireline & Testing in SugarLand, Texas, USA are investigating this the-ory and its implications. Preliminary worksuggests that resistivity anisotropy may berelated to grain-size variation and thereforeto permeability anisotropy. Less dramatic,

but equally illuminating, is that resistivityanisotropy in the water leg will usually bedifferent from that in the oil leg, as well asdifferent in the invaded zone and uninvadedformation. These linkages between resistiv-ity and permeability anisotropies are thefuel for exciting work. Resistivity anisotropy,once a bothersome headache, may one dayprovide clues to deeper understanding ofpermeability. JMK

31. Moran and Gianzero, reference 27.

Lling MG, Rosthal RA and Shray F: Processing Modeling 2-MHz Resistivity Tools in Dipping, Lanated, Anisotropic Formations, Transactions of thSPWLA 35th Annual Logging Symposium, Tulsa,Oklahoma, USA, June 19-22, 1994, paper QQ.

32. Leake J and Shray F: Logging While Drilling KeepHorizontal Well on Small Target, Oil & Gas Jour89 (September 23, 1991): 53-59.

33. Bittar MS and Rodney PF: The Effects of RockAnisotropy on MWD Electromagnetic Wave Resitivity Sensors, Transactions of the SPWLA 35thAnnual Logging Symposium, Tulsa, Oklahoma,USA, June 19-22, 1994, paper PP.

34. Hagiwara T: Response of 2 MHz Resistivity Deviin Thinly Laminated Formations (Anisotropic Restivity and EM Log Interpretation), paper SPE 284presented at the 69th SPE Annual Technical Confence and Exhibition, New Orleans, Louisiana, USSeptember 25-28, 1994.

35. Klein JD: Induction Log Anisotropy Corrections,The Log Analyst34, no. 2 (March-April, 1993):18-27.

36. Klein JD: Saturation Effects on ElectricalAnisotropy, The Log Analyst, submitted for publition.

37. Capillarity is the action by which surface tensiondraws fluid into the interstices of a material.

56 Oilfield Revie

n

Electrical anisoin a horizontal Gu

of Mexico well. Thorizontal and vecal resistivities,derived from CDRmeasurements, sa more pronouncanisotropy in thesand than in theshale. At most, shanisotropy reach2.5, whereas sanresistivity, ranginfrom 2 to 10 ohmhas an anisotropabout 5. The cauresistivity anisotris suspected to be

variation in grainsize, and thereforin irreducible wasaturation.

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Oilfield Anisotropy Its Origins and Electrical Characteristics

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