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4 Oilfield Review

No More Waiting: Formation Evaluation While Drilling

Bob AdolphChris StollerPrinceton, New Jersey, USA

Mike ArcherChevronLafayette, Louisiana, USA

Daniel CodazziTamir el-HalawaniPatrick PerciotGeoff WellerClamart, France

Mike EvansSugar Land, Texas, USA

Jeff GrantHouston, Texas

Roger GriffithsAbu Dhabi, United Arab Emirates

Don HartmanGerald SirkinDevon Energy CorporationHouston, Texas

Makoto IchikawaJapan Oil, Gas and Metals National Corporation (JOGMEC)Chiba, Japan

Graham ScottNexen Petroleum U.K. LimitedAberdeen, Scotland

Ian TribeAberdeen, Scotland

David WhiteCambridge, England

For help in preparation of this article, thanks to FrançoiseAllioli, Clamart, France; Sonny Auld, Emma Jane Bloor andSonny Johnston, Sugar Land, Texas; Zoila Cedeño, Ivor Gray, Bart Hughes and Russ Neuschaefer, Houston;and Chatham Grimmer, Youngsville, Louisiana.adnVISION, APS (Accelerator Porosity Sonde), arcVISION,DecisionXpress, DSI (Dipole Shear Sonic Imager),EcoScope, EcoView, ECS (Elemental CaptureSpectroscopy), ELANPlus, GeoFrame, geoVISION, GVR(geoVISION resistivity), Minitron, Orion, Platform Express,RAB (Resistivity-at-the-Bit), RST (Reservoir SaturationTool), SpectroLith, TDT (Thermal Decay Time), TeleScopeand WellEye are marks of Schlumberger.

Accurate and timely formation evaluation is an essential element of the exploration

and production business. In the past, operators had to compromise between the

real-time advantages of logging-while-drilling tools and the more comprehensive

formation evaluation of wireline techniques. A new, integrated logging-while-

drilling tool, along with powerful interpretation software, sets a new standard in

safety and efficiency, and reduces formation evaluation uncertainty.

Exploration and production companies havebeen anticipating a faster, safer and morecomprehensive way to assess the productivepotential of oil and gas reservoirs and toaccurately place productive wells using logging-while-drilling (LWD) tools. Until recently, basicformation properties, such as resistivity andporosity, in addition to drilling-relatedmeasurements, such as inclination, vibration andannular pressure, were acquired by stackingindividual measurement tools in long bottomholeassemblies (BHAs). These assemblies can takesubstantial time to make up and break downwhen tripping into and out of a well. Perhapsmore importantly, longer distances between bitand sensors cause measurement delays and force engineers and geoscientists to wait forinformation that, in many cases, could

immediately influence drilling procedures andtarget identification.

Logging-while-drilling priorities, identifiedthrough an industry survey, included reducingthe distance from the bit to LWD sensors.Reducing this distance decreases environmentaleffects on measurements and reduces waitingtime for acquisition and interpretation of dataneeded for key decisions.1 Along with improvedtool reliability and increased real-time data-transmission rates to surface, survey respondentsalso expressed a desire to eliminate chemicalradioactive sources in LWD tools.

The wait for improved capabilities is over.Scientists and engineers at Schlumberger havedeveloped an integrated LWD tool that meetsthese requirements and delivers importantdrilling and logging measurements. These include

1. Weller G, Griffiths R, Stoller C, Allioli F, Berheide M,Evans M, Labous L, Dion D and Perciot P: “A NewIntegrated LWD Platform Brings Next-GenerationFormation Evaluation Services,” Transactions of theSPWLA 46th Annual Logging Symposium, New Orleans,June 26–29, 2005, paper H.

2. Rotary valve pressure-pulse generators, which alternatelyrestrict and open the flow of the drilling fluid, causevarying pressure waves to be generated in the drillingfluid at a carrier frequency that is proportional to the rateof interruption. Downhole sensor-response data aretransmitted to the surface by modulating this acousticcarrier frequency.

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Autumn 2005 5

measurements already obtained by existing LWDtools and new measurements, previously providedonly by wireline-conveyed technologies thatprovide information about formation lithologyand fluids. An innovative tool design reduces thelength of the entire measurement section to asingle 26-ft [7.9-m] collar and offers a sourcelesslogging option that reduces risk to personnel, theenvironment and the well.

This article briefly reviews the history ofmeasurements-while-drilling (MWD) and LWDtechnologies, along with their advantages andlimitations. We introduce the new EcoScopemultifunction logging-while-drilling service anddescribe its measurements and the obstacles

overcome during its development. Case studiesdemonstrate the early impact of this newtechnology and its associated interpretationsoftware in Gulf of Mexico, North Sea and MiddleEast reservoir exploitation.

Advances Above the BitThe technological progression of takingmeasurements while drilling has been steady butsomewhat limited by the difficulties oftransmitting data to the surface in the boreholeenvironment. Commonly, analog data from LWDsensors are converted to binary form downhole.By using a flow-restricting mechanism in thedrilling-fluid flowstream, data are transmitted by

producing positive or negative pressure pulses.These pressure pulses are transmitted throughthe mud column inside the drillpipe, read at thesurface by pressure sensors, and then recordedand processed.

Another type of data transmission uses rotaryvalves with a modulator that generates acontinuous pressure wave to carry information.2

Recent developments in this technology haveresulted in data-transmission rates as high as fourtimes the industry average that are much lesssusceptible to drilling and mud-pump noise, anddownhole motor stalls. This technology is used inthe Orion high-speed telemetry platform and the


TeleScope high-speed telemetry-while-drillingservice (above).

The first MWD tools were developed in theearly 1970s to measure drilling-relatedproperties, such as inclination and azimuth,which are essential in directional drilling.3

Important additional measurements such astorque, weight on bit (WOB) and temperatureallow drillers and drilling engineers to monitordownhole drilling performance parameters inreal time rather than infer these parametersfrom surface measurements. Generally, real-timeMWD measurements are monitored to helpoptimize the drilling process, avoid drillingproblems and monitor the well trajectory toensure that the intended target is reached.4

These early measurements improved industryknowledge of dynamic drilling processes. As aresult, drilling became more efficient, less risky,and often less expensive. For example, there arenow fewer catastrophic borehole failures thatforce companies to sidetrack or abandon wells.Borehole quality has improved, decreasingcementing costs and problems. Reducedborehole rugosity also improves the quality offormation evaluation from both wireline andLWD devices.

The first LWD measurements were developedin the 1980s to identify penetrated strata and, inmany cases, to confirm the location of the bitwith respect to the formation rather than havingto rely only on the measured depth. Thiscapability facilitated well-trajectory changes toavoid hazards and to penetrate the targetedreservoir.5 LWD also served as an alternative wayto acquire basic formation data in areas wherewireline logging proved difficult, such as inhighly deviated and horizontal wells, or in wellswith problematic boreholes. Another primeobjective of logging the borehole while drillingwas to measure formation-fluid properties beforethe drilling process—particularly invasion ofdrilling fluids—significantly disturbed thereservoir near the wellbore.

Borehole imaging techniques have beendeployed on wireline since the 1960s. As while-drilling data-transmission rates improvedduring the last decade, similar techniques havebecome an important part of LWD operations.6

For example, real-time images from LWD toolssuch as the RAB Resistivity-at-the-Bit or the GVRgeoVISION resistivity sub are used to assessformation bedding, identify fractures, assist information evaluation, and to direct geosteering

and geostopping operations.7 As LWD measure-ments have improved and become morenumerous, they are now increasingly used to helpoperators make crucial drilling decisions anddetermine the state of stress around theborehole.8 In addition, LWD technology is playinga role in both completion and stimulation design.

Challenges While DrillingChanges in the near-wellbore environment fromdrilling time to wireline logging time andinherent differences in tool designs must betaken into account when comparing LWDmeasurements with those of wireline logs.9

However, there is usually one indisputable fact:the near-wellbore region is less disturbedimmediately after bit penetration than days orweeks later, when wireline logging occurs.

The number of LWD measurements continuesto grow, but wireline logs are still preferred inmany areas, especially where drilling-rig costsare moderate, wellbore inclination is low, andborehole conditions are satisfactory. In addition,the range and versatility of wireline measure-ment and sampling capabilities are compellingreasons to use wireline tools.

Until recently, several measurements that aidin the identification of formation fluids—gas, oiland water—were not deployed in LWD systems.One example is the thermal neutron capturecross section measurement, which determinesthe probability that a thermal neutron iscaptured by nuclei in the formation. The captureof neutrons results in the emission of gammarays. The measurement of the decay of thegamma ray signal over time is commonly used todetermine the average thermal neutron capturecross section, or sigma, of the formation, whichhelps characterize the fluids in the pore spacenear the wellbore.10 Formations that contain alarge percentage of high-salinity water have highsigma, Σ , values, because chlorine [Cl] has alarge thermal neutron capture cross section,while formations containing oil, gas or freshwater generally exhibit lower sigma values. Thisis similar to the typical conductivity response,allowing petrophysicists to use sigma for thecalculation of water saturation, Sw. This isespecially useful in low-resistivity pay zones,where resistivity-based calculations are often mis-leading and may result in missed pay (next page).

The sigma measurement is relatively shallowcompared with deep-resistivity measurements,so mud-filtrate invasion often reduces itseffectiveness. Consequently, measuring sigmabefore significant invasion occurs yields a morerepresentative description of reservoir fluids and

6 Oilfield Review

> LWD and MWD continuous-wave telemetry technology. The TeleScope high-speed telemetry-while-drilling collar contains a turbine that generates the power for the EcoScope multifunction logging-while-drilling tool and eliminates the need for lithium batteries. As the TeleScope modulator rotaryvalve turns, it alternately restricts and opens the flow of drilling mud through the collar, generating acontinuous pressure wave that transmits the telemetry signal.

