Water saturation,lithology, and porositythrough casing

RSTPro

Designed for reservoir complexityWithin the last decade, nearly every aspectof reservoir management has grown incomplexity. What once was the exceptionis now routine: multiple-tubing and gravel-pack completions, secondary and tertiaryrecovery, highly deviated wellbores, andthree-phase production environments.

The RSTPro* Reservoir Saturation Toolhelps you manage complexity by deliver-ing reliable, accurate answers in today’schallenging production environment. Runon the PS Platform string, with its suite ofcased hole reservoir evaluation and produc-tion logging services, the RSTPro tool usespulsed neutron techniques to determinereservoir saturation, lithology, porosity, andborehole fluid profiles. This information isused to identify bypassed hydrocarbons,evaluate and monitor reserves in mixed-salinity and gas environments, perform formation evaluation behind casing, anddiagnose three-phase flow independentlyof well deviation.

Pulsed neutron technologyAn electronic generator in the RSTPro toolemits high-energy (14-MeV) neutrons inprecisely controlled bursts. A neutroninteracts with surrounding nuclei, losing

energy until it is captured. In many of theseinteractions, the nucleus emits one or moregamma rays of characteristic energy, whichare detected in the tool by two high-efficiencyGSO scintillators. High-speed digital signalelectronics process and record both thegamma ray energy and its time of arrival relative to the start of the neutron burst.

Exclusive spectral analysis algorithmstransform the gamma ray energy and timedata into concentrations of elements (relative elemental yields).

Hardware configurationsThe RSTPro tool is available in two configurations:

• The 111⁄16-in-diameter tool passes through23⁄8-in tubing, eliminating the need to pulltubing or kill the well.

• The 21⁄2-in-diameter tool runs through tubing as small as 31⁄2 in for operations in larger casing sizes below. Focusedshielding enhances far-detector sensitivityto the formation and near-detector sensitivity to the borehole.

The neutron generator, gamma raydetectors, and associated electronics are identical for the two tools. A flask for the slim tool extends operation above 300 degF and to 20,000 psi.

Applications� Evaluate and monitor reservoir

saturation at any salinity

� Locate bypassed hydrocarbons,depleted reservoirs, and gas zones

� Evaluate formations behind casing

� Determine production fluid pro-files in horizontal, deviated, andvertical wells

� Detect water entry and flowbehind casing

� Determine gravel-pack quality

Benefits� Reduced costs and downtime,

with no need to pull tubing orkill the well

� Ability to log flowing wells,avoiding reinvasion

� Accuracy of reserve estimatescomparable to that from coreanalysis

� Option of performing full forma-tion evaluation after completion

� Reliable production logginganswers regardless of boreholeflow regime

� Increased wellsite efficiencythrough fully combinable hardware

� Time savings of one-trip measurement availability

Features� Neutron generator delivers high

output and precise burst control.

� Near and far detectors useexclusive gadolinium oxy-orthosilicate (GSO) scintillatorsto acquire high-quality gammaray spectra.

� Full spectral analysis and cali-bration methods reduce environ-mental bias.

� Extensive laboratory and model-ing characterization ensuresaccuracy in a wide range ofenvironments.

� Dewar flask extends operationabove 300 degF [150 degC].

� Hardware combines fully withthe PS Platform* productionservices platform and SCMT*Slim Cement Mapping Tool.

Rigging up the RSTPro tool before running a survey.

Job planning and log quality controlRSTPro services form a powerfulframework for evaluating, monitoring,and diagnosing reservoir performance.To achieve maximum benefits, theRSTPro job planner is used to developa detailed job design. Convenient displayssummarizing hardware performance andmeasurement quality are defined for eachtool mode and monitored during the job.The RSTPro quicklook application isavailable at the wellsite to monitor measurement quality and repeatability.

Formation sigma, porosity, and borehole salinityIn sigma mode, the RSTPro tool measuresformation sigma, porosity, and boreholesalinity using an optimized Dual-Burst*thermal decay time sequence.

The two principal applications of thismeasurement are saturation evaluation,which relies on measurement accuracy,and time-lapse monitoring, where sensi-tivity is determined by measurementrepeatability.

A new degree of accuracy in the forma-tion sigma measurement is achieved bycombining high-fidelity environmental

correction with an extensive laboratorycharacterization database. The accuracyof RSTPro formation sigma is 0.22 cu forcharacterized environments and has beenverified in the Callisto and AmericanPetroleum Institute industry-standardformations. Formation porosity and bore-hole salinity are either computed in thesame pass or input by the user.

