40 Oilfield Review

Nuclear Magnetic Resonance Logging While Drilling

R. John Alvarado Houston, Texas, USA

Anders Damgaard Pia Hansen Madeleine Raven Maersk OilDoha, Qatar

Ralf HeidlerRobert HoshunJames KovatsChris Morriss Sugar Land, Texas

Dave Rose Doha, Qatar

Wayne Wendt BPHouston, Texas

For help in preparation of this article, thanks to Emma JaneBloor, Jan Morley, Marwan Moufarrej and CharlesWoodburn, Sugar Land, Texas, USA; Kevin Goy, Doha,Qatar; Mohamed Hashem, Shell, New Orleans, Louisiana,USA; Martin Poitzsch, Clamart, France; Joe Senecal,Maersk Oil, Doha, Qatar; and Brett Wendt, ConocoPhillips,Houston, Texas.CMR (Combinable Magnetic Resonance), CMR-200, CMR-Plus, IDEAL (Integrated Drilling Evaluation andLogging), MDT (Modular Formation Dynamics Tester),PowerDrive, PowerPulse, proVISION and VISION are marks of Schlumberger.

Innovative drilling and measurements technologies now provide increasingly

comprehensive borehole and formation-evaluation data in real time. Recent

developments in nuclear magnetic resonance logging while drilling are helping

operators make more informed drilling and completions decisions, reduce risk and

nonproductive time and optimize wellbore placement and productivity.

Nuclear magnetic resonance (NMR) logging whiledrilling (LWD) represents a significant advance-ment in geosteering and formation-evaluationtechnology, bringing the benefits of wireline NMRto real-time drilling operations. Critical petro-physical parameters, such as permeability andproducibility estimates, can now be obtainedwhile drilling, providing information that helpspetrophysicists, geologists and drillers achieveoptimal wellbore placement within a reservoir.

Real-time while-drilling measurements areespecially important in high-cost and time-sensitive drilling environments. With rig costsrunning as high as USD 175,000 per day, errors in well placement, formation evaluation or well-completion design can result in significant additional well costs or the drilling of expensive sidetracks.1

In this article, we review basic NMR con-cepts, introduce developments in NMR loggingwhile drilling and discuss how operators areusing this technology for wellbore placement andformation evaluation in real time.

Development of Wireline NMRIn the decade that NMR logs have been available,they have undergone continual improvement.2

The CMR Combinable Magnetic Resonance toolfamily, beginning with the introduction of theCMR-A service in 1995, provided measurementsof effective porosity, bound-fluid volume (BFV),permeability and T2 distributions, a conceptdescribed later in this article. The CMR-200Combinable Magnetic Resonance tool introducedadvances in electronics that provide an increasedsignal-to-noise ratio (S/N) while shorter echo

1. Aldred W, Plumb D, Bradford I, Cook J, Gholkar V,Cousins L, Minton R, Fuller J, Goraya S and Tucker D:“Managing Drilling Risk,” Oilfield Review 11, no. 2(Summer 1999): 2–19.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.Bratton T, Edwards S, Fuller J, Murphy L, Goraya S,Harrold T, Holt J, Lechner J, Nicolson H, Standifird W

and Wright B: “Avoiding Drilling Problems,” OilfieldReview 13, no. 2 (Summer 2001): 32–51.

2. Kenyon B, Kleinberg R, Straley C, Gubelin G and Morriss C: “Nuclear Magnetic Resonance Imaging—Technology for the 21st Century,” Oilfield Review 7, no. 3(Autumn 1995): 19–33.Allen D, Crary S, Freedman B, Andreani M, Klopf W,Badry R, Flaum C, Kenyon B, Kleinberg R, Gossenberg P,Horkowitz J, Logan D, Singer J and White J: “How toUse Borehole Nuclear Magnetic Resonance,” OilfieldReview 9, no. 2 (Summer 1997): 34–57.Allen D, Flaum C, Ramakrishnan TS, Bedford J, Castelijns K,Fairhurst D, Flaum C, Gubelin G, Heaton N, Minh CC,Norville MA, Seim MR and Pritchard T: “Trends in NMRLogging,” Oilfield Review 12, no. 3 (Autumn 2000): 2–19.For more on the history and development of NMR logging:Dunn KJ, Bergman DJ and LaTorraca GA: NuclearMagnetic Resonance—Petrophysical and LoggingApplications, Seismic Exploration No. 32. Amsterdam,The Netherlands: Pergamon Press (2002): 3–10.

3. Allen et al (2000), reference 2.

Summer 2003 41

spacing, on the order of 200 µs, improved petro-physical measurement quality, including totalporosity. Further improvements led to the CMR-Plus logging tool with high-speed capability toacquire data at logging rates up to 2400 ft/hr[730 m/hr] for full porosity logging and 3600 ft/hr[1100 m/hr] for bound-fluid logging, rates three tofive times faster than the CMR-200 tool.3

To date, more than 7000 CMR logging jobshave been performed. For many applications,NMR measurements are superior to other loggingtechniques and can provide critical answers to

questions concerning the presence, type and producibility of reservoir fluids. For many operators, NMR logging has become a routineservice in typical logging programs.