Power-generation module

Electronics module

Transmission module


Autumn 2005 7

therefore a better sigma baseline, informationgreatly desired by petrophysicists.

Another method to assess formation fluids isthe measurement of hydrogen index (HI), thefoundation of neutron porosity measurements.Borehole size, temperature and mud propertiescan have significant effects on neutron porosityreadings. Wireline techniques use eccentereddevices to minimize these borehole effects.However, LWD tools are typically centered in theborehole, making borehole corrections evenmore important in ascertaining the correctneutron porosity value. In the absence ofaccurate caliper or standoff data, corrections forborehole size are at best inaccurate, causing thecalculated neutron porosity to be too low.

From a Reliable SourceUntil recently, americium-241 beryllium [AmBe]chemical sources were the only source ofneutrons in LWD tools. As the drilling industryattains higher rates of penetration (ROP), the statistical precision of LWD nuclear measurements and the associated recordingrates may be limiting factors in exploitingpotential gains in ROP. Moreover, the use ofchemical sources entails health, safety andenvironmental concerns.11

The first radioactive sources were used inwell logging during the mid-20th century tomeasure subsurface formation properties andallow calculation of porosity.12 During the last50 years, oilfield service companies have takenextraordinary steps to limit exposure toradioactive emissions by developing detailedprocedures for storing, handling and usingradioactive sources.13 The storage, regular testingand eventual disposal of these sources are highlyregulated by nuclear authorities.14

Occasionally, logging tools that house thesesources get stuck in boreholes because ofborehole problems or irregularities, such askeyseats. When radioactive sources cannot berecovered and are left in a well, operators are required to employ special well-plugging and monitoring procedures to minimizeenvironmental impact. The loss of the well andthe operations themselves, which may includedrilling a monitoring well, can be extremelycostly. The expectations are that worldwideregulation of radioactive logging sources willbecome more stringent and that the costsassociated with their use will increase.

There are other, more complicated problemswhen using radioactive logging sources in LWDcollars. The process of loading and unloading thesource is more time-consuming than with wireline

tools, increasing operating time. In addition, morepersonnel are typically required to make up andbreak down LWD equipment.

Schlumberger LWD tool designs include anannular loading system that allows the sources tobe fished through the drillpipe using wireline,eliminating the need to leave sources in the toolwhen the tool becomes stuck downhole. Whilethis procedure adds time to the fishingoperations, it reduces the risk of source damage,therefore reducing the risk to the environment.This annular design allows 85% of sources to berecovered, whereas stuck BHAs are recoveredonly 35% of the time.15 Even with this reducedrisk, operators have had to choose betweenretrieving the sources and spending more timetrying to recover the entire BHA.

3. http://www.oilonline.com/news/features/dc/20050118.BACK_TO_.16901.asp (accessed August 17, 2005).Bonner S, Burgess T, Clark B, Decker D, Orban J,Prevedel B, Lüling M and White J: “Measurements at theBit: A New Generation of MWD Tools,” Oilfield Review 5,no. 2/3 (April/July 1993): 44–54.

4. http://www.oilonline.com/news/features/oe/20050314.Scope_of.17389.asp asp (accessed August 17, 2005).

5. Bargach S, Falconer I, Maeso C, Rasmus J, Bornemann T,Plumb R, Codazzi D, Hodenfield K, Ford G, Hartner J,Grether B and Rohler H: “Real-Time LWD: Logging forDrilling,” Oilfield Review 12, no. 3 (Autumn 2000): 58–78.Borland W, Codazzi D, Hsu K, Rasmus J, Einchcomb C,Hashem M, Hewett V, Jackson M, Meehan R andTweedy M: “Real-Time Answers to Well Drilling andDesign Questions,” Oilfield Review 9, no. 2 (Summer1997): 2–15.

6. Cheung P, Hayman A, Laronga R, Cook G, Flournoy G, Goetz P, Marshall M, Hansen S, Lamb M, Li B, Larsen M,Orgren M and Redden J: “A Clear Picture in Oil-BaseMuds,” Oilfield Review 13, no. 4 (Winter 2001/2002): 2–27.Pike B: “Logging History Rich with Innovation,” Harts E&P 75, no. 9 (September 2002): 52–55.

7. Rohler H, Bornemann T, Darquin A and Rasmus J: “TheUse of Real-Time and Time-Lapse Logging-While-DrillingImages for Geosteering and Formation Evaluation in theBreitbrunn Field, Bavaria, Germany,” paper SPE 71733,presented at the SPE Annual Technical Conference andExhibition, New Orleans, September 30–October 3, 2001.

> Using the average thermal neutron capture cross section, or sigma, to calculate water saturation. Sigma increases as thechlorine [Cl] content in the formation increases, for example in high-salinity formation water. Sigma is useful for calculatingwater saturation, Sw, because it offers a contrast in readings between hydrocarbon and typical formation water (top). WhileArchie’s equation was derived empirically and uses exponents that must be estimated, the sigma response equation is simpleand linear (bottom). Bulk sigma, Σbulk, is measured, and water sigma, Σwater, can be calculated from the Cl concentration in theformation water. Hydrocarbon sigma, ΣHC, is usually estimated or taken from oil-analysis data. Solid-grain sigma, Σgrain, can bedetermined if the various mineral fractions are known. Porosity, Φ, is calculated from log data. Using capture spectroscopy data from logs provides mineral-fraction information.

(Σ − Σ ) + Φ (Σ − Σ )

Φ (Σ − Σ )= .

Lithology

Fluid Gas Oil WaterFresh Increasing salinity

Σ 0 5 10 15 20 25 30 35 40 45 50

Sandstone = 4.3

Dolomite = 4.7

Calcite = 7.1

Anhydrite = 12

Clays

8. Bargach et al, reference 5.9. Hansen P and Shray F: “Unraveling the Differences

Between LWD and Wireline Measurements,”Transactions of the SPWLA 37th Annual LoggingSymposium, New Orleans, June 16–19, 1996: T1–T12.

10. For more on the sigma measurement: Adolph B, Stoller C,Brady J, Flaum C, Melcher C, Roscoe B, Vittachi A andSchnorr D: “Saturation Monitoring With the RSTReservoir Saturation Tool,” Oilfield Review 6, no. 1(January 1994): 29–39.

11. Kurkoski PL, Holenka JM and Evans ML: “RadiationSafety and Environment for Measurement-While-Drilling:A Different Approach,” paper SPE 23264, presented atthe SPE Health, Safety and Environment in Oil and GasExploration and Production Conference, The Hague,November 11–14, 1991.

12. http://www.spe.org/spe/jsp/basic/0,,1104_1714_1003934,00.html (accessed August 9, 2005).

13. Aitken JD, Adolph R, Evans M, Wijeyesekera N,McGowan R and Mackay D: “Radiation Sources inDrilling Tools: Comprehensive Risk Analysis in theDesign, Development and Operation of LWD Tools,”paper SPE 73896, presented at the SPE InternationalConference on Health, Safety and Environment in Oil and Gas Exploration and Production, Kuala Lumpur,March 20–22, 2002.

14. http://www.nssihouston.com/disposal.html (accessedAugust 17, 2005).

15. Aitken et al, reference 13.


Learning While DrillingTechnological progression is mainly evolutionarybut sometimes it is revolutionary.16 Wireline andLWD devices improve with every generation asenabling technologies are developed, andknowledge is captured and put to use.

Unlike wireline logging devices, LWD toolsensors and electronics must endure theenormous shock and vibrational forcesassociated with drilling. Tool reliability hascontinually improved over the years becausesophisticated testing methods have greatlyreduced tool failures caused by shock andvibration. Today, real-time monitoring of shockand vibration allows drilling engineers to alterdrilling parameters and BHA configurations toreduce these forces, thereby extending bit lifeand avoiding damage to the BHA, which includesLWD equipment.17 Another way for tooldevelopers to reduce LWD equipment failures isto reduce the number of LWD collars, therebydecreasing the number of weak points andrequired connections in the string. This can beaccomplished through designs that integratesensors to reduce tool lengths, enabling multiplesensors to be packaged in a single collar.

Advances in electronics and improvedunderstanding of measurement physics have ledto efficiency and reliability breakthroughs, forexample, those achieved with the PlatformExpress integrated wireline logging tool.

Recently, LWD technologies have also evolvedtoward the integration of more measurements inshorter tools. Improved tool reliability, wellsiteefficiency gains and measurements taken closerto the bit are the result.

The time and distance gap between the bitand LWD sensors often forces petrophysicistsand geologists to wait for hours before they canuse data from all available sensors to make athorough interpretation. There are alsoimportant formation evaluation concerns. Largedifferences between the times that varioussensors cross a given depth can lead to depthdiscrepancies between measurements and tointerpretation ambiguities. In addition, invasioneffects change during the period when thevarious staggered measurements pass a givenpoint in the wellbore. Changes in the near-wellbore region caused by invasion of drillingfluids can be significant in only minutes, makingthe prospect of colocating measurements evenmore attractive. Measurements acquired at thesame depth and time, and under the sameconditions, allow a more meaningful directcomparison, with less uncertainty fromenvironmental effects. Geosteering and geo-stopping operations are also enhanced whensensors are located closer to the bit.

Effective geosteering depends on timelyinformation from subsurface sensors to guidedecisions about well trajectory. For example,

during operations to keep a horizontal well abovethe oil/water contact, drilling tens of feet withoutcrucial information can lead to drilling into water, resulting in premature water production.Moreover, when a well reaches total depth (TD),a long distance between the bit and theuppermost sensors necessitates the drilling ofextra footage, or rathole, to accommodate thelengthy BHA so that all measurements can beacquired. While this practice allows the logging ofimportant strata—often reservoir rock—it addstime, risk and expense to drilling operations.

