28

ndersidsonre-ge-g-

ectellne

A Multipurpose ServiceThe RST service was introduced in June,1992 with a through-tubing pulsed neutrontool capable of providing both carbon-oxy-gen ratio (C/O) and sigma reservoir satura-tion measurements.1 Interpretation of eithermeasurement, under suitable formation andborehole conditions, provides quantitativeoil saturation. The high-yield neutron gener-ator and high-efficiency dual-detector sys-

The Many Facets of Pulsed Neutron Cased-Hole Logging

ign and fast, efficient gamma ray

ervoir saturation tool that is capable

through casing and more. Lithology

ons and flow profiles are some of the

by this multipurpose tool.

Ivanna AlbertinHarold DarlingMehrzad MahdaviRon PlasekSugar Land, Texas, USA

Italo CedeñoCity Investing Company Ltd.Quito, Ecuador

Jim HemingwayPeter RichterBakersfield, California, USA

Marvin MarkleyBogota, Colombia

Jean-Rémy OlesenBeijing, China

Brad RoscoeRidgefield, Connecticut, USA

Wenchong Zeng Shengli Petroleum AdministrationChina National Petroleum CorpoChina

■■The multipurposeRST service. Car-bon-oxygen ratio,inelastic and capture spectra,sigma, boreholeholdup, porosity,water and oilvelocities, andborehole salinityare some of themeasurements thatcan be made withRST equipment.

For help in preparation of this article, thanCannon, Wireline &Testing, Sugar Land, TCruz, GeoQuest, Quito, Ecuador; Steve GGeoQuest, Bakersfield, California, USA; Mand Susan Herron, Schlumberger-Doll Resfield, Connecticut, USA; Chris Lenn and CSchlumberger Cambridge Research, Cambland; and Chris Ovens, GeoQuest, AberdeIn this article, CNL (Compensated Neutron(Combinable Production Logging Tool), ELLog Analysis), FloView, FloView Plus, FMIFormation MicroImager), Phasor (Phasor InRST (Reservoir Saturation Tool), SpectroLit(Thermal Decay Time) and WFL (Water Flomarks of Schlumberger.1. For a detailed description of the RST too

and the latest scintillation detector techAdolph B, Stoller C, Brady J, Flaum C, MRoscoe B, Vittachi A and Schnorr D: “SMonitoring With the RST Reservoir SatuOilfield Review 6, no. 1 (January 1994)Sigma is a measure of the decay rate of trons as they are captured.

2. Holdup is a measure of the volumetric each phase in the borehole. Water holdholdup plus gas holdup equals unity. Flholdup multiplied by area and by veloc

To manage existing fields as effectively aefficiently as possible, reservoir enginemonitor movement of formation fluwithin the reservoir as well as productifrom individual wells. Pressure measuments play a vital role in reservoir manament. However, these data need to be aumented by other measurements to detfluid movement within the producing wand the surrounding formation. O

Advanced neutron generator des

detectors combine to make a res

of detailed formation evaluation

determination, reservoir saturati

comprehensive answers provided

Bureauration

ks to Darrelexas; Efrainarcia, ichael Herronearch, Ridge-olin Whittaker,ridge, Eng-en, Scotland. Log), CPLTAN (Elemental (Fullbore duction SFL),

h, TDT w Log) are

l hardwarenology:

elcher C,aturation ration Tool,”

Oilfield Review

recently introduced cased-hole logging tool,the RST Reservoir Saturation Tool, providesabundant single-well data to help reservoirengineers locate bypassed oil and detectwaterflood fronts, fine-tune formation evalu-ation and monitor production profiles.

tem provide higher gamma ray count rates,and hence better statistics, than previousgenerations of pulsed neutron devices. Thishas led to the development of many otherapplications, including spectroscopy mea-

: 29-39.thermal neu-

percentage ofup plus oilow rate equalsity.

29Summer 1996

surements, accurate time-lapse reservoirmonitoring and evaluation in difficult log-ging environments such as variable forma-tion water resistivity and complex lithology.

Other features of the tool design allowseveral auxiliary measurements such asborehole salinity and thermal neutronporosity. The tool comes in twodiameters—the 111/16-in. RST-A tool and21/2-in. RST-B tool. Both use the same typeof neutron generator, detectors and electron-ics. However, the larger diameter RST-B toolincorporates shielding to focus the neardetector towards the borehole and the fardetector towards the formation, allowinglogging in flowing and unknown boreholefluids and also providing a borehole holdupmeasurement.2 More recent applications forthe RST-A tool include WFL Water Flow Logmeasurements and separate oil and waterphase velocities in horizontal wells—PhaseVelocity Log (PVL) measurements.

Essentially the RST service provides threetypes of measurements:• reservoir saturation from C/O or sigma

measurements• lithology and elemental yields from

analysis of inelastic and capture gammaray spectra

• borehole fluid dynamics from holdup,WFL and PVL measurements.This article summarizes the many facets of

RST logging and reviews several examples.

Reservoir SaturationReservoir saturation is derived from C/O orinferred from sigma measurements (see “Sat-uration Monitoring, South American Style,”next page). Inelastic gamma ray spectra areused to determine the relative concentrationof carbon and oxygen in the formation. Ahigh C/O indicates oil-bearing formations; alow ratio indicates water-bearing forma-tions. Sigma is derived from the rate of cap-ture of thermal neutrons—mainly by chlo-rine—and is measured using capturegamma rays. Saline water has a high valueof sigma, and fresh water and hydrocarbonhave low values of sigma. As long as forma-tion water salinity is high, constant andknown, water saturation Sw may then becalculated.

Carbon-oxygen—Carbon-oxygen ratio ismeasured in two ways. A ratio (C/Oyields) isobtained from full spectral analysis of car-bon and oxygen elemental yields. A secondC/O (C/Owindows) is obtained by placingbroad windows over the carbon and oxygenspectral peak regions of the inelastic spec-trum. The C/Oyields is the more accurate ofthe two ratios, but lower count rates and,therefore, poorer statistics make it less pre-

Precise

Alpha processing

Imprecise

Accurate Inaccurate

Yields

Windows ■■Accuracy and precision. Alpha processing combinesthe accuracy of theelemental yieldscomputation of oilvolume (bottom left)with the precision ofthe windowsapproach (top right).The result is an oilvolume that is bothaccurate and pre-cise (top left).

cise than the C/Owindows. Conversely,C/Owindows is often less accurate but has bet-ter statistics and so is more precise. Eachratio is first transformed to give an oil vol-ume, and then the two oil volumes arecombined using an alpha processingmethod to give a final oil volume with goodaccuracy and good precision (top ). Thetransforms of C/O ratio to volume of oil usean extensive data base covering multiplecombinations of lithology, porosity, holesize, casing size and weight, as well as a

correction for the carbon density of thehydrocarbon phase.

