api pit calibrationof mwd gamma ray tools b...

SPWLA Twenty-Ninth Annual Logging Symposium, June 5-8, 1988

API PIT CALIBRATIONOF MWD GAMMA RAY TOOLS

Thomas M. Bryant and Tyrone D. GageTeleco Oilfield Services, Inc.

Meriden, Ct.

ABSTRACT

The responses of MWD gamma ray sensors and full MWD tools rangingin size from 6 3/4” to 9 1/2” outside diameter have recently, forthe first time, been measured in a series of tests conducted inthe API gamma radiation pit and in the larger KUTh test pitfacilities in Houston.

Purposes of these tests included the determination of:

1) the relationship between isotopic mixture and collar thick-ness with respect to both attenuation and spectral biasingof gamma radiation.

2) the ratio of API gamma radiation units to MWD counts persecond (cps) for both Geiger-Mueller and scintillatorsensors, and for each sensor type and drill collar sizecombination.

3) the influence of borehole size on both MWD cps responseand the API/MWD cps relationship.

4) the effect of sensor or tool positioning (eccentricity).

5) the equivalent API rating of environmental calibratorsused to emulate the API gamma calibration pit.

Additional tests were also conducted under controlled conditionsto either derive or verify the magnitudes of background radiationtransmitted through the environmental calibrator, the air-to-waterattenuation factor, and the effect of mud potassium concentrationon both Geiger-Mueller and scintillator sensor responses.

Relationships based upon these empirical data for sensorresponse and borehole effects have been developed. Observedresponses in the potassium, uranium, and thorium zones verifythe roughly exponential attenuation of radiation intensity withincrease in collar thickness. Results of an isotopic sensitivityanalysis confirm the previously observed relationship betweenspectral biasing and isotopic mixture, and further confirm nodifferences due to sensor type.

B

-1-


These tests provide a documented characterization of both sensorand sensor-in-collar responses. Incorporation of the derivedrelationships into existing quality assurance and standardiza–

-b

tion procedures provides the capability for a calibrated MWDAPI-equivalent gamma ray measurement, directly linked to theindustry standard.

INTRODUCTION

Since the introduction of MWD gamma ray (GR) logging at thebeginning of this decade, differences between gamma ray measure-ments made by MWD and those made by the traditional wirelinetechnique have been recognized. The discrepancies have usuallybeen largely attributed to the major differences between thetwo methods of measurement, namely, attenuation and spectralbiasing due to the MWD drill collars, loggin speeds, and

8differences in standard conditions (Coope, 1 83; Baker et.al.,1987).

Certain characteristics of either MWD logging or the MWD systemitself result in gamma ray measurements superior to theirwireline counterpart, whereas others have often produced a logdeemed inferior either qualitatively or quantitatively.

Positive aspects of MWD ~amma ray measurements include slowerlogging speeds, acquisition of measurements in an essentiallygauge hole, and measurements made before little, if any,

?

alteration of the formation has occurred due to physical andchemical interaction with drilling fluid. These positiveaspects are frequently manifested by MWD GR logs of superiorbed resolution and character.

Aspects of MWD GR measurements commonly deemed inferior includepoor character and bed resolution at high penetration rates, inheavy muds, and with large collars and/or large holes; inadequatecalibration techniques; poor depth control; and a variety ofmeasurement units with undocumented or otherwise uncertainrelationships to the wireline API standard.

Most of the major factors contributing to differences betweenMWD and wireline gamma ray measurements have been previouslyinvestigated and discussed. These include: the effects oflogging speed and the theoretical attenuation due to collarthickness (Coope, 1983); borehole correction (Jan and Campbell,1984); tool calibration, the air-to-water attenuation factor,and variable background attenuation factors (Meisner et al.,1985); tool calibration, standard conditions, logging speeds,

and attenuation (Baker et al .,1987).

-2-

,


In an effort to further contribute to an understanding of the,- factors which particularly influence the MWD gamma ray measure-

ments, a test program was initiated to empirically examine not Bonly the above-mentioned major factors, but also some of thosewhich are either generally regarded as minor or have simply notbeen addressed in detail in previous publications.

Objectives of this test program were:

1) Utilizing the University of Houston’s API gamma calibrationpit, determine- a gamma ray reference calibration, in termsof API units to MWD counts per second (CPS), for both theMWD scintillator and Gei er-Mueller sensors, in bothcentered and eccentered !sidewalled) positions.

2) Transfer the reference calibrations to portable, MWDsource calibrators which contain an isotopic mixturespectrally balanced to the API pit.

3) Using the new KUTh calibration pits, examine the effectsof attenuation and spectral biasing for MWD gamma ray toolsof varying sizes, thicknesses, and sensor types.

4) Determine the influence, if any, of MWD tool positioningon gamma ray measurements.

5) Quantify the effects of air-to-water attenuation andvariable background attenuation for each sensor and MWDtool .