One of the prime motivations for runningLWD devices is to gather information about thereservoir as early as possible. However, real-timesoftware tools that perform data quality controland comprehensive formation evaluation usingLWD data have been lacking in the past. Equallyimportant is the need to use formationparameters—depth, net thickness, porosity, Sw

and permeability—to characterize potentialreservoirs, including net pay and reserveestimates, and to define completion intervals.

Since the development of the first LWDmeasurements, steady progress has been madein addressing tool limitations. Many of theseproblems have recently been overcome throughthe development of innovative LWD technology.

Sourceless LWD Nuclear Measurements In 1995, Schlumberger and the Japan Oil, Gasand Metals National Corporation (JOGMEC),previously Japan National Oil Corporation(JNOC), collaborated to develop a ground-breaking pulsed-neutron measurement for LWDapplications.18 The pulsed-neutron generator(PNG) design was ruggedized in the early 1990s.In addition to the potential for eliminating anuclear logging source, the PNG provides manyimportant LWD measurements, many of whichwere established in wireline formationevaluation. (left). From this collaboration, newLWD measurements became feasible, includingsigma, neutron-gamma density (NGD) andcapture spectroscopy to accurately calculatemineral fractions (see “New LWD MeasurementsUsing a PNG,” next page).

8 Oilfield Review

> Pulsed-neutron generator. Instead of a radioactive AmBe source, apulsed-neutron generator (PNG) is used in some logging devices togenerate high-energy neutrons. Within the PNG, the Minitron neutrongenerator device (top) comprises a deuterium reservoir, an ion source, anaccelerating column and a target. The reservoir releases deuterium gaswhen heated. The ion source uses a hot cathode electron source and apulsed grid to ionize and partially dissociate deuterium and tritium. Thehigh voltage accelerates the ions, forcing them to collide with a tritium-permeated target. When deuterium bombards tritium, the resulting fusionreaction (bottom) produces 14-MeV neutrons. The high acceleratingvoltage, on the order of 100 kV, requires special techniques to protect thePNG from destructive electrical arcs and mechanical shocks. To preventarcing, the space surrounding the Minitron device in the PNG assembly isfilled with insulating sulfur hexafluoride gas.

np+

Deuteriu

Deuteriumreservoir

m2H1

n np+

Tritium3H1

Helium4He2

n np+ p++ + + Kinetic

energyn

Neutron1n0

Ion source Accelerating column Target

Highvoltage

e-D+

Ion beam

Cathode Grid

14 M

eV

n

16. Zimmerman T: “The Innovator’s Choice,” Oilfield Review 14, no. 1 (Spring 2002): editorial page.

17. Ashley DK, McNary XM and Tomlinson JC: “ExtendingBHA Life with Multi-Axis Vibration Measurements,”paper SPE/IADC 67696, presented at the SPE/IADCDrilling Conference, Amsterdam, February 27–March 1, 2001.

18. Evans M, Adolph R, Vildé L, Morriss C, Fisseler P, Sloan W, Grau J, Liberman A, Ziegler W, Loomis WA,Yonezawa T, Sugimura Y, Seki H, Misawa RM, Holenka J, Borkowski N, Dasgupta T and Borkowski D:“A Sourceless Alternative to Conventional LWD NuclearLogging,” paper SPE 62982, presented at the SPEAnnual Technical Conference and Exhibition, Dallas,October 1–4, 2000.


Autumn 2005 9

Porosity, Spectroscopy and SigmaThe high-energy neutrons emitted by thepulsed-neutron generator (PNG) lose energythrough elastic and inelastic scattering withnuclei in the formation. While inelasticscattering plays a significant role in the initialslowing down of the neutrons, the subsequentenergy loss is dominated by the presence ofhydrogen. Neutrons quickly lose a largefraction of their energy in collisions withhydrogen and are reduced to thermal energylevels, which are nine orders of magnitudelower than their initial energy levels.

The scattered low-energy neutrons arecounted by two sets of neutron detectors atdifferent spacings from the source. Thesedetectors record their number as a functionof time. The hydrogen content of theformation dominates the detector count rates.

A thermal neutron keeps migrating throughthe formation, undergoing multiple collisionswith formation nuclei. Eventually, a collisionwill result in the absorption of the neutron bya formation nucleus, also called neutroncapture, resulting in the emission of capturegamma rays.

The energies of the capture gamma raysdepend on the type of nucleus that capturedthe neutron. Therefore, the gamma rayenergies measured by two gamma raydetectors reflect the elements within theformation. The data from these detectors arerecorded as a function of time and energy.The capture gamma ray spectra recorded bythe gamma ray detector positioned closer tothe source are analyzed, and concentrationsof the formation elements are derived.

The time-decay spectrum of the capturegamma rays of the same detector is used to

determine sigma. Unlike most wireline toolsthat use a two-detector method to correct forborehole, casing and cement effects, the newLWD method uses a single detector. This ismade possible because the LWD collaressentially fills the borehole and displaces thedrilling fluid, reducing borehole effects. Also,most wireline sigma tools are runpredominantly in cased hole and thereforerequire correction and compensation for thepresence of casing and cement.

Neutron-Gamma Density (NGD)The physics of the NGD measurement issimilar to that of the gamma-gamma densitymeasurement. In the NGD case, high-energyneutrons emitted from the PNG create asecondary source, or cloud, of gamma raysfrom inelastic reactions in the formationsurrounding the source. This serves as thesource of gamma rays for a conventionalgamma-gamma density measurement. Thesegamma rays are detected by a far-spaceddetector. Two dominant competing effectsinfluence the gamma ray signal observed atthe far gamma ray detector.

The first effect relates to the transport ofthe fast neutrons from the source to thepoints of gamma ray generation in theformation. Hence, the extent of the gammaray source varies as a function of the size ofthe fast neutron cloud around the neutronsource. The size of this cloud is mainlydetermined by the hydrogen content of theformation. It is therefore necessary to correctthe count rates observed in the gamma raydetector for the varying extent of the neutroncloud. This is accomplished by measuring theepithermal neutron flux at a distance that iscomparable to the slowing-down length ofneutrons from the PNG.1 The epithermal fluxis an excellent indicator of the size of theneutron cloud.

The second effect relates to the transport ofthe gamma rays in the formation (above).Once the gamma rays have been generated inthe formation, they are attenuated as theytravel to the far-spaced detector by the samephysical mechanism—Compton scattering—that determines the response in the traditionalgamma-gamma density measurement. Hence,the response is mainly influenced by thedensity of the formation.

1. Slowing-down length is the average distance from thesource—in this case, the PNG—at which neutronsreach thermal energy levels.

New LWD Measurements Using a PNG

> Neutron-gamma density. The physics of theneutron-gamma density measurement issimilar to that of the traditional gamma-gammadensity measurement. Fast neutrons from thePNG create a secondary source of inelasticgamma rays in the formation surrounding thesource. This serves as the source of gammarays for a conventional gamma-gammadensity measurement. The size of thesecondary source depends on the transport offast neutrons into the formation. Thus, thegamma ray signal observed at the far-spaceddetector exhibits neutron transport effectsthat must be compensated for in deriving aneutron-gamma density measurement. Thecompensation is obtained by measuring theepithermal neutron count rate at a distancecomparable to the typical slowing-down lengthof high-energy neutrons.

Gamma raydetector

Gamma ray

PNG neutronsource

NeutrondetectorNeutron


The five-year collaboration betweenSchlumberger and JOGMEC began with nuclearmodeling, a research mockup and a researchwireline tool, all emulating a pulsed-neutronsystem in drill-collar geometry. The researchwireline tool performed about a dozen openholefield tests to verify modeled performance andexplore tool capabilities. Based on the researchteam’s success, the collaboration moved to thenext phase—development of an experimentalprototype LWD tool. The demanding while-drilling environment requires significantengineering development, even for experimentalprototype tools. The development team producedthe LWD experimental Porosity Evaluation Tool(xPET) using a PNG as the neutron source.

The xPET PNG generates 100,000,000neutrons per second at energies of 14 MeV, anoutput about five times higher and energies

about three times higher than traditional AmBelogging sources. It uses a fusion reaction toproduce neutrons by accelerating tritium anddeuterium ions into a tritium-loaded target. APNG does not generate neutrons withoutelectrical power and has been exempted by theNuclear Regulatory Commission (NRC) fromspecial precautions for abandonment in wells inthe USA.

10 Oilfield Review

> Comparing xPET data with wireline data. In atest well (above), the xPET measurementscompare closely with Platform Express and APSAccelerator Porosity Sonde measurementsacross medium-porosity sandstone and low-porosity limestone (top right). In the same testwell, the xPET spectroscopy data are comparedwith ECS Elemental Capture Spectroscopyoutputs (bottom right). Schlumberger andJOGMEC scientists were encouraged by theclose agreement between datasets.

Neu

tron

poro

sity

, %

20

0

40

APS epithermal neutron porosityxPET epithermal neutron porosity

Platform Express thermal neutronporosity

Form

atio

n de

nsity

, g/c

m3

2.5

3.0

2.0

Platform Express densityxPET neutron-gamma density

Sigm

a, c

u

25

50

0540 560 580 600 620 640 660 680

Depth, ft

Gam

ma

ray,

gAPI

0

100

200Platform Express gamma rayAPS sigmaxPET sigma

Calcium40

200

Silicon40

200

Aluminum40

200

Sulfur20

100

Iron

0

20

10

Min

eral

type

, % b

y w

eigh

t

xPET toolECS tool

Min

eral

ogy,

% b

y dr

y w

eigh

t

100

80

60

40

20

0300 400 500 700 800 900 1,000 1,100 1,200 1,300600

Depth, ft

ClayQuartz + feldspar + micaCarbonate

19. Evans et al, reference 18.20. Aitken et al, reference 13.21. The PNG contains tritium and there are small stabilization

sources in some of the detectors. These small sourcesdo not require any special handling while inside the tool.