Carbon-oxygen ratios are generated forthe near and far detectors. These two ratiosare used to give water saturation and bore-hole oil holdup (above).

Sigma—Sigma is a measure of how fastthermal neutrons are captured, a processtypically dominated by chlorine. Henceformation sigma may be considered a mea-

■■Water saturation, Sw, and borehole oil holdup, Yo, crossplot. Far car-bon-oxygen ratio (FCOR) is more influenced by formation carbon, andnear carbon-oxygen ratio (NCOR) is more influenced by borehole car-bon. A crossplot of FCOR versus NCOR (crosses) can, therefore, be usedto determine water saturation and borehole oil holdup. Overlying thecrossplot is a quadrilateral whose end points are determined from anextensive data base that depends on environmental inputs such aslithology, casing size and hydrocarbon carbon density. The cornerscorrespond to 0 and 100 % Sw and 0 and 100 % Yo. Interpolation pro-vides Sw and Yo at each depth.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Near carbon/oxygen ratio

Far

carb

on/o

xyge

n ra

tio

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

x

xxxxxxxxxxx

xxxxxx

xxxxx

Sw=0%, Yo=0%

Sw=100%, Yo=0%

Sw=100%, Yo=100%

Sw=0%, Yo=100%

30 Oilfield Review

Fanny field, situated among the oil fields east of

the Andes mountains, in the Oriente basin,

Ecuador, was discovered in 1972 and is presently

operated by City Investing Company Ltd. (below). Differential compaction of sands and shale

probably created the structural high that forms

the field. Primary production is from the M-1

sandstones of the Upper Cretaceous Napo with

secondary production from the Lower U sand-

stones of the Lower Cretaceous Napo.

There are six wells in Fanny field and these are

coupled to three others from the adjoining 18B

field drilled by the national oil company of

Ecuador, PetroProduction. Total output is 4000

BOPD of 22.2° API oil with a fluctuating water cut

of between 37% and 91%. Production is by

hydraulic pump.

Fanny-1 was completed as a commingled pro-

ducer in 1978 and after 18 years it was still pro-

ducing about 150 BOPD with 90% water cut from

two zones in the M-1 sand body. The high water

cut prompted City Investing to investigate.

A 111/16-in. RST-A tool was run with the well shut-

in to record carbon-oxygen ratio, formation

sigma, borehole sigma, thermal neutron porosity

and borehole salinity measurements.

■■Fanny-1 RST log results. ELAN Elemental Log Analysis interpretation of Sw and lithology (track 3) shows theoriginal openhole water saturation. Since then the oil-water contact has risen to 7752 ft (track 2) shown by theRST Sw of nearly 100% through the bottom section of the M-1 sand. The high carbon-oxygen ratio from 7702to 7709 ft is a coal seam. Very little of M-1 above the oil-water contact is depleted and the Lower U sand alsoshows high hydrocarbon saturation.

Saturation Monitoring, South American Style

Formation sigma and thermal neutron porosity

improved on the original formation evaluation by

providing a better estimation of shale volume in

the silty, sometimes radioactive, sandstones,

and also more accurate lithology identification.

The final interpretation showed that high water

production was caused by a rise in the oil-water

contact to 7752 ft [2363m] (above). It also

showed that other sections of the M-1 sand were

still at original water saturation and identified

two virgin oil zones.

Tests on the interval 7710 to 7720 ft [2350 to

2353 m] confirmed the RST results with a produc-

tion rate of 900 BOPD at only 10% water cut. The

two new zones were also tested and they pro-

duced 1300 BOPD at 4% water cut.

The old perforations were cement squeezed

and the well, reperforated and recompleted, is

now producing 1000 BOPD with low water cut—

a sixfold production increase.

South America

Quito

Tigre

Tumaco

Tiputini

EsmeraldasBalao

Fanny

E C U A D O R

7750

8400

7700

Sw RST<<SwOHSw RST<<Sw OH

Lith.inelastic

RST

Depth,ft

Sand

M-1 sand

Lower U sand

ClayLime Combined Model

0 p.u. 100Fluid Analysis

50 p.u. 100Far C/R

0 0.25

Near C/R-0.10 -0.15

Near C/RFar C/R

GR 10 API 110

SP from OH120 mV 30

Sigma 0 c.u. 30

Caliper6 in. 16

100 0p.u.

TotalPorosity

Sw from the RST

100 p.u. 0

25 0p.u.

WaterOil

Bound waterCalciteCoalSilt

QuartzClay

WaterOil

Bound water

■■Fanny field location.

31Summer 1996

3. For more on the dynamic parameterization algorithmapproach:Plasek RE, Adolph RA, Stoller C, Willis DJ and BordonEE: “Improved Pulsed Neutron Capture Logging WithSlim Carbon-Oxygen Tools: Methodology,” paper SPE30598, presented at the 70th SPE Annual TechnicalConference and Exhibition, Dallas, Texas, USA, Octo-ber 22-25, 1995.

■■Time-lapse logging in California. This logis from a well in the middle of a field that isproduced by heating the oil in place withsteam. Steam takes a narrow path fromone wellbore to another and will, therefore,not flush out all the heavy oil. After sometime, the steam needs to be redirected toproduce bypassed oil. RST time-lapse dataare used to monitor steam location andchanges in oil saturation.

There has been little change in oil satura-tion of the upper intervals X100 to X190 ft(track 2). The lower interval, X200 to X270ft, shows some oil movement. Steam hasbeen turned off in the zone X195 to X205 ftwhich has resaturated with water (track 3).

Bound Water

Irreducible Water

K-Feldspar

Quartz

Clay

Gamma Ray

Depth,ft

Formation Water

Phasor Oil Volume

Steam/Air 1993

Steam/Air 1995

SO from Core

0 p.u. 100

Porosity from Core

100 p.u. 0

X100

X200

X300

SW (11/7/93)100 p.u. 0-90 mV 120

DCAL-10 in. 0

DIT-E SO (11/7/93)0 p.u. 100

RST SO (11/27/93)0 p.u. 100

RST SO (4/16/94)0 p.u. 100

RST SO (1/30/96)0 p.u. 100

0 API 300

SP

sure of the chlorine content or salinity ofthe formation, and tracks openhole resistiv-ity curves.

