Triaxial Induction—A New Angle for an Old …...Triaxial induction resistivity is rejuvenating an old measurement. Formation resistivity, the fundamental property log analysts use

64 Oilfield Review

Triaxial Induction—A New Angle for an Old Measurement

Barbara AndersonConsultantCambridge, Massachusetts, USA

Tom BarberRob LeveridgeSugar Land, Texas, USA

Rabi BastiaKamlesh Raj SaxenaAnil Kumar TyagiReliance Industries Limited Mumbai, India

Jean-Baptiste Clavaud Chevron Energy Technology CompanyHouston, Texas

Brian CoffinHighMount Exploration & Production LLCHouston, Texas

Madhumita DasUtkal UniversityBhubaneswar, Orissa, India

Ron HaydenHouston, Texas

Theodore KlimentosMumbai, India

Chanh Cao MinhLuanda, Angola

Stephen WilliamsStatoilHydroStavanger, Norway

For help in preparation of this article, thanks to Frank Shray,Lagos, Nigeria; and Badarinadh Vissapragada, Stavanger.AIT (Array Induction Imager Tool), ECS (Elemental CaptureSpectroscopy Sonde), ELANPlus, FMI (Fullbore FormationMicroImager), MR Scanner, OBMI (Oil-Base MicroImager),OBMI2 (Integrated Dual Oil-Base MicroImagers) and Rt Scanner are marks of Schlumberger.Excel is a mark of Microsoft Corporation.Westcott is a mark of Acme United Corporation.

A new induction resistivity tool provides 3D information about formations far from the

wellbore. It improves the accuracy of resistivity measurements in deviated wells and

in dipping beds, and can measure formation dip magnitude and direction without

having to make contact with the wellbore. The tool’s highly accurate triaxial

resistivity measurement means fewer missed opportunities and better understanding

of the reservoir.

Triaxial induction resistivity is rejuvenating anold measurement. Formation resistivity, thefundamental property log analysts use to evaluateoil and gas wells, was the first measurementacquired with wireline logging tools. As theequipment to provide resistivity measurementsevolved, induction resistivity logging became thestandard measurement technique for acquiringformation resistivity. However, the accuracy oftool response at high resistivities and in deviatedwells or dipping reservoirs was limited by thephysics of the measurement. A new toolovercomes many of the limitations of previousinduction logging techniques. This 3D triaxialinduction measurement enables petrophysiciststo better understand and evaluate the types ofreservoirs where, before the new technology,hydrocarbons could have easily beenunderestimated or overlooked.

The resistivity story began a century ago,when Conrad Schlumberger developed atechnique for measuring the resistivity of thesubsurface layers of the Earth. His experimentsdemonstrated a practical application withcommercial possibilities. The concept waspromising enough that he formed a business

venture to put the technique into practice.1 OnSeptember 5, 1927, with equipment designed andbuilt by Henri-Georges Doll, the first electricallogging experiment, a measurement of formationresistivity, was conducted in a well in thePechelbronn oil region, France’s only large oilfield (next page, bottom).2

The fledgling oil and gas industry adoptedthis electrode-based resistivity measurement,and, with modifications, used it to identifyhydrocarbon deposits. Porous, permeable zoneswith high resistivity indicated the potential foroil or gas; low resistivity suggested the presenceof salt water. Then, in the 1940s, Doll introducedthe principles of induction resistivity logging tothe industry.3 This technique acquired formationresistivity in wells without a conductive path,notably in oil-base mud, overcoming a majorlimitation of electrode-based measurements.

The process of measuring formationresistivity is not as simple as taking a directreading from a tool or a measurement from Point A to Point B; however, in the past half-century, great strides have been made inaccurately measuring this critical parameter.Because induction logging tools provide

1. Gruner Schlumberger A: The Schlumberger Adventure.New York City: Arco Publishing, Inc., 1982.

2. Oristaglio M and Dorozynski A: A Sixth Sense: The Lifeand Science of Henri-Georges Doll Oilfield Pioneer andInventor. Parsippany, New Jersey, USA: The HammerCompany, 2007.

3. Doll HG: “Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil-BasedMuds,” Petroleum Transactions, AIME 1, no. 6 (June 1949): 148–162.

4. For more on induction tool response: Gianzero S andAnderson B: “A New Look at Skin Effect,” The Log Analyst 23, no. 1 (January–February 1982): 20–34.

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apparent formation resistivity by taking ameasurement from a large volume of materialbeyond the borehole, all the components withinthat sensed region influence the final reading.Some of these interactions can negatively impactthe quality and accuracy of the measuredresistivity value.4 This is especially true when thelayers are not perpendicular to the axis of the

tool, as is the case with dipping beds anddeviated wells. Because of the effects of adjacentconductive layers, the resistivity measured byinduction logging tools in dipping beds may beconsiderably lower than the true resistivity,resulting in an underestimate of the hydrocarbonin place. Heterogeneity between the subsurfacestrata, and even within individual layers, alsoaffects tool response.

To account for these and other effects, loganalysts first used manual corrections and laterdeveloped computer-based, forward-modelingand inversion techniques to more closelyapproximate the true formation resistivity.However, they could not resolve all theunknowns—particularly formation dip. Despitethese unresolved errors in the measurement, the

Rh

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> The first resistivity log. The first carottage électrique (electrical coring) from a well in France’s Pechelbronn oil field was recorded on September 5,1927. The equipment to provide this resistivity log was based on tools used for surface mapping. The log is scaled in ohm.m, as are modern resistivitylogs. The high-resistivity interval correlated with a known oil sand in a nearby well, validating the use of log data to evaluate wells.

High resistivity

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industry has successfully discovered much of theworld’s hydrocarbon resources using inductionlogging tools. Unfortunately, some reservoirshave been overlooked or underestimatedbecause of the measurement limitations.

Another difficult formation property forinduction tools to contend with is electricalanisotropy—variations in properties that changewith the direction of the measurement.5

Anisotropy is prevalent in shales as well as in theparallel bedding planes of laminated sand-shalesequences. When the beds are thinner than thevertical resolution of the induction logging tool,the measurement becomes a weighted average ofthe properties of the individual layers,dominated by the elements with the lowestresistivities. This phenomenon may mask thepresence of hydrocarbons.

The effects of anisotropy on the inductionresistivity measurement have been known sincethe 1950s, but until recently there has been noway to resolve the horizontal and verticalcomponents.6 By taking a 3D measurement—inessence a tensor rather than a scalar approach—these types of ambiguities and errors can be fullyresolved. However, sensors with the ability tomeasure induction resistivity in three dimensionsin tensor form had been beyond the limits ofexisting hardware. Similarly, the processingrequired to model and invert the measurementwas extremely time-consuming, even when usingsupercomputers or distributed networks.7

Many of the limitations inherent in inductionlogging have now been overcome with the Rt Scanner triaxial induction service. Currentlyavailable computational-processing power hasbeen combined with a new tool design to createa step change in the evolution of inductionlogging. This new tool is solving problems andproviding the industry with answers to questionsthat have plagued log analysts and geologistsfrom the beginning of well logging.

Three primary applications of triaxial induc -tion tools are accurate resistivity measure mentsin dipping formations, identi fication andquantification of laminated pay intervals and anew structural dip measurement that requires nopad contact. This article describes how thesemeasurements are made and demonstrates theirapplications. Also included are case studies fromAfrica, India and North America.

Induction Resistivity BasicsA two-coil array demonstrates the physics of atraditional uniaxial induction resistivity measure -ment. Alternating current excites a transmittercoil, which then creates an alternating-electromagnetic field in the formation (left).8

This field causes eddy currents to flow in acircular path around the tool. The ground loops ofcurrent are perpendicular to the axis of the tooland concentric with the borehole. They are atleast 90° out of phase with the transmittercurrent, and their magnitude and phase dependon the formation’s conductivity.

The current flowing in the ground loopgenerates its own electromagnetic field, whichthen induces an alternating voltage in thereceiver coil. The received voltage is at least 90°out of phase with the ground loop and more than180° out of phase with the transmitter current.Induction resistivity from the formation is derivedfrom this voltage, referred to as the R-signal.Direct coupling of the tool’s primary transmitter

66 Oilfield Review

> The concept of induction resistivity. The basic physics of the inductionresistivity measurement is represented by a two-coil array. A continuousdistribution of currents, generated by the alternating-electromagnetic fieldof the transmitter (T), flows in the formation beyond the borehole. Theseground loops of current generate electromagnetic fields that are sensed bythe receiver coil (R). A phase-sensitive detector circuit, developed originallyfor land-mine detection during World War II, separates the formation signal(R-signal) from the directly coupled signal coming from the transmitter (X-signal). The R-signal is converted to conductivity, which is then convertedto resistivity. (Adapted with permission from Doll, reference 3.)