22. Weller G, el-Halawani T, Tribe I, Webb K, Stoller C, Galvin S and Scott G: “A New Integrated LWD PlatformDelivers Improved Drilling Efficiency, Well Placementand Formation Evaluation Services,” paper SPE 96652,presented at the Offshore Europe Conference,Aberdeen, September 6–9, 2005.


Autumn 2005 11

The xPET tool characterization used MonteCarlo modeling, laboratory measurements andfield testing to produce extensive databases.These databases were essential in developing thealgorithms for the HI, NGD, sigma and spectro-scopy measurements (previous page).19

Risk AssessmentBefore moving PNG technology to the high-riskenvironment of the LWD BHA, radioactive safetyconsiderations required extensive evaluation anda full risk analysis. Between the two mostcommonly used logging sources—neutron-emitting AmBe and gamma ray-emitting cesium [137Cs]—the AmBe source presented asignificantly greater risk during operations(below left).20 The half-life of americium [241Am]is 432 years, compared to 30.2 years for 137Cs.Moreover, 241Am decays to neptunium [Np],which continues to emit high-energy alphaparticles and has a half-life of more than2 million years.

In rare cases, tools that contain a radioactivelogging source must be abandoned in a well. Thesource, or its radioactive isotope, becomes amajor environmental concern when source half-life exceeds the long-term corrosion and damageresistance of the protective source encapsulationand of the BHA. The neutrons emitted by theAmBe source are more difficult to shield and aremore damaging to living cells than the gammarays and low-energy beta particles emitted by137Cs. For these reasons, the AmBe neutronsource presents a greater risk to the environmentin LWD operations than the 137Cs gamma raysource. Therefore, substituting the PNG for theAmBe source is a great advantage.

While this replacement required developing aporosity measurement with the same response asthe LWD standard AmBe-based neutron porositymeasurement, it also allowed the addition of newmeasurements that significantly enhanceformation evaluation. As the collaborationbetween Schlumberger and JOGMEC made clear,

a PNG could perform well in the LWDenvironment. In addition, formation densitycould now be measured using only a PNG source,providing nuclear formation evaluation for LWDthat contains no chemical logging sources.

Safer, Faster, Smarter LoggingIn January 2001, a Schlumberger team began aproject to incorporate this “sourceless” LWDlogging concept into an integrated LWDmeasurement collar. This new LWD tool nowprovides real-time data for comprehensiveformation evaluation, efficient and safe drillingoperations and accurate well placement.21 TheEcoScope multifunction logging-while-drillingservice eliminates many of the drawbacksassociated with previous LWD technologies. Itacquires new measurements for petrophysicistsand geologists, and provides unsurpassed safetyand efficiency for drilling personnel (below).22

The EcoScope LWD collar is 26 ft long and has a nominal diameter of 63⁄4 in. [17.15 cm].

> Radioactive chemical sources used in well logging. Most density logging tools use a 137Cschemical source, with a half-life of 30.2 years. In a first step, 137Cs decays to an excited state of137Ba through beta-emission of an electron from the nucleus (top). The resulting excited state ofbarium [137Ba*], with a half-life of 2.6 minutes, decays to its stable state through the emission of asingle gamma ray with an energy of 662 keV. Most neutron logging tools use an 241AmBe chemicalsource (bottom). This source relies on a nuclear reaction between high-energy alpha particles—4He nuclei—and 9Be to generate energetic neutrons. 241Am serves as the source of the alphaparticles as it decays to an excited state of neptunium [237Np*]. The 237Np* nucleus reaches itsground state through the subsequent emission of a 60-keV gamma ray. A small fraction of the alphaparticles emitted by the 241Am react with the 9Be surrounding the 241Am nuclei. The reaction resultsin the formation of a short-lived excited state of carbon 13C [13C*], which emits a neutron andtransforms itself into an excited state of 12C [12C*]. 12C* reaches its stable state through the emissionof a high-energy gamma ray. The neutron production in this source is very inefficient. A typicalsource emits about 4x1010 alpha particles in a second and results in about 2x107 neutrons/s.

Cs137 Ba*137 Ba137 γ (662 keV)

β_Cesium Source Reaction

γ (60 keV)

γ (4.4 MeV)

Am241 Np*237 Np237

α (5.5 MeV)

Be9 C*13 C*12 C12n (4 MeV average)

Americium-Beryllium Source Reaction

> The EcoScope integrated measurements collar.New EcoScope measurements include thesourceless neutron-gamma density,spectroscopy, sigma, shock and vibration,inclination, annular pressure and azimuthalgamma ray. The collar is 26 ft in length, and thefarthest measurement is less than 16 ft [4.9 m]from the bottom of the collar.

Neutron-gamma densitySpectroscopySigmaPorosity

Ultrasonic caliper

Azimuthal density,photoelectric factorand density caliper

InclinationAnnular pressurewhile drilling

Three-axis shockand vibration

Azimuthal naturalgamma ray

57088schD04R1.qxd 1/12/06 4:57 PM Page 11

It has a maximum flow rate of 800 galUS/min[3.03 m3/min], can tolerate a maximum doglegseverity while rotating of 8° per 100 ft [30.5 m], amaximum dogleg severity while sliding of 16° per100 ft, and can be run in hole sizes ranging from83⁄8 to 97⁄8 in. [21.3 to 25.1 cm].

Schlumberger experts understood theimportance of establishing backward compati-bility for industry acceptance, so the option ofacquiring a standard density measurement wasincluded in the tool design.23 The Cs-baseddensity option allows the measurement ofphotoelectric factor (PEF) for lithologydetermination. Also acquired are a densitycaliper for log-quality control and hole volume,and density and PEF images for structuralanalysis. The neutron-gamma density is a newmeasurement that compares favorably withprevious density measurements.

The risks associated with the use of LWDchemical sources, especially AmBe, highlight theimportance of a viable PNG source. The safety ofthe PNG is inherent in the EcoScope tool design,as the PNG is powered directly and exclusively bya turbine generator in the TeleScope tool that isenergized by mud circulation.

The challenges of developing such a devicewere considerable. At an early stage,Schlumberger scientists and engineers analyzedthe risks and decided to include a 137Cs source inthe EcoScope tool to supply standard density andPEF measurements and their associated boreholeimages. The position of the 137Cs source waschanged to facilitate quick source loading and toimprove the precision and accuracy of the densitymeasurement. The 137Cs source is loaded from theside of the EcoScope collar, a procedure thattakes, on average, a third of the time needed forloading with the annular method. Also, there is noAmBe source to load. The 137Cs source positionhas been optimized for increased count rates andenhanced density response at high ROP.24

The PNG produces the high-energy neutronsneeded to measure thermal neutron porosity(TNPH), best thermal neutron porosity (BPHI)and formation sigma, or Σ bulk. For consistencyand backward compatibility, the EcoScope TNPHresponse is similar to TNPH from the adnVISIONAzimuthal Density Neutron tool, and these twomeasurements have shown good agreementduring tests. However, because the PNG producesfive times the number of neutrons at three timesthe energy of an AmBe source, the EcoScopeTNPH measurement is statistically more precise,reads more deeply into the formation, and is lessaffected by borehole rugosity. Both the EcoScope

TNPH and adnVISION neutron porosities exhibitthe same formation-density effects. This is themain cause of the familiar high neutron porosityreadings in shale.

Because of the higher neutron energy andthe increased distance of the far-spaceddetector from the neutron source, the EcoScopeneutron porosity measurements are influencedmore by the formation density than is theadnVISION measurement. In the BPHIcalculation, most of the density effects areremoved, resulting in an HI measurement thathas a larger dynamic range, is more precise athigh porosities and has smaller lithology effectsthan the TNPH response (below). The EcoScopeBPHI measurement matches TNPH in cleanzones and gives greater consistency from well towell. The input required for the BPHI densitycorrection can be obtained either from the Cs-source density or from NGD. The advancedneutron measurements from the new LWD tool

give operators more flexibility in designing LWDlogging programs. The PNG provides the optionof eliminating radioactive chemical loggingsources from the entire operation.

The EcoScope tool also includes a propa-gation resistivity measurement, which has thesame measurement principle as the previous-generation LWD resistivity tool, the arcVISIONArray Resistivity Compensated tool. Themeasurement is made at two differentfrequencies—2 MHz and 400 kHz—using tworeceivers and five transmitters with spacingsfrom 16 to 40 in. [41 to 102 cm]. The similaritybetween the EcoScope resistivity measurementand those of previous tools offers distinctadvantages in formation evaluation and allowsthe industry to exploit existing resistivity-modeling breakthroughs. In the EcoScope tool,the resistivity and nuclear measurement sectionsinterleave, a major design innovation thatenabled the shorter, integrated collar.

12 Oilfield Review

> Comparing thermal neutron porosity measurements from the previousadnVISION tool and the new EcoScope tool. The EcoScope thermal neutronporosity, TNPH, compares favorably with the adnVISION TNPH, even indense shales, which are simulated by the alumina formation (top). The bestthermal neutron porosity, BPHI, is an HI measurement that shows a largerdynamic range than the previous TNPH (bottom) and will read lower inshales than the standard TNPH.