Dance of the ProtonsNMR logging measures the magnetic moment ofhydrogen nuclei (protons) in water and hydro-carbons. Protons have an electrical charge andtheir spin creates a weak magnetic moment.NMR logging tools use large permanent magnetsto create a strong, static, magnetic-polarizingfield inside the formation. The longitudinal-relaxation time, T1, describes how quickly thenuclei align, or polarize, in the static magneticfield. Full polarization of the protons in pore fluids takes up to several seconds and can be

done while the logging tool is moving, but thenuclei must remain exposed to the magnetic fieldfor the duration of the measurement. The rela-tionship between T1 and increasing pore size isdirect, yet inverse, to formation fluid viscosity.

A series of timed radio-frequency (rf) pulsesfrom the logging-tool antenna can be used tomanipulate proton alignment. The aligned protonsare tilted into a plane perpendicular to the staticmagnetic field. These tilted protons precessaround the direction of the strong induced magnetic field. The precessing protons createoscillating magnetic fields, which generate aweak but measurable radio signal. However,since this signal decays rapidly, it has to beregenerated by repeatedly applying a sequenceof radio-frequency pulses. The precessing protonsin turn generate a series of radio-signal pulses orpeaks known as spin echoes. The rate at whichthe proton precession decays, or loses its align-ment, is called the transverse-relaxation time, T2.

T1 and T2 processes are affected predomi-nantly by interaction between pore-fluidmolecules, or bulk-relaxation characteristics,and from pore-fluid interactions with the grainsurfaces of the rock matrix, also known as surface-relaxation characteristics. In addition, inthe presence of a significant magnetic-field gradient within the resonant zone, there is relax-ation by molecular diffusion that influences onlyT2 processes.4

NMR While DrillingFollowing the widespread acceptance of wirelineNMR, development and field-testing of LWD NMRtools began in the late 1990s.5 Research anddevelopment efforts and lessons learned fromwireline-conveyed NMR logging ultimately led tothe introduction of the proVISION real-timereservoir steering service in 2001, capable of

providing precise high-resolution NMR measure-ments under the harsh conditions typicallyencountered while drilling. Similar to the CMRtool, the proVISION LWD tool delivers measure-ments that include mineralogy-independentporosity, bound-fluid volume (BFV), free-fluidvolume (FFV), permeability, hydrocarbon detec-tion and T2 distributions.

Flexible design allows engineers at the well-site to modify the measurement sequence andoperational characteristics of the tool for one ofthree drilling modes: rotating, sliding or station-ary. The tool can be programmed manually or set to switch automatically based on drilling conditions (below). Engineers can program thetool to measure T1, T2, or both simultaneously.Although both measurements can generate NMRformation-evaluation data, the proVISION sys-tem relies primarily on T2 measurements, whichproduce higher statistical repeatability and ver-tical resolution.

Both T1 and T2 measurements sample anexponential time evolution process. T1 measure-ments sample an exponential buildup and T2

measurements, an exponential decay. The T1

measurement consists of a few samples on thisbuildup, each of which requires an additionalwait time depending on the point measured. TheT2 measurement, on the other hand, captures thecomplete decay within a single Carr-Purcell-Meiboom-Gill (CPMG) measurement after onlyone wait time, resulting in a greater number ofechoes per measurement. Thus the T2 measure-ment can be taken more quickly leading to eithera higher sample rate or to more averaging and,therefore, enhanced data quality.

For LWD NMR measurements to be availablein real time, they must be transmitted to the surface by mud-pulse telemetry. From the rawmeasurements performed by the tool, an optimal

signal-processing algorithm is implementeddownhole to perform the critical T2 inversionprocess. As a result of this inversion, importantpetrophysical measurements can be derived in real time, namely: lithology-independentporosity, T2 spectral distributions, bound- andfree-fluid volumes, permeability and informationabout fluid saturations and characteristics.However, because of telemetry bandwidth limita-tions, real-time data transmission is limited tomagnetic resonance-derived porosities, BFV,FFV, motion-dependent quality control parame-ters and T2LM, or logarithmic mean of the T2 dis-tribution. These are used in conjunction with thestandard formation evaluation and survey mea-surements to optimize wellbore placementwithin the reservoir.

Transmission of T2LM, BFV or FFV and poros-ity allows calculation of permeability using theSchlumberger-Doll Research (SDR) or Timur-Coates equations.6 Although T2 distributionsthemselves can be provided in real time, teleme-try bandwidth limitations require prioritizationof data; less critical information is stored inmemory for later processing.

Data are transmitted to surface in real timeby the PowerPulse MWD telemetry system. Aswith other VISION Formation Evaluation andImaging While Drilling LWD tools, maximumenvironmental conditions for the proVISION toolare 300°F [150°C], 20,000 psi [138 MPa], anddogleg severity of 8°/100 ft [8°/30 m] while rotat-ing and 16°/100 ft [16°/30 m] while sliding.

The proVISION opposing-dipole magnetdesign produces a symmetric magnetic field. Thevertically oriented tubular samarium-cobalt permanent magnets are stable within the operat-ing temperature range of the tool. A predictableand repeatable NMR measurement is produced(next page, top).