The raw sigma measurement contains con-tributions from the borehole as well as theformation. To isolate the formation sigma,the neutron generator is pulsed in a dualburst pattern: a short burst followed by along burst. Near-detector measurements arestrongly influenced by the borehole environ-ment and hence borehole sigma— espe-cially for the short neutron burst measure-ment. Far-detector measurements areinfluenced more by formation sigma—espe-cially the long neutron burst measurement.

Raw sigma measurements are also affectedby neutron diffusion and environmentalvariables related to the borehole, casing,cement and formation. At the heart of thecorrection process for these effects is a database detailing thousands of combinations ofborehole sizes, casing types, formations ofdiffering porosity and lithology, and bore-hole and formation salinities. Instead of try-ing to define the response to these variablesby a single set of equations with fixedparameters, a dynamic parameterizationalgorithm uses the data base to compute thecorrected response in real-time, duringacquisition (see “The Sigma Data Base,”next page).3

Time-lapse—Once carbon-oxygen mea-surements or sigma measurements havebeen interpreted to produce saturation logs,these measurements may be repeated later tomonitor reservoir fluid movement such asoil-water contacts, secondary recovery pro-cesses or hydrocarbon depletion (right ).Good precision is important for time-lapse

(continued on page 34)

The Sigma Data Base

■■The SchlumbergerEnvironmental EffectsCalibration Facility,Houston, Texas, USA.Over 4000 measure-ments were made inmore than thirty forma-tions of differing lithol-ogy and porosity, withdifferent combinationsof formation salinities,borehole salinities, andcompletions to producethe sigma data base.

■■EUROPA facility, Aberdeen, Scotland.Diffusion, borehole and lithology effects must be

considered when transforming raw pulsed neu-

tron capture measurements to actual physical

quantities. These effects are difficult to account

for in direct analytical approaches across the

entire range of oilfield conditions. Therefore, an

extensive data base of laboratory measurements

is used to correct for these effects in real time.1

Over several years, the data base was acquired

for the RST-A, RST-B and TDT-P logging tools at

the Schlumberger Environmental Effects Calibra-

tion Facility (EECF), Houston, Texas (above andright). This enables raw tool measurements to be

referenced to calibrated values of formation

sigma, borehole salinity and formation porosity

for a variety of environmental conditions. Each

tool was run in over 30 formations of different

lithologies and porosities. Formation and bore-

hole fluid salinities were varied and different

completions were introduced into the borehole

representing different casing sizes and cement

thicknesses.

Altogether more than 1000 formation-borehole

combinations were measured for each tool. Mod-

eling was used to extend the range of available

sandstone formations. To date, the data base con-

tains over 4000 points.

The sigma values of the database formations

are calculated classically

∑ = (1-Φ) ∑ ma + Φ Sfl∑ fl

where Φ is the formation porosity, ∑ ma is

matrix sigma, Sfl is the formation fluid saturation

and ∑ fl is fluid sigma.

Porosity of the EECF tank formations was deter-

mined by carefully measuring all weights and vol-

32 Oilfield Review

1. Plasek RE et al, reference 3, main text.2. McKeon DC and Scott HD: “SNUPAR—A Nuclear

Parameter Code for Nuclear Geophysics Applications,”Nuclear Physics 2, no. 4 (1988): 215-230.

umes of the rocks, fluids and tanks used. CNL

Compensated Neutron Log measurements veri-

fied the porosity values and the homogeneity of

the formations.

Matrix sigma values were determined by gross

macroscopic cross-section measurements pro-

vided by commercial reactor facilities and by pro-

cessing complete elemental analyses through

Schlumberger Nuclear Parameter (SNUPAR)

cross-section tables.2

Water salinity was determined by a calibrated

titration procedure and then converted into fluid

sigma again using SNUPAR cross-section tables.

Algorithm—RST Sigma Processing

A three-step sequence is performed to translate

raw log measurements into borehole salinity,

porosity, corrected near and far sigma and forma-

tion sigma (next page, top).The first step is to correct the near and far

detector time-decay spectra for losses in the

detection and counting system, and for back-

33Summer 1996

ground radiation. Typically the background is

averaged to improve statistics.

The next step is to generate the apparent quan-

tities from the spectra, such as near and far

apparent formation sigmas. These quantities are

not environmentally corrected.

The third step is to apply transforms and envi-

ronmental corrections to the apparent tool quanti-

ties to arrive at borehole salinity, porosity and

formation sigma. The technique uses dynamic

database parameterization that handles both the

transformation and environmental corrections.

Accuracy

A series of benchmark measurements has been

made to assess the accuracy of the algorithm

used with the data base to compute borehole

salinity, porosity and formation sigma (below).These benchmark measurements include repro-

cessing the entire data base as well as logging in

industry standard facilities such as the EUROPA

sigma facility in Aberdeen, Scotland (previouspage, top right) and the API porosity test pit,

at the University of Houston, in Texas.

Database points were reprocessed with the

dynamic parameterization algorithm and the

results were compared with the assigned values.

STEP 3

STEP 1Correction to Spectra

Counting loss correctionsBackground adaptive filtering

Background subtraction

STEP 2

Transform from Apparent toCorrected Quantities

ExternalKnowledge(Optional)Porosity

Borehole salinity

ToolCalibration

Near/far ratio

Data Base

InputTime decay spectra

Compute Apparent QuantitiesNear apparent borehole sigma SBNAFar apparent formation sigma SFFANear/far capture count rate ratio TRAT

EnvironmentalParametersBorehole size

Casing size/weightLithology

OutputsBorehole salinity BSAL SIBFPorosity TPHICorrected near and far sigma SFNC SFFCFormation sigma SIGM

0

0

5

10

15

20

25

30

35

Assigned sigma, c.u.

Mea

sure

d si

gma,

c.u

.

LimestoneSandstoneDolomite

60

Mea

sure

d si

gma,

c.u

.

Assigned sigma, c.u.50403020100

60

50

40

30

20

10

0

-1.5 0.0 1.5Deviation from assignedsigma, c.u.

5 10 15 20 25 30 35Sigma, c.u.

250

200

150

100

50

00 10 20 30 40 50

Bor

ehol

e sa

linity

, kpp

m N

aCl

41 p.u.18 p.u. 0 p.u.