1.5

1.0

2.0

0.5

0

2.5

3.0

Nea

r/fa

r corre

cted

ratio

Best thermal neutron porosity (BPHI)

Nea

r/fa

r corre

cted

ratio

1.5

1.0

2.0

0.5

0

2.5

3.0

0 10 20 30 40 50 60 70Porosity, %

EcoScope tool in limestoneadnVISION tool in limestoneEcoScope tool in Al2O3

adnVISION tool in Al2O3


23. Aitken et al, reference 13.24. Weller et al, reference 1.25. Li Q, Liu CB, Maeso C, Wu P, Smits J, Prabawa H and

Bradfield J: “Automated Interpretation for LWDPropagation Resistivity Tools Through Integrated ModelSelection,” Transactions of the SPWLA 44th AnnualLogging Symposium, Galveston, Texas, June 22–25, 2003,paper UU.

26. Barson D, Christensen R, Decoster E, Grau J, Herron M,Herron S, Guru UK, Jordán M, Maher TM, Rylander Eand White J: “Spectroscopy: The Key to Rapid, ReliablePetrophysical Answers,” Oilfield Review 17, no. 2(Summer 2005): 14–33.

27. Ashley el al, reference 17.28. Hutchinson M and Rezmer-Cooper I: “Using Downhole

Annular Pressure Measurements to Anticipate DrillingProblems,” paper SPE 49114, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, September 27–30, 1998.Aldred W, Cook J, Bern P, Carpenter B, Hutchinson M,Lovell J, Rezmer-Cooper I and Leder PC: “UsingDownhole Annular Pressure Measurements to ImproveDrilling Performance,” Oilfield Review 10, no. 4 (Winter 1998): 40–55.

29. The EcoScope annular pressure measurements includea real-time dynamic pressure measurement whenpumps are running, and a static pressure measurementwhen pumps are off. A real-time clock battery suppliespower for the static measurement. Static data are sentto surface once the pumps are turned back on.

30. Inaba M, McCormick D, Mikalsen T, Nishi M, Rasmus J,Rohler H and Tribe I: “Wellbore Imaging Goes Live,”Oilfield Review 15, no. 1 (Spring 2003): 24–37.Breton P, Crepin S, Perrin J-C, Esmersoy C, Hawthorn A,Meehan R, Underhill W, Frignet B, Haldorsen J, Harrold T and Raikes S: “Well-Positioned SeismicMeasurements,” Oilfield Review 14, no. 1 (Spring 2002): 32–45.Bargach et al, reference 5.

Autumn 2005 13

The interpretation of resistivity data is muchmore complex in highly deviated and horizontalwells than in vertical wells. For this reason, atremendous amount of work has gone intoadvanced processing techniques for LWDresistivity data, such as inversion, to resolve trueformation resistivity (Rt) and improve thecalculation of fluid-volume fractions within the reservoir.25

True formation resistivity can be combinedwith the new and traditional nuclearmeasurements from the EcoScope tool toproduce a comprehensive and more quantitativeformation evaluation. This is accomplished in theEcoView interpretation software, specificallydesigned for stand-alone, PC-based interpre-tation of EcoScope data. EcoView softwaresupports two-dimensional (2D) and three-dimensional (3D) visualization of the boreholegeometry and geologic or borehole image data. Inaddition, the software contains the robustmethodology of the DecisionXpress petrophysicalevaluation system.26

The petrophysical interpretation in EcoViewsoftware assumes that EcoScope data arecollected shortly after drilling and that invasionis therefore negligible. EcoScope data acquiredlater in the drilling process, such as during areaming pass, may require more sophisticatedlog-interpretation applications to validate thepetrophysical results. This includes using theSchlumberger GeoFrame integrated reservoircharacterization system.

The integration of geological, petrophysicaland borehole geometry data in one platform givesasset teams improved information for formationevaluation. At the wellsite, EcoView softwareutilizes drilling-related measurements from theEcoScope tool to help improve decision makingfor well placement and drilling optimization.

Safer, Faster, Smarter DrillingThe EcoScope LWD tool acquires triaxial shockand vibration, annular pressure, continuousinclination and ultrasonic- and density-caliperdata. These measurements are monitored in realtime, allowing constant assessment of drillingperformance and hole quality.

Analysis of downhole vibration and shock isessential for optimizing drilling operations andextending BHA component life, including bits,

downhole motors and LWD and MWD systems. InMWD tools, strain gauges measure torque, whileaccelerometers measure axial and lateral shock,which together allow computation of vibration.27

Vibration data help characterize the vibrationmechanism, or the combination of mechanisms,causing the shocks. These can include bitbounce, stick/slip, bit whirl and BHA whirl. Oncethe main cause of the vibration is identified,drilling procedures or parameters may bechanged to correct problems. For example,changing weight on bit (WOB) or rotation ratecan have an enormous effect on vibration.Vibration may also be reduced by changes to theBHA, such as changing to a different bit oradding roller reamers, or changing the mudsystem, such as by increasing mud lubricity.Efforts to minimize vibration and shock oftenimprove ROP and hole quality.

The annular pressure measurement from theEcoScope tool helps drillers and drillingengineers identify and avoid potential drillingproblems.28 In complex drilling environments,such as extended-reach and deepwater wells,real-time annular pressure data help manageequivalent circulating density (ECD). By keepingECD within a tolerable window, drillingengineers can prevent lost circulation andborehole instability. These problems can lead tocost overruns and the potential loss of a well.Moreover, continuous monitoring of annularpressure provides information on solidssuspension, kicks, and swab and surge pressures,promoting the implementation of specificpractices to further optimize drilling operations.29

Caliper measurements during drilling offerimmediate and crucial feedback on wellborestability and borehole shape. Caliper data arealso used to assess well conditions before therunning of casing, and for hole-volumecalculations that are used to estimate requiredcement volume. Historically, hole-size data havebeen difficult to acquire from an LWD platformbecause mechanical calipers, such as those used by wireline tools, are impractical in thedrilling environment.

The EcoScope tool acquires two independentcaliper datasets. Two ultrasonic sensors make a16-sector standoff measurement that is used toprovide the azimuthal borehole diameter whenthe BHA is rotating. When the tool is sliding, themeasurement is taken in opposing directions,perpendicular to the tool axis. An azimuthal

16-sector caliper measurement is also generatedfrom the density-standoff measurement and isacquired when the drillstring is rotating. Thedensity-based caliper measurement requires theuse of the 137Cs source.

In addition to assessing drilling performance,the EcoScope development team focused on wellplacement. Today’s exploration requirementsdemand that drillers hit reservoir targets withprecision and avoid drilling and productionhazards, and that they do so efficiently. Giventhese demands, the most important measure-ments in MWD are related to wellbore position.Wellbore azimuth and inclination data are criticalin allowing directional drillers to adjust welltrajectories to accommodate new geologicalinformation from real-time LWD measurements.30


For optimal directional steering control,borehole trajectory measurements must be takenas close as possible to the bit. Reducing the timelag between taking actions and seeing thequantitative effects of those actions givesdirectional drillers immediate feedback andenhances drilling control. For this reason, thenew EcoScope tool measures continuousinclination 7 ft [2.1 m] above the bottom of thecollar. Borehole orientation data now arrive more quickly and are more relevant to the bitposition, translating into improved directionaldrilling control.

New LWD Technology in the Gulf of MexicoThe positive impact of LWD technologies in theGulf of Mexico is indisputable. With high rigcosts, efficiency improvements in all aspects ofdrilling operations produce enormous dividendsfor operators.

EcoScope technology represents a major stepforward because it expands and improvesformation evaluation while drilling. It reducesthe rig time associated with BHA makeup andbreakdown, allows faster rates of penetrationwithout compromising data quality, andincreases real-time data-transmission rates withthe TeleScope tool. The EcoScope tool designplaces a vast array of measurements much closerto the bit than ever before, minimizingenvironmental and invasion effects on log dataand reducing the time required for crucial datato reach the experts.

Devon Energy Corporation in the Gulf ofMexico ran the EcoScope LWD tool to investigateits operational impact in directional wells in theGulf of Mexico. After drilling one well, Devon ranSchlumberger wireline tools to compareequivalent measurements with EcoScope data.Wireline-conveyed measurements included arrayinduction, standard density, epithermal-neutronporosity, sigma and neutron capture spectro-scopy. A detailed comparison of the datasetsdisplayed excellent agreement in the shale sections.The log comparison in the permeable sandsshowed differences, which have been interpretedas caused by the invasion of synthetic oil-basemud filtrate into the water-bearing sands. Thisinvasion occurred during the time that elapsedbetween drilling and wireline logging (right).

On another well, the EcoScope measure-ments were compared with those of theadnVISION tool placed 50 ft [15 m] above the 26-ft EcoScope collar. During the drilling of thiswell, the BHA was pulled out of the hole because

14 Oilfield Review

> Comparison of EcoScope and wireline data in the Gulf of Mexico.EcoScope and wireline measurements overlay in shales where invasioneffects are negligible. However, in the more porous and permeablesandstones, the EcoScope and wireline data show differences as a resultof oil-base mud-filtrate invasion. True resistivity, Rt, readings from bothlogging methods read similarly because those measurements read beyondthe invasion-affected volume. The effect of the oil-base mud filtrate is seenin the wireline bulk density and shallow resistivity curves. The wireline andEcoScope neutron porosity measurements are in good agreement. Track 1shows a comparison of gamma ray readings and a caliper curve, Track 2displays a comparison of Rt data and the wireline flushed-zone resistivitycurve, and Track 3 contains overlays of sigma, hydrogen index and densitymeasurements made by EcoScope and wireline tools.

X,050

X,100

X,150

X,200

X,250

X,300

X,350

0 200gAPI

Gamma Ray - EcoScope

0 200gAPI

Gamma Ray Wireline

6 16in.

Caliper

6 16in.