The interaction of the rf field and static mag-netic field produces a resonant region, or shell,with a diameter of 14 in. [36 cm] and height of6 in. [15 cm] (next page, bottom). Magnetic-fieldstrength within the shell is approximately 60gauss, with a field gradient of about 3 gauss percentimeter. The width of the measurement shellallows formation measurement in slightlyenlarged or deviated wellbores and when the toolis eccentered. The formation depth of investiga-tion (DOI) varies with borehole diameter. Forexample, in an 81⁄2-in. diameter borehole, the DOIis 23⁄4 in. [7 cm]. At a drilling rate of 50 ft/hr[15 m/hr], vertical resolution is 3 to 4 ft [0.9 to1.2 m] after data stacking.

42 Oilfield Review

> The proVISION tool pulse-sequence parameters. The tool’s programmabilityis demonstrated in this triple-wait-time acquisition sequence that was usedto evaluate oil-productive (upper set) and oil- and gas-productive (lower set)intervals in a deepwater Gulf of Mexico well, USA.

Anticipatedproductivity

Wait time,sec

Repetitions Number ofechoes

Oil 6.000.600.04

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40

50030020

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Summer 2003 43

For geosteering purposes, field engineers canplace the tool directly behind the downholemotor or PowerDrive rotary steerable system ordirectly above the bit sub. To further enhancegeosteering capabilities, the proVISION antennasection, which contains the permanent magnets,is located at the bottom of the tool, placing themeasurement point as close to the bit as possible.

The existence of powerful magnets within thebottomhole assembly (BHA) has the potential toadversely affect azimuthal magnetic-surveyinstruments used for determining spatial coordi-nates of the borehole. However, Schlumbergerengineers have demonstrated through modelingand experimentation that the axially symmetricmagnetic field of the proVISION tool has littleinfluence on azimuthal magnetic measurement.Since the magnitude of magnetic-field interfer-ence is small and directly proportional to theintensity of the magnetic field produced by theproVISION tool, errors are significant only whenthe proVISION tool is placed directly above thesurvey instrument. Based on numerical modelsand physical measurements, Schlumberger engineers have developed survey correction algorithms for NMR magnetic interference.These algorithms are included in the IDEALIntegrated Drilling Evaluation and Logging well-site software.

4. For more on T2 relaxation mechanisms: Kenyon et al andAllen et al (2000), reference 2.

5. Prammer MG, Drack E, Goodman G, Masak P, Menger S,Morys M, Zannoni S, Suddarth B and Dudley J: “TheMagnetic Resonance While-Drilling Tool: Theory andOperation,” paper SPE 62981, presented at the SPEAnnual Technical Conference and Exhibition, Dallas,Texas, USA, October 1–4, 2000Drack ED, Prammer MG, Zannoni SA, Goodman GD,Masak PC, Menger SK and Morys M: “Advances in LWDNuclear Magnetic Resonance,” paper SPE 71730, pre-sented at the SPE Annual Technical Conference andExhibition, New Orleans, Louisiana, USA, September 30–October 3, 2001. Horkowitz J, Crary S, Ganesan K, Heidler R, Luong B,Morley J, Petricola M, Prusiecki C, Speier P, Poitzsch M,Scheibal JR and Hashem M: “Applications of a NewMagnetic Resonance Logging-While-Drilling Tool in aGulf of Mexico Deepwater Development Project,”Transactions of the SPWLA 43rd Annual LoggingSymposium, Oiso, Japan, June 2–5, 2002, paper EEE.Morley J, Heidler R, Horkowitz J, Luong B, Woodburn C,Poitzsch M, Borbas T and Wendt B: “Field Testing of aNew Magnetic Resonance Logging While Drilling Tool,”paper SPE 77477, presented at the SPE Annual TechnicalConference and Exhibition, San Antonio, Texas, USA,September 29–October 2, 2002.

6. Akbar M, Vissapragada B, Alghamdi AH, Allen D, Herron M, Carnegie A, Dutta D, Olesen J-R, Chourasiya RD, Logan D, Stief D, Netherwood R, Russell SD and Saxena K: "A Snapshot of CarbonateReservoir Evaluation," Oilfield Review 12, no. 4 (Winter 2000/2001): 20–41.

> The proVISION tool design. Housed within a 37 ft [11.3 m] long, 63⁄4-in.[17.1-cm] diameter drill collar, the tool’s outside diameter is 73⁄4 in. [19.7 cm].When configured with no external upsets and with wearbands in place, thetool can be run in boreholes ranging from 83⁄8 in. up to 105⁄8 in. diameter. On-site field engineers may attach a screw-on stabilizer to reduce lateral motionand centralize the tool in a borehole. Telemetry connections on both ends ofthe tool assembly allow configuration to any section of a bottomhole assem-bly (BHA). The tool is turbine-powered, rather than battery-powered, and canaccommodate flow rates ranging from 300 to 800 gal/min [1136 to 3028 L/min].

Tubularsamarium-cobalt

magnetsOptional stabilizer

Mud flow

Magnetic field (14-in. diameter x 6-in. height)

> Cross sections of the proVISION tool. The axial section through theantenna (left) illustrates the symmetric tool design. The dark blue bars arehollow cylindrical magnets. Lines of constant field strength (blue) indicate agradient magnetic field that decays away from the tool. The section throughthe coaxial wound antenna coil is shown in black. The interaction of theantenna and the magnets produces a cylindrical resonant shell (red stripes)that is 6 in. [15 cm] long, 0.4 in. [10 mm] thick, with a 14-in. [36-cm] diameter ofinvestigation. The transverse section through the coaxial wound antenna coil(right) illustrates the axisymmetric resonant shell (red). The resonant shell isthe only place the measurement is made—no measurement is madebetween the tool and the resonant shell or from the resonant shell fartherinto the formation. The formation depth of investigation (DOI) in an 81⁄2-in.[21.5-cm ] diameter borehole is 23⁄4 in. [7 cm].

