API Calibration Facility for Nuclear...

API Calibration Facility for Nuclear Logs

WILLIAM B. BELKNAP,~: JOHN T. DEW AN,^ C. V. KIRKPATRICK,~ WILLIAM E. M O T T , ~ A. J. PEARSON,** AND W. R. RABSON***

ABSTRACT

The work of the API National Subcommittee on Revis~on of RP 53 Recom.~ne7~cletl P r c ~ c t ~ & for Stccncle~rcl Cal.lr',rcctzo7~ and For?)?. for Nftcleccr Logs IS explained 111 tliis papel Accoinplishnieiits of tliis Subcommittee ~nclude a revlsed recommended standard nuclear log lieadilig and form, a defin~tion of standard gamma-ray and neutlon log units fol measunng radiatlon, the tles~gii and coiist~uctloii of a nuclear log calibration

I. INTRODUCTION

Nuclear logglng methods wele introcluced commei- cially to the petroleum industry about 20 years ago. Their utilizat~on has steadily increased u n t ~ l today a t least one nuclear log IS run 111 nearly every well dr~l led. Table 1 is presented to slio\v the number of nuclear logs l u n d u l ~ n g recent yeais In splte of this wide use, those pelsons usillg nucleai logs have found it a very com- phcated - lf not a t t ~ m e s imposs~ble - task to make coniparlsons between logs run by the vanous logg~iig companies l~ecause of the absence of standardrzation. T h ~ s lack of s tandard~za t~on IS demonstrated by the numelous dlffereiit u n ~ t s used for measuring radiation, d~fferent procedures for checklng and calibrating logging ~nstruments , d~fferent log headrngs, and d~fferent scales used by the logging companies. The exlstlng differences In log nieasureinents have severely l ~ m ~ t e d the use of nuclear logs In areal studies and critical correlation

The response of the present-day gamma-ray logs is calibrated in a u n ~ t which, so f a r a s the petroleum ~ndus t ry a s a whole 1s concerned, is ideiit~fiable only w ~ t h the logg~ng company making the log A survey of eight logging companies offerlng nuclear logs showed seven d~fferent units be~iig used for measuring gamma radia- t ~ o n . Some of these u n ~ t s have identical names, but a r e not the same unit of measure since the logging companies use d~fferent ca l~bra t lng techniques. In current usage a re the following diverse units of gamma-ray rneasurement. counts/mlnute, connts/second, radiatlon units, inicrogralns rad-eq./ton, and ni~croloentgens/

*Phllllps Petroleum Company. Bartlesvllle, Okla iSchlumberger WeU Surveying Corp , Houston. Texas

i Un~verslty of Houston, Houston, Texas Gulf Research and Development Co P~ttsburgh, Pa

**The Atlantlc Refinlng C o , Dallas, Texas ***Pan Geo Atlas Corp , Houston, Texas

f a c ~ h t y , and a recommended standard procedure for presenting cal~bratlon data. Deta~led des~gii and eiigl- neering data on the calibration fac~l i ty a r e presented. The development of standard nuclear log units of measurement and calibiation of-logs in these units is a s~giiificant forward step in this field. I t \vil1 enable petroleum engineers and g e o l o ~ s t s to compare dliectly nuclear logs run by different logg~ng companles

hour. completely l a c k ~ n g is a common approach w h ~ c h can be used by all involved to relate these units to each other a s well a s to the phys~cal properties of the formation being measured. There have been attempts by some of the operating companies to ~nter-relate units through the use of field data, but as yet a n uil-to-date set of factors fo r converting gamma-I ay readmgs from one company's log to that of another company has not been publ~shed By and large, the work of developing conversion factors has proved to be time-consum~ng, expensive, and usually of temporary value

A s i m ~ l a r ploblem exists with present-day neutron logs The suivey showed SIX different units being used for neutron measurement by seven logglng companles These ~nclude the following counts/second, inches per standard unit, counts per mmute, environmental u n ~ t s , ~nches per 100 standard units, and standard counts per second

( The lack of standard~zation and need for calibration of nuclear logs resulted in the Steerlng Comm~ttee on Product~on Practice of the D ~ v ~ s ~ o i i of Production, '

American Petroleum Institute, appo~nt ing a national subcomm~tteea In June, 1956, to ievle\v RP 35 Rec- ol,fnlemdetE Prcictfce for Stc~)lclurd Raclior~ct~uity Log F o r ~ , (Tei i ta t~ve) I' This Subcommittee was ~iistlucted to accumulate all available suggest~ons for revlsion of R P 55. and to develop a standard practice tha t would create unifornnty and allow d ~ r e c t comparison of radloactlve logs I t was selected to provlde representation from both 011 and logging companies from throughout the United States

The Subcommittee investrgated the e x ~ s t ~ n g coiid~tions of s tandardizat~on of log head~ngs, forms, u n ~ t s fo r

aSee Appendlx A for membership bThe first ed~tlon of R P 33 was adopted and publ~shed In November

1948.