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field in the receiver coil, the X-signal, combineswith the formation R-signal; however, the directlycoupled signal is out of phase with thecontribution from the formation. This phasedifference, detected using phase-sensitivecircuitry, permits the rejection of the X-signal andmeasurement of the R-signal.

Conversion of the R-signal voltage toconductivity was first accomplished by equationsbased on the Biot-Savart law, which assumes themajor contribution of a single ground loop willhave a maximum value at the midpoint of thetransmitter and receiver coils.9 Schlumbergermathematicians later developed equations—based on the complete solution for Maxwell’sequations—that provided more accurate measure -ments.10 This solution can be visualized using asimplified version of Maxwell’s equations—theBorn approximation—which is an acceptedmethod of determining the source and location of the formation signal. For the two-coil axial array, the response is essentially a toroid shapesurrounding the tool and perpendicular to its axis,with maximum values near the midpoint of thetransmitter and receiver (right).11

In vertical wells with thick homogeneoushorizontal beds, standard resistivity loggingtools, such as the AIT Array Induction ImagerTool, work reasonably well. These uniaxial toolsmeasure apparent resistivity, Ra, in a horizontalplane, which is equivalent to horizontallymeasured resistivity, Rh. Resistivity measured ina vertical plane, Rv, cannot be measured withuniaxial induction tools in a vertical well.

Because the ground loops of induction toolsintersect a huge volume of the formation, theymay traverse a path that includes several differentlayers with varying electrical proper ties.Anisotropy results in a resistivity measurementthat changes based on the direction of themeasurement. This limitation in the measurementwas one of the factors that led to the developmentof the Rt Scanner tool.

The Impetus for Triaxial MeasurementsAlthough the concepts underlying triaxialinduction measurements first appeared in theliterature in the mid 1960s, the tools to make thismeasurement were not developed. There werethree main reasons for the delay: a triaxial toolcould not be built with the existing technology,the data processing required was beyond thecapability available at the time, and the tool’sresponse to conductive fluids in the borehole could be much larger than the signal from the formation.

Interest in triaxial induction was renewedchiefly because of the recognized limitations ofuniaxial resistivity measurements in two areas:anisotropic reservoirs and bedding planes thatare not perpendicular to the axis of the tool.12

Although both of these limitations wereidentified in the 1950s, there was then no direct

method of measuring anisotropy with aninduction logging tool, and the solution tonegative effects of real or relative dipping bedson induction resistivity was not trivial.13 Astechnology advanced, measurement under -standing, processing power and tool design allplayed key roles in solving for these effects,

5. For more on anisotropy: Anderson B, Bryant I, Lüling M,Spies B and Helbig K: “Oilfield Anisotropy: Its Origins andElectrical Characteristics,” Oilfield Review 6, no. 4 (October 1994): 48–56.Tittman J: “Formation Anisotropy: Reckoning with ItsEffects,” Oilfield Review 2, no. 1 (January 1990): 16–23.

6. Kunz KS and Gianzero S: “Some Effects of FormationAnisotropy on Resistivity Measurements in Boreholes,”Geophysics 23, no. 4 (October 1958): 770–794. Moran JH and Gianzero S: “Effects of FormationAnisotropy on Resistivity-Logging Measurements,” Geophysics 44, no. 7 (July 1979): 1266–1286.

7. Anderson B, Druskin V, Habashy T, Lee P, Lüling M, Barber T, Grove G, Lovell J, Rosthal R, Tabanou J,Kennedy D and Shen L: “New Dimensions in ModelingResistivity,” Oilfield Review 9, no. 1 (Spring 1997): 40–56.

8. For a detailed explanation of induction theory: Moran JHand Kunz KS: “Basic Theory of Induction Logging andApplication to Study of Two-Coil Sondes,” Geophysics 27,no. 6, part I (December 1962): 829–858.

9. The Biot-Savart law describes the magnetic field generated by an electric current.

10. Maxwell’s equations, named for physicist James ClerkMaxwell, are a set of four partial differential equationsthat explain the fundamentals of electric and magneticfield relationships.

11. Habashy T and Anderson B: “Reconciling Differences inDepth of Investigation Between 2-MHz Phase Shift andAttenuation Resistivity Measurements,” Transactions ofthe SPWLA 32nd Annual Logging Symposium, Midland,Texas, June 16–19, 1991, paper E.

12. Moran and Gianzero, reference 6.13. For the theoretical solution to Maxwell’s equations as

applied to induction logging: Moran and Kunz,reference 8. Anderson B, Safinya KA and Habashy T: “Effects of Dipping Beds on the Response of Induction Tools,” paper SPE 15488, presented at the SPE Annual Technical Conference and Exhibition, New Orleans,October 5–8, 1986.

> Born approximation for a uniaxial induction logging tool. The sensed regionfor uniaxial induction tools is a toroid shape (red), perpendicular to the tool.The maxima are located approximately at the midpoint between the transmitter(T) and receiver (R). This rendering shows the Born approximation of the fullsolution to Maxwell’s equations. The shape is valid for thick beds andhomogeneous, isotropic formations. This region sampled by the uniaxialinduction tool corresponds to only one of the nine modes measured by thetriaxial Rt Scanner tool.

T

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ultimately resulting in the development of atriaxial induction tool (below left).

Developing such a tool involved understandingthe effects of the borehole on the measurement.14

There is a great sensitivity to eccentricity in theborehole: the more conductive the mud, thegreater the effect. The sensitivity results in theformation signal being overwhelmed by theborehole signal. This situation, the effects ofwhich can be two orders of magnitude greater fortriaxial tools than for uniaxial induction tools,would have been an insurmountable obstaclewithout intensive computer modeling.

Iterative modeling allowed various triaxialtool designs to be tested without having to buildand test physical tools. Final tool design includeda sleeve with electrodes connected to aconductive copper mandrel. This configurationreturned the borehole currents through the tool,reducing the large signals caused by thetransverse eccentricity to a level equivalent tothat of the AIT tool. The correction for boreholeeffects could then be handled in a mannersimilar to that used for the AIT measurement.15

After engineers solved for borehole effects,tool response to various geometrical scenarioswas investigated. For most of their history,induction measurements have had to contendwith geometry, both in the borehole and in the

formation. Geometry was regarded by inter -preters as a major nuisance or, at best, somethingto be coped with.16 However, after the AIT tool’sresponse was modeled, tool designers discoveredthat the formation-geometry effects are thestrongest contributor to the induction signal.When properly resolved and modeled, geometrynow provided a key to accurate measurement offormation resistivity. In addition, dipping beds—those that are not perpendicular to the axis ofthe logging tool—could be properly measured.

Dipping beds are the result of geologicaltilting of formations, deviation of the wellboretrajectory from vertical, or combinations of both.Fast analytical codes, developed in the 1980s,estimate resistivity in dipping beds using datafrom uniaxial induction tools, but the processing

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> Rt Scanner triaxial induction service. The RtScanner tool comprises a triaxial transmitter,three short-spacing axial receivers for boreholecorrections and six triaxial receivers. Electrodeson the tool and the Rm sensor in the bottom nose,which measures the mud resistivity, are alsoused for borehole corrections. An internal metalmandrel (not visible in the drawing) provides aconductive path for borehole currents to returnthrough the electrodes on the exterior of the tool.

Electronics housing

Triaxial transmitter

Three short uniaxialreceivers for boreholecorrection

Six triaxial receivers

Metal mandrel

Sleeve with shortelectrodes

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Electrode > Three-dimensional arrays. The Rt Scanner service produces a nine-elementarray for each transmitter and receiver pair. Traditional induction measurementsare made by passing current through coils that are wrapped around the axisof the tool, also called the z-axis (blue), which induces current to flow in theformation concentrically around the tool. Triaxial induction tools also includecoils that are wrapped around the x-axis (red) and y-axis (green), whichcreate currents that flow in planes along the tool’s x- and y-axes. The x, y andz components of the transmitter couple with the x, y and z receivers. Forvertical wells with horizontal beds, only the xx, yy and zz couplings respond tothe conductivity (σ) of the formation. In deviated wells or wells with dippingbeds, all nine components of the array are needed to fully resolve theresistivity measurement. The multiple triaxial transmitter and receiver pairsgenerate 234 conductivity measurements for each depth frame.

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relies on inputs from other sources.17

Unfortunately, the uniaxial measurement maybecome unreliable or provide nonuniquesolutions when external data sources are used.

All these issues posed problems for uniaxialinduction tools. In most cases, there was notenough information to fully correct the data.Triaxial induction tools, however, make thenecessary measurements to resolve the ambi -guities and properly measure the resistivity ofanisotropic reservoirs, correct for nonuniformfiltrate invasion, correct for the effects of dipping beds and deal with geometrical effectson the measurement.18

Triaxial Resistivity Theory Previous induction logging tools, such as thosefrom the AIT family, measure horizontalresistivity (uniaxially). The Rt Scanner toolmeasures in three dimensions (triaxially).Although the physics of measurement aresimilar, triaxial tools are much more complex(previous page, bottom right).