Mud

flow

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Making MeasurementsThe proVISION tool operates in a cyclic moderather than a continuous mode. The operatingcycle consists of an initial polarization wait timefollowed by the transmission of the high-frequency rf pulse and then the reception of thecoherent echo signal, or echo train. The cycle ofpulsing and echo reception is repeated in succession until the programmed number ofechoes has been collected. Typically, the acquisi-tion is defined by the Carr-Purcell-Meiboom-Gill(CPMG) sequence. An initial 90° pulse followedby a long series of timed 180° pulses character-izes the CPMG sequence. The time intervalbetween the successive 180° pulses is the echospacing and is generally on the order of hundredsof microseconds.

To cancel the intrinsic noise in a CPMGsequence, the CPMGs are collected in pairs. Thefirst of the pair is a signal with positive phase.The second of the pair is collected with an 180°phase shift, also known as the negative phase.The two CPMG sequences are then combined togive a phase-alternated pair. Compared with theindividual CPMG sequence, the combined orstacked CPMG sequence has an improved S/N.

Measurements of T1 and T2 and their distri-butions are key elements of NMR logging. Theprimary T1 quantity measured is signal amplitudeas a function of polarization recovery time. Theprimary T2 quantities measured are echo-signalamplitudes and their decay. Pulse parameterssuch as echo spacing, wait times and the NMRmeasurement cycle define all aspects of the NMRmeasurement and are completely programmablein the proVISION tool.

Drillstring Dynamics and NMR MeasurementsNMR measurements are not instantaneous. Toolmovement may cause the resonant or excitedregion to move during data acquisition (aboveleft). The proVISION tool is equipped with sen-sors that measure the amplitude and velocity oflateral motion, and instantaneous revolutionsper minute (rpm).

Tool movement can affect both T1 and T2 mea-surements. Motion-induced decay primarilyaffects long T2 values, resulting in faster echodecays that may reduce the accuracy of NMRmeasurement, particularly in light hydrocarbonand carbonate formations. These motion effectsare most severe when the measurement shell isthin in relation to the tool displacement, oftenresulting in movement of the resonant shell outof the region of investigation, even for small toolmovements. A high-gradient static magnetic field

44 Oilfield Review

> Effect of lateral motion on the proVISION NMR measurement. The tool iscentered in the borehole at the beginning of the measurement cycle (left).Subsequent to the initial polarization, drillstring motion causes the tool to restagainst the borehole wall, partially outside the polarized region (right). Ideally,the tool would not move during the course of a CPMG pulse-echo sequence.However, lateral motion of the tool during rotation causes the measurementshell, or resonant region, to move out of the polarized region of investigation.This can result in T2 amplitude and distribution errors.

Borehole wall

Resonant region

Polarizedregion

Reso

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regi

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13 1⁄2 in. 13 1⁄2 in.

> Lateral drillstring motion plots. During the 20-sec time interval, the lower left and right panels showexamples of benign and severe motion recorded by the proVISION tool while rotary drilling. Intervalsof motion amplitude less than 1 mm (bottom left) correspond to the low-rpm intervals shown (top) andrepresent a nearly stationary condition. Violent motion occurs during the remaining time intervals,when the tool is spinning freely and has lateral motion amplitudes up to 5 mm.

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Summer 2003 45

results in a thin measurement shell, whichrapidly decays with distance away from the tool.In contrast, the proVISION tool has a low gradi-ent design that results in a thick measurementshell and insensitivity to tool motion.

Since lateral motion can potentially shortenT2 decay rates, understanding this motion is critical for developing data quality-control techniques. To assess motion-induced effects,engineers must know the frequency, amplitude,trajectory and timing of the motion.7 Rapid-sampling accelerometer and magnetometer systems measure real-time drillstring motion(previous page, bottom). Motion data are pro-cessed in 20-sec snapshots. Raw snapshot dataare compressed and can be stored in memory,while the processed results are recorded contin-uously to provide an uninterrupted log of lateralmotion. The theoretical maximum T2 valueresolvable during motion is calculated and a flagindicating NMR data quality is transmitted withthe real-time data set.

Motion data obtained with the proVISIONtool have broad independent utility. These datacan alert the driller to excessive lateral motion,an unfavorable resonant mode or excessiveshocks allowing corrective action to be taken toreduce potential BHA or drill-bit damage and tooptimize drilling rates, improving drilling effi-ciency. Timely response to excessive drillstringmotion can also minimize borehole enlargement(above right).

Optimizing Well ProductivityProper well placement and completion designare key to optimizing productivity. To accomplishthis, drillers must place wellbores in the mostproductive part of a target reservoir, and engi-neers must design completions to maximize oilproduction and recovery while simultaneouslylimiting water production. Real-time LWD NMRlogging provides the data necessary for informeddecision-making.