■■Processing accuracy. Benchmark measurements were made to assess the accuracy of the algorithm in computing formation and borehole sigma, porosity and bore-hole salinity. Sigma measured with the RST-A tool versus assigned database sigma (left) shows average errors are small—0.22 c.u. Sigma measured at the EUROPAfacility in Aberdeen (middle) again shows excellent agreement with the assigned values. Comparison of RST-A tool sigma (right) versus borehole salinity shows that corrected sigma is independent of borehole salinity—vital for time-lapse surveys or log-inject-log operations. In the crossover region (shaded area), formation sigmaapproaches or even exceeds borehole sigma. Historically, pulsed neutron capture tools erroneously identify the borehole decay as formation sigma and formation decayas borehole sigma in this region. However, the RST dynamic parameterization method solves this long-standing problem, correctly distinguishing between formation andborehole sigma components.

■■Simplified RST sigma processing.

34 Oilfield Review

Per

mea

bilit

y, m

d

Dispersed clay, %0 0. 2 0.4

500

400

300

200

100

0

60030 p.u.

20 p.u.

10 p.u.

20 p.u. 15% Calcite

techniques, which by definition look at dif-ferences from one log to another over aperiod of several months. RST data can begathered at logging speeds nearly three timesthose of previous-generation tools for thesame precision.4

LithologyAssessing reservoir deliverability andenhancing zone productivity rely on a thor-ough understanding of the rock matrix. Forexample, clay content dramatically affectspermeability (above ).5 Elemental yieldsderived from RST spectroscopy measure-ments provide the input to determine clayand other mineral content and henceimprove understanding of the rock matrix.

Elemental yields—Neutrons interact withthe formation in several ways. Inelastic andcapture interactions produce spontaneousrelease of gamma radiation at energy levelsthat depend on the elements involved. Mea-surement of the gamma ray spectra pro-duced by these interactions can then beused to quantify the abundance of elementsin the formation. Elemental yields are oftenused in various combinations or ratios to aidcomplex lithology interpretation, to deter-mine shale volume or to augment incom-plete openhole data (see “Making Full Useof RST Data in China,” page 36).

4. For more details on time-lapse monitoring see sec-tions on precision and auxiliary measurements: Plasek RE et al, reference 3.

5. Herron M: “Estimating the Intrinsic Permeability ofClastic Sediments from Geochemical Data,” Transac-tions of the SPWLA 28th Annual Logging Symposium,London, England, June 29-July 2, 1987, paper HH.

6. Roscoe B, Grau J, Cao Minh C and Freeman D: “Non-Conventional Applications of Through-TubingCarbon-Oxygen Logging Tools,” Transactions of theSPWLA 36th Annual Logging Symposium, Paris,France, June 26-29, 1995, paper QQ.

■■Effect of clay andcalcite on perme-ability. A smallpercentage of clayhas a dramaticeffect on perme-ability. Calcite alsoreduces perme-ability. So to deter-mine a well’s pro-ducibility or thecause of any for-mation damage, itis important tounderstand themineralogy.

At high neutron energies, inelastic interac-tions dominate. After a few collisions, neu-tron energy is reduced below the thresholdfor inelastic events. The probability of aninelastic interaction occurring is also rea-sonably constant for all major elements.

As neutrons slow to thermal energy levels,capture interactions dominate. Some ele-ments are more likely to capture neutronsthan others and so contribute more to thecapture gamma ray spectrum.

Inelastic and capture gamma ray spectraare recorded by opening counting windowsat the appropriate time after a neutron burstfrom the RST neutron generator. Tool designallows not only for much higher gamma raycount rates than previous generation tools,but also for gain stabilization that enableslower gamma ray energy levels to berecorded for both inelastic and capturemeasurements. A major advantage of this isthe inclusion of the inelastic gamma raypeaks on the spectrum at 1.37 MeV formagnesium and at 1.24 MeV and 1.33 MeVfor iron.6

A library of standard elemental spectra,measured in the laboratory for each type oftool, is used to determine individual ele-mental contributions (next page).

SpectroLith interpretation—SpectroLithprocessing is a quantitative mineral-based

The algorithm does exceptionally well in match-

ing the assigned values. For example, the aver-

age errors for formation sigma were 0.22 capture

units (c.u.) for the RST-A tool and 0.20 c.u. for

the RST-B tool.

The EUROPA facility is an independent sigma

calibration facility partially funded by the UK

Atomic Energy Authority with major support from

a consortium of 15 oil companies and govern-

ment agencies. The RST-A tool was run in all the

openhole formations and several cased-hole for-

mations. A smaller number of measurements

were made with the RST-B tool. Both tools read

the true formation sigma over a wide range of

lithologies, porosities, formation and borehole

fluids, borehole sizes and completions. Even in

the difficult crossover region, where formation

sigma approaches or exceeds borehole sigma,

the errors are small and the tool does not lock on

to the wrong sigma component.

Both EUROPA and the University of Houston API

pits were used to check porosity readings. The

agreement between the two sets of porosities

was excellent.

Precision

Key to time-lapse monitoring techniques is

repeatability or precision. Time-lapse uses differ-

ences in measured quantities to monitor, for

example, the progress of waterflooding, the

expansion of gas caps and the depletion of reser-

voirs. The RST tool has been benchmarked to log

nearly three times faster than previous genera-

tion tools for the same level of precision.3

3. For examples of repeatability—precision—see: Plasek et al, reference 3, main text.

7. Herron SL and Herron MM: “Quantitative Lithology:An Application for Open and Cased Hole Spec-troscopy,” Transactions of the SPWLA 37th AnnualLogging Symposium, New Orleans, Louisiana, USA,June 16-19, 1996, paper E.

8. See Roscoe B et al, reference 6.

Iron

ChlorineSilicon

Titanium

Calcium

Sulfur

HydrogenGadolinium

Oxygen

Inelastic Spectra

Capture Spectra

Silicon

Iron

Calcium

Magnesium

SulfurBackground

Carbon

Energy, MeV1 2 3 4 5 6 7 8

Rel

ativ

e co

unts

1 2 3 4 5 6 7 8Energy, MeV

Rel

ativ

e co

unts

35Summer 1996

■■Elemental stan-dards for the RST-Atool. Lower gammaray energy levelsare recorded by theRST tools than byprevious generationpulsed neutron tools.This allows mea-surement of elemen-tal contributionsfrom elements suchas magnesium andiron. Elementalyields are processedfrom standard spec-tra obtained usinglaboratory measure-ments. Shown arethe standards forinelastic (top) andcapture (bottom)spectra for the1 11/16-in. RST-A tool.

lithology interpretation derived from elemen-tal yields. Traditional lithology interpretationrelied on measurements of elements such asaluminum and potassium to determine claycontent. Aluminum, especially, is difficult tomeasure and requires a combination of log-ging tools; the interpretation is also complex.