6) Quantify the influence of radioactive (potassium) muds.

DETERMINATION OF THE API/MWD CPS RELATIONSHIP

Two types of MWD gamma ray sensors were employed in the APIgamma radiation calibration tests. The scintillation sensorconsists of a cylindrical sodium iodide (NaI) cystal coupledto a photo-multiplier tube. The Geiger-Mueller sensor consistsof five, concentrically aligned ionization tubes. Each sensoris contained within a 4.00” outside diameter beryllium copperpressure housing. The test sensors were the same as thosedeployed with our commercial MWD tools.

Each of the sensor types logged the 4.892” internal diameterAPI gamma calibration pit in a series of tests which variedsensor positioning.

The sidewalled responses for both sensor types were within thestatistical error of the centered responses. Hence, the effect

-3-


of sensor positioning is essentially non-measurable for thissmall hole size.

Results of these tests are given in Table 1.

RATING THE PORTABLE SOURCE CALIBRATORS

The MWD gamma ray sensors and tools (i.e., the monel drill collarscontaining the sensors centralized within their bore) arecalibrated by means of portable, wrap-around, aluminum environ-mental calibrators which contain a mixture of natural potassium(K), uranium (U), and thorium (Th) salts dispersed within apolymeric resin. Two different source calibrator sizes, of 4.0”and 10.0” internal diameter, allow for calibration of both thesensors and tools, the latter ranging in size from 6.75” to 9.50”.

Calibration of sensors and tools serves a dual purpose, in thatit:

1) provides a means to compare the characteristics of any onesensor and/or sub to that of a reference standard, and

2) allows for gamma radiation as acquired by a MWD tool to beexpressed in API units which are directly related to theAPI calibration pit. This is achieved by assigning thesource calibrator an API “rating”, and by determining anAPI/MWD cps relationship for the tool when centralized

--lL

within the calibrator.

The procedure for assignment of an API rating to the sourcecalibrators involves:

1) borehole size correction of the API/MWD cps relationship(determined in the calibration pit) from 4.892” to the 4.0”and 10.0” diameters of the calibrators,

2) adjustment of this relationship for the attenuation dif-ference between water and air, and

3) correction of the MWD cps determined with the calibratorin place, for the effect of variable transmission of back-ground radiation.

Borehole Size Correction

The newer, outdoor gamma radiation pits at the University ofHouston consist of two sets of boreholes each possessing zonesof different isotopic mixes. Referred to as KUTh #l and KUTh #2,

-4-


each set contains a borehole of 6.0”, 8.0”, and 12.0” internaldiameters, as depicted in Figure 1. These sizes allowed for

‘P the sensor housings to be run in all three boreholes. The 6.75” Btools were run in the two larger boreholes, and the remainingtool sizes were run only in the 12.0” hole.

The relationship of response to borehole size was determinedas:

log cps (da) - (da - db) * (-m)CPS (db) = 10

where:

cps (db) = gamma radiation, cps, for borehole of diameter b“,where b < a

cps (da) =?aroma radiation, cps, for borehole of diameter a“

m= ogarithmic change in cps per inch of hole diameter,for water (8.4 ppg)

The value of the lo~arithmic change in cps per inch of boreholediameter is a function of isotopic mixture and fluid density.For these water-filled pits, m ranges from a minimum of 0.22 forthe Thorium zone to a maximum of 0.29 for the Potassium zone,with the High Mix value of m having the value of 0.0238.

The effect of fluid density on the value of m could not be.&% determined within the constraints of the test facilities.However, this influence has been previously determined using aMonte Carlo analysis.

Water to Air Attenuation Factors

It is intuitively obvious that the magnitude of gamma radiationobserved at a detector will increase with a decrease in thedensity of the medium separating it and a source. Hence, theAPI/MWD cps ratio determined in a water-filled pit must be cor-rected in order to accurately reflect a similar relationshipdetermined in air. This procedure is necessary, for example,for the assignment of an API value to the source calibrators.The correction factor is termed the water-to-air attenuationfactor, or WAF.

Determination of WAF values involved measuring a background-corrected source radiation for both sensors and tools whencentered in a source calibrator and alternatively placed inwater and air.

The WAF values for the sensors were found to be dependent onthe size of the source calibrator,4,00” and 0.750 for the 10.00”.

equalling 0.976 for the

-5-


WAF values were also determined for the various tool sizes andthicknesses. In practice, the tools are assigned an API/MWD cps _value that is measured in air with the 10.00” source calibrator.The equivalent relationship for a tool centered in a 10.00” holefilled with water is determined by dividing the in-air measuredvalue by the appropriate WAF value. WAF values for the toolswere found to be essentially the same, equalling 0.84 (with anerror of 4%).