Determining which intervals of a reservoirshould be completed requires an estimate of awell’s productivity index (PI). Traditionally, thisquestion has been addressed after completion ofdrilling, wireline logging and production testing.The PI is based on a permeability profile, whichis the product of reservoir permeability and vertical thickness. These measurements areobtained from well logs, formation tests, or both.

For more than a decade, operators havesought real-time estimates of permeability andPI. In 1994, BP engineers successfully experi-mented with real-time PI determination methods

at their Wytch Farm project located in the southof England. Geological studies of the Sherwoodsandstone oil reservoir established that reservoirproductivity is a function of permeability, andthat permeability is controlled by grain size and porosity. Core data were used to create permeability bulk-density transforms for eachgrain-size class and these, in turn, were used toestimate PI. As drilling progressed, a permeabil-ity log was generated in real time using grain sizeobtained from sieve analysis of drill cuttings andcombining porosity measurements from a litho-density-neutron logging tool. Petrophysiciststhen calibrated the model against offset wells.

Engineering and petrophysical teams usedthese early real-time permeability-productivityestimates to model and optimize a well’s eco-nomic potential in several ways. Decisions toadjust well trajectory were based on real-timeproductivity predictions. By optimizing perfora-tion intervals, the team maximized productionand minimized the potential for water break-through. These data were used to estimatereserves remaining in wells where intervals hadbeen plugged back for water shutoff.8

At Wytch Farm, BP’s method was relativelysimple to implement. The Sherwood sandstone isnot highly cemented and grain size, porosity andpermeability have a clearly defined relationship.Also, well cores were available for model calibra-tion. In many other reservoirs, the petrophysicalcharacteristics are less straightforward. Whilesimilar processes might provide comparableresults while drilling in more complex reservoirs,the petrophysical community wanted a moreaccurate and complete formation-evaluationsolution. NMR in real time can provide this information and help in optimizing wellboreplacement and completion design.

7. Speier P, Crary S, Kleinberg RL and Flaum C: “ReducingMotion Effects on Magnetic Resonance Bound FluidEstimates,” Transactions of the SPWLA 40th AnnualLogging Symposium, Oslo, Norway, May 30–June 3, 1999,paper II.

8. Blosser WR, Davies JE, Newberry PS and Hardman KA:“Unique ESP Completion and Perforation MethodologyMaximises Production in World Record Step-Out Well,”paper SPE 50586, presented at the SPE EuropeanPetroleum Conference, The Hague, The Netherlands,October 20–22, 1998.Harrison PF and Mitchell AW: “Continuous Improvementin Well Design Optimises Development,” paper SPE30536, presented at the SPE Annual TechnicalConference and Exhibition, Dallas, Texas, USA, October 22–25, 1995. Hogg AJC, Mitchell AW and Young S: “Predicting WellProductivity from Grain Size Analysis and Logging WhileDrilling,” Petroleum Geoscience 2, no. 1 (1996): 1–15.

> An example of extreme stick-slip. The upper graph shows instantaneous rotation (rpm). At about8 sec into the time interval, the BHA becomes stuck for about 7 sec until the continued buildup in torquereleases the BHA and the stored energy accelerates the drillpipe to over 300 rpm after which the BHAbecomes stuck again. The lower graph shows the number of cumulative rotations. The number of rota-tions increases until the BHA becomes stuck, at which point the topdrive continues turning and buildsseven wraps in the drillstring before the BHA breaks free. The BHA releases the built-up energy, andinertia causes it to overrotate and advance six wraps ahead of the topdrive, potentially unscrewingsections of the BHA.

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NMR in Real Time Modern NMR logs measure mineralogy-independent porosity and provide an estimate ofpermeability and bound-fluid volumes. They canalso detect the presence of hydrocarbons. Whencombined with other LWD measurements, NMR data can be used to generate potential production estimates in real time.

In 2002, BP engineers applied the proVISIONsystem on a deepwater project in the Gulf ofMexico, USA (right). During drilling with oil-basemud, real-time NMR logs were obtained in threeseparate 81⁄2-in. diameter wells. The proVISIONpulse sequence consisted of a single wait timeand burst sequence. A relatively long wait time of12 sec was used to ensure adequate polarizationof the light hydrocarbons that were expected in this reservoir. Six hundred echoes were col-lected after the long wait time. The burst sequence consisted of 20 echoes following a 0.08-sec wait time. Echoes were collected withspacing of 0.8 and 1.2 msec. The overall NMRcycle time was about 30 sec at a drilling rate ofapproximately 70 ft [21 m] per hour. This combi-nation of cycle time and rate of penetration(ROP) gave a depth sample rate of about 0.75 ft[0.23 m] per phase-alternated pair.

To determine BFV, a T2 cutoff of 90 msec waschosen. This T2 cutoff value was based on experi-ence with wireline NMR measurements in thisfield. Evaluation by the petrophysical team indi-cated that neutron, density and NMR porositywere in agreement through the sandstone, whichhas a porosity of about 28 p.u. In addition to NMRdata, the proVISION data set provided the opera-tor with drilling performance, lateral motion anddownhole RPM logs to detect erratic drilling con-ditions, such as stick-slip motion, and allowedthe driller to take corrective actions, potentiallyextending the life of the bottomhole assemblyand optimizing ROP.