Exceptional measurement repeatabilitymakes the RSTPro tool more sensitive tominute changes in reservoir saturationduring time-lapse monitoring. The gainsin repeatability and tool stability are theresult of higher neutron output and sensorregulation loops. At the typical loggingspeed of 900 ft/h [275 m/h] for time-lapsemonitoring, RSTPro repeatability is 0.21 cu.

Inelastic C/O ratioThe inelastic C/O ratio is derived fromthe net inelastic spectrum acquired in thetool’s inelastic/capture (IC) mode. Theprimary application of inelastic C/O ratioanalysis is saturation evaluation andmonitoring in environments with freshwater or unknown salinity.

Energy windows used in conventionalC/O analysis contain too little information

to reliably resolve the C and O signalsfrom the overlapping lithology and completion background. The RSTPro tooluses elemental standards, which repre-sent the tool’s spectral response to indi-vidual elements, to decompose the netinelastic spectrum into a sum of inelasticyields. Full spectral analysis takes advan-tage of the unique energy signature ofeach standard and is a more accuratemethod for determining a C/O ratio freeof environmental bias. New RSTPro elemental standards extend C/O meas-urement sensitivity in the presence of gasand in formations with lower porosity.

A straightforward model partitions theC and O yields into contributions frompore fluids, formation rock, and borehole.Endpoints defining minimum and maxi-mum expected values of the near- andfar-detector C/O ratios are determinedthrough characterization measurementsin the laboratory. The model is invertedto solve for the oil volume: the saturationis multiplied by the porosity.

The spectral C/O advantageIn IC mode, RSTPro electronics bin thegamma ray energy into a spectrum of

Focused toolShielding in the 21⁄2-in-diametertool enhances far-detector sen-sitivity to the formation andnear-detector sensitivity to theborehole, enabling carbon/oxygen ratio (C/O) logging inflowing wells.

Gamma ray detectorsThe exclusive RSTPro GSOdetectors are more efficientthan detectors that usesodium iodide (NaI) crystalsto detect gamma rays, andthey produce a spectroscopy-quality spectrum at muchhigher count rates thandetectors that use bismuthgermanate (BGO). UnlikeBGO, which can degrade substantially above 120 degF[50 degC], GSO can withstandtemperatures to 300 degFwithout the protection of a flask.

Flasked toolA Dewar flask enables theslim tool to operate at highertemperatures and pressures.

256 channels. Details of the spectrumshape carry valuable information aboutthe tool and its environment. By analyzingthe full energy spectrum, the RSTProtool achieves a more accurate and robustcalculation of environmental effects, tem-perature corrections, and tool calibration.

Spectral analysis takes advantage of theunique energy signature of each elementalstandard to accurately decompose theacquired gamma ray energy spectruminto elemental yields.

• Environmental effects: Conventionalenergy windows are insufficient to reliably solve for C and O amid changinglithology and completion contributions.As shown on the figure to the right,only a relatively small fraction of Cgamma rays is in the C energy window(yellow). The spectral C contribution is easier to resolve because it relies onthe shape of the C elemental standardacross the full energy spectrum.

• Temperature corrections: In all commonscintillators, energy resolution degradeswith temperature. RSTPro spectralanalysis corrects both the spectral- and the windows-derived C/O ratios forthe effects of temperature degradation.

Obtaining the most accurate C/O answer—faster than everThe oil volume computed from a spectralC/O ratio is more accurate, while the oilvolume computed from a windows C/Oratio has better statistical precision. Inpractical terms, better accuracy meansbeing closer to the correct answer. Betterprecision translates into improved

measurement repeatability and faster logging speeds.

RSTPro alpha processing combines the spectral- and windows-derived oilvolumes to benefit from both methods,resulting in a saturation answer that isboth highly accurate and can be acquiredat faster speeds. Even further gains inlogging speed are achieved in monitoringprojects with the patented recycled alpha

technique, with an accuracy of reservesestimates comparable to that from core analysis.

Reservoir saturationFormation sigma and the inelastic C/Oratio are the two measurements most frequently used in reservoir evaluationand saturation monitoring through casing.Because oil and fresh water have nearlythe same neutron capture cross section(sigma), formation water salinity must be sufficiently high and of known valueto compute saturation from sigma. If theformation water is fresh or the salinity isunknown, saturation is derived from thesalinity-independent C/O ratio measure-ment. Where both measurements can beused, sigma is preferred primarilybecause it can be measured at faster logging speeds. In some cases, such as a waterflood with a different connatewater salinity, sigma and C/O ratio meas-urements can be combined to identifyinjection water breakthrough.