Background attenuation factors

The radiation measured by either sensor or MWD tool, whencentralized in the source calibrator, consists of radiationoriginating from both the source and the background. A smallportion of the background radiation is not attenuated by thethicknesses of the calibrator, drill collar, and sensor housing.The magnitude of background radiation that does not reach thesensor is determined by application of what is termed the back-ground attenuation factor, or BAF. This relationship may beexpressed:

BAF = NS - NsbNb

where:BAF = background attenuation factor, dimensionlessNs = source-only radiation, cps

Nsb = source plus transmitted background radiation, cpsNb = background-only radiation intensity, cps

BAF values were determined in a test in which background radi-ation was measured with the particular sensor or tool on itsown and with a “dummy” calibrator in place. This dummy cali-brator is identical to its source counterpart except that itcontains only inert resin.

Conducted in Texas, Louisiana, Connecticut, Scotland, and Norway,these BAF tests indicate that BAF values are a function of thedimension of the annulus between the outside diameter of thesensor or tool and the internal diameter of the calibrator;the drill collar thickness; and the local environment. Varia-tions average 5% by tool size and 10 to 13% by location (and a3% variation by sensor type, which is within statistical error).

Representative BAF values are shown in Table 2.

-6-


For the tool sizes and thicknesses utilized in these KUTh pit

P tests, the potential error in API/MWD cps values which wouldresult by not using BAF factors ranged from a minimum of 10% Bfor the 6.75” RGD tool to a maximum of 15% for the 9.50” DGtool .

The variability of BAF values indicates that calibrations beperformed with either the application of locally-derived valuesor with both source and dummy calibrators (the latter obviatesBAF value determinations).

Assignment of API Values to the Source Calibrators

The above tests and determinations provide all the necessarytools for assigning an API rating to the source calibrators.The results, shown in Table 3, exhibit very good agreement bysensor type and source size.

Assignment of API/MWD Cps Values to MWD Tools

Prior to logging the KUTh pits, each of the sensor and drillcollar combinations was calibrated with respect to the 10.00”source calibrator for the purpose of determining each tool’sAPI calibration factor. This procedure involves measurement ofbackground radiation (a five minute survey time for the scin-tillator, fifteen minutes for the G.M.) and source radiation(three minutes for the scintillator, ten for the G.M).Repeated twice, this procedure determines both the source plustransmitted background response and the API conversion factor,viz.:

API_CF (lO/air) = APINs

where:API_CF (lO/air) = the API to MWD cps relationship for a tool

centered in a 10.0” borehole filled withair, API/MWD cps

API = API value assigned to the 10.0” calibratorNs = source-only response for tool centered in

the 10.0” source, MWD cps

This determination of the API conversion factor differs fromthe procedure normally employed by our commercial facilities.That procedure uses fifteen minute survey times while the sourcecalibrator is in place. As a consequence of the reduced timesused during the KUTh pit tests, error ranging from 2 to 9% wasintroduced into our equivalent API “formation” values.

-7-

1,


The API calibration factor is then divided by the air-waterattenuation factor, yielding the value of the API/MWD cpsrelationship for a 10.0” borehole filled with water. Following ?

this, the API/MWD cps value for any size borehole filled withwater is determined by:

109 API/MWD CPS (10.()/8.4) + (dh - 10.0) ● (-0.0238)

API/MWD CPS (dh/8.4) = 10

where:API/MWD CPS (dh/8.4) = the value of the API to MWD cps

ratio for a borehole of diameterdh” filled with water (8.4 ppg).

API/MWD CPS (dh/10.0) = the value of the API to MWD cpsratio for a borehole of diameter10.0” filled with water (8.4 ppg).

The API_CF values, and their equivalents for the 12.0” KUThpits, are given in Table 4. The API values for these pits,given in a subsequent section, are derived using these factors.

THE EFFECTS OF TOOL POSITION, ATTENUATION AND SPECTRAL BIASING

The KUTh Calibration Pits

The major portion of the study concentrated on response relation- =ships determined in the 12.0” KUTh #1 and KUTh #2 radiation pits.The fifty-six (56) planned logging runs varied tool outsidediameter and thickness, sensor type, and tool positioning(centered versus sidewalled), of which there were twenty-eight(28) combinations. Tool outside diameters were 4.00” (thesensor pressure housin s), 6.75”, 8.25” and 9.50”; collar thick-

!nesses ranged from 1.1 O“ to 2.750”.

Six (6) zones of different radiation spectra have been includedin the KUTh pits; details of the weight fractions of the majorspecies are given in Table 5 (Andersen, 1986).

Figures 2 and 3 illustrate the centered responses for thepressure housing, 6.75” RGD, and 9.50” DG tools for both thescintillator and Geiger-Mueller sensors in the KUTh pits.These responses include both source and background radiation.

Effects Due To Tool Positioning

The goal of determining radiation responses for varying toolsizes in both centered and sidewalled positions required theconstruction of several sets of special stabilizers. However,

-8-


subsequent to almost stickin~ a tool in one of the pits, theiruse was abandoned. Of the fifty six log in~ runs, only eighteenm !were definitely centered and only six de inltely sidewalled. BAlthough limited in number, sufficient data was provided todetermine the effects due to tool positioning.