Bit Size

0.2 200ohm.m

True Resistivity - EcoScope

0.2 200ohm.m

True Resistivity - Wireline

0.2 200ohm.m

Flushed-Zone Resistivity-Wireline

1.85 2.85g/cm3

Bulk Density - EcoScope

1.85 2.85g/cm3

Bulk Density - Wireline

0.45 -0.15volume ratio

Hydrogen Index - EcoScope

0.45 -0.15volume ratio

Hydrogen Index - Wireline

60 0cu

Sigma - EcoScope

60 0cu

Sigma - Wireline

Dept

h, ft


Autumn 2005 15

of lost mud returns. The proximity of EcoScopenuclear data to the bit permitted the operator torun a liner without having to prepare for andperform a wireline logging run to identify whatwas in the bottom sandstones, savingUS$ 250,000 (left). Previous LWD nuclear toolswould not have been close enough to the bit toproperly characterize the entire sand interval.

Kicking Off in the North SeaIn the central North Sea, Nexen Petroleum U.K.Limited tested the full range of drilling andformation evaluation measurements in EcoScopetechnology.31 To recover attic hydrocarbons,Nexen designed an updip sidetrack of an existingwell to penetrate the reservoir sands at an angle of 35° and to produce the untapped hydro-carbons from an 81⁄2-in. [21.6-cm] borehole. High-quality formation evaluation was crucial to placethe well within the reservoir and to complete themost productive intervals. Other objectives forNexen were to assess the quality of the newEcoScope measurements and to determine andquantify any safety and efficiency advantages.

To perform a meaningful comparisonbetween EcoScope data and previous LWD poros-ity measurements, Nexen and Schlumbergerincluded an adnVISION tool above the TeleScopetool, located above the EcoScope collar. TheEcoScope tool acquired standard gamma ray,resistivity from 2-MHz and 400-kHz phase andattenuation measurements, Cs-density measure-ment, neutron porosity and caliper data tocharacterize the reservoir and to compare these measurements with the correspondingmeasurements from the adnVISION tool. Theobjective of the LWD operation was to fullyevaluate EcoScope standard LWD measurements,and assess the potential value of the advancedEcoScope measurements, such as sigma, capturespectroscopy, neutron porosity and neutron-gamma density generated from the PNG.

Two gas-bearing reservoir sand units werepenetrated by the sidetrack well, logged one totwo hours after the bit by the EcoScope sensorsand then logged again with the adnVISION stringone to two hours after the EcoScope collar passedthe interval (left). The log data comparedfavorably, but showed differences because ofrapid invasion into the reservoir. While theinherent time lapse between the measurementcollars during drilling helped to characterize theinvasion process, another, more definitive,

31. Weller et al, reference 22.

>Measurements closer to the bit. An operator in the Gulf of Mexico ran theadnVISION and EcoScope tools to compare these LWD tools. Lost-circulationproblems forced the driller to pull the BHA out of the hole before the lowestsandstone of interest could be logged by the adnVISION tool. Without theEcoScope tool in the BHA, the lowest sandstone at X,100 ft would have beenmissed by the previous technology. Because the EcoScope measurementswere taken close to the bit, this tool was able to properly characterize thesandstone and a wireline logging run was avoided. The EcoScope toolindicates significantly more gas (yellow shading) in the apparent gas-bearingsands than the adnVISION tool, which was logged approximately four hoursafter the EcoScope tool. Track 1 contains gamma ray, caliper and ROP data.Track 2 compares the resistivity measurements from the EcoScope tool.Track 3 displays the adnVISION nuclear measurements, while Track 4 showsthe EcoScope nuclear measurements.

X,000

Dept

h, ft

X,050

X,100

0 150gAPI

Gamma Ray

500 0ft/h

0.2 200ohm.m

Phase Shift Resistvity 40-in.

0.2 200ohm.m


0.2 200ohm.m


0.2 200ohm.m


0.2 200ohm.m


0.02 20ohm.m

Attenuation Resistvity 40-in.

0.02 20ohm.m


0.02 20ohm.m


0.02 20ohm.m


0.02 20ohm.m


0 100

Photoelectric Factor Bottom

1.65 2.65g/cm3

Bulk Density Bottom

0.6 0ft3/ft3

Thermal Neutron Porosity

-0.8 0.2

Density Correction Bottomg/cm3

adnVISION Tool

100

2.65g/cm3

Bulk Density Bottom

0ft3/ft3

Thermal Neutron Porosity

0.2

Density Correction Bottomg/cm3

EcoScope Tool

0

1.65

0.6

-0.8

5 25in.

Horizontal Hole Diameter

5 25in.

Vertical Hole Diameter

EcoScope Tool

Photoelectric Factor Bottom

Crossover Crossover

Rate of Penetration5-ft Average

> Elapsed time from drilling to logging for two LWD tools. The EcoScopesensors lagged the bit by 1 to 2 hours. The adnVISION tool passedcorresponding depths 2.5 to 4 hours after the bit. Time lag between the twotools varies because of changes in the rate of penetration.

Dept

h, ft

10,500

10,450

10,400

10,350

10,300

10,550

10,600

10,650

10,700

Time after bit, h0 1 2 3 4 5

EcoScope tool adnVISION tool


comparison was made 20 hours after drillingduring a reaming pass. This pass showed goodagreement between the measurements (left).

Petrophysical and productivity analyses usingEcoView interpretation software were completedover the zones of interest. EcoView software usesSpectroLith lithology processing of spectra fromneutron-induced gamma ray spectroscopy.Elemental yield information from the EcoScopespectroscopy data allows the determination oflithology, including accurate clay fractions.32

Knowing the lithology makes it possible tocompute matrix properties—for example graindensity, grain PEF, grain sigma and grain neutronvalues—for use in petrophysical evaluation.33

The EcoView analysis identified twoproductive sand intervals with low clay volume.The upper sand at a depth of 10,360 ft [3,158 m]is 125 ft [38 m] thick, and the lower sand at adepth of 10,660 ft [3,249 m] is 63 ft [19 m] thick.The sands exhibit porosities of at least 15%,calculated peak permeabilities of more than1 Darcy and a good estimated productivity. Oncompletion, the lower sand was perforated andproduced 11,000 bbl [1,748 m3] per day ofcondensate and 54 million ft3 [1.5 million m3]per day of gas. The upper sand interval will becompleted after the lower interval startsproducing excessive water.

The EcoScope LWD collar also recordedborehole images to determine the structural dipof the interval. Both gamma ray and densityimages were computed and interpreted usingdip-picking capabilities in WellEye three-dimensional borehole data viewer software,which is used to view borehole images andperform dip computations. While drilling thissidetrack, drillers encountered both stick/slipproblems and excessive vibration. Theseoccurrences were captured on the driller’sdisplay, helping Nexen correlate drillingproblems to specific lithologies within thesection and potentially mitigating problems infuture wells (left).

Safe and efficient operations are extremelyimportant to Nexen. The EcoScope tool supportsthis goal; unlike other LWD collars, it does notrequire personnel to handle lithium batteries,because the TeleScope turbine generator,energized by circulating drilling mud, suppliesits power. Also, the new, integrated, single-collardesign has eliminated the handling of othercollars when tripping into and out of the well.The single source can be removed quicklybecause it is accessed from the side of the collar,and the single collar can be racked straight back

16 Oilfield Review

> Time-lapse logging. EcoScope and adnVISION log data are compared fromthe drilling pass and from a reaming pass 20 hours later. While the differencebetween the two datasets from the drilling pass shows clear effects from adynamic invasion process, the reaming pass data overlay. This suggests thatthe invasion of mud filtrate had stabilized by the time the reaming pass wasmade. In addition, the time difference between the EcoScope pass and theadnVISION pass was substantially less during the reaming pass because theBHA had moved through the zone more quickly during reaming than duringdrilling. Track 1 shows gamma ray and caliper data. Tracks 2 and 3 displaythe comparison of bulk density and neutron porosity readings, respectively.Log data from drilling and reaming passes are displayed. Track 4 contains theEcoScope density image.

10,400

10,450

10,500

Mea

sure

d de

pth,

ft

150

Gamma Ray

gAPI

0 150

Gamma Ray

300

gAPI

6

Caliper

in. 16

1.95 2.95g/cm3

1.95 2.95g/cm3

1.95 2.95g/cm3

1.95 2.95g/cm3

Bulk Density adnVISION Drilling

0.45 -0.15ft3/ft3

0.45 -0.15

0.45 -0.15

0.45 -0.15

Neutron Porosity EcoScope Drilling

DensityImage

Low High

Bulk Density EcoScope Reaming

Bulk Density adnVISION Reaming

Bulk Density EcoScope Drilling

ft3/ft3

ft3/ft3

ft3/ft3

Neutron Porosity adnVISION Reaming

Neutron Porosity EcoScope Reaming

Neutron Porosity adnVISION Drilling

> A wealth of drilling data. Presenting drilling data along with lithologicinformation allows drilling experts to identify troublesome formations andmitigate future problems when drilling equivalent strata. Caliper andborehole-shape data are presented along with the gamma ray curve in Track1. The depth track contains lithology information from the capturespectroscopy data. Track 2 shows the PEF image data, acquired from thestandard Cs-based density measurement section on the EcoScope collar.Track 3 contains BHA rotational information, and Track 4 shows tool-vibrationdata. Interestingly, the vibration levels decrease in the sands and increase inthe shales. The last track displays annular temperature and pressure data.

500 ft/h

Rate of Penetration5-ft Average

6 in.

Density Caliper,Average Diameter

6 in.

Bit Size

0 gAPI

Gamma Ray

in.

Density Caliper,Vertical

in.

Density Caliper,Vertical

Mineralogy andHole Shape

EcoScope

Photoelectric Effect c/min

Instantaneous CollarRotational Speed,

Minimum

c/min

Instantaneous CollarRotational Speed,

Maximum

c/min

Rotational Speed

gn

S-Axis RMS Vibration

ft-lbf

Torsional (Rotational)RMS Vibration

gn

Lateral Vibration

psi

DownholeAnnular Pressure

°F

DownholeAnnular Temperature

6 in.