290 BELKNAP, DEWAN, KIRKPATRICK, MOTT, PEARSON, AND RABSON

measuling lac l~a t~on , and c a l ~ b i a t ~ o n procedures Early In thls study it \\.as agreed tha t development of conversion factors between vailous logg~ng compan~es' gamma-ray and neutron logs would not be a sat~sfactoly solut~on to the problem I t was recognized tha t fu tu ie developments would greatly Increase the ~nipoltance of neutron logglng and in tens~fy the need fol a solutlon to the standardization problem.

111 the original RP :'3, gamma-ray and neutron logs weie classed a s radloactlv~ty logs The Subcomni~ttee changed thls tltle to nuclear logs because the term "nuclear" IS more descrlptlve

Leadlng to the objective of standartl~zlng nuclear logs, the Subcommittee acconlpllshed the follownig

1 Designed a standaid log heading and folm 2. Established a standard API Gamma-Ray U n ~ t . 3 Established a standaid A P I Neutron U n ~ t . 4. Des~gned and promoted construction of a callbra-

t ~ o n facllity fol nucleai logs 5 Developed a standard cal~brat lon piocedure

These accompl~shments resulted in pubhcat~on In Sep- tember 1959 of the 2nd ed~tlon of RP 3.3 Recom?laended Prrictlce for St4~7tda~d Cnl16ri~tz07~ (~71d Form for Nziclecir Logs The rev~sed edltlon contalns the lecommended form for nuclear logs and de ta~ ls of procedures to be followed for callbratlng nuclear logg~ng tools in standard A P I unlts

The purpose of thls paper 1s to plesent In detall the technical aspects of planning, constluct~ng, and operat- Ing the cal~bratlon fac~l l ty 2 The factors involved ~n arrlvlng a t sultable s t a n d a i d ~ z ~ n g m e d ~ a for nucleal logs, viz , gamma-ray and neutron c a l ~ b i a t ~ o n plts, a r e outhned. Detalls of the s t u d ~ e s iequ~red to de te lm~ne the slzes of plts needed and materials used t h e i e ~ n a r e presented Constluct~on of the fac l l~ ty IS descr~bed to ~ l lus t ra te the many problems ~nvolved In selecting, measuslng, processing, and placlng m a t e i ~ a l s 111 the plts The various tests, ~ n c l u d ~ n g core analyses, spectral analyses, chemlcal analyses, log checks, etc a r e presented. The operation, financing, callbrat~on procedure, conversion from ex ls t~ng u n ~ t s to API ulnts, and records n ~ a ~ n t a ~ n e d by the Un~versl ty of Houston a r e described.

Table 1

Total Nuclear Logging Operations'

F~nal ly , poss~ble f u t u ~ e uses of the faclllty a s well a s ~ t s value to the Unlverslty a re discussed

11. FACTORS INVOLVED IN GAMMA-RAY STANDARL)IZATION

Year

Gamma-ray L o g g ~ n g Principles

'Reference; are a t the end of the paper

'Comp~led from data supl~lied by the following compames Blrdwell. Inc. Frontler Perforators, Inc . Creat Lakes Petroleum Serv~ces, Inc , Lane- Wells Company. McCullough Tool Company. Perforating Guns Atlas Corporation. Ram Guns. Inc . Schlumherger Well Surveying Corporat~on. Weles, Inc . The Western Conlpany. Worth Well Surveys. Inc

Nucleal Logs Run* Outslde

U S. U. S. Total

Wells Drllled Outs~de

U S U S. Total

The alrn of gamma-ray logglng a s currently practiced IS to obtaln a measure of the total number of gamma l a y s emltted per second by a unit welght of f o l m a t ~ o n lock The gamma lad~a t lon of natural mateilals 1s due almost e n t ~ l e l y to the decay products of uranium, the

Total Nuclear Opela t~ons In Teims of Wells Dr~lled, Percent

Outslde U. S. U. S Total

decay products of tlior~uni, and the rad~oactlve soto ope of potass~uni Hence,- the gamma-lay actlvlty, A , of a secl~mentaly rock can be cons~dered a hneai function of the amounts of uranluni, t h o l ~ u m , and potasslum piesent, 1.e .

A = AuWu + ATILWTIL + AKWK (1) I.l 'he~eu~ Au, ATJL, and A,- represellt the spec~fic actlvl- tles (gamma rays per second or~glna t lng e ~ t h e r d~rec t ly 01 ~ndliect ly from a gram of the element) and I,i'u, li'TJ,. and IYK the corresponding fractional abundances of uianlum, thonum, and potass~um, respect~vely. F o r all practical purposes In well logglng, ~t can be assumed tha t the source mateilals a r e un~foini ly d ~ s t r ~ b u t e d