The Quest for Carbonate EvaluationHydrocarbons in the Al Shaheen field, offshoreQatar, are currently produced from threeCretaceous formations, the Kharaib, Shuaiba andNahr Umr. The Kharaib and Shuaiba reservoirsare carbonate, while the Nahr Umr comprisesthin sandstones (next page, top).

Maersk Oil operating the Al Shaheen field incooperation with Qatar Petroleum is developingthese complex formations with extended-reachhorizontal wells that occasionally exceed30,000 ft [9144 m] measured depth (MD) while

only 3000 ft [914 m] in true vertical depth.9 Insuch wells, drillpipe cannot be rotated in thehole with logging cable attached. Frictionaleffects eventually prohibit sliding beyond about13,000 ft [3962 m]. Thus, wireline-conveyed log-ging tools are typically unable to reach the far-thest part of a horizontal section. LWD tools areconveyed over the entire length of the boreholewhile providing data for geosteering and primaryformation evaluation.

NMR techniques can help determine reser-voir fluid flow and permeability characteristics.These characteristics may vary significantly withchanges in geologic facies. Detection of faciesvariation is critical to reservoir understandingand optimal wellbore placement. Often, particu-larly in carbonate reservoirs, the lack of consis-tent relationships between porosity andpermeability on a reservoir scale limits LWD

46 Oilfield Review

> Formation analysis in deepwater Gulf of Mexico, USA. The proVISION resistivity-independent oil-indicator information, bound-fluid volume data and permeability data are integrated with wireline log-derived water-saturation information to deliver key producibility estimates while the well is beingdrilled. Tracks 1 through 4 are available as real-time data channels. Changes in the signature of therecorded mode T2 distribution (Track 5) confirm the oil/water contact. The hash marks in the depthtrack are NMR raw-data sample points.

XX650

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Summer 2003 47

petrophysical characterization using porositylogs. Conventional wireline-conveyed NMR logginghas improved the characterization of geologicfacies and other petrophysical carbonate proper-ties such as permeability (bottom).

Drilling extended-reach wells in the AlShaheen field is challenging. Rotary steerableBHAs are typically used for directional controlin the drilling of the long horizontal sections. Thepetrophysical team was concerned aboutdiminished LWD NMR data quality due tomotion-dependent T2 decay resulting fromthe typically high levels of BHA shock, stick-slipand lateral tool motion during drillstring rota-tion. With ROPs occasionally in excess of500 ft/hr [152 m/hr], further data-quality losswas expected.

Carbonate rocks typically have lower surface-relaxation times, which leads to extended T2

times. Since much of the important petrophysi-cal information is contained in the later echoes,acquisition sequences in carbonates typicallyrequire a longer wait time and a greater numberof echoes than in clastic formations. It wasunknown whether the late T2 components typi-cally seen in the Al Shaheen carbonate rockswould be detected under the expected difficultdrilling conditions.

Engineers attempted to alleviate as manyvariables as possible during prejob planning. Toimprove the S/N, raw echo stacking was alsoplanned. Since facies changes typically occurover tens or hundreds of feet in extended-reachwells, and the detection of small-scale variationswas not the main objective, a loss of resolution inexchange for improved S/N was acceptable.

The world’s first proVISION deployment in acarbonate reservoir was in an extended-reach,81⁄2-in. diameter horizontal well, drilled to morethan 24,000 ft [7315 m] MD with water-base mud.A rotary steerable assembly controlled trajectorywhile LWD NMR data were obtained in real timealong the entire borehole length.

Limited amounts of core material were avail-able from this particular section of the Shuaibareservoir. Historically, carbonate facies identifi-cation and interpretation were based on a com-bination of drill cuttings, thin sections and log

9. Damgaard A, Hansen P, Raven M and Rose D: “A NovelApproach to Real Time Detection of Facies Changes inHorizontal Carbonate Wells Using LWD NMR,”Transactions of the SPWLA 44th Annual Symposium,Galveston, Texas, USA, June 22–25, 2003, paper CCC.

TURKEY

YEMEN

SYRIA

IRAQ IRAN

OMAN

SAUDI ARABIA

AFGHANISTAN

PAKISTAN

SAUDI ARABIA

QATAR

UNITED ARAB EMIRATES

IRAN

Al Shaheenfield

0

0 50 100 150 miles

100 200 300 km

> Location of the Al Shaheen fieldoperated by Maersk Oil Qatar AS incooperation with Qatar Petroleum.

> Identifying changes in the Shuaiba limestone reservoir with wireline NMR data. The NMR data showa large decrease in free fluid, an increase in bound fluid (Track 3, shown shaded yellow) and a decreasein NMR permeability (Track 2) from a depth of XN010 to XN070. It would be difficult, if not impossible,to identify these changes with standard porosity (Track 3, neutron porosity in blue and bulk density inred) and gamma ray logs (Track 1, solid green curve).

