PDK-100

Enhance formation evaluation and reservoir monitoring with proven pulsed neutron capture technology

In June 1963, the first Neutron LifetimeLog® was run in a well near Humble, Texas,by Lane Wells Co., a predecessor of BakerAtlas. Since then, thousands of pulsed neutron logs have been used by successivegenerations of petroleum engineers and log analysts. The commercial success of

the Neutron Lifetime Log and its continu-ing enhancement, such as the PDK-100®

service, is a tribute to the dedication ofBaker Atlas’ scientists and engineers and the management’s commitment to progress in the petroleum industry.

The PDK-100® (pulse and decay 100 channels)instrument provides high-resolution formation data to help you assess theextent of your hydrocarbon reserves.

The PDK-100 data can be used to optimize hydrocarbon recovery, whetheryou are producing a single well or an entire field. By periodically monitor-ing reservoir fluid saturations and gas/oil/water contact levels, you can con-tinually refine the reservoir model, impacting decisions concerning dailywell-site operations to initiating secondary or tertiary recovery. The PDK-100 data can also help you avoid premature well or field abandonment byidentifying bypassed hydrocarbons that could be recovered economicallyfrom the reservoir.

The PDK-100 instrument responds to both inelastic and capture gammarays, allowing you to determine sigma (Σ), the macroscopic neutron absorp-tion cross section of the formation, in a variety of borehole fluid conditionsand completion hardware configurations. Full-spectrum recording, digitaldata transmission, and continuous tool monitoring ensure data quality andreliability in a variety of cased and openhole environments. The unique full-spectrum recording provides additional reservoir and borehole informationto further enhance your formation evaluation.

ApplicationsReservoir monitoring of gas/oil/watercontact levels

Formation evaluation (fluid saturations and porosity)

- Through casing

- When openhole logs are not available

- Through drill pipe when openhole logs cannot be run due to holeconditions

Hydrocarbon typing — differentiationbetween gas and oil

Flood monitoring

Log-inject-log surveys

Preabandonment logging to locatebypassed hydrocarbons

Locating hydrocarbons trapped between tubing and casing strings

Identifying water channeling

The PDK-100 instrument provides valuableinformation for developing accurate 3-D reservoir depletion models.

System Overview

The PDK-100 instrument uses a fast (14 MeV)neutron accelerator and two scintillationgamma ray detectors to measure sigma (Σ),the macroscopic neutron absorption crosssection of subsurface formations.

The instrument responds to both inelasticand capture gamma rays. The inelasticgamma rays, produced by fast neutron collisions during the accelerator pulse, areused primarily to infer the type of fluidpresent in the formation. The capturegamma rays are generated by the absorp-tion of slow or “thermal” neutrons afterthe pulse. The sensitivity of the captureprocess to salinity produces a log responsesimilar to a conductivity measurement.These measurements allow for calculatingthe formation water saturation and monitordifferent fluid contact points behind thecasing, through tubing or drill pipe, as wellas in open hole.

The PDK-100 instrument measures a com-plete 100-channel decay spectrum for eachdetector. The formation Σ is computed fromthe rate at which these spectra decay towardthe background level. A spectrum that ex-hibits rapid decay is produced by a high-Σformation, such as a shale or high-porosityzone with high-salinity formation fluid.Conversely, a low-Σ formation, such as low-porosity reservoir rocks, gas zones, or low-salinity formation fluids, is represented bya slowly decaying spectrum. The PDK-100instrument can record all 100 channels ofspectral data for future in-depth analysis.

A single-level PDK-100 decay spectrum for twodifferent formations — clean limestone and a shale. A unique advantage of the PDK-100system is that the entire spectrum is recordedover 100 channels, providing a detailed delin-eation of decay for each detector.

Pulsing and Timing ModesThe PDK-100 instrument fires short burstsof fast neutrons into the formation at a rate of 1,000 times per second. Gammarays, induced by neutron interactions in the borehole and the formation, are mea-sured and sorted into 100 equal-width gates for each of the two detectors. Twocomplete decay curves are thus obtained,each representing the neutron absorptionrates in both the borehole and the forma-tion. The short duration of the pulse andthe large number of counting gates provideenhanced delineation of all the pertinentfeatures, decay process, as well as superiorstatistical precision.

The method used for computing the for-mation Σ from the short-spaced detector is based on a single exponential decayprocess. The borehole contribution to thetool response is minimized by delaying the analysis until the rapidly diminishing borehole component of the decay curve has died away. Compensation for any residual borehole signal, as well as diffu-sion effects, can readily be accomplished by using corrections based on extensive sets of laboratory and modeling data.Background activation is eliminated fromthe decay curve by subtraction. The mea-surement of this background activity takesplace within a 4-ms counting gate thatoccurs after every 28 neutron pulses. Thismethod for determining the formation Σprovides an accurate log and, yet, is farmore robust and reliable than more com-plex iterative algorithms based on dual-exponential or diffusion models.