The Rt Scanner tool consists of a collocatedtriaxial transmitter array, three short axial

receivers and three collocated triaxial receiverarrays. The triaxial transmitter coil generatesthree directional magnetic moments in the x, yand z directions. Each triaxial receiver array hasa directly coupled term and two terms cross-coupled with the transmitter coils in the otherdirections. This arrangement provides nineterms in a 3x3 voltage tensor array for any givenmeasurement. All nine couplings are measuredsimultaneously. An advanced inversiontechnique extracts resistivity anisotropy, bed-boundary positions and relative dip from thetensor voltage matrix. The receiver arrays arelocated at different spacings to provide multipledepths of investigation.

The Born approximation for the triaxialinduction tool’s response provides a graphicalrepresentation for the solution of the equationsrepresenting the sensed region (above). Theuniaxial induction tool’s response was shownearlier to have a single toroid shape; the triaxialtool delivers nine responses superimposed oneach other. The zz term from the Rt Scanner toolis essentially the same response as thatmeasured by the uniaxial induction tool.

Collocation of the coils is an importantfeature of the Rt Scanner tool: when thetransmitter or receivers are not at the sameposition, the spacings for the cross-terms will bedifferent from those of the direct terms. Becausethe entire ensemble of measurements is madewithin a single depth frame, no measurements

14. Rosthal R, Barber T, Bonner S, Chen K-C, Davydycheva S,Hazen G, Homan D, Kibbe C, Minerbo G, Schlein R, Villegas L, Wang H and Zhou F: “Field Test Results of anExperimental Fully-Triaxial Induction Tool,” Transactionsof the SPWLA 17th Annual Logging Symposium, Galveston, Texas, June 22–25, 2003, paper QQ.

15. For details on Rt Scanner design and modeling: Barber T, Anderson B, Abubakar A, Broussard T, Chen K-C, Davydycheva S, Druskin V, Habashy T, Homan D, Minerbo G, Rosthal R, Schlein R and Wang H:“Determining Formation Resistivity Anisotropy in thePresence of Invasion,” paper SPE 90526, presented atthe SPE Annual Technical Conference and Exhibition,Houston, September 26–29, 2004.

16. Moran and Gianzero, reference 6.17. Barber TD, Broussard T, Minerbo G, Sijercic Z and

Murgatroyd D: “Interpretation of Multiarray Logs inInvaded Formations at High Relative Dip Angles,” TheLog Analyst 40, no. 3 (May–June 1999): 202–217.

18. During the drilling process, fluids from the drilling mudleave the wellbore and enter permeable formations. Themud filtrate alters the electrical characteristics of theformation around the wellbore. The depth of filtrate inva-sion, and its associated geometry, may be unpredictable.

> Born approximation for a triaxial induction tensor voltage array. The Born response function for a triaxial induction tool ismuch more complex than that for a uniaxial induction tool. There are nine elements, one for each component of the tensorvoltage array. Each transmitter-receiver pair has positive (red) and negative (blue) responses. The surfaces represent theregions where 90% of the signal measured by the receiver coil originates. Each of the nine components is superimposed atthe measure point of the tool. The xx, yy and zz elements are derived from the direct coupling of a triaxial transmitter and itsassociated triaxial receiver. The other six elements represent cross-coil responses. The zz response (bottom right ) is theonly one measured by the simpler uniaxial induction tool.

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have to be depth-shifted to form the measure -ment tensors. When all nine components are atthe same spacing and location, the matrix can bemathematically rotated to solve for relativeformation dip. A change from one coordinatesystem to another is also greatly simplifiedbecause it involves a simple transformation, andall measurements are made along the samecoordinate system as well as at the same depth.Collocation is especially important when beddingplanes are not perpendicular to the relativeposition of the tool.

Power in the ProcessingCollocated orthogonal transmitter and receiverpairs made the triaxial resistivity measurementfeasible, but advancement in processing powerwas the enabler that spurred the development ofthe tool. Even in the late 1990s, triaxial inductionwas referred to as a theoretical concept, prima -r ily because the computing power needed tomodel and develop fast processing codes was notreadily available.19 Moore’s law, the observationthat computing power doubles every two years, isevidenced in the progression that has occurredwith induction resistivity logging.

The first induction resistivity tools convertedconductivity measured downhole to an analogvoltage that was measured at the surface. The loganalyst read the resistivity from the logs andapplied corrections from charts to account forthe effects of adjacent beds and filtrate invasion,generally ignoring borehole effects. Boreholecorrection charts were then developed based ongeometrical-factor curves obtained from labora -tory measurements made in plastic pipesimmersed in waters of varying salinity.20 In themid 1980s, these empirically derived charts werereproduced using computer modeling.

70 Oilfield Review

0–2,500 –2,000

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0 500 1,000 1,500 2,000 –2,500 –2,000 –1,500 –1,000 500 0 500 1,000 1,500 2,000 –2,500 –2,000 –1,500 –1,000 500 0 500 1,000 1,500 2,000

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> Modeling the triaxial induction response. A 1D horizontally layered,transversely isotropic (TI) model was used to validate the triaxial inductionresponse to known conditions (bottom right ). The five layers used in themodel consist of two low-resistivity homogeneous layers, a high-resistivityhomogeneous layer, and two anisotropic layers with high- and low-contrast beds. The first measurement is conducted with a vertical tool inhorizontal beds (top left ). The zz (blue) and yy (green) components react tothe resistivity of the beds, but the xx and all cross-components are zero.Prior to inversion, none of the curves indicates the correct horizontal (pinkdash) and vertical (black dash) conductivity. Next, the model well isdeviated 75° (Θ) and the tool position is rotated 30° (Φ) from the high sideof the wellbore. All nine components become active (center) and nonereads the same as the vertical model. The zz (blue) componentcorresponds to a uniaxial induction measurement, and although it is similarto the curve in the vertical response model, the curve’s shape andamplitude have changed. The data are then rotated mathematically (topright ) to zero the yx and yz (green dash) cross-coil contributions. Theangle of rotation required to zero these components corresponds to therelative dip of the beds. Finally, the data are inverted, correcting for bedthickness and deviation, and converted from conductivity to resistivity(bottom left ). In the three lower layers, which are homogeneous, Rv (blue)and Rh (red) are equal and match the input resistivity. In the laminatedlayers, the curves separate as a result of anisotropy.


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The manual process of correcting inductionlog data was carried out sequentially: applyborehole corrections, correct for shoulder-bedeffects and correct for invasion. With the advent ofdata recorders, log data could be processed usingcomputers. Codes were developed to perform 1Dcorrections automatically, first at mainframe-equipped computing centers and then asprocessing power continued to grow, at thewellsite using computer-equipped logging units.

Advances in computer technology renderedthe manual corrections obsolete, but there was aproblem in the methodology. The codes weredeveloped assuming horizontal, homogeneousbeds, and corrections were applied with thesame linear approach used by log analysts.However, the ground loops produced by inductiontools intersect and interact with all the mediathey come into contact with in a complex,nonlinear fashion.21 The sequential approach,used for decades, was found to be inadequate.

This situation was improved when fast 2Dasymmetric forward-modeling codes weredeveloped in the mid 1980s. They revealed justhow inaccurate sequential chartbook correc tionswere for determining the true resistivity, Rt,especially in thin beds invaded by mud filtrate.Development of the AIT tool was a result oflessons learned from those models. Since then,various techniques have been applied to obtainRt, including iterative forward modeling andinversion.22 Models have been developed thatinclude 1D corrections as well as corrections forinvasion and nonhorizontal bedding (2D) and

nonlinear invasion in tilted reservoirs (3D). Onlyrecently has advanced computer-processingpower enabled inversion codes that fully correctthe induction measurement. These codes allowsimulations to be run in hours instead of weeks.If Moore’s law holds true, hours for processinginduction measurements will eventually bereduced to seconds.

Induction resistivity data, acquired with atriaxial tool, could now be processed in areasonable time frame. All the pieces of thepuzzle were available; the next step was to putthe triaxial tool to the test.

Testing the CodeTo test the validity of the acquisition andinversion algorithm for triaxial induction data, a1D horizontally layered, transversely isotropic(TI) model was constructed (previous page).Five layers simulated a complex reservoircomprising two low-resistivity sands, a high-resistivity sand, an anisotropic low-resistivityshale and a laminated sand-shale sequence.

This simulated reservoir included featuresthat present limitations for uniaxial resistivitytools. The testing proved that a triaxial resistivity measurement overcomes theselimitations and provides accurate resistivity inchallenging environments.

The outputs of the processing are trueresistivity corrected for dip in the nonlaminatedlayers and a shale-affected resistivity inlaminated layers. Rv is provided from theprocessing, although it is equivalent to Rh in theisotropic intervals.