A recent detailed study of cores showedthat a linear relationship exists between alu-

minum and total clay concentration. Ofmore importance, it also showed that sili-con, calcium and iron can be used to pro-duce an accurate estimation of clay withoutknowledge of the aluminum concentration.7The concentrations of these three elementscan be obtained from RST spectroscopymeasurements.

In addition, carbonate concentrations—defined as calcite plus dolomite—can bedetermined from the calcium concentration

alone with the remainder of the formationbeing composed of quartz, feldspar andmica minerals.

SpectroLith interpretation involves threesteps:• production of elemental yields from

gamma ray spectra• transformation of yields into concentra-

tion logs• conversion of concentration logs into

fractions of clay, carbonate and frame-work minerals.

Borehole FluidThe producing wellbore environment mayinclude a combination of oil, water and gasphases in the borehole as well as flowbehind casing. Borehole fluid interpretationis primarily based on fluid velocities andborehole holdup. The RST equipmentmakes these measurements using severalindependent methods, with enough redun-dancy to provide a quality control crosscheck:• The WFL Water Flow Log measures water

velocity and water flow rate using theprinciple of oxygen activation. Thismethod detects water flowing inside andoutside pipe, and in up and down flow.

• The Phase Velocity Log (PVL) measuresoil and water velocities separately byinjecting a marker fluid, which mixes andtravels with the specified phase. Thismethod may be applied to up and downflow, but only fluids in the pipe aremarked and therefore detected.

• Two-phase—oil and water—boreholeholdup may be measured in continuouslogging mode with the RST-B tool.8

• Three-phase—oil, water and gas—bore-hole holdup is currently an RST-A stationmeasurement based on a combination ofC/O and inelastic count rate ratio data.

• Borehole salinity is one of the computa-tions made as part of the sigma and poros-ity log and may be used to compute aborehole water holdup with either theRST-A or the RST-B tool.

(continued on page 39)

36 Oilfield Review

Gu Dao and Sheng Tuo are typical of the Shengli

complex of oil fields about 200 km [125 miles]

southeast of Beijing near the Bo Hai Gulf, China

(right).1 Both fields have a similar deltaic deposi-

tional environment, with alternating sand-shale

sequences. Thin, tight, calcareous streaks within

the depositional sequences are common. Reser-

voir layer thickness varies from more than 10 m

[31.2 ft] to less than 1 m [3.1 ft] and each layer is

produced separately.

For more than 30 years, many of these eastern

Chinese oil fields have been under water injec-

tion to maintain pressure and improve sweep of

the heavy hydrocarbons. The water injection pro-

gram uses a mix of the low-salinity connate water

and fresh surface water, which has resulted in

variable and unknown water resistivity in many

reservoirs.

In order to efficiently manage the waterflood

enhanced oil recovery program and maximize oil

recovery, it is essential to know the waterflood

sweep efficiency, determine residual or remain-

ing oil saturation, and pinpoint zones bypassed

by the recovery scheme.

Hydrocarbon saturation evaluation from open-

hole resistivity logs, run in newly drilled infill

wells, is difficult because the formation water

resistivity is variable and most of the time

unknown. Reservoir saturation monitoring with

sigma measurements is impractical, as there is

little contrast between the oil and water sigmas

and, in any case, the water sigma is unknown.

These constraints leave carbon-oxygen measure-

ments as the only viable option.

The Shengli oilfield operators—Shengli

Petroleum Administration Bureau, China National

Petroleum Corporation (SPAB-CNPC)—decided to

run the 21/2-in. RST-B tool for many reasons:

•The shielded dual-detector system alleviates

the effect of a changing or unknown borehole

oil holdup, as well as the effect of waxy

deposits on the casing.

•Through-tubing logging, while the well was

flowing, avoids formation damage and also

increases operational efficiency in a multiwell

campaign.

•The 51/2-in. casing inside 81/2-in. borehole

completion produces a thick cement sheath

that reduces measurement sensitivity. The RST

tool has a high-energy, high-yield neutron gen-

erator and an efficient detection system that

provide better statistics in thick cement than

the previous-generation pulsed neutron tools.

• An additional pass in sigma mode provides

data useful to accurately evaluate shaliness,

especially in wells with scarce openhole data.

• Measurements such as neutron porosity and

count rates can also be recorded to aid inter-

pretation when gas is present.

Evaluation with Scarce Openhole Data

Key to the interpretation of carbon-oxygen data is

a knowledge of lithology to account for matrix

carbon, and effective porosity to calculate oil sat-

uration. A typical Sheng Tuo well illustrates the

benefits of additional data provided by the RST

tool (next page). For this well the openhole data

were limited to sonic and gamma ray logs.

Sonic and gamma ray data do not provide

enough lithology information to account for matrix

carbon. For example, carbonates cannot be distin-

guished from tight siliclastic streaks. Sonic-

derived porosity may also be inaccurate if lithol-

ogy and formation fluids are unknown, and also, if

the sands are unconsolidated and the compaction

factor is unknown. The gamma ray curve alone is

unsuitable for accurate shale volume evaluation

because the reservoir sands are rich in micas and

feldspars—both radioactive minerals.

To augment the limited openhole data, an RST

sigma-mode pass provided sigma for shale vol-

ume estimation and thermal neutron porosity

(TPHI) for effective porosity evaluation. The

inelastic-capture data were analyzed in detail not

only for the carbon-oxygen ratio (C/O), but also for

elemental yields to provide other ratios. For exam-

ple, the ratio of iron to silicon (IIR) is indicative of

shale volume if kaolinite and heavy minerals are

not present; the ratio of silicon to silicon-plus-cal-

cium (LIR) may be used as a lithology indicator;

and the ratio of chlorine to hydrogen (SIR) gives a

formation salinity indicator.

The initial volume of oil was computed from the

openhole resistivity data in 1994 assuming that all

sands were at connate water resistivity. The 1995

RST carbon-oxygen evaluation computed remain-

ing oil. A decrease in oil between the two may be

due to reservoir depletion, but could also be due

to an overly optimistic openhole evaluation if the

reservoir water was not at connate salinity, but at

the fresher floodwater salinity.