Gas identificationThe weighted inelastic ratio (WINR) is a shaly gas indicator formulated from a combination of inelastic and capture data acquired in the tool’s sigma mode.Changes in porosity and lithology causechanges in the capture count rate similarto those that occur in gas. The inelastic

Energy, MeV1 2 3 4 5 6 7 8

Toolbackground

Calcium

Carbon

Iron

Oxygen

SiliconCounts

Spectral channel0 50 100 150 200 250

100,000

80,000

60,000

40,000

20,000

0

Counts

Tool

IronCalciumSiliconOxygenCarbon

Carbon Oxygen

background

Elemental yields from spectral analysis.

The RSTPro electronics bin the gamma ray energy into a spectrum of 256 channels.

count rate at the far detector is sensitiveto gas but relatively insensitive to lithologyand liquid-filled porosity. WINR curvemovement from the shale baseline indicates that formation gas is present.Borehole gas shifts the overall baseline.The WINR curve gives the productionengineer a fast method to evaluate smallor marginal shale-laminated gas zonesthat may otherwise be overlooked.

Formation evaluation behind casingIn many complex lithologies and clays, a better understanding of the matrix isnecessary to produce a credible fluidanalysis. RSTPro below- and behind-tubingmeasurements of capture elemental yieldscan be input to SpectroLith* lithologyprocessing of spectra from neutron-induced gamma ray spectroscopy toolsto produce a quantitative, mineral-basedlithology interpretation derived from single-tool elemental yields. A modifiedgeochemical oxides closure model trans-forms the capture yields into elementalconcentrations. An exclusive core data-base was used to develop the lithologyinterpretation that converts concentrationsto fractions of clay, carbonate, andframework quartz.

Production diagnosis Complex flow regimes occur in the multi-phase production of horizontal and highlydeviated wells. Slotted liners, uncementedcompletions, and gravel packs partitionflow in ways that traditional productionlogging sensors alone cannot address.RSTPro pulsed neutron techniques supplymeasurements of three-phase holdup andwater velocity to determine productionrates in wells where conventional meth-ods do not work. The axially symmetric111⁄16-in-diameter tool is ideal for theseapplications. Centering the tool in theborehole optimizes sensitivity in hetero-geneous flow regimes such as segregated or bubbly flow.

Three-phase holdupThe RSTPro TPHL* three-phase fluidholdup log from pulsed neutron measure-ments is a continuous measurement ofholdups for oil, water, and gas. The near-and far-detector C/O ratios acquired in ICmode are used to solve for the boreholeoil fraction, and the near- to far-detectornet inelastic count rate ratio (NICR)determines the borehole gas fraction.Because of its favorable borehole sensi-tivity, TPHL logging has speeds signifi-cantly faster than for the accompanyingcentered-tool saturation answer.

Water velocityThe RSTPro WFL* Water Flow Log meas-ures water velocity by using the principleof oxygen activation. Neutron bursts asshort as 0.067 s enable the RSTPro toolto measure water velocities from 3 to 500 ft/min. Additional gamma ray toolsare used for higher velocities or forsimultaneous measurements of up anddown velocity. Exceptional control of theneutron generator makes possible theultrashort bursts required to detect fastvelocities. Gamma ray energy discrimina-tion and tool shielding reduce the back-ground from stationary activation,improving sensitivity in low-signal envi-ronments such as flow behind casing.

Water or oil velocity in deviated flowThe RSTPro PVL* Phase Velocity Logmeasures the velocity of the oil or water

phase within a deviated well with segregated flow. An oil- or water-solublemarker is released to the wellbore froman ejector tool located below the RSTProtool to mix with either the oil or waterphase. The marker contains an elementwith a large neutron-capture cross section.The neutron generator pulses and eachdetector records a time-decay spectrumthat provides a sigma measurement. Thepulse and detect sequence is repeated to give multiple sigma measurementsthat are plotted against time. When theoil or water that contains the marker isflowing across the detector, the detectorrecords a high borehole sigma. Thevelocity of the oil or water phase is calculated from the distance between theejector and detector and the elapsed timebetween ejection and detection of thesigma peak.

–0.1

0

0.1

0.2

0.3

0.4

0.5

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0.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Near-detector C/O ratioFa

r-de

tect

or C

/O ra

tio

C/O Ratio Crossplot Overlay Across Internal Gravel PackEndpoints: 33-pu sandstone, 121/4-in borehole, 95/8-in casing, 51/2-in internal gravel pack

Endpoints defining minimum and maximum expected values of the near- and far-detector C/O ratios are characterized for the RSTPro tool in a wide range of completions, from conventional open hole and single casing to multiple tubing, gravel pack, and blast joints.

The Schlumberger Environmental Effects Calibration Facility (EECF) in Houston, Texas, comprises the largestcollection of natural and artificial formations and borehole environments in the industry. For environments thatfall outside of this collection, customized response characterization campaigns can be designed to achievedesired accuracy.