Ultrasonic Caliper,Average Diameter

0

16

16

150

-16 16

16 -16

0 200

0 200

0 200

0 10

0 5,000

0 10

0 1,500

0 250

10,600

10,650

10,700

10,750

10,800

10,850

16


Autumn 2005 17

when it is pulled out of the hole. Incorporatingthe PNG has reduced personnel exposure toradiation during source loading and unloading.Using an LWD string that contains no chemicallogging sources remains an attractive option forNexen in the future.

In extrapolating the time-savings made onthis sidetrack well to typical future operations,the EcoScope tool will save an estimated 6 hoursof total rig time. The major time savingsidentified included one hour because LWDengineers no longer have to load and unload theAmBe radioactive source. Also, the single,shorter EcoScope collar reduces the pickup andlaydown times, saving approximately 11⁄2 hours,and shorter repeat sections logged at 450 ft/h[137 m/h] saves another hour per run.34 Nexenviews these new efficiencies as significant,amounting to over US$ 65,000 in rig-time savings.

Finding Sand in the Gulf of MexicoChevron has deployed EcoScope technology toimprove formation evaluation and operatingefficiency in a stratigraphically complex, faultedstructure in the Gulf of Mexico in water depthsgreater than 400 ft [122 m]. The field features aneast-dipping fault and a stratigraphic pinchout tothe north. The main reservoir comprisesPleistocene sand lobes, which may or may not becontinuous and connected. Despite detailedmapping with extensive 3D seismic surveys, thislateral, large-scale heterogeneity complicatesdrilling, development and recovery strategies forChevron. Six directional production wells havebeen drilled from a single platform, interceptingthe sands at angles between 30° and 60° to drainas many productive sands as possible.

Drilling and completion challenges aresignificant in this field. Abnormal pressures andlost-circulation zones exist because the reservoirsands are permeable and unconsolidated.Complicating matters further, the shales areprone to swelling and cause excessive torque anddrag, frequently resulting in stuck pipe. Chevroncontinues to search for the optimal mud systemto mitigate these problems, and has used threedifferent mud systems, including both water-baseand oil-base mud systems. Also, Chevron installsfrac-pack sand-control completions because thesands are unconsolidated.35

In this area, the logging program iscontingent on a variety of factors—includinglocal reservoir complexities, control from offsetdata and anticipated borehole problems—and isusually a combination of both LWD and wireline-conveyed methods. The most commonly run LWDmeasurements are gamma ray and resistivity,

while wireline techniques are used to acquireneutron and density data, measure reservoirpressures, acquire reservoir-fluid samples andcollect sidewall cores. A key Chevron objective inrunning the wireline tools is to identify fluidtypes and fluid-contact levels within the varioussand lobes, since both vary considerably fromsand to sand.

In February 2005, Chevron ran the EcoScopecollar while drilling, and a standard suite ofwireline logs, plus a DSI Dipole Shear SonicImager tool and sidewall cores after the wellreached TD. The wireline tools were run 48 hoursafter the EcoScope sensors, giving Chevroninsight into invasion processes in the Pleistocenesands (above).

32. Herron SL and Herron MM: “Quantitative Lithology: AnApplication for Open and Cased Hole Spectroscopy,”Transactions of the SPWLA 37th Annual LoggingSymposium, New Orleans, June 16–19, 1996, paper E.Grau JA, Schweitzer JS, Ellis DV and Hertzog RC: “A Geological Model for Gamma-Ray SpectroscopyLogging Measurements,” Nuclear Geophysics 3, no. 4(1989): 351–359.

33. Herron MM, Herron SL, Grau JA, Seleznev NV, Phillips J,El Sherif A, Farag S, Horkowitz JP, Neville TJ and Hsu K:“Real-Time Petrophysical Analysis in Siliciclastics fromthe Integration of Spectroscopy and Triple-ComboLogging,” paper SPE 77631, presented at the SPE AnnualTechnical Conference and Exhibition, San Antonio,Texas, September 29–October 2, 2002.

Herron SL and Herron MM: “Application of NuclearSpectroscopy Logs to the Derivation of Formation Matrix Density,” Transactions of the SPWLA 41st AnnualLogging Symposium, Dallas, June 4–7, 2000, paper JJ.

34. The EcoScope tool obtains two data points per foot at a450 ft/h [137 m/h] ROP.

35. Gadiyar B, Meese C, Stimatz G, Morales H, Piedras J,Profinet J and Watson G: “Optimizing Frac Packs,”Oilfield Review 16, no. 3 (Autumn 2004): 18–29.

> Chevron EcoScope field data from the Gulf of Mexico. The EcoScope datawere acquired immediately after the bit penetrated an oil- and gas-bearingsandstone from XY,864 ft to XY,926 ft and before significant invasionoccurred. Track 1 contains gamma ray and ultrasonic caliper data. Tracks 2and 3 show resistivity and porosity data, respectively. Track 4 displays theEcoScope sigma measurement. Sigma acquired prior to invasion matchedcore-analysis results and represents an excellent baseline sigma log forcomparison with future sigma data from tools such as the RST ReservoirSaturation Tool. Time-lapse sigma logging allows reservoir engineers tomonitor reservoir drainage and identify bypassed reserves.

XY,800

XY,850

XY,900

Mea

sure

d de

pth,

ft

in.6 16 0.1 1,000ohm.m 1.65 2.65g/cm3 60 0cu

SigmaBulk Density

0.6 0ft3/ft3

Neutron Porosity

0.6 0ft3/ft3

Best Neutron Porosity

Gas Effect

Bit Size

gAPI0 150

Gamma Ray

in.6 16

Ultrasonic Caliper

Washout

0.1 1,000ohm.m

0.1 1,000ohm.m

0.1 1,000ohm.m

0.1 1,000ohm.m

EcoScope Resistivity 16-in.






The Chevron petrophysicist responsible forevaluating this complex field assessed theEcoScope measurements. The computed porosityfrom EcoScope data and sigma responsesmatched core-fluid analysis. For Chevron, sigmafrom an LWD tool represents perhaps the mostsignificant EcoScope breakthrough because theEcoScope tool supplies an efficient way to obtaina baseline sigma measurement instead of having

to make a separate wireline run after casing isset. This avoids having to wait several weeks forinvading fluid to dissipate before acquiring sigmadata by running tools such as the RST ReservoirSaturation Tool or the TDT Thermal Decay Timetool on wireline in cased hole. As the reservoir isproduced, the original baseline sigma logs can becompared with subsequent sigma logs to identifybypassed reserves and to refine reservoir fluid-flow models.

Invasion, borehole-rugosity and tool-stickingeffects were visible on the wireline formationevaluation data. Chevron and Schlumbergercompared the processed results from EcoScopedata with the results computed using thewireline measurements (above). As anticipated,there was a considerable difference between themoved fluid analyses from each set ofmeasurements caused by invasion of drilling

18 Oilfield Review

> Comparing EcoScope data with similar wireline measurements. A comparison of the wireline and EcoScope datareveals the impact of invasion and tool pulls. Tracks 1, 2 and 3 compare the data from the EcoScope integrated LWDtool with wireline measurements. The wireline fluid-volume analysis in Tracks 4 and 5 shows a significantpercentage of flushed hydrocarbon, while the EcoScope tool, which logged immediately after the bit cut theformation, indicates that the near-wellbore region is virtually unflushed (Tracks 6 and 7). During the wireline loggingrun, the toolstring pulled, causing erroneous log data across the sandstone at XY,970 ft. Formation evaluation usingthe EcoScope data identified the sand as productive.

XY,850

XY,900

XZ,000

XY,950

Mea

sure

d de

pth,

ft

Gamma RayEcoScope

150gAPI0

Caliper Wireline16in.6

Ultrasonic CaliperEcoScope

16in.6

150gAPI0

0.01 100ohm.m


0.01 100ohm.m


0.01 100ohm.m


0.01 100ohm.m


0.01 100ohm.m


0.1 1,000ohm.m

Wireline Resistivity 90-in.

0.1 1,000ohm.m


0.1 1,000ohm.m


0.1 1,000ohm.m


0.1 1,000ohm.m


0.6 0ft3/ft3

Best Neutron Porosity

0.6 0ft3/ft3

Neutron Porosity EcoScope

1.65 2.65g/cm3

Bulk Density EcoScope

1.65 2.65g/cm3

Bulk Density Wireline

0.6 0ft3/ft3

Neutron Porosity Wireline

01

ELANPlus Volumes

Illite

Bound Water

Quartz

Gas

Water

Oil

Moved Hydrocarbon

Moved Water

Gas

Oil

Water

Irreducible Water

Moved Hydrocarbon

Moved Water

00.5

ELANPlus Volumes

01

ELANPlus Volumes

00.5

ELANPlus Volumes

Illite

Bound Water

Quartz

Gas

Oil

Water

Irreducible Water

Moved Hydrocarbon

Moved Water

Gas

Oil

Water

Irreducible Water

Moved Hydrocarbon

Moved Water

ft3/ft3 ft3/ft3 ft3/ft3 ft3/ft3

WirelineVolumetric Analysis

WirelineFluid Analysis

EcoScopeVolumetric Analysis

EcoScopeFluid Analysis

Tens

ion

Gamma RayWireline

Irreducible Water


Autumn 2005 19

fluids. The wireline data were greatly affected bytool-sticking problems and failed to identify apotential reservoir sand. The EcoScopemeasurements taken immediately after the bitpenetrated this sandstone indicated that thelower zone was productive.

