the many facets of pulsed neutron cased-hole logging/media/files/resources/oilfield...28 oilfield...
TRANSCRIPT
28
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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