For the two laminated layers, Rv and Rh arenot equal, and the curves have separation basedon the degree of anisotropy. Neither Rh nor Rv

provides the true resistivity of the modeledreservoir in the case of laminated sections, buttechniques have been developed to provide theresistivity of the sand layers.

True ResistivityThe true resistivity of a formation, Rt, is acharacteristic of an undisturbed, or virgin,region. Much study and research have beencarried out in the name of acquiring this elusivemeasurement. The measurement of inductionresistivity in a virgin zone is predicated on somedegree of homogeneity, consistent perpendicularbeds and isotropic reservoirs. In nature, this israrely the case.

The concept of vertical and horizontalresistivities evolved early in the development ofelectrical logging. Measured apparent resis tivity,Ra, of stacked rock layers differs with changes inthe measurement direction. If the measurementis made parallel to the layers, the result issimilar to measuring resistors in parallel—thelowest resistances dominate (above). For aparallel resistor circuit, more current flowsthrough the smaller resistors, and each resistor

19. Anderson BI: Modeling and Inversion Methods for theInterpretation of Resistivity Logging Tool Response. Delft,The Netherlands: Delft University Press, 2001.

20. Moran and Kunz, reference 8. 21. Anderson, reference 19.22. Howard AQ: “A New Invasion Model for Resistivity Log

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

> Direction matters. Under the right conditions, the deep-induction response to a homogeneous, isotropic bed (left ) is the same as that to an anisotropic,laminated bed (center). This occurs when beds are thinner than the vertical resolution of the measurement. For the 90-in. deep-induction array, thevertical resolution is 1 to 4 ft [0.3 to 1.2 m]. Horizontal resistivity (Rh) measurements are analogous to parallel resistor circuits, so the resistivity value of thelaminated bed is primarily influenced by the layer with the lowest resistivity, Rshale. With standard induction tools, hydrocarbon-bearing sand layers caneasily be overlooked. Vertical resistivity (Rv) is analogous to a series resistor circuit (right ), and its value is dominated by the layer with the highestresistivity. A large difference between Rv and Rh indicates anisotropy.

1,800

Depthft

Computed Deep Induction

ohm.m0.2 2,000

1,810

1,820

1,830

1,840

1,800

Depthft

1,810

1,820

1,830

1,840

Computed Deep Induction

Model Rt Profile Model Rt Profile Model Rh-Rv Profile

Rh Rv

ohm.m0.2 2,000

Horizontal Resistivity, Rh

Vertical Resistivity, Rv

ohm.m0.2 2,000R

sandR

shale

Rsand

Rshale

Rshale

Rsand

Rsand


divides the current according to the reciprocalof its resistance.

When the measurement is made across thestack, the measured resistance is similar tomeasuring resistors in series. In an electricalseries circuit, the resistance values are addedtogether. Higher resistance, which is the case forthe layers containing hydrocarbon, is dominant.

The concept that the measured resistancedepends on the direction in which it is made isreferred to as electrical anisotropy. Since welllogging began in vertical wells with stacks ofmore or less horizontal layers, the resistivityparallel to the layers was called the horizontalresistivity, Rh, and the resistivity measuredacross the layers was called the verticalresistivity, or Rv. In an isotropic, thick sand Rh =Ra = Rv. If, however, the thickness of the beddinglayers is less than the tool’s vertical resolution,the Rh measurement is analogous to the parallelelectrical circuit.

Most of the technology for determiningformation resistivity measured the horizontalcomponent, giving rise to difficulties inevaluating thin layers comprising shale andhydrocarbon-bearing sands. For a uniaxialinduction measurement the formation currentsflow in horizontal loops, and the resultingsensitivity is to the horizontal resistivity. Formost laminated reservoirs, Rh ≠ Rv. Based on theparallel circuit analogy, Ra will be similar invalue to that of the layer with the lowerresistivity, usually the shale. Therein lies theproblem with interpreting induction resistivity inlaminated reservoirs: the dominant nature of theless-resistive layers masks the more-resistivelayers that may have hydrocarbon potential. Theresult is that pay zones may be overlooked orunderestimated.23 The Rv /Rh ratio is a usefulmeasurement for determining the level ofanisotropy, and when the ratio is higher than 5, it alerts the log analyst to look for potentiallaminated-pay reservoirs.

For a laminated sand-shale sequence, theportion of the reservoir that is of interest is thesand. Although Rv does not provide the actualresistivity of the hydrocarbon-bearing sand layer,Rsand, it can be combined with othermeasurements to derive it. The shale effectsmust be removed from the volumetricmeasurement to obtain the resistivity of the sandlayers (above). Calculating Rsand from Rh and Rv

requires a secondary source to determine thevolume of shale before its effects can beeliminated. Shale volume can be obtained fromseveral sources, including the ECS ElementalCapture Spectroscopy sonde. Once determined,Rsand can be used to calculate water saturation,Sw, using Archie’s equation. The full derivation ofthe formula for Rsand and Sw in the presence ofanisotropy can be found in the literature.24

72 Oilfield Review

> Hidden saturation. Rh and Rv are outputs from the Rt Scanner tool. The resistivity of the sand layers can beresolved from these measurements in combination with fractional volumes of sand and shale. For this example,the conventional induction tool would have measured Rh = 2.3 ohm.m. Rv from the triaxial induction measurementis 12.8 ohm.m. The volume fractions, Fshale and Fsand, could come from an ECS Elemental Capture Spectroscopytool. Because shales often exhibit anisotropy without the presence of sand laminations, two different shalevalues are used in this example: vertical Rshale-v is 2 ohm.m and horizontal Rshale-h is 1 ohm.m. These values shouldbe determined within an anisotropic shale interval. This method gives an Rv /Rh ratio in the shale of 2, comparedwith the 5.6 ratio of the entire sand-shale sequence. Solving the equations (right ) for Rsand yields a value of 20 ohm.m.The 2.3 ohm.m measured by a conventional induction tool would considerably underestimate the hydrocarbon volume.

Rsand

Rsand

Rsand

Rshale-h

Rsand

Rshale-h

Rshale-h

Rshale-v

Rshale-v

Rshale-v

Rsand

Rsand

Rshale–h = 1 ohm.m

Rshale–v = 2 ohm.m

Rv = 12.8 ohm.m

Rh = 2.3 ohm.m

1

Rh

= +Fsand

Rsand

Fshale

Rshale-h

Rv = +x xFsand Rsand

Fshale = 40%

Fsand = 60%

Rsand = 20 ohm.m

Fshale Rshale-v


Summer 2008 73

The calculation of Rsand and Sw in the sandfraction is typically carried out usingpetrophysical analysis software. However, Excelspreadsheets have been developed to manuallyconvert Rv and Rh to water saturation, Sw.25

The two major limitations of uniaxialinduction tools, incorrect resistivity in dippingbeds and anisotropy effects, are overcome by thetriaxial induction measurement. More accurateresistivity leads to more accurate Sw, whichenables petrophysicists to correctly evaluatehydrocarbon reservoirs. Properly characterizinglaminated sands means fewer missed low-resistivity reservoirs. True resistivity in deviatedwells and dipping beds means more accuratevolumetric analysis. Ultimately, more oil and gascan be discovered and produced from reservoirs.The following case studies demonstrate howtriaxial resistivity measurements are used toevaluate difficult-to-interpret oil and gas wells.

True Resistivity in Deviated WellsAn AIT tool was run offshore Angola in a well thatwas deviated 60°. The formations encounteredincluded two 30-ft [10-m] sands with highresistivity. A 30-ft interval is generally within thevertical resolution of this uniaxial tool andtherefore should provide a reasonable Rt readingfrom the deepest induction measurement, the90-in. array. However, because of effects of welldeviation on the measurement, the 90-in.resistivity reading was lower than the actual Rt.

An Rt Scanner tool was then run over thesame interval. Inverting the data and correctingfor the effects of dip produced resistivity valuesthat were more accurate than those of the AITtool (above right). The corrected resistivity fromthe Rt Scanner tool is five times greater than thedeep resistivity value from the AIT tool.

Although the water saturation calculatedwith the resistivity from either tool wouldindicate the presence of hydrocarbons, reservecalculations would be substantially different.Higher hydrocarbon saturations and volumescalculated using outputs from the Rt Scannertool would affect production-facility design, long-term infrastructure planning and recoverabilitydecisions in the event of secondary and tertiaryrecovery programs. Having an accurate Rt valuehas enormous implications, especially formarginal reservoirs, where critical go/no-godecisions based on less-accurate data wouldunderestimate hydrocarbon in place.