For the sensor pressure housing only, the effect of radialdisplacement from a centralized position was observed and maybe approximated by:

0.0813 * ( (dh - dt)/2 )EF = 0.933 * e

where:EF = eccentricity factor, dimensionless, having a minimum

value of 1.00= base of Napierian logarithms

d; = hole diameter, in.dt = collar outside diameter, in.

The observed differences between centered and sidewalled sensorresponses varied from non-measurable in the API calibration pit,within statistical error in the 6.0” pit, and approached 30%in the 12.0” pit. A separate inhouse test utilizing the sensorpressure housings and the 10.0” source calibrator verified boththe effect and the approximation equation.

In several of the remaining positioning test runs which involvedthe MWD tools, differences which exceeded statistical error by2 to 3% were observed. It may be concluded, then, th&t=theposition of a MWD tool in a borehole has no particular influenceon the accuracy of gamma ray measurements.

Effects Due to Attenuation and Spectral Biasing

The composition of the potassium, uranium, and thorium zones ofthe KUTh radiation pits provides an outstandin

Yopportunity to

examine the effects of attenuation and spectra biasing causedby the MWD drill collar. The isotopic mixtures comprising thesezones is such that responses within any one zone may be attributedto that zone’s predominant isotope.

Following subtraction of background (adjacent barren zones)radiation, the observed K, U, and Th responses of each sensorpressure housing and each sensor-drill collar combinationwere used in a matrix operation to determine their particularisotopic sensitivities. These isotopic sensitivities aredetailed in Table 6.

-9-


These sensitivities suggest that the relatively higher energylevel potassium radiation is far more capable of passing throu h ~the variable thickness of the MWO collar than is either that o!uranium or thorium. Hence, the MWD collar spectrally biases

iaroma radiation in favor of the high energy potassium radiation.nd conversely, the MWD collar spectrally biases gamma radiation

to the detriment of uranium and thorium radiation, which becauseof their relatively lower energy levels are preferentiallyscattered or absorbed by the collar.

The relative decrease in sensitivity for each of these isotopeswith increasing collar thickness is clearly evidenced in theirrespective attenuation factors (Table 6). For MWD drill collarsrangin

!in thickness from 1.100” to 2.750”, the attenuation for

each o these isotopes ranges from approximately two and a halfto ten times. On the whole, attenuation factors with respect topotassium are the smallest, and those for uranium and thoriumare approximately the same.

The isotopic sensitivities determined in the K, U, and Th zoneswere used to predict responses in both the High and Low Mixzones. Table 7 presents a comparison between predicted andobserved responses for these zones. Except for a few caseswhich may involve some positional or depth-related error, theagreement is excellent.

Spectral biasing favoring the relatively higher radiationenergies is clearly evident in the MWD API values of thePotassium zone given in Table 8. It may also be seen that theAPI value as given by the thickest MWD collar is essentiallythe same as that given by the thinnest collar. Given thata wireline sensor pressure housing is both smaller and thinnerthan its MWD counterpart, the MWD API values given herecorroborate the previous observations (Coope; Baker) thatlithologies relatively richer in potassium will produce highervalues when logged with MWD than with wireline.

Table 8 also gives the MWD API values for the Uranium andThorium zones of the 12.0” pit. The trend of decreasin API

Yvalues in both these zones is clear evidence of the dri 1 collarabsorption of these lower radiation energies. These trendsare in agreement with and substantiate both observations (Coope)and results of modelling indicating that zones deplete inpotassium or relatively richer in uranium or thorium will yieldlower equivalent API values when logged with MWD.

Table 9 gives MWD API values for each sensor and sensor-drillcollar combination for the High and Low Mix zones.

-1o-


THE EFFECT DUE TO MUD POTASSIUMpm

Since a gamma ray sensor detects gamma radiation regardless ofits source, it is necessary to distinguish formation-relatedfrom non-formation-related gamma radiation. The primary sourcesof non-formation gamma radiation are potassium compounds whichare commonly added to drilling fluids for the purpose ofinhibiting shale hydration and dispersion. The most commonlyemployed potassium mud additives are potassium chloride (KC1),potassium hydroxide (KOH), and potassium lignite (K-Li ).These chemicals may be used in concentrations ranging ! rom afew lbs/bbl to over 50 lbs/bbl.

The effect of potassium concentration on the observed MWD

!aroma radiation has historically been deemed relatively minorCoope) or not addressed in detail (Jan and Campbell). Both

our in-house tests and our field experience indicate that thiseffect may frequently be of considerable ma nitude, in agreement

!with the conclusion of Cox and Raymer (1976 .