The 100-channel raw data spectrum showsthe excellent delineation of the decay pro-cess. The counts in the first six channelsoccur during the neutron pulse and consistprimarily of gamma rays produced duringinelastic collisions with atomic nuclei inthe borehole and formation. The next 10 to20 channels are strongly affected by neu-tron capture within both the borehole andthe formation. Beyond 400 µs, the decayprocess is governed primarily by neutroncapture in the formation.

PDK-100 short-spaced spectrum — SS detectorresponse provides an enhanced thin-bedresponse and superior statistical precision.

PDK-100 and openhole neutron/densityresponses in zones of low-porosity, gas, oil, and water-saturated rock

Field Applications

The PDK-100 log presentations and curvedescriptions are provided in a documentaccompanying this brochure.

Example 1This example shows a typical PDK-100response in zones of low porosity, gas, oil,and water-saturated rock.* In the low-porosity zone, SGMA goes low while RINincreases due to the high density and lowhydrogen index of the formation. The con-firmation that this zone is tight is the low-porosity readings of the openhole neutronand density logs shown at the right.

In the gas zone, SGMA again moves to lowvalues. In this case, however, a dramaticdrop in the RIN curve and the separation of the LONG SPACE (LS) and SHORT SPACE(SS) count rates reveals the presence of gas.This gas interval is confirmed by the cross-over in the neutron and density porositylogs.

In the oil zone, SGMA remains low and RINstays relatively high throughout the zone.Here the SS and LS count rates help distin-guish this zone from the low-porosity for-mation. Note that in this oil-filled porosity,the neutron and density overlay. The wetzone is inferred by a high value of RINalong with a relatively high SIGMA readingindicating formation salinity. Also, the neu-tron and density porosity curves almoststack in the shaly zone.

*Adapted from example in paper by Scheibal et al.,“Differentiation of Hydrocarbon Type in LaminatedPliocene/Pleistocene Turbidite Sands via Inelastic PulsedNeutron Capture Data” (SPE 24737, 1992 SPE AnnualTechnical Conference, Washington, D.C.)

RIN is high in a low ø zone.

Neutron/Densitylow øLS/SS crossover

LS/SS crossoverand separation

No crossoverof LS/SS

LS & SSstack

Low Sigma

LOWPOR

A

RIN is high in a high ø oil zone.

Neutron/Densityoverlay, high ø

OIL

C

RIN is high in ahigh ø water zone.

WET

D

RIN is low in a high ø gas zone.

Neutron/Densitycrossoverhigh ø

Neutron/Densityhigh ø

GAS

B

Low Sigma

High Sigma

Low Sigma

15GAMMA RAY

65SIGMA DPHI

60 (su) 0 60 (pu) 0

16RIN=INELASTIC RATIO RESPONSE

6

LONG (CPS)0 3800 60 0

NPHI(pu)

SHORT (CPS)0 10000

(API)

Example 2This is a SEARCH Time Lapse analysisshowing the use of PDK-100 measurementsto monitor reservoir depletion over time.SEARCH is a cased-hole exploration serviceanalysis program developed by Baker Atlasto calculate fluid saturation, reservoirporosity, and shaliness from PDK-100 andopenhole log data for monitoring reservoirdepletion and gas/oil/water contact levels.The six-track display includes a gamma raycorrelation curve in track 1; three water saturation curves in track 2; porosity andfluid analysis in tracks 3, 4, and 5; and atotal volume analysis in track 6. The totalvolume analysis and fluid volumes shownin tracks 5 and 6 are based on openholeporosity and resistivity data. The PDK-100data were used to monitor changes inhydrocarbon and water saturations over 13 years of production.

SEARCH Time Lapse analysis of PDK-100 andopenhole log data help monitor hydrocarbonand water saturation changes over a 13-yearperiod.

SHALE SANDSTONE HYDROCARBON WATER

DEPTH

X100

GAMMA RAY-API

0 100

SW-RT ORIGNAL

0 100

SW-PDK YEAR 130 100

SW-PDK YEAR 100 100

FLUID POR.YEAR 13

50 0

FLUID POR.YEAR 10

50 0

FLUID POR.ORIGINAL

50 0

SHALE

0 100

POROSITY100 0

SAND100 0

Track 1 Track 2 Track 3 Track 4 Track 5 Track 6

X200

Example 3The examples below demonstrate the abili-ty of the PDK-100 system to perform underdifferent borehole conditions. The upperzone (3A) shows a high GOR oil zone ontop of water measured through a string ofcasing and stuck drill pipe. SGMA, RIN, SS,

GasSand

GasSand

OilSand

WaterSand

GasSand

GammaRay

7-5⁄8” csg

31⁄2” DP

Sigma

GasFlag RIN

RPOR

SS

LS

GammaRay

75⁄8” csg

Sigma

RIN

RPOR

SSLS

31⁄2” DP

Oil/WaterContact

OilSand

X10

X950

X900

X850

X800

X750

3B3A

X150

X200

X250

X300

X350

X100

and LS combine to provide the neededinformation. The lower interval (3B)reveals gas sands above and below the bottom of the casing. Again, the measure-ment is made through a string of stuckdrill pipe. SGMA and RIN both read low in the gas zones where the LS and SScount-rate curves separate.