An additional consideration is that the cost ofdrilling deepwater prospects has limited thenumber of wells that can be drilled to evaluate aprospective reservoir. Petrophysicists and

geologists must construct reservoir models withsurface-acquired data validated by fewer actualwells. It is absolutely crucial that these modelsbe calibrated to the most accurate dataavailable, because the luxury of drilling step-outand infield wells to refine the models isprohibitively expensive. It is more cost-effectiveto use accurate triaxial induction resistivity datacorrected for dip and deviation to improvereservoir understanding with the very first well.

Anisotropy in Deepwater Turbidites E&P companies cannot afford to underestimatereserves or miss opportunities. Unfortunately,laminated sand-shale sequences have been

overlooked because of the effects of anisotropy.Examples of laminated reservoirs are turbiditeand fluvial deltaic sediments. The term “low-resistivity pay” has been applied to these types of environments.

Anisotropy-related suppression of theresistivity values measured by traditionalinduction logging tools is the predominantreason for the low resistivity. But even whenthese reservoirs are correctly identified, they aredifficult to evaluate. In practical terms, usingconventional resistivity measurements to calcu -late hydrocarbon reserves may result inunderestimates of more than 60% compared withanalysis using Rv and Rh.26 The Krishna-Godavari

23. Boyd A, Darling H, Tabanou J, Davis B, Lyon B, Flaum C,Klein J, Sneider RM, Sibbit A and Singer J: “The Lowdown on Low-Resistivity Pay,” Oilfield Review 7, no. 3 (Autumn 1995): 4–18.

24. Clavaud JB, Nelson R, Guru UK and Wang H: “FieldExample of Enhanced Hydrocarbon Estimation in ThinlyLaminated Formation with a Triaxial Array Induction Tool:A Laminated Sand-Shale Analysis with AnisotropicShale,” Transactions of the SPWLA 46th Annual LoggingSymposium, New Orleans, June 26–29, 2005, paper WW.

25. Clavaud et al, reference 24.26. Saxena K, Tyagi A, Klimentos T, Morriss C and Mathew A:

“Evaluating Deepwater Thin-Bedded Reservoirs with Rt Scanner,” presented at the 4th PetroMin Deepwaterand Subsea Conference, Kuala Lumpur, June 20–21, 2006.

> Correcting induction resistivity for deviation. Correct resistivity is a critical parameter for accuratecalculation of hydrocarbon in place. This 60° deviated well has two hydrocarbon-bearing zones ofhigh resistivity. The AIT resistivity (Track 2, green dash) from the 90-in. induction array measures 100 ohm.m in the upper lobe (X,940 to X,990) and as low as 20 ohm.m in the lower lobe (Y,000 to Y,050).After dip correction, the resistivity values from the Rt Scanner tool (Track 3, red) are higher:approximately 500 ohm.m in the upper sand and 100 ohm.m in the lower section. In the lower 100 ft(Y,100 to Y,200), Rh (Track 3, blue) is significantly less than Rv (red), indicating anisotropy. This anisotropy(yellow shading) suggests a potential laminated sand-shale sequence; further analysis of this intervalmay reveal additional hydrocarbon potential.

Y,200

Depthft

Gamma Ray

0 gAPI 150

ohm.m ohm.m

AIT Resistivity

Rt Scanner Resistivity

X,9001 10 100 1,000 1 10 100 1,000

Y,000

Y,100

Caliper

6 in. 16

10-in. array

20-in. array

30-in. array

60-in. array

90-in. array

90-in. array

Rh

Rv


basin, off the east coast of India, is a deepwaterexample of a thin sand-shale turbidite sequence(above). Reliance Industries experienced initialsuccess in the area, but evaluating the reservoirpotential in the presence of anisotropy made insitu hydrocarbon volume difficult to quantify.

Thin beds, by definition, are reservoir layersthat are thinner than the vertical resolution ofthe tool. The thicknesses of the sand-shale-siltsequences of the Krishna-Godavari basin were inthe millimeter range, well below the minimum 1-ft [0.3-m] resolution available from inductiontools, and even less than the 1.2-in. [3-cm]vertical resolution of porosity devices. Logs

acquired using conventional tools did not provideenough information to evaluate the anisotropiczones (above right). The interval above X,X65 m,where cleaner, productive sandstone sectionsend, has resistivity values of 1 to 2 ohm.m. Withsuch low resistivity, hydro carbon productionwould not be expected.

74 Oilfield Review

> Krishna-Godavari basin off the east coast ofIndia. The KG-1 well is located in the KG-DWN-98/3 block. The laminations in this core example(above) are about a millimeter [0.04 in.] thick,typical of the turbidite sequences found in theKrishna-Godavari basin. The minimum verticalresolution for induction tools is 0.3 m. Evaluationand calculation of recoverable hydrocarbon aredifficult because of the low-resistivity, anisotropicnature of the reservoir.

INDIA

PAKISTAN

AFGHANISTAN C H I N A

SRI LANKA

KG-DWN-98/3

> Underestimated reserves. Typical of logs run in the field, the ELANPlus analysis calculateshydrocarbon (Track 5, red) in the sands (Track 6, yellow), but the volumes are low, considering thenet footage. Above X,X65 m the water saturation and hydrocarbon volumes indicate little oil or gaswould be produced. But, this zone is known to be a laminated sand-shale turbidite sequence. Atriaxial induction tool can help determine the degree of anisotropy and the hydrocarbon potential.

X,X45

Depth

m

Sigma

Resistivity

0.2 ohm.m 1000 cu 50

0 gAPI 150

6 in. 16

Sw

EffectivePorosity

X,X50

X,X55

X,X60

X,X65

X,X70

X,X75

X,X80

90-in. Array

Gamma Ray

Caliper

0.2 ohm.m 100

60-in. Array

0.2 ohm.m 100

30-in. Array

60 % 0

Neutron Porosity

60 % 0

Crossplot Porosity

1.65 g/cm3 2.65

Bulk Density

0.2 ohm.m 100

20-in. Array

0.2 ohm.m 100

10-in. Array

Crossover Hydro-carbon

Montmorillonite

Bound Water

Quartz

Gas

Water

100100

50 0%

%%

Lithology

00

Anisotropiczone


Summer 2008 75

For its KG-1 well, Reliance acquired high-resolution log suites and OBMI Oil-BaseMicroImager data (below). The OBMI imagesrevealed thin laminations, corroborated by thecore. A synthetic resistivity log was generatedfrom the high-resolution OBMI data, which

indicated anisotropy. The AIT resistivitymeasurement was 1 to 2 ohm.m. The Rt Scannertool was added to the logging program because ofthe low AIT resistivity measurements in thelaminated reservoir.

The log data from the Rt Scanner toolindicated a high degree of anisotropy in thereservoir and provided an accurate measurementof sand resistivity. Several promising zones,denoted by an Rv /Rh ratio greater than 5, wereidentified as areas for further evaluation. In the

> Logs and core from the KG-1 well. The core at right shows fine laminations, which can be seen on the OBMI image (Track 4). All fiveAIT curves (Track 2) overlay, but the spiky nature of the reconstructed resistivity from the OBMI data (green) indicates laminations. Thisis because the OBMI tool has better vertical resolution. Curves from the density-neutron tools (Track 3) are separated over most of theinterval, indicating high shale content. There are a few places where the density and neutron cross (yellow shading), indicating thepossibility of light oil or gas, but these zones are less than a meter [3 ft] thick. Low resistivity measurements from the AIT tool and littlesand content would result in a pessimistic evaluation of hydrocarbon production in this interval.

in. m

Bit Size Depth

6 16

in.

Caliper

6 16

cu

Formation Sigma

0 50

%

Neutron Porosity

60 0

g/cm3

Bulk Density

OBMI Image

Conductive Resistive

0° 360°240°120°

1.65 2.65

gAPI

Gamma Ray

0 150

ohm.m

OBMI Data

Resistivity

0.2 200

ohm.m

90-in. Array

0.2 200

ohm.m

60-in. Array

0.2 200

ohm.m

30-in. Array

0.2 200

ohm.m

20-in. Array

0.2 200

ohm.m

10-in. Array

0.2 200

73

74

75

76

77

78

79

Crossover


KG-1 well, zones where the Rv /Rh ratio is below 5lack laminations. Corroboration by core datavalidated the Rt Scanner measurement (above).

The ELANPlus advanced multimineral loganalysis identified approximately 8 m [26.2 ft] of

quality reservoir using conventional inter -pretation techniques. After the triaxial inductiondata over the complete logging interval wereincorporated into the analysis, the net-paythickness, using 7% porosity and 80% water

saturation for cutoffs, was increased by 35%.Calculated reserves values were 55.5% higherthan those previously obtained using traditionallogs and petrophysical evaluation programs(next page).