Free Fluid

Bound Fluid

Bins 1-2

Bins 7-8

API

Gamma Ray

1000

Timur-Coates Permeability

msec

T2LM Image Orientation

NMR T2 Distribution

60000.3

29 ohm-m

Deep Image

0

Photoelectric Effect

122

XM900

Depth,ft

XN000

Sliding–no image

Sliding–no image

Bin 6

Bin 5

Bin 4

Bin 3

SDR Permeability

m3/m3

Neutron Porosity

00.6

g/cm3

Bulk Density

2.71.7

ft3/ft3

CMR Free Fluid

00.6

m3/m3

Total CMR Porosity

00.6

3.45

11.2

516

.67

19.4

922

.77

2?.1

430

.38

34.4

237

.57

40.3

242

.?7

4?.2

750

.05

54.8

362

.42

82.8

822

115.

12

U BR L U

analysis. The borehole was expected to penetratemultiple carbonate facies with varying perme-abilities and producibility characteristics.Maersk Oil hoped to gain significant reservoirinformation in real time from the proVISION

tool, including differentiating various carbonatefacies along the wellbore path and comparingLWD NMR log quality with that of selected intervals of wireline-conveyed NMR logs.

As expected, a high level of downhole shockand stick-slip occurred. ROP was variable, some-times exceeding 500 ft/hr. Because of tool motionand fast ROP, NMR LWD data had a moderatedegree of noise compared with a wireline-

48 Oilfield Review

> A clear image of borehole trajectory. The LWD resistivity image (Track 5) shows the wellbore trajec-tory encountering an overlying marl. The NMR data clearly show a bimodal T2 (Track 4) with the shortT2 peak, centered at 6 msec, coming from the argillaceous material above the borehole from XX329 toXX429 ft, and the longer T2 peak, centered at 200 msec coming from the limestone below the borehole.Lateral changes in the limestone are also indicated. Facies 3 occurs from XX460 to XX474 ft and XX488to XX500 ft, characterized by the lower T2LM value (Track 4).

Gamma Ray

Binned NMR PorosityEarly Late

API 100 00 500

ROP

XX200

XX300

XX400

XX500

XX600

ft/hrBFV–NMR

6000msec3 U R LB U

NMR T2LM Image Orientation

NMR T2 Distribution

Timur-Coates Permeability

SDR Permeability Total NMR Porosity

Thermal Neutron Porosity

2.71.7

Bulk Density

Free Fluid

Bound Fluid

g/cm3

Summer 2003 49

conveyed NMR log. However, data stackingimproved the S/N. Results from multiple MDTModular Formation Dynamics Tester runs pro-vided data to estimate fluid mobility and adjustthe constants in NMR permeability equations.

Analysis based on NMR permeabilities,porosities, T2LM, bound-fluid volumes and free-fluid volumes discerned three distinct porositysystems. The team used changes in T2 characterto map facies variation along the borehole

(previous page). A low bound-fluid volume and ahigh ratio of free to bound fluid typify Facies 1(above). Facies 2 has moderate bound-fluid vol-ume and a lower bound- to free-fluid ratio. Theaverage T2 of Facies 2 is shorter than that of

> Facies 1 from LWD NMR. The LWD data shown indicate an interval of clean carbonate where the T2(transverse relation time) distribution (Track 4) contains a significant percentage of late T2 values. Thesolid blue line is an empirically determined T2 cutoff that is used to partition the T2 distribution into afast component representing bound fluids and a slow component indicating the free fluids. The redtrace represents the T2LM distribution. The T2LM is generally well above the T2 cutoff value, indicatingthat most of the fluid in the pore space is free fluid. The total porosity computed from the NMR data,shown as a dashed black line in Track 3, is in agreement with the conventional limestone matrix neu-tron porosity in blue, and with the formation bulk density displayed in red. The yellow area representsthe bound-fluid volume, while light green indicates the portion of the total porosity that is filled withfree fluids, or the effective porosity. The longest T2 times indicate the largest pores, while the shortestare attributed to the smallest pore sizes. Large pores appear to make up a significant portion of thetotal porosity, with only a small percentage comprising small and very small pores.

Free Fluid

Bound Fluid

Gamma Ray

API

Early Late

1000 0500

ROP

ft/hr 6000msec3

NMR T2LM

NMR T2 Distribution

Timur-Coates Permeability

SDR Permeability

Binned NMR Porosity

BFV–NMR

Total NMR Porosity

Thermal Neutron Porosity

2.71.7

Bulk Density

g/cm3

XX800

XX900

XY000

XY100

Facies 1 and the complete data spectrum isshifted to shorter T2 values. Facies 3 is typified byhigh bound-fluid volume and a low ratio of free tobound fluid. In Facies 3, the T2 spectrum isshifted farther toward shorter values. Thin sections made from cuttings confirmed the faciessignificance of the LWD NMR T2 response.

LWD NMR porosity agreed with density poros-ity in Facies 1 and 2 with an average 3 p.u. deficitin Facies 3 believed to be due to a percentage offaster-decaying T2 signals. LWD NMR data indi-cate different T2 decay rates for each of the threefacies, allowing clear differentiation; this wouldnot have been possible with neutron-porositymeasurements alone (left).

To improve confidence that the LWD NMRdata were identifying petrophysical changes inthe carbonate facies, the team had to rule out thepossibility that the interpreted T2 response wasbeing dominated by motion-induced T2 decay.Measured lateral velocity data were used to con-firm that the T2 data were accurate and correctly indicating changes in the carbonate facies (nextpage, top left). This particular data set shows alarge amount of T2 data acquired even at ele-vated lateral velocities. The current proVISIONdesign does not directly allow compensation fordownhole tool motion in the T2 decay measure-ment. However, highlighting intervals of increasedtool motion can be used as a log-quality indicator.