Neutron generatorThe RSTPro advanced neutron generatorproduces up to 3 × 108 neutrons per second.Increased output improves measurementrepeatability and enables faster loggingspeeds. The neutron burst profile exhibitsexceptionally sharp rise and fall times.The resulting “square” burst is the key to thoroughly removing capture gammaray contamination and its associatedsalinity dependence from the inelasticgamma ray spectrum used to computethe C/O ratio.

Tool calibration Tool-to-tool variability is minimizedthrough full spectral calibration, whichtransforms acquired spectra to match theresponse of the primary calibration tool.

Characterized for accuracy For RSTPro measurements such as formation sigma, porosity, and saturationbased on the C/O ratio, accurate parame-terization of environmental effectsrequires tool response characterization in known wellbore environments.

The Schlumberger EnvironmentalEffects Calibration Facility in Houston,Texas, comprises the largest collection of natural and artificial formations andborehole environments in the industry.For environments that fall outside thiscollection, customized response charac-terization campaigns can be designed to achieve the required accuracy.

RSTPro environmental corrections for formation sigma and porosity arecomputed dynamically using a weightedmultiple linear regression technique.Tool response is characterized in morethan 1,700 laboratory measurements,including openhole and air-borehole configurations.

Endpoints defining minimum and maximum expected values of the near-and far-detector C/O ratios are character-ized for the RSTPro tool in a wide rangeof completions, from openhole and con-ventional single-casing configurations tomultiple tubing, gravel-pack, and blastjoint completions.

Energy

γγγ

A B C 20 us 20 us 60 us

n

t

n

t

γ

~1,500 us

t

γ

~30 s to 100 s

t

n

t

In IC mode, the RSTPro tool measures the burst-on spectrum (A) and the early (B) and late (C) burst-off capture spectra. Relative inelastic yields for C, O, Si, Ca, and Fe are computed from the net inelastic spectrum(A – kB). Relative capture yields for H, Cl, Si, Ca, S, Fe, Ti, and Gd are computed from the late capture spectrum(C). Capture yields from rock elements such as Si, Ca, and Fe are input to SpectroLith interpretation for thedetermination of lithology volumes.

In sigma mode, the RSTPro tool runs an optimized Dual-Burst timing sequence and measures the time decayof the population of capture gamma rays after each burst. The time decay of late capture (long burst) gammarays is dominated by the macroscopic sigma of the formation and is used to deduce water saturation if thewater salinity is known and sufficiently high. The time decay of early capture (short burst) gamma rays is pri-marily sensitive to borehole salinity. The ratio of near-to-far detector capture count rates (TRAT) is used tocompute the thermal neutron formation porosity. The ratio of far-to-near detector burst-on count rates (IRAT)provides a gas indicator that is independent of thermal neutron absorbers.

In WFL mode, neutron burst-on and -off times can be selected to optimize sensitivity to the expected velocityand flow rate. Burst-on times are adjustable from 0.067 s for very fast flow to 34.1 s for very slow flow. Burst-off times can be set from 1.067 s to 135.5 s. During the burst-off period, the RSTPro tool measures the time dis-tribution of the activation gamma rays. After the background due to activation of stationary elements in thewellbore is removed, the remaining time distribution signal is analyzed for the presence of a peak. If it isdetected, its position is used to estimate the average water velocity. The area under the time distribution withthe background subtracted is proportional to the amount of water flowing by the tool and can be used for flowdetection behind casing.

POCKET

Electronics

Photomultiplier tube

GSO detector (far)

Electronics

Photomultiplier tube

GSO detector (near)

Shielding

Neutron generator

Far

Near

111/16 -in tool 21/2 -in tool

Arrangement of detectors, photomultiplier tubes, and neutron generators in RSTPro tools.

www.slb.com/oilfield 06-PR-001 July 2006*Mark of SchlumbergerCopyright © 2006 Schlumberger. All rights reserved.Produced by Schlumberger Marketing Communications

Specifications

111⁄16-in Slim Tool 21⁄2-in Focused Tool 21⁄8-in Flasked Tool

Pressure, psi [kPa] 15,000 [103,420] 15,000 [103,420] 20,000 [137.900]Temperature, degF [degC] 300 [150] 300 [150] 6 h at 400 [200]†

Diameter, in [cm] 1.710 [4.34] 2.505 [6.36] 2.125 [5.40]Min. restriction, in [cm] 1.813 [4.605] 2.625 [6.668] 2.250 [5.715]Tool length, ft [m]‡ 23.1 [7.04] 22.2 [6.77] 33.7 [10.27]Tool weight, lbm [kg] 100 [45] 208 [94] 243 [110]†Higher temperatures are possible for shorter holding times. Consult the RSTPro job planner.‡RSTPro tool components only, excluding common telemetry cartridge.