76 Oilfield Review

> Anisotropy using Rv /Rh ratio. The Rt Scanner service provides an Rv /Rh ratio (Track 1, black) that is above 5 inseveral intervals (red arrow). These zones correspond to laminations in the core (left ). In intervals where the Rv /Rhratio is low (black arrow), the core has few or no laminations (right). Throughout this section, Rh (Track 3, blue)rarely measures above 2 ohm.m, although the Rv (red) and Rsand (black) curves are measuring much higher. Thedensity-neutron logs (Track 4) indicate hydrocarbon (red shading) below 100 m but do not provide much help inevaluating the reservoir above 100 m. Although the Rh values suggest little productive potential, the higher values ofRsand indicate hydrocarbon.

Density-Neutron

%

Neutron Porosity

1.65 g/cm3

Bulk Density

2.65

60 0

%

Crossplot Porosity

60 0

Thin beds are

visible in core.

From Rt Scanner

tool, the Rv /Rh

ratio = 9. This

zone has high

electrical

anisotropy.

No thin beds

are visible in

the core.

The Rv /Rh ratio

is low. This zone

has negligible

electrical

anisotropy.

80

90

100

110

120

m

Depth

0

Rv /Rh Ratio

20

8 in.

Bit Size

18

0 gAPI

Gamma Ray

100

8 in.

Caliper

18

Bad Hole

0.2 ohm.m

Rsand

200

0.2 ohm.m

Rv

200

0.2 ohm.m

Rh

200

0.2 ohm.m

90-in. Array

200

0.2 ohm.m

60-in. Array

200

0.2 ohm.m

30-in. Array

200

0.2 ohm.m

20-in. Array

200

0.2 ohm.m

10-in. Array

Resistivity

200


Summer 2008 77

> Incorporating Rt Scanner data. The AIT curves (Track 2) are approximately 1 ohm.m with a few 2-ohm.m sections. Rh(Track 3, blue) is equivalent to the AIT 90-in. curve. Rv (red) measures above 10 ohm.m in several intervals. Rsand (black),calculated from the Rt Scanner outputs, is used as an input for water saturation, Sw. Water saturation from the Rt Scanneroutputs (Track 5, red) is lower than the Sw from AIT data (blue). This finding indicates that more hydrocarbon is in thereservoir than originally computed.

0

Rv /Rh Ratio

m

Depth

30

40

50

60

70

20 0.2 ohm.m

90-in. Array

200

8 in.

Bit Size

18

8 in.

Caliper

18

Bad HoleDensity-Neutron

Montmorillonite

Bound Water

Quartz

Gas

Water

0.2 ohm.m

60-in. Array

200

0.2 ohm.m

30-in. Array

200

0.2 ohm.m

Rsand

200

0.2 ohm.m

Rv

200 60 %

Neutron Porosity

0

60 %

Crossplot Porosity

0

1.65 g/cm3

Bulk Density

2.65

100 %

AIT Sw

0 100 %

Lithology

0

100 %

Rt Scanner Sw

0

0.2 ohm.m

Rh

200

0.2 ohm.m

20-in. Array

200

0.2 ohm.m

10-in. Array

Resistivity

200


Resolving Anisotropy in West AfricaInterpretation of electrically anisotropic reser -voirs has been difficult with traditionalpetro physical analysis techniques. Klein et alwere the first to propose a framework for usinggraphical crossplots to evaluate these reservoirs.27

The technique was further adapted to incorporatedata from additional logging tools, includingnuclear magnetic resonance (NMR) and triaxialinduction resistivity.28 The original Klein plotsassume a layering of isotropic, macro- andmicroporous material, and layering of coarse-grain and fine-grain sands—a condition that doesnot commonly occur in laminated sand-shalesequences surrounded by anisotropic shales.Compaction, which typically increases with depth,has been shown empirically to increase the levelof shale anisotropy (right).

To account for the more-realistic scenario ofanisotropic shales, a modified Klein plot hasbeen developed that graphically solves for Rv andRh while adjusting for shale anisotropy.29 Becauseanisotropic shales can create false expectationsof low-resistivity pay if not accounted forproperly, NMR data are also used to differentiatelaminated shales from sand-shale sequences.NMR tools measure free-fluid volume, or porosity,in the reservoir. Shales usually have high fluidvolumes, but the fluid is bound to the clays thatmake up the shales. By incorporating the NMRporosity, which ignores the fluids in the shales,log analysts can identify laminated sand-shalesequences with hydrocarbon potential whileeliminating laminated shale sequences from the analysis.

The modified Klein plots are similar todensity-neutron crossplots, and an anisotropicshale point can be graphically determined fromthem (below). Because of their characteristicshape, these modified crossplots are referred toas butterfly plots. From them, log analystsgraphically choose parameters, perform quality

checks and assess the potential for productionfrom laminated reservoirs.

Logs from an offshore West Africa welldemonstrate the modified Klein plot technique.30

The addition of NMR data further enhanced theevaluation. The operator elected to run the Rt Scanner tool, MR Scanner expert magnetic

78 Oilfield Review

> Klein plots. The traditional Klein plot (left ) does not take shale anisotropy into account. The modified butterfly plot (center) includes shale anisotropy andcan be partitioned into pay and nonpay regions, pivoting at the shale point. The crossplot Rv and Rh data fall into specific regions that can be analyzedquickly (right ). The water point (blue circle) indicates 100% water saturation. The shale point indicates 100% shale.

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

No shale anisotropyWater With shale anisotropy Water

Nonpay

Shale Pay

Water

Fshale Fshale

Rshale-v = 1

Rshale-h = 1

Shale

Rshale-v = 10

Rshale-h = 1

Shale

Rsand Rsand

> Anisotropy in sands and shales. As compaction (red) increases—thetypical case with deeper depositional environments—the clay porositydecreases and the shale Rv /Rh ratio increases. Triaxial induction tools alonecannot distinguish between compaction-induced shale anisotropy and thatmeasured in a laminated sand-shale sequence. And, while the NMR tool isbeneficial in identifying zones with movable fluids and differentiatinganisotropic shales from laminated sand-shale sequences, the volume of sandand shale must be determined from other sources, such as the ECS tool.

0

2

4

6

8

1

3

5

7

9

R v/R h

0 10 20 30

Porosity, %

40 50 60

Compaction


Summer 2008 79

resonance service, and density-neutron and OBMItools. In one zone, the triaxial inductionmeasurement resulted in an 80% increase in net-to-gross pay calculation and increased the calcu -lated net hydrocarbon interval by 15 ft [5 m]—from 23 to 38 ft [7 to 11.6 m] compared withcalculations using conventional logs and traditionalpetrophysical techniques (above).

The butterfly plots identified the shale pointand distinguished the anisotropic shales fromanisotropic sand-shale-silt sequences. Based ontheir Rv /Rh ratio, nonproductive shale intervalsexhibited anisotropy that was similar to that ofthe sand-shale laminated sequences. This case

study demonstrates how NMR data can be usedwith triaxial induction data to differentiatenonproductive shales from potentially productivesand laminations.

Another West Africa example featured twovery different shale types, and modified Kleinplots differentiated reservoir-quality rock fromshales. Two hydrocarbon-productive intervals

27. Klein JD, Martin PR and Allen DF: “The Petrophysics ofElectrically Anisotropic Reservoirs,” The Log Analyst 38,no. 3 (May–June 2007): 25–36.

28. Fanini ON, Kriegshäuser BF, Mollison RA, Schön JH and Yu L: “Enhanced, Low-Resistivity Pay, ReservoirExploration and Delineation with the LatestMulticomponent Induction Technology Integrated withNMR, Nuclear, and Borehole Image Measurements,”paper SPE 69447, presented at the SPE Latin Americanand Caribbean Petroleum Engineering Conference,Buenos Aires, March 25–28, 2001.

29. For more on the use of modified Klein plots: Cao Minh C,Clavaud J-B, Sundararaman P, Froment S, Caroli E, Billon O, Davis G and Fairbairn R: “Graphical Analysis ofLaminated Sand-Shale Formations in the Presence ofAnisotropic Shales,” World Oil 228, no. 9 (September2007): 37–44.

30. Cao Minh C, Joao I, Clavaud J-B and Sundararaman P:“Formation Evaluation in Thin Sand/Shale Laminations,”paper SPE 109848, presented at the SPE AnnualTechnical Conference and Exhibition, Anaheim,California, USA, November 11–14, 2007. This paper is one of a three-part series. See also:Cao Minh C and Sundararaman P: “NMR Petrophysics in Thin Sand/Shale Laminations,” paper SPE 102435,presented at the SPE Annual Technical Conference andExhibition, San Antonio, Texas, September 24–27, 2006. Cao Minh C, Clavaud JB, Sundararaman P, Froment S,Caroli E, Billon O, Davis G and Fairbairn R: “GraphicalAnalysis of Laminated Sand-Shale Formations in thePresence of Anisotropic Shales,” Transactions of theSPWLA 21st Annual Logging Symposium, Austin, Texas,June 3–6, 2007, paper MM.