The potassium test involved determining the responses of severalsensor and drill collar combinations to varying concentrationsof mud potassium. These responses, depicted in Figure 4, areessentially independent of MWD tool size and hole size. Asopposed to potassium contained in a formation, potassium in adrillin

!fluid has a larger effect on observed response because

its inf uence occurs primarily within the drill collar bore,which may be considered an ideal “4fi” geometry. The potassiumin the mud in the annulus, by contrast, contributes a ne ligibleeffect, !since these gamma rays are attenuated by the CO1 ar.

The effect due to mud potassium concentration is greatlyinfluenced by sensor type. The effect for the sclntillator isnearly ten times that for the Geiger-Mueller, viz.:

cps(K) = 2.77 * (% K) for the scintillator

cps(K) = 0.29 * (% K) for the Geiger-Mueller

Given the concentrations of any of the above potassium additives,it is a relatively simple matter to determine both the weightpercentage of potassium and its portion of the total observedgamma radiation. For instance, potassium muds are frequentlyused while drilling the Tertiary claystones of the North Sea.A representative 9.5 ppg potassium mud may contain 20 lbm/bblKC1 and 4 lbm/bbl KOH, contributing 2.86% and 0.88% potassium,respectively.

-11-


Not only is it necessary to identify and determine the magnitudeof gamma radiation produced by radioactive muds, it is also -$essential that this contribution be removed both before conver-sion to API units and, particularly, before borehole correction.

The above potassium mud, for instance, contributes 61 API whenlogged with our 9.50” RGD tool, while drilling 17.50” holethrough 80 API formations. At standard conditions, this non-formation radiation would be equivalent to 100 API. The effectis magnified logarithmically with increase in hole size.

THE EFFECTS OF STANDARD CONDITIONS AND CORRECTION ROUTINES

Any quantitative comparison between MWD and wireline gamma raymeasurements must be made subsequent to correction for environ-mental effects (borehole size, mud density, mud potassium, toolposition). Such comparisons are facilitated by similar MWDand wireline standard conditions.

We have defined our standard conditions to be similar to thatcommonly used in the wireline industry, that is, the equivalentcentered response in a 8.00” borehole filled with a 10.0 ppg mud.

Our environmental correction procedure works as follows:

1) effects of mud potassium radiation are removed before anyother correction

T

2) the potassium-corrected response (in cps) is corrected tothe equivalent response which would be obtained in a 8.0”borehole filled with mud of the relevant density

3) this value is then normalized to that expected for a 8.0”borehole filled with 10.0 ppg mud

4) the value obtained in step 3 is multiplied by the valueof the API correction factor at standard conditions togive the corrected API value.

We estimate the accuracy of our environmental corrections tobe within 1% to 3% of the equivalent centered wireline correc-tions. Our corrected MWD gamma ray measurements will readconsiderably higher than wlreline eccentered corrections,probably by the similar difference between these and thecentered wireline corrections,

-12-

SPWLA liventy-Ninth Annual Logging Symposium, June 5-8, 1988

CONCLUSIONS

Testing of MWD gamma ray sensors and tools at the Universityof Houston’s gamma radiation pits has provided some new insightsand empirical data relating to differences between MWD andwireline measurements.

These tests empirically confirm prior observations concerningattenuation and spectral biasing which are responsible formajor differences between MWD and wireline gamma ray measure-ments.

Factors which may account for large differences (up to of 60%)include spectral biasing, mud potassium, and inappropriatecomparisons which do not involve similar standard conditionsand correction routines.

Factors which may account for errors in the range of severalpercent to in excess of 30% are variable background and air-water attenuation factors.

Clearly, it is important to identify these factors, to quantifytheir effects, and to either remove, minimize, or, as in thecase spectral biasing, simply appreciate their influence.

-ACKNOWLEDGEMENTS

The authors wish to thank the management of Teleco OilfieldServices for permission to publish this paper.

REFERENCES

American Petroleum Institute, RP 33, Recommended Practice forStandard Calibration and Format for Nuclear Logs, 1974.

Andersen, J., personal communication re arding analysis of!samples from KUTh calibration mode s, to W.W. Given,

chairman, API gamma calibration task group, 1986.

Baker, C., Fristad, P., and Seim, P., Reservoir Evaluation withMWD Logs, SPWLA Twenty-eighth Annual Logging Symposium,London, June 1987.

Coope, D.F., Gamma Ray Measurement-While-Dril ling, The LogAnalyst, Jan. 1983.

-13-

Sf’WLA Twenty-Ninth Annual Logging Symposium, June 5 8, 1988

cox, J.W. and Raymer, L.L., The Effect of Potassium-Salt Muds on Gamma Ray, and Spontaneous Potential Measurements, SPWLA Seventeenth Annual Logging Symposium, June 1976.

Jan, Yih-min and Campbell, J.R., Borehole Correction of MWD Gamma Ray and Resistivity Logs, SPWLA Twenty-fifth Annual Logging Symposium, June 1984.