The additional RST data proved invaluable. For

example, in the Gu Dao and Sheng Tuo fields in

general, sigma responds primarily to changes in

matrix sigma and therefore provides the best shale

indicator. The lithology indicator ratio LIR was

used to identify the tight calcite streaks at X201 m

and X218 m.

Interpretation of the salinity indicator ratio (SIR)

is more complicated. However, when the forma-

tion water volume remains constant, SIR responds

directly to formation fluid salinity and can be used

to determine the progress of injection water—

approximately the case in the large reservoir

between X220 m and X245 m.

■■Location of Gu Dao and Sheng Tuo fields.

Making Full Use of RST Data in China

C H I N A

Hong Kong

TAIWAN

Shanghai

Qingdao

M O N G O L I A

Beijing

Sheng Tuo Gu Dao

Beijing

Shengli Complex

Bo Hai Gulf

1. Olesen J-R, Chen Y, Zeng W, Zhu L and Zhang Z:“Remaining Oil Saturation Evaluation in Water FloodedFields Under Variable Formation Water Resistivity,” to bepresented at the 1996 International Symposium on WellLogging Techniques for Oilfield Development, Beijing,Peoples Republic of China, September 17-21, 1996.

37Summer 1996

• The inelastic count rate ratio (CRRA) from the

near and far detector is sensitive to porosity

and gas content.

For example, in one Gu Dao well, the upper

sand body, X103 m to X109 m, shows the pres-

ence of gas (next page, top). Sigma and CRRA

scales were chosen so that the curves overlay in

clean gas-free formations. In the upper sand they

show negative separation as both sigma and

CRRA are driven lower by the presence of gas.

Similarly, TPHI shows a reduced neutron porosity

when compared to the true formation porosity

taken from the openhole interpretation of 1990.

No gas was apparent on the 1990 openhole

logs, so it is assumed that reservoir pressure has

declined below bubblepoint allowing gas to come

out of solution. Tests indicate that this is a water-

bearing zone with some gas, confirming the RST

interpretation.

Determining Water Resistivity and Flood Index

Interpreting openhole logs of newly drilled wells

in reservoirs that have been partially or fully

flooded is challenging. Water resistivity, Rw ,

often varies continuously from the relatively high

value of fresh floodwater to the low value of the

more saline connate water. If connate water

resistivity is used for Rw , then hydrocarbon satu-

ration will be optimistic in partially flooded

zones.

However, by combining openhole and RST data

a continuously varying Rw may be calculated

leading to true hydrocarbon saturation. The eval-

uation may be taken further if floodwater resistiv-

ity is known and constant. In this case, the total

volume of water may then be split into connate

and floodwater.

Reservoir saturation acquisition timing is criti-

cal to the interpretation. It must be late enough

after well completion to allow drilling fluids to

dissipate, but before significant hydrocarbon

depletion occurs. Four weeks has proven ade-

quate for Gu Dao and Sheng Tuo fields.

Water resistivity is computed using standard

openhole interpretation methods. Openhole logs

provide Rt, Rclay, Vclay and effective porosity,

Φeff. Water saturation comes from RST interpre-

■■Formation evaluation with additional RST data. Volumetric analysis (track 4) shows remaining hydrocarbonsaturation determined from RST carbon/oxygen ratio. The 1994 openhole fluid curve indicates more oil due toeither depletion or an overly optimistic evaluation. A comparison of RST porosity (TPHI), cased hole CNLCompensated Neutron Log porosity (NPHI), and sonic transit time (DT), shows good agreement (track 3),especially when NPHI is put on a sandstone scale—3 to 4 p.u. shift to the left. The lithology indicator (LIR) isabout 1 for siliclastics and decreases for carbonates (track 2). Two tight calcite streaks can be seen at X201and X218 m. The salinity indicator (SIR) responds to formation salinity if porosity and hydrocarbon saturationare approximately constant (track 2). The iron indicator (IIR), gamma ray and sigma (track 1) follow the sametrend, and each may be used for shale volume calculation under the correct conditions. Gamma ray indicationof shale will be pessimistic if radioactive sands are present—for example, those containing micas andfeldspars. Clays, except for kaolinite, contain iron. Sigma responds to formation matrix and fluids. Sigma fluidis almost the same when oil and fresh water are present, so sigma responds primarily to changes in matrix. In Gu Dao and Sheng Tuo, sigma has proved to be the best shale indicator.

X200

X250

Depth, m

IIR

0 2.5

SIGM

0 c.u. 50

GR

100 API 250

LIR

0.625 1.25

SIR

-0.5 ppk 3.5

DT

150 µsec/ft 50

TPHI

60 p.u. 0

NPHI

60 p.u. 0

Openhole Analysis

0 p.u. 100

Openhole Fluid 1994

100 p.u. 0

Shale

Bound Water

Quartz

Calcite

RST Oil 1995

Water

In the shaly lower section of the reservoir,

salinity is high and probably at connate level,

indicating minimal depletion. The middle section

is the cleanest, most permeable section and

shows a progressive drop in salinity. The water-

flood front has reached this section. The upper

section shows an intermediate salinity and shale

content, and also a smaller discrepancy between

RST saturation and openhole saturation. Flooding

has reached this section, but is not complete.

Similar results have been seen with other RST

logs in these fields.

Identifying Gas-Bearing Zones

Carbon/oxygen ratio responds to the carbon con-

centration in pore space. In gas-bearing zones,

carbon concentration is low, so C/O is low. Low

C/O can easily be misinterpreted as a water-bear-

ing zone. However, several auxiliary measure-

ments can help identify gas-bearing intervals:

• Gas sigma is much lower than water sigma or

oil sigma; therefore, at comparable shale lev-

els, the RST sigma measurement will be lower

in gas-bearing reservoirs.

• Hydrogen index is also low in gas-bearing

zones. Therefore, neutron porosity measure-

ments such as RST porosity (TPHI) underesti-

mate formation porosity.