> Modified Klein plot in action. The crossplot of Rv and Rh values is shown in the butterfly plot (right ). The log analyst selects thedata points that fall in the hydrocarbon region (magenta), in water-productive regions (blue) and at the shale point (green). Thecolor-coding along the resistivity track (Track 3) of the ELANPlus log corresponds to the data points manually selected by the loganalyst. Points that are not selected (black) are not presented. The water saturation values change (Track 5, yellow shading) whenRsand (red) is used rather than the uniaxial resistivity, Rh (black). The interval above 700 m has significant anisotropy (Track 4, green)but little hydrocarbon. One of the advantages of the modified Klein plots is the ability to quickly identify these nonproductive zones.

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

Fshale

0 0.5 1.0

Neutron Density Rh, Rv, Rsand, Rsh Anisotropy

500

Dep

th, m

600

700

800

900

1,000

1,100

1,200

1,300

40 30 20 10 100

0 5 10 15

Water Saturation

100 50 0101

102

Sw Rsand

Sw Rh

Rshale-v = 3.27

Rshale-h = 0.51

Shale

Fshale

Rsand


were separated by a nonproductive shale section,but a zone with similar characteristics hadproduction potential (below). Triaxial inductiondata were instru mental in properly evaluatingthe well. In the upper interval, the sand countincreased by 54% and the net-to-gross ratio by

70% compared with values obtained withconventional techniques. In the lower interval,the increase was not as pronounced because thesands were not as heavily laminated. Still, thenet-to-gross ratio was approximately 20% greaterafter incorpo rating the triaxial induction data

(next page, top left). The nonproductiveanisotropic shale was identified and eliminatedfrom further analysis. The MR Scanner toolprovided an independent verification of netfootage of hydrocarbon.

80 Oilfield Review

> Variable shale anisotropy. These examples are from intervals with twodifferent shale types that were logged with Rt Scanner, density-neutron,OBMI and MR Scanner tools. The NMR tool and the density-neutron toolswere used as sand-shale indicators (Track 1). Anisotropy is present, asindicated by the separation between Rv and Rh (Track 3) and the Rv /Rh ratiocurve (Track 4, green shading). Rh ranges from 1 to 2 ohm.m, whereas Rsand(Track 7, red) is consistently greater than 10 ohm.m in the upper interval.Because higher resistivity corresponds to greater hydrocarbon volume,

the calculated hydrocarbon (HC) volume (Track 9) is greater when calculatedusing Rsand (red) than uniaxial induction resistivity (black). In the upper log,the anisotropy values (Track 4, green) from X,680 to X,720 look similar tothose from Y,760 to Y,820 in the lower log. Although there is high anisotropy inboth intervals, it is the result of anisotropic shales in the lower log, nothydrocarbon. The butterfly plots quickly isolate and identify thesenonproductive zones from the pay zone (magenta) as shown on theELANPlus plots.

Phisand

Phisand NMR Rv , Rh Anisotropy

OBMIGR

T2Fsand

Fsand NMR

Rt Scanner Rsand

NMR Rsand NMR Fluids HC VolumePa

y Zo

nes

X,700

X,740

Dep

th, m

Dep

th, m

X,660

X,620

0.5 10 0.4 0.2 0 0 0 0 0 0 0 0.2 0.4 0 0.2 0.410 1000.5 110 100 1,0005 10 1510 100

40 m

Shale

Cutoff

Sand

Oil

OBM

Water

NMR Fluids

0 0.2 0.4

Oil

OBM

Water

Phisand

Phisand NMR Neutron Density

Neutron Density

Rv , Rh Anisotropy

OBMIGR

T2Fsand

Fsand NMR

Rt Scanner Rsand

NMR Rsand HC Volume

Pay

Zone

sPa

y Zo

nes

Y,850

Y,900

Y,800

Y,750

0.5 10 0.4 0.2 0 0 0 0 0 0 0 0.2 0.410 1000.5 110 100 1,0005 10 1510 100

10 m

Shale

Cutoff

Sand

Rt ScannerData

AIT Data

NMR Data

Rt ScannerData

AIT Data

NMR Data

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

101

Rh, ohm.m

R v, o

hm.m

10–1

10–1

100

101

102

103

100

102

103

Fshale

Rsand

Rshale-v = 1.24

Rshale-h = 0.52

Shale

Fshale

Rshale-v = 2.54

Rshale-h = 0.58

Shale

Rsand


Summer 2008 81

In the final analysis, hydrocarbon net footageand net-to-gross ratio were more accuratelyquantified from data derived from the Rt Scanner tool and information from the MR Scanner service. Compared with traditionalAIT induction results, there were significantgains in calculated reserves. Modified Klein plotswere also shown to be a powerful quicklook toolfor the log analyst.

Induction Dipmeter The final two case studies demonstrate the utilityof dipmeter data derived from the Rt Scannerservice. Using induction measurements toprovide formation dip is not new—the conceptwas first patented in the 1960s—but there hadbeen no practical application. Triaxial inductiontools provide dipmeter data as a natural by-product of their standard data processing.

Traditional dipmeter tools are equipped withseveral pads that measure small resistivitychanges occurring along the borehole wall.Software programs correlate similar readingsfrom adjacent sensors and pads to compute thedip magnitude and direction of the formationbedding planes. Data from the sensors on thepads produce an electrical image of the wellborefrom which structural dip, stratigraphic featuresand fractures can be visualized and manuallyidentified using software applications.

Dipmeter tools have a vertical resolution lessthan 0.5 in. [1.3 cm], whereas a triaxial inductiontool has a vertical resolution measured in feet.Although fine details cannot be resolved with theaccuracy of the FMI Fullbore FormationMicroImager or OBMI and OBMI2 tools, the Rt Scanner service can provide structural dip.

Dipmeter imaging tools require a conductivemud system to acquire readings, which are thenconverted into images. Because the electricalinsulating properties of oil-base-mud drillingsystems create difficulty in acquiring data,engineers developed solutions, such as the OBMIand the OBMI2 tools, to overcome the problem.Pad contact with the formation is critical,especially when tools are used in oil-base muds.

Hole conditions, such as washouts andrugosity, make pad contact difficult and degradethe quality of the measurement. This is true inboth oil-base and water-base muds. Tools loggingin deviated wells can experience floating pads,caused by the weight of the tool collapsing thecaliper arms and preventing the pad fromcontacting the borehole wall. In addition,irregular tool motion negatively affects thequality of the images.

The Rt Scanner tool is insensitive to boreholeconditions such as rugosity and washouts, and itcan log up or—with a modified caliper—down.By contrast, because of the need to push the pads against the borehole wall, dipmeter toolsalmost always log in an upward direction. Theexception is drillpipe-conveyed FMI tools run inhorizontal wells.

Conventional dipmeter tools take theirmeasurements at a very shallow depth ofinvestigation, which is the region most affectedby the drilling process (below). A triaxial

> Padless dipmeter. The triaxial induction measurement senses a very large volume (left ). The conventional dipmeter tool (right ) provides a high-resolutionimage but sees a small electrical diameter. It must also make contact with the borehole wall to acquire usable data.

Dip

Azimuth

Electrical diameter90 in.

Rh

Rv

Rh

Rv

Dip

Azimuth

Interval—143 m (top) NMR ToolRt Scanner ToolAIT Tool

Summary of Results

Hydrocarbon (HC), m

Net to gross (NTG)

Net change, HC/NTG

8.2

0.26

12.6

0.44

54%/70%

12.5

Interval—163 m (bottom) NMR ToolRt Scanner ToolAIT Tool

Hydrocarbon, m

Net to gross

Net change, HC/NTG

18.0

0.47

20.6

0.57

14%/21%

21.3


induction tool surveys the region beyond thenear-wellbore and is less affected by the drilling-induced damage. Induction-derived dipmeterdata are also available from multiple arrays. Theability to compare dips from different depths ofinvestigation is useful for quality control,although variations in the dips may result from distortions in the bedding planes away fromthe wellbore.31

Because the Rt Scanner tool requires noconductive fluid to acquire data, structural dipcan be obtained in wells where it was difficult orimpossible in the past. Induction-deriveddipmeter data do not replace information fromconventional dipmeter imaging tools, butcomplement their measurement, as for example,when bad borehole conditions degrade the dataacquired with pad contact devices.

The workflow for generating dip informationis part of the data inversion and correctionprocess. Bed boundaries are defined usingborehole-compensated raw data that have beencorrected for tool rotation. As a first-orderapproximation to define bed boundaries, asecond derivative technique produces a squaredlog from the induction array (above). Thesquared log has sharper boundary edges thanconventional smoothed curves, and the sharptransition points are used to determine where tooutput dip information.