Meisner, J., Brooks, A., and Wisniewski, W., A New Measurement- While-Drilling Gamma Ray Log Calibrator, SPWLA Twenty-sixth Annual Logging Symposium, Dallas, June 1985.

ABOUT THE AUTHORS

Thomas M. Bryant is currently Senior Staff Drilling Engineer for Teleco Oilfield Services in Meriden, Ct. He graduated from Syracuse University in 1970 with a BS in geolo y.

8 Tom

worked for NL Petroleum Services for l/2 years on overseas assignments involving drilling fluid, geological, and drilling engineering, before joining Teleco in 1984. He is a member of SPWLA, SPE, and AAPG.

Tyrone D. Gage is currently Electronic Test Engineer for Teleco Oilfield Services in Meriden, ct. His education includes attending the U.S. Air Force Academy and graduating from Penn State University with a BS in electrical engineering. He works with automated electronic test systems, radiation detection, and he has authored several electronic service manuals.

-14-

PIT *1 PIT +2

CommercialReady Mix

L \\y/,\\\.——

Ground Level

Barren Poured Concrete ‘Zones’———

Low Activity Concrete

———

Figure 1. The KUTh pits located at the University of Houston.Diagram courtesy of the API Gamma Calibration TaskGroup.

z0000

SPWLA llventy-Ninth Annual Logging Symposium, June 5-8, 1988

. . . . . . . . . . . . . . . . . . . . . . . Scintillator ...........................

A E c D F G HT;;t Sy;IJ~y Active Barren C ~D Error API/cps Comment

. Cps Cps Cps %+

300 279.45 24.76 254.69 0.72 0.7853 Centered; 300 279.38 24.87 254.51 0.72 0.7858 “3 300 280.79 24.83 255.96 0.72 0.7814 “4 300 272.39 24.76 247.63 0,73 0.80775 300 280.99 24.91 256.09 0.72 0.7810 Sid;wall6 300 281,23 24.36 256.87 0.72 0.7786 “

279.04 24.75 254.29 0.72 0.7865 300 secave.

. . . . . . . . . . . . . . . . . . . . . . . Geiger-Mueller .........................

A B c 0 F G HTest S;;;~y Active Barren C ‘D Error API/cps CommentNo. Cps Cps Cps %+

900 22.16 2.73 19.43 0.29 10,293 Centered1: 420 21.95 2.88 19.08 0.43 10.482 Sidewall

Table 1, API gamma calibration pit test data.+ Statistical error reported at 95% confidence.

Tool Scintillator Geige~~~uellerBAF

4.00’! PH4.00” PH6.75” RGD+7.75” RGD6.75” DG+8.25” RGD7.75” DG9.50” RGD8.25” DG9.50” DG

;.;::*

0:8440.8560.8570,8590.8640.8670.8710.875

;.:::*

0:8640.8670.8730.8740.8810.8850.8860.897

Table 2. Background attenuation factors (BAF) for sensorpressure housings (PH) and the MWD tools used inthe KUTh tests. The MWD tools are listed, from topto bottom, in order of increasing collar thickness.All BAF values were determined in air using the 10.0”source calibrator, except those denoted with *,applicable for the 4.00” calibrator. These BAFvalues are representative only for Connecticut.

+ RGD (Resistivity-Gamma-Di rectional) andDG (Directional-Gamma) are marks of Teleco OilfieldServices, Inc.

P“%


B

A B c D E GDetector S:;;:y APIlcps API/cps WAF D:E Ns F!G

Type 4.892”8( 4.oo”& API/cps Cps APIsees 8.4ppg 8.4ppg

. . . . . . . . . . . . . . . . . . . 4.00’’ Source Calibrator ...................

Scin. 300 0.7865 0.7490 0.9756 0.7307 372.3 272G.M. 900 10.2930 9.8019 11 9.5629 28.6 273

. . . . . . . . . . . . . . . . . . . 10. 00’’ Source Calibrator ...................

API/cps10.0” &8.4 ppg

Scin. 300 0.7865 1.0406 0.7500 0.7806 292.4 228G.M. 900 10.2930 13.6180 11 10.2160 22.3 227

&- Table 3. API rating of source calibrators.

. . . . . . . Scintillator ...... ....... Geiger-Mueller .....

API/MWO CPS ..... API/MWD Cf)S .....Tool Ns”” “io.o”& lo.o”& 12.o”& Ns itili:*&lo.oI*& 12.oII&

Air 8.4ppg 8.4ppg Air 8.4ppg 8.4ppg

292.4 0.780 1.039 0.932 22.26 10.243 13.653 12.236%GD 103.9 2.194 2.611 2.340 7.62 29.921 35.606 31.9106DG 85.5 2.667 3.173 2.844 6.14 37.134 44.189 39.6028RGD 69.0 3.304 3.932 3.524 5.13 44.444 52.889 47.399!33:D 50.8 4.488 5.341 4.787 3.42 66.667 79.333 66.812

46.7 4.882 5.810 5.207 3.35 68.060 80.991 72.5839DG 29.9 7.625 9.074 8.132 2.04 111.765 133.000 119.193

Table 4. API/MWD cps relationships. All values of API/cpswere obtained usin

Ithe 10.0” source calibrator

rated at 228 API. s = source radiation, cps.