To examine the effects of downhole toolmotion on LWD NMR data, wireline CMR mea-surements acquired after drilling were comparedwith real-time proVISION data. Porosity, FFV,BFV, T2LM and NMR permeabilities all comparefavorably (next page, right). The CMR data wereacquired over limited intervals for comparison,primarily in the proximal part of the well thathad been open to invasion the longest. SomeCMR logged intervals displayed a small decreasein T2LM values consistent with the additional fil-trate invasion time prior to wireline logging.None of the LWD NMR intervals indicated anyidentifiable motion-induced T2 decay. The favor-able comparison of the late T2 components indi-cates that downhole lateral tool motion is not adominant T2 decay mechanism in this data set.

The proVISION system was configured totransmit porosity, T2LM and FFV in real time toallow use of measurements for geological charac-terization and to aid geosteering. Although furtherevaluation will be required to completely under-stand the NMR T2 response in carbonate rocks,the team working in the Al Shaheen field demon-strated that carefully interpreted LWD NMR datacan be used to help detect variation in carbonatefacies and their petrophysical characteristics.

50 Oilfield Review

> Contrasting NMR data with resistivity images. An LWD resistivity image log is shown in Track 5. Theimage is scaled such that conductive formations are dark and more resistive formations are light withno absolute scale. The resistivity image shows a significant change in the formation resistivity whilethe porosity remains more or less constant, implying a possible textural change. The NMR log over theinterval identified as Facies 2 indicates some large pores. The T2LM is above the cutoff value, but witha broad distribution of pore sizes resulting in a significant percentage of the total porosity being occu-pied by bound fluid. The estimated permeability of Facies 2 is lower than that of Facies 1 (see figure,page 49 ). The NMR log over the interval identified as Facies 3 indicates few, if any, large pores. TheT2LM is below the cutoff value, and most of the total porosity is occupied by bound fluid. The estimatedpermeability of Facies 3 is lower than that of Facies 1 or 2.

Free Fluid

Bound Fluid

Gamma Ray

Binned NMR PorosityEarly Late

APIft/hr

ROP

100 5000 0BFV–NMRTimur-Coates Permeability

SDR Permeability

U R LB UImage Orientation

6000msec3

NMR T2LM

NMR T2 DistributionTotal NMR Porosity

Neutron Porosity

Bulk Density

2.71.7 g/cm3

XX400

XX500

XX600

XX700

XX800

XX900

XX000

Facies 2

Facies 3

Summer 2003 51

The Next GenerationThe proVISION system has demonstrated its ability to acquire real-time logs in both clasticand carbonate reservoirs, potentially identifyingless obvious or otherwise undetected facieschanges. Even for longer T2 components in carbonate formations drilled at elevated ROP,the tool delivers sufficient data resolution forfacies determination and for permeability andbound- to free-fluid volume calculations. TheLWD proVISION tool provides essential real-time reservoir information and data useful formaking geosteering decisions in complex reservoir settings.

Severe stick-slip and BHA shock are oftenassociated with drilling long horizontal sections.Bottomhole shock, combined with high ROP, mayincrease noise in the data sets. However, fielddata demonstrate that the proVISION tool is sufficiently robust to handle these conditionsand provide reliable T2 data.

Future generations of NMR tools hold greatpromise. The industry can look forward to the con-tinued evolution of LWD NMR technology, which isexpected to provide drilling engineers and petro-physical teams with significant advancements inreal-time formation evaluation for geosteering andproductivity optimization. —DW, SP

> Lack of motion-induced decay. The data acquired in this field showno apparent reduction in T2 values associated with the lateral velocityof the LWD NMR tool, implying that in this well, tool motion does notaffect T2 decay.

0

50

100

Late

ral v

eloc

ity, m

m/s

T2LM, ms150 225 3000 75

150

200

> Agreement of wireline CMR and proVISION data. The wireline NMR poros-ity is seen to follow the same trend as the LWD NMR porosity with a smallsystematic shift to lower porosity (Track 1). This difference in total porosity isinfluenced by the differing depth of investigation of the tools and the difference in mud-filtrate invasion related to the formation exposure time.Computed bound-fluid volumes are in agreement (Track 1). The vertical, orspatial, resolution of the LWD NMR tool is reduced because of the high levelof stacking utilized to increase the S/N. Likewise, the physics of measurementimposes a temporal, or time, resolution limit on the LWD tool relative to thatseen with the wireline sensor. The overall effect is a smoothing of the T2 dis-tribution over time and depth. The T2LM of the LWD NMR is shown overlaidon the CMR data (Track 2). Considering the difference in tool design, acquisi-tion parameters, environmental conditions, and the time lapse betweendrilling and drillpipe-conveyed wireline logging, the comparison is excellent.

Free Fluid

Bound Fluid

0500 290

ROP

ft/hrCMR Porosity

NMR T2LM

NMR T2 Distribution

290

NMR T2 Distribution

CMR T2LM CMR T2LM

XX250

XX300

XX350

XX400

NMR Porosity

CMR–BVF

NMR–BVF