Formation evaluation and differentiation between gas, oil, and water canbe achieved through drill pipe and casing using the 1.70-in. OD PDK-100instrument.

Borehole and Diffusion CorrectionsCorrection to the raw Σ measurement maybe required for quantitative analysis. Forinstance, if the neutron population is notuniformly distributed within the boreholeand the formation, the measured Σ valuemay be high. Thermal neutrons tend tomove from regions where their concentra-tion is high into regions where their con-centration is relatively low. This movementof neutrons in and out of various regionsdue to concentration differences is called“diffusion.”

Diffusion can have a significant influenceon the apparent rate of decay of thermalneutrons. As a general rule, thermal neu-trons tend to move away from the boreholeand out into the less populated regions ofthe formation. This phenomena reduces thecount rate and tends to make the formation

appear more absorptive, with a larger mea-sured Σ than the actual “intrinsic” value.The diffusion effect is more important forformations with low intrinsic cross sections.

Another influence on the raw Σ measure-ment is that various regions in the formationand the borehole absorb thermal neutronsat different rates. The standard SGMA mea-surement is made by waiting a sufficientamount of time after the neutron pulse sothat most of the counts in the detector willresult from neutrons that have spent mostof their time in the formation rather thanin the borehole. In many wells, boreholeand diffusion effects are insignificant, androutine formation evaluation can be per-formed with proper interpretation parame-ters. Under some circumstances, however, it is necessary to make corrections prior toquantitative log analysis. These correctionsare particularly important in well monitor-ing applications where borehole fluidschange from one logging run to the next.

Borehole- and diffusion-correction algo-rithms and charts for the PDK-100 havebeen determined from extensive sets of laboratory measurements and modeling calculations. The top figure shows the combined effects of borehole and diffusionon the PDK-100 instrument in 7-in. liquid-filled casing.† The corrected value, SGNT, is determined from the measured Σ, SGMA,and analyst input of borehole fluid infor-mation. PDK-100 borehole fluid indicators,such as RBOR, can be used to guide the analyst. SGNT is not dependent on any “initial guesses” as required by simultane-ous optimization solution techniques. Thecalculations are robust and produce reliableresults. The log interval in the bottom fig-ure shows three PDK-100 logging passesthrough the same interval with differentborehole fluids.† The raw SGMA measure-ments are shown just to the right of thedepth track, with the corrected SGNT valuespresented in the far right track. The bore-hole and diffusion-corrected SGNT is avail-able from your Baker Atlas GeoscienceCenter or as postprocessing on many fieldunits. Corrections can also be applied toPDK-100 log data from your files.

†Log example and surface plot are from a 1990 paper by Murdoch et al., “Diffusion Corrections To Pulsed Neutron Capture Logs: Methodology,” Trans., SPWLA Thirty-First Annual Logging Symposium, Lafayette, LA.

SGMA-SGNTdeparture surfacefor 8-in. (203-mm)borehole/7-in. (178-mm) casing(fresh cement) plotted as a func-tion of SGMA andborehole salinity

PDK-100 responsein three differentborehole fluids —diesel, 42k ppm salt water, and175k ppm saltwater

Instrument Specifications

Temperature Rating 340°F (171°C)

Pressure Rating 16,000 psi (110.3 MPa)

Min. Hole Diameter 2.25 in. (57.2 mm)

Tool Diameter 1.70 in. (43.2 mm)

Length 32.9 ft (10.03 m) including CCL and GR

Weight 148 lb (67.1 kg)

Recommended Logging Speed 20 ft/min (6.1 m/min)

Vertical Resolution 25 in. (635 mm) given proper formation contrast above and below zone of interest

Depth of Investigation 11 in. (279 mm), estimated for a 7.88 in. (200 mm) water-filled borehole with a nominal 20% porosity

Wireline Requirements Single conductor or 7-conductor cable

Detector Scintillation

Source Pulsed neutron 14 MeV, 1000 Hz

Measure PointsSS 8.0 ft (2.4 m) from bullplug (zero point)GR 20.0 ft (6.1 m)CCL 29.4 ft (9.0 m)

CCL MP

29 ft-5.0 in.(8.97 m)

32 ft-10.8 in.(10.03 m)

GR MP

20 ft-0.0 in.6.10 m

SS MP

8 ft-0.0 in.(2.44 m)

1.70 in.(43.2 mm)

© Copyright 1992 Baker Hughes Incorporated. All rights reserved. L99-059 Rev. 8/99 9666A 2M TGI

www.bakerhughes.com

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