Pit No. Zone Midpoint %K ppm U ppm ThDepth(ft)

Uranium 7.50 0.3 16.6; Thorium 17.50 0.3 6;:;

High Mix 25.25 5.4 ;:: 23.8; Potassium 7.50 4.5 1.0 2.4

Low Mix 17.50 1.1; High Mix 22.50 5.4 ;:: 2:::1,2 Barrens - 0.2 0.8 2.3

-%

Table 5. Radioactive species content of the KUTh radiationpits. Data from the analyses of J.Anderson, andis presented here courtesy of the API GammaCalibration Task Group.

--’%

) )

CPSo 100 200 300

CPSo 4 8 12 16 20

I

Figure 2. Centered radiation responses for the 4.00” sensorpressure housing (PH), 6.75” RGD, and 9.50” DGtools, in the KUTh #1 12.0” pit. Scintillatorresponses are on left, Geiger-Mueller on right.Top zone is Uranium, middle is Thorium, and thethin zone on bottom is a High Mix used for depthverification.

WI

“w

m

CPS

o 50 100 150 200 250

9DG 6RGD

f

CPSo 5 10 15 20 25

, t

i

Figure 3. Centered radiation responses for the 4.00” sensorpressure housing (PH), 6.75” RGD, and 9.50” DGtools, in the KUTh #2 12.0” pit. Scintillatorresponses are on left, Geiger-Mueller on right.Top zone iS potassium, middle is LOW Mix, and on

bottom is the High Mix.

)

I

SPWLA Twenty-Ninth Annual Logging Symposium, June 5-8,1988

B

. . . . . . . . . . . . . . . . . . . . . Scintillator ............................

Response by Zone Isotopic Contribution AttenuationCps per unit of Factors

Tool K u Th %k ppm U ppm Th K U Th

72.52 193.37 234.58 1;.;; 12.09 3.91 - - -;!GD 28.94 66.27 80.54 4.14 1.34 2.49 2.92 2.926DG 22.91 54.09 65.79 5:15 3.38 1.10 3.15 3.58 3.578RGD 17.71 42.12 52.24 3.98 2.63 0.87 4.07 4.60 4.499RGD 13.78 30.25 37.70 3.10 1.90 0.63 5.23 6.37 6.238DG 13.37 28.13 35.24 3.01 1.75 0.59 5.38 6.89 6.679DG 8.09 17.85 23.89 1.82 1.11 0.40 8.91 10.86 9.82

. . . . . . . . . . . . . . . . . . . . Geiger-Mueller ...........................

Response by Zone Isotopic Contribution AttenuationCps per unit of Factors

Tool K u Th %k ppm U ppm Th K U TH

5.23 14.35 17.42 1.17 0.90 0.29 - - -HGD 1.93 4.82 6.29 0.43 0.30 0.10 2.70 2.98 2.766DG 1.83 3.88 5.08 0.41 0.24 0.08 2.83 3.71 3.438RGD 1.30 2.90 3.93 0.29 0.18 0.07 4.00 4.96 4.399RGD 0.99 2.03 2.97 0.22 0.13 0.05 5.24 7.12 5.808DG 0.74 2.00 2.85 0.17 0.13 0.05 7.08 7.18 6.049DG 0.48 1.18 1.87 0.11 0.07 0.03 10.81 12.12 9.35

Table 6. Isotopic sensitivity (in terms of cps response)for sensors and MWD tools, as a function of sensortype. Response inputs for the K, U, and Th zonesmay incorporate some positional errors, and do notinclude background radiation. Calculated attenuationfactors for each isotope and tool are listed at right.


“-%

. . . . . . . . . . . Scintillator ................

. . . . . . . High Mix Zone ........ ..... Low Mix Zone .....

A B c D E F G H ITool Pred. Error Ohs. Error Other Pred. Error Ohs. Error

Cps Cps Cps Cps % Cps Cps Cps Cps

249.30 2.73 38.44 1.07 38.90 1.08;[GD 90.36 2.02 83.09 1.93 3.7 14.02 0.79 14.69 0.816DG 72.92 1.81 65.59 1.72 5.2 11.30 0.71 11.27 0.718RGD 57.00 1.51 53.65 1.46 0.7 8.82 0.59 8.62 0.599RGD 42.76 1.31 38.53 1.24 3.9 6.62 0.51 6.50 0.518DG 40.03 1.20 35.18 1.13 6.3 6.23 0.47 6.58 0.489DG 25.48 0.62 21.97 0.57 9.1 3.94 0.24 3.84 0.24

. . . . . . . . . . . Geiger-Mueller ................