Schlumberger consulting log analysts, usingthe EcoScope log data and ELANPlus advancedmultimineral log analysis processing, performed

the formation evaluation. Log-derived porositiesand permeabilities compared closely withporosities and empirical permeabilities derivedfrom sidewall core samples. In addition, fluidtypes identified in the cores correlated well withthe ELANPlus fluid calculations using EcoScopedata, particularly considering the significantinvasion that had taken place between the time

the EcoScope measurements were taken and thesidewall cores were acquired (above).

From the ELANPlus producibility results andinput from Chevron, including fluid propertiesand reservoir pressure, analysts computed a log-based flow profile. It shows the relativecontribution of each of the two sand intervals.The lower zone was completed in March 2005,

> EcoScope formation evaluation results versus core-analysis data. Effective porosity from EcoScopedata compares well with core analysis from sidewall cores taken during a wireline run (Track 4). Theempirically derived core permeabilities displayed in Track 5 reasonably match log-derived permeabilitycomputed during ELANPlus advanced multimineral log analysis processing in GeoFrame software. Thefirst three tracks contain EcoScope field data. Proposed completion intervals are shown on the right sideof the depth track. Track 4 displays lithology, measured core porosities, computed effective porosity andfluid-volume fractions. Track 5 shows core permeabilities along with computed log-derivedpermeabilities, and Track 6 contains calculated mobility for each fluid type. Track 7 shows the log-basedflow profile, which predicts the relative production from each completion interval.

Mea

sure

d de

pth,

ft

XY,800

XZ,050

XY,850

XY,900

XY,950

XZ,000

Bit Size16in.6

Gamma Ray150gAPI0

Caliper16in.6

Washout

0.2 2,000ohm.m

Resistivity 40-in.

0.2 2,000ohm.m

Resistivity 34-in.

0.2 2,000ohm.m

Resistivity 28-in.

0.2 2,000ohm.m

Resistivity 22-in.

0.2 2,000ohm.m

Resistivity 16-in.

Crossover

0.6 0ft3/ft3

Density Porosity

0.6 0ft3/ft3

Neutron Porosity

1 0

Volumetric Analysis

Mixed Layered Clay

Bound Water

Quartz

Calcite

Gas

Oil

Water

Moved Hydrocarbon

Moved Water

1 0Core Porosity

ft3/ft3

0.1 10,000mD

Permeability Water

0.1 10,000mD

Permeability Gas

0.1 10,000mD

Permeability Oil

0.1 10,000mD

Intrinsic Permeability

1 0

Core Permeabilityft3/ft3

0.1 10,000mD/cP

Gas Mobility

0.1 10,000mD/cP

Water Mobility

0.1 10,000mD/cP

Oil Mobility

1 0ft3/ft3

Water Flow

1 0ft3/ft3

Oil Flow

Gas

Oil

Water

1 0ft3/ft3

Gas Flow

Perfo

ratio

ns

ft3/ft3

Irreducible Water

57088schD04R1.qxd 1/12/06 4:57 PM Page 19

and four months later was producing 3,000 bbl[477 m3] of oil and 8 million ft3 [226,535 m3] ofgas per day through a slotted sleeve. The slidingsleeve remained closed across the upper zonebecause of the production capacity limitations onthe platform. However, according to the analysis,the upper zone is expected to contribute morethan three times the production volume observedfrom the lower zone.

Formation Evaluation While Drilling CarbonatesRecently, a major national oil company ran theEcoScope tool while drilling a Middle Eastcarbonate reservoir. EcoScope technology helpeddrilling and reservoir teams avoid high-permeability water zones, and helped identifyhigh-salinity, low-resistivity pay zones that addedsignificant production revenues.

The operator also ran adnVISION technologyto compare measurements. Before drilling avertical pilot hole, the operator predicted thatthe vertical distance between the deepestpotential pay zone and the water-producinginterval would be 26 ft. The EcoScope measure-ments were close enough to the bit to supplycritical data to the directional driller and assetteam to successfully geostop the well just 2 ft[0.6 m] above the water layer. A full evaluation ofthe carbonate reservoir was accomplished within21 ft [6.4 m] of TD (above left). Currently, onlythe EcoScope collar can make this manymeasurements so close to the bit.

This heterogeneous reservoir contains low-resistivity pay zones, which are notoriouslydifficult to interpret using standard resistivity-based methods. Erroneous water-saturationcalculations come from low-resistivity pay zonesthat have water-filled microporosity and oil-filledmacroporosity. The microporosity essentiallyforms a conductive path, producing resistivityvalues that underestimate the hydrocarbonpercentage within the pore spaces. This type ofreservoir can often be evaluated more accuratelyusing sigma instead of resistivity because sigmais a volumetric measurement. The contrast islarge between sigma hydrocarbon readings—from 1 to 12 capture units (cu) for gas andaround 20 cu for oil—and Middle East formation-water sigma readings—often greater than 90 cu.Until now, the sigma measurement has not beenavailable in LWD technologies and could neverhave been acquired with wireline tools beforeextensive mud-filtrate invasion had occurred.

20 Oilfield Review

> Placing LWD sensors closer to the bit. The EcoScope LWD tool provides a complete formation evaluation within 21 feet of the bit. The diagramsummarizes the Middle East case, demonstrating the importance of havingmeasurement sensors as close as possible to the bit. In this example, theoperator stopped drilling just 2 ft above a water-producing layer, and wasstill able to evaluate the low-resistivity oil zone, Unit B.

70

60

50

40

30

20

10

0

Interunit dense

Unit B,low-resistivityoil zone

Unit A,low-resistivityoil zone

Predictedinterunitdense thickness

Predicted Unit Cwater zone

Tool

leng

th, f

t

WellTD

24 ftto bottomof Unit B

Complete formationevaluation within21 ft of well TD

6 3/4-in.EcoScope Tool 2 ft

Expected top of water zone

ResistivityNeutronDensityCaliperGamma ray

> Finding productive low-resistivity pay using sigma. Several different combinations of saturationexponent, n, and cementation factor, m, were used (Tracks 1 and 2) to compute Sw from Archie’sequation, and these were compared to the linear sigma solution for Sw. Total porosity and hydrocarbonvolumes from both resistivity and sigma solutions are shown in Track 3, while lithology and fluidvolumes are presented in Tracks 4 and 5. The resistivity-based answer (Track 4) was pessimistic,while the sigma-based Sw (Track 5) more accurately predicted the observed production from the zone.

1 01 0

IlliteIllite

0.4 0

0.4 0ft3/ft3

0.4 0Bound WaterBound Water

AnhydriteAnhydrite

CalciteCalcite

DolomiteDolomite

GasGas

OilOil

WaterWater

SigmaResistivity

ft3/ft3ft3/ft31 10 0

ft3/ft31 0

ft3/ft31 0

ft3/ft31 0

ft3/ft31 0

ft3/ft31 0

ft3/ft3 01

ft3/ft3 01

ft3/ft31 0

ft3/ft31 0

ft3/ft31 0

ft3/ft31 0

ft3/ft3

ft3/ft3

ft3/ft3 ft3/ft3

ELANPlus VolumesELANPlus VolumesHydrocarbon Volume Using Sigma

Total Porosity

Hydrocarbon Volume Using Resistivitym = 2.0, n = 2.0

From Sigma From Sigma

m = 2.2, n = 2.0

m = 1.7, n = 2.0

m = 2.3, n = 2.0

m = 2.1, n = 2.0

m = 2.0, n = 2.0

m = 1.9, n = 2.0

m = 1.8, n = 2.0

Water Saturation

m = 2.0, n = 1.5

m = 2.0, n = 2.5

m = 2.0, n = 1.0

m = 2.0, n = 2.0

Water Saturation


Autumn 2005 21

Because the EcoScope sigma measurementwas taken during the time when invasion isconsidered to be negligible, it was especiallyhelpful in identifying low-resistivity productivezones that would have been bypassed if evaluatedwith resistivity-based methods. Formationevaluation using the sigma measurementrevealed significantly more oil in the low-resistivity pay zone than was indicated byconventional resistivity analysis. When the wellwas put on line, the zone produced approximately70% oil (previous page, bottom).

At Bit’s End The development of the EcoScope LWD tool hasmoved the industry closer to the ultimate goal offormation evaluation independent of the mode ofconveyance. This tool increases efficiency,improves safety and reduces formation-evaluation uncertainty for operating companies.

The integrated single-collar design, higherallowable penetration rates during drilling andquicker source loading improve efficiency. Thesingle-collar design and the elimination of theAmBe source reduce tool- and source-handlingrequirements, and decrease safety andenvironmental risks.

The new LWD nuclear measurements intro-duced with EcoScope technology use a PNG thatis powered by a downhole mud turbine from theTeleScope MWD tool. EcoScope technology nowallows quantitative formation evaluation to occurmuch closer to the bit than do previous LWDtools. Moreover, interactive, PC-based EcoViewsoftware facilitates immediate and integratedinterpretation of EcoScope data (above).Because of these advantages, the EcoScope toolis making a major impact in oil-producingregions around the world. It has been run morethan 100 times worldwide.

From the beginning, the development of theEcoScope LWD collar was based on input fromthe exploration and production industry. Thecollaboration of Japan Oil, Gas and MetalsNational Corporation and Schlumberger helpedproduce the first PNG-based porosity device forLWD applications. Schlumberger engineers andscientists designed an integrated-measurementscollar that contributes to safer, more efficientdrilling operations. Asset teams now receivehigh-quality formation-evaluation and drillingdata that are acquired closer to the bit than everbefore, and operators no longer have to wait foranswers that, until recently, came only from theend of a wireline cable. —MGG

> EcoView integrated, stand-alone formation evaluation software specifically developed for EcoScope data visualization and analysis.

3D welltrajectory display

Petrophysics plot

Crossplot analysis

Answer plot of interpretation results

Expanded view