38 Oilfield Review

X100

X125

Depth, m

Openhole Sw 1990

100 p.u. 0

Cased Hole Sw 1995

100 p.u. 0

RST Gas Indicator

5.75 1.75

SIGM

-10.0 c.u. 30.0

Gas

Openhole Analysis

0 p.u. 100

Shale

Bound Water

Quartz

Calcite

RST Oil 1995

Water

Radius of Bit

0 in. 10

Borehole Fluid

Casing Wall

Assumed CementSheath

Formation

Openhole Porosity

50 p.u. 0

O.H. Fluid Volume 1990

50 p.u. 0

RST Fluid Volume 1995

50 p.u. 0

TPHI from Sigma mode

0.5 p.u. 0

RST Oil 1995

tation. The flood index is determined as a linear

interpolation between floodwater resistivity and

connate water resistivity.

In a Gu Dao field example, connate and floodwa-

ter salinities are 8.5 ppk and 3 ppk, respectively

(below left). The lower section, X296 to X303 m,

is shaly and water-bearing. The middle section,

X287 m to X296 m, is the cleanest and is separated

from the lower section by a thin, clean, sand streak

where the oil-water contact is situated.

The clean midsection has the highest permeabil-

ity and provides a preferential conduit for water-

flooding. The discrepancy between RST-derived

and openhole hydrocarbon saturation is due to the

inadequate Rw estimation for the openhole evalua-

tion. True hydrocarbon saturation is 40% as shown

by RST data and not 60%. Water resistivity, com-

puted from a synthesis of RST and openhole data,

indicates that fresh waterflooding has increased

Rw from the connate water value of 0.35 ohm-m to

about 1 ohm-m. The flood-index calculation con-

firms that the cleanest levels of this reservoir have

been heavily flooded.

The shalier upper sand section shows general

agreement between RST-derived and openhole

hydrocarbon saturation. Because of the increase in

shaliness and the related decrease in permeability,

waterflooding essentially bypasses this section

and little hydrocarbon sweep is achieved.

Campaign Success

The Shengli oilfield RST campaign has shown that

hydrocarbon monitoring in waterflooded fields with

varying salinity is a viable procedure. In addition,

ancillary RST measurements complement open-

hole information, improving both formation evalua-

tion and detection of gas-bearing intervals. Also,

the combination of openhole and RST data

acquired within one month is a powerful tool for

evaluating the waterflooding process. During the

course of the campaign, RST data contributed to

the achievement of the SPAB-CNPC engineers’ goal

of maintaining oil output while controlling water

production. RST results showed a large amount of

remaining hydrocarbon, especially in the massive

sands of the Sheng Tuo oil field.

X290

X300

Depth,m

Radius of Bit

0 10

Borehole Fluid

Casing Wall

Assumed Cement Sheath

Formation

RST-derived Rw

0 2

Cased Hole RST Sw

100 p.u. 0

Flood Index

2 0

Openhole Porosity

50 p.u. 0

O.H. Fluid Volume 1994

50 p.u. 0

RST Fluid Volume 1995

50 p.u. 0

Nonmovable Oil

Remaining Oil RST1995

Flood Water

Openhole Analysis

0 p.u. 100

Shale

Bound water

Quartz

Nonmovable oilOpen Hole 1995

Movable RST Oil 1995Water

■■Gas detection. Inelastic count rate ratios of near-to-far detector counts and sigma are both affected by gas(track 2). Negative separation of these curves indicates gas. RST porosity, TPHI, also reads lower in gas (track3). Although no gas was shown on the openhole logs, it is assumed that solution gas has accumulated in thefully depleted zone between X100 m to X109 m. Tests indicate that the layer is mainly water and gas.

■■Water resisitivity, Rw, and flood index. A floodindex can be calculated from variable Rw (track 2)computed from RST and openhole data collectedbefore any hydrocarbon depletion and after invasionfluids have dissipated (track 3).

39Summer 1996

WFL measurements—Water flow logging,introduced with the last-generation TDTThermal Decay Time service several yearsago, is now available with the RST service.The RST neutron generator providesimproved burst control, which allows detec-tion of water velocities up to 500 ft/min[150 m/min] with the far detector alone. Inaddition, the introduction of energy discrimi-nation and shielding between neutron gener-ator and detectors results in a significantimprovement in the signal-to-noise ratio, andextends sensitivity to low flow conditions.

Oxygen molecules in water are activatedby a burst of neutrons producing a radioac-tive cloud. The cloud moves with the wateralong the borehole, emitting gamma rays asactivated oxygen decays back to its steadystate (top right). As the cloud passes, gammarays are first detected by the near detectorand then by the far detector of the RSTsonde, producing a characteristic peak inthe count rate of each. The time betweenneutron burst and cloud detection—time-of-flight—and the distance between neutrongenerator and detector give water velocity.Other detectors can be added farther awayin the tool string to detect extremely highwater velocities. The RST equipment canalso be turned upside-down to detect down-ward flow.

In addition, the volume of activated oxy-gen is proportional to the volume of waterflowing by the detectors. The profile of thedetected signal carries information aboutthe mean water velocity, water holdup andwater flow rate. These quantities are relatedin that the water velocity, water holdup andeffective cross-sectional area of the pipe canbe combined to compute the water flowrate (see “Production Logging in the SanJoaquin Basin,” next page).

PVL—Phase velocity logging has beendeveloped for horizontal wells where strati-fied flow is present. Like WFL logging, thePhase Velocity Log measures time-of-flight.Gadolinium has a very high thermal neutroncapture cross section and is injected into theproducing borehole (bottom right ). Theinjection fluid is designed to mix with eitherthe water or oil phase only. Gadolinium actsas a sink, sucking in thermal neutrons and

■■WFL Water Flow Log service. A short burst of neutrons interactswith oxygen in the surrounding water forming an oxygen isotopewith a half-life of 7.1 sec. As the activated oxygen decays back toits steady state, gamma rays are emitted. In flowing water thecloud of activated oxygen, and hence gamma rays, travels alongat the water velocity. Characteristic increases in count rate areseen as the cloud passes the various detectors. The distancebetween neutron generator and detector and the time-of-flightgive water velocity. The initial cloud volume is proportional to theamount of oxygen present and hence volume of water. The areaunder the gamma ray peak as the cloud passes a detector is,therefore, also proportional to the volume of water flowing by(water holdup)—allowing for effects of diffusion and decay rate.Combining water velocity and holdup gives water flow rate.

16O+n p+16N β+16O* 16O+γ Half-life ~7.1sec

Minitron Oil

Water

Casing

Near Detector Far Detector Additional Detector

■■Phase VelocityLogging (PVL). A strong neutronabsorber isinjected into theappropriate phaseof producing fluid.This is subse-quently detected,allowing a time-of-flight measure-ment that gives the velocity of thatphase.