. . . . . . . High Mix Zone ........ ..... Low Mix Zone .....

Tool

~;GD6DG8RGD9RGD8DG9DG

A B c D E F G H I TPred. Error Ohs. Error Other Pred. Error Ohs. ErrorCps Cps Cps Cps % Cps Cps Cps Cps

18.32 0.73 17.74 0.73 2.82 0.29 3.36 0.326.52 0.31 5.81 0.30 1.5 1.00 0.12 1.00 0.125.58 0.24 5.73 0.24 0.87 0.09 0.94 0.104.14 0.20 4.14 0.20 0.64 0.08 0.60 0.083.07 0.15 2.87 0.15 0.47 0.06 0.46 0.062.72 0.16 2.57 0.16 0.42 0.06 0.49 0.071.72 0.09 1.65 0.09 0.26 0.04 0.32 0.04

Table 7. Comparison of predicted versus observed responsesfor the KUTh High and Low Mix zones, for each sensorpressure housing and each MWD tool, as a function ofsensor type. Statistic~~t;j;or determined at the99% confidence level. “ refers to error thatexceeds that attributed to the predicted and observedvalues.

‘-%


KUTh #2 Potassium ZoneB

&’-

Tool

;IGD6DG8RGD9RGD8DG9DG

Tool

~!lGD6DG8RGD9RGD8DG9DG

Tool

&D6DG8RGD9RGD8DG9DG

Scintillator Geiger-Mueller

Ohs.+ API Ohs.+ APICps Cps

72.52 67.6 5.23 64.028.94 67.7 1.93 61.622.91 65.2 1.83 72.517.71 62.4 1.30 61.613.78 66.0 0.99 66.113.37 69.6 0.74 53.78.09 65.8 0.48 57.2

KUTh #1 Uranium Zone

Scintillator Geiger-Mueller

Ohs.+ API Ohs.+ APICps Cps

193.37 180.2 14.35 175.666.27 155.1 4.82 153.854.09 153.8 3.88 153.742.12 148.4 2.90 137.530.25 144.8 2.03 135.628.13 146.5 2.00 145.217.85 145.2 1.18 140.6

KUTh #1 Thorium Zone

Scintillator

Ohs.+ APICps

234.58 218.680.54 188.565.79 187.152.24 184.139.08 187.135.24 183.523.89 194.3

Geiger-Mueller

Ohs.+ APICps

17.42 213.26.29 200.75.08 201.23.93 186.32.97 198.42.85 206.91.87 222.9

Table 8.The effects of spectral biasing for MWD tools in the12.0” KUTh Potassium, Uranium, and Thorium zones.MWD API values assigned using the relationships fromTable 4. + indicates observed response less averagebackground-of surrounding barren zones.


Tool

UGD6DG8RGD9RGD8DG9DG

Tool

I!GD6DG8RGD9RGD8DG9DG

KUTh #2 High Mix Zone

. . . . Scintillator . . . . . . Geiger-M~eller . . .

Pred. API Error+ Pred. API Error+Cps +-API Cps +-API

249.30 232.3 18.32 224.2 14.890.36 211.4 ::: 6.52 208.1 19.272.92 207.4 10.2 5.58 221.0 20.357.00 200.9 10.7 4.14 196.2 20.342.76 204.7 12.7 3.07 205.1 25.940.03 208.4 13.1 2.72 197.4 24.725,48 207.2 13.5 1.72 205.0 28.0

KUTh #2 Low Mix Zone

.... Scintillator ... ... Geiger-Mueller ...

Pred. API Error+ Pred. API Error+Cps +-API Cps +-API

38.44 35.8 2.82 34.5 4.514.02 32.8 ;:; 1.00 31.9 5.311.30 32.1 2.8 0.87 34.5 5.38.82 31.1 0.64 30.36.62 31.7 ::: 0.47 31.4 N6.23 32.4 3.5 0.42 30.5 6.43.94 32.0 3.3 0.26 31.0 7.4

“-’l

‘--i

Table 9. Response of MWD tools in the 12.0” KUTh High andLow Mix zones, in terms of MWD API values.+ indicates error at the 99% confidence level andincludes error for both the predicted responseand error in the API/cps conversion factor.

-m


B

35

30

25

20CPS

15

10

5

0

3.5

3.0

2.5

2.0CPS

1,5

1.0

0.5

0.0

10 10 20 30 40 50 60 70 80 90

KCL (LBS/BBL)

o’ 10 20 30 40 50 60 70 80 90

KCL (LBSJBBL)

Figure 4. The effect of radioactive mud potassium ionconcentration on the response of both scintillator(top) and Geiger-Mueller sensors.

api pit calibrationof mwd gamma ray tools b...

Documents