Next, the rotated, borehole-corrected curvefrom a single array is output with an initialestimation of conductivity, bed dip and boreholeazimuth. Typically a 20-ft [6.1-m] window isinverted, but this depends on how rapidly the dipis changing. Rv, Rh and bed boundaries arerefined with this inversion step. The softwareagain solves for dip and azimuth for the best fitover the entire window. The program then movesone-half the window length and inverts with agenerous overlap of the previous interval toeliminate edge effects. This process continuesover the entire logged interval. The result isborehole-corrected, dip-corrected resistivityalong with structural dip and borehole azimuth,which are presented using conventional tadpolesand azimuth plots.

Dipmeter in Air and WaterIn the USA, an Rt Scanner tool providedformation dip and direction in an air-drilledprospect well. Air is used instead of drilling fluidin formations that react with the drilling mud orin hard-rock areas where conventional drillingtechniques are less effective. Because there is noliquid in the wellbore, conventional dipmetertools do not work—including the OBMI tool.

For the well in question, two intervals withvery different characteristics are shown (nextpage). The zone from X,X00 to X,X50 ft hasconsistent 15° dip oriented to the south-southeast with little variation. Although difficult

to see, there are three independent measure -ments from three depths of investigationpresented. Throughout the interval, the tadpolesfrom all three measurements overlay, indicatingagreement among the different datasets.

In a deeper interval, the data show very high-angle formation dips, which corroborated thegeologists’ interpretation and expectations. Suchhigh-angle dips—approaching 70°—might beconsidered questionable were it not for core datafrom nearby wells showing similar charac -teristics. An unconformity can clearly beidentified on the log at Y,Y40 ft. Also, despiteconsiderable hole rugosity in the Y,Y00 to Y,Y50interval, the dipmeter data are available; a padcontact tool may have been affected by thecondition of the borehole.

In a second example, the operator, drillingwith water-base mud, ran the Rt Scanner tool in adeepwater Gulf of Mexico exploration well. TheFMI tool was run for comparison. The well wasdeviated 60°, and the true formation dip,corrected for well deviation, was approximately30°. A comparison of the data derived from FMImeasurements and data from the Rt Scanner tool

82 Oilfield Review

31. Amer A and Cao Minh C: “Integrating Multi-Depths ofInvestigation Dip Data for Improved Structural Analysis,Offshore West Africa,” presented at the Offshore Asia Conference and Exhibition, Kuala Lumpur, January 16–18, 2007.

> Steps in the process, induction to dipmeter. Dipmeter information from the triaxial induction tool is an automatic output of the processing used for dipcorrection and calculating Rv (red) and Rh (blue). In block intervals, the raw data (Track 1) are corrected for borehole effects and then inverted. Bedboundaries are identified from square logs (black curve), which are the result of a second derivative technique, output to show the bed boundaries. The dipis calculated where resistivity changes are apparent. Homogeneous, isotropic intervals produce no dips because there are no step changes of resistivityin the interval. After each section is fully processed, succeeding intervals are computed with a 25% overlap to eliminate bed-boundary effects.

300

200

100

Dep

th

0–500 0 0 10 100 1,000500

R-signal, mS/m Resistivity, ohm.m

1,000 1,500 –500 0 0 10 100 1,000500

R-signal, mS/m Resistivity, ohm.m

1,000 1,500 0 10 100 1,000

Resistivity, ohm.m

25% overlap

xx

xy

xz

yx

yy

yz

zx

zy

zz

Square log

xx

xy

xz

yx

yy

yz

zx

zy

zz

Square log

RhRv

RhRv

RhRv


Summer 2008 83

> The first induction dipmeter in an air-drilled well. The results of the dipmeter log from the Rt Scanner tool (Track 3, top) in anair-drilled well show excellent agreement at all three depths of investigation: 39 in., 54 in. and 72 in. [99 cm, 137 cm and 183 cm].Deeper in the well, the high-angle dip data (Track 3, bottom) rapidly transition to low-angle dip at about Y,Y40, indicating apossible unconformity. Dip as high as 70° agrees with core data from nearby wells. The hole rugosity and enlarged hole sections(Track 1, blue shading) do not affect the Rt Scanner measurement, but it would have been difficult to acquire valid data in thissection using tools that rely on pad contact.

X,X00

X,X50

Y,Y00

in.

Caliper

ft

Depth

244

Y,Y50

in.

Bit Size

244

72-in. Array

121 ohm.m

54-in. Array

121 ohm.m

39-in. Array

121 ohm.m

gAPI

Gamma Ray

2000

ohm.m

90-in. Array

1,0001

ohm.m

ohm.m

ohm.m

ohm.m

Rv, 72-in. Array

%

Neutron Porosity

%

Density Porosity

1,000 –10

–10

1 30

30

Rh, 72-in. Array

1,0001

Rh, 54-in. Array

1,0001

Rv, 54-in. Array

1,0001

ohm.m

Rv, 39-in. Array

1,0001

ohm.m

deg

Rh, 39-in. Array

Quality, 39-in. Array



1,000

90

0

0

01

0

12

12

12

ohm.m

10-in. Array

Dip, 72-in. Array

True Dip

Quality [5.15]

Quality [15.20]

1,0001Bad Hole

Q Flag, 54-in. Array




shows excellent agreement (above). A low-resistivity laminated pay section, present in thiswell, could easily be overlooked usingconventional methods. Incorporating the triaxialresistivity data in the logging suite identified thepotentially productive zones.

Future DevelopmentsAlthough many enhancements have been added toinduction logging tools since the first commercialtool was introduced more than 50 years ago, thebasic theory of the measure ment has changedlittle. Advancements in computer simulations andmodeling have greatly improved the industry’sunderstanding of the measure ment. The triaxial

induction measure ment of the Rt Scanner toolbrings new information to the petrophysicist, suchas dip-corrected resistivity, laminated-reservoirproperties and induction-derived dipmeter data,as discussed in this article.

This advanced technology has opened newpossibilities and presented new needs to theindustry. Development of fast inversion routinesapplied at the wellsite would provide moreaccurate resistivity measurements for calcu -lating water saturation in real time. Thisadditional information would improve the abilityto make informed decisions, such as inidentifying optimum locations for measuringpressure and taking fluid samples. Also,laminated sand-shale sequences that may havepotential as hydrocarbon reservoirs could beidentified more quickly and reliably.

Potential application has been shown forincorporating seismic data with inductionmeasurements.32 Although the concept is promis -ing, it remains unclear whether multiple deep

imaging of formations can be extended to resolveseismic structures from surface-acquired data.

Commercial processing of triaxial data iscurrently limited to 1D inversion and includesthe assumption that invasion does not impact themeasurement. By using 2D and 3D inversion, theinvasion effects can be determined, including thedip of the invasion.33 This is a nontrivial task;currently it takes a week to process 100 ft [30.5 m]of data on a high-end PC compared with half aminute for 1D inversion. Commercial imple -mentation will require time and innovation both in the processing software and in hard -ware configurations.

Resistivity is the oldest wireline loggingmeasurement, but interest has been renewed inthis technology because of the triaxial inductiontool. This advance presents exciting possibilitiesfor petrophysical evaluation and the potential tolocate and produce previously bypassed pay. –TS

84 Oilfield Review

> Gulf of Mexico example. This high-angle Gulf of Mexico well had 30° dip and thinly laminated sands (Track 9). The induction-derived dipmeter data(Track 8, green) show excellent agreement with the FMI data (red) in both direction and magnitude of dip. This zone includes a low-resistivity pay intervalfrom X,820 to Y,000. The conventional resistivity data used to compute water saturation indicate little hydrocarbon content (Track 6, green). Using thetriaxial induction data to compute water saturation (Track 7, green) yields considerably more oil volume.

X,750

Depth

ft

Shale

Lithology

X,800

X,850

X,900

X,950

Y,000

Y,050

Y,100

Fsand

Gamma Ray

gAPI

ft3/ft3 1.51.5

Bound Water

% 050 deg

Rt Scanner Dip

QualityFMI Image

900

Bound Water

% 050

Bulk Density

g/cm3 2.651.65

Neutron Porosity

% 060

Sand Laminated Sw

Clay-Bound Water Clay-Bound Water

ELANPlus SwRh

ohm.m 2000.2

Rv

ohm.m 2000.2

90-in. Array

ohm.m 2000.2 Water

% 050

Water

% 050

Total Porosity

% 050

Total Porosity

AIT SaturationRt Scanner

Saturation

% 050

Quality

deg

FMI Dip

Quality

900

Quality

32. Amer and Cao Minh, reference 31.33. Abubakar A, Habashy TM, Druskin V, Davydycheva S,

Wang H, Barber T and Knizhnerman L: “A Three-Dimensional Parametric Inversion of Multi-ComponentMulti-Spacing Induction Logging Data,” ExtendedAbstracts, SEG International Exposition and 74th AnnualMeeting, Denver (October 10–15, 2004): 616–619.


Documents

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