Oil

Water

Oil-miscible marker RST tool

Phase Velocity Sonde

0 10 20 30 40 50 60 70 80Time, sec

Start of injection

90

Marker signal

decrease at X430 ft. The temperature also drops

at this point. The combination of decrease in flow

rate and temperature can occur only if the forma-

tion is taking fluid—a thief zone. Conventional

openhole logs and the mud log suggest that there

is a highly resistive, low porosity carbonate in

this interval. The FMI Fullbore Formation

MicroImager tool shows what has been inter-

preted as a calcite healed fracture. This fracture

has most likely been opened by acid treatment

and has created the thief zone.

Production Logging in the San Joaquin Basin

■■WFL Water Flow Log. The flow profile indicates that most of the gas production is from X350 to X370 ft (tracks 2 and 3). Below this depth is a complex profile of thief zone and water recirculation. WFL stationary read-ings determined the water production profile, and temperature and pressure (track 1) aided the interpretation.

The next significant event in the flow profile

occurs across the short perforated interval X350 to

X370 ft. Here, a large increase in spinner flow rate

and a change in slope of the pressure data indicate

an influx of gas. The WFL log shows doubling of the

water flow rate across the same interval.

1. Water recirculation occurs, usually in deviated wells,when water and oil are present. Water can flow up withthe oil on the upper side of the well and down on thelower side in a continuous cycle. A thief zone occurs when a perforated zone has a lowerformation pressure than the borehole, causing flowfrom borehole to formation.

C A L I F O R N I A

U S A

Taft

Elk hillsBakersfield

Fresno

Coalinga

San Andreas Fault

Elk Hills is one of the largest oil fields in the San

Joaquin basin about 20 miles [32 km] west of Bak-

ersfield, California, USA (below). The field forms

part of the Naval Petroleum Reserve No. 1 and is

operated by Bechtel Petroleum Operations, Inc.

for the Department of Energy. Although Elk Hills

was discovered in 1911, production was limited

until the 1974 oil crisis resulted in opening up the

field to full production in 1976. The field has pro-

duced over 1.1 billion barrels of oil and a signifi-

cant quantity of gas, and now produces about

60,000 BOPD of medium-gravity crude.

Earlier this year, Bechtel wanted to determine

the flow profile and quantify the zonal contribu-

tions to oil, water and gas production from a well

in which production from a waterflooded sand

reservoir was commingled with production from a

shaly interval. A production log consisting of tem-

perature, pressure and spinner was run and sta-

tionary WFL Water Flow Log measurements were

taken with the RST tool.

The flow profile turned out to be complex,

showing a zone of water recirculation near the

bottom and a thief zone above (right).1

A combination of spinner and WFL data located

the recirculation zone. The spinner indicated down

flow, while the WFL data indicated a small

amount of water flowing up. The temperature log

also showed a strong anomaly over this interval.

The flow profile shows a net flow of oil from this

zone simply because a recirculation zone requires

multiphase flow.

Both spinner and WFL data show an increase in

flow above the recirculation zone before an abrupt

X200

X400

X600

X800

Gas

Oil

Water

Downhole Flow Rate, B/D

Water Flow Log, B/D

Pressure

Depth,ft

Temp

0 3000

01050 1300psi °F 211206 3000

Thief zone

Water Flow Stations

Recirculating water zone

■■ Location ofElk Hills field,Kern County,California.

40 Oilfield Review

changing the borehole sigma. The detectionof this change provides a time-of-flight mea-surement for the marked phase.

Two-phase borehole holdup—The twodetectors of the RST sonde provide two car-bon-oxygen measurements that are suffi-cient to solve for formation water saturation(SW ) and borehole oil holdup (YO ) (seecrossplot, page 29 ). Four points may bedefined on a plot of far carbon-oxygen ratioversus near carbon-oxygen ratio to give aquadrilateral:• Water in the formation and water in

the borehole (SW = 100, YO = 0)• Oil in the formation and water in the

borehole (SW = 0, YO = 0)• Water in the formation and oil in

the borehole (SW = 100, YO = 100)• Oil in the formation and oil in the

borehole (SW = 0, YO = 100).

The exact position of these points dependson lithology, porosity, hydrocarbon carbondensity, hole size, casing size, casing weightand sonde type—RST-A or RST-B sonde.

With the larger RST-B sonde, the quadrilat-eral is wide since the far detector is shieldedto be more sensitive to the formation andthe near detector shielded to be more sensi-tive to the borehole. This provides good sep-aration of the signals and a good boreholeoil holdup measurement in addition to a for-mation saturation measurement. The slim-mer RST-A sonde is not focused and, there-fore, requires knowledge of the boreholefluids to separate the formation and bore-hole signals.9

Three-phase holdup—A combination ofRST measurements can be used to computethree-phase holdup. Gas holdup is indicatedby the inelastic near-to-far count rate ratio.The near and far C/Oyields depend on gas,water and oil holdups. By combining thesemeasurements and applying two condi-tions—the sum of the holdups must equalunity and also the sum of the saturationsmust equal unity—three-phase holdups maybe calculated. The RST measurement ofborehole sigma can also be combined withthis analysis to enhance the holdup calcula-tion if the water salinity is known.

41Summer 1996

GR RST

FloView toolFlow regimeWater holdup

RST Reservoir Saturation ToolOil holdupGas indicator

FloView Plus tool

Phase Velocity LogMarker injection for oiland/or water velocity

WFL Water Flow LogWater velocityWater holdupWater flow rate index

CPLT

CPLT CombinableProduction Logging ToolPressure and temperature

Fluid markerinjector

Spinner

Total flow rate

Gamma raydetector

■■The next generation production logging tool string.

9. For an alternative method of measuring boreholeholdup with the RST-A tool: Roscoe B et al, refer-ence 6.

10. Schnorr DR: “Determining Oil, Water and Gas Saturations Simultaneously Through Casing by Com-bining C/O and Sigma Measurements,” paper SPE35682, presented at the SPE Western Regional Meet-ing, Anchorage, Alaska, USA, May 22-24, 1996.

Comprehensive Cased-Hole EvaluationSince commercialization of the RST servicefour years ago, many applications havebeen developed. With the addition of lithol-ogy interpretation, phase velocity loggingand three-phase holdup, the tool is rapidlybecoming a comprehensive cased-holeevaluation service.10 A future OilfieldReview article will explain in more detailsome of these new services, including newproduction logging combinations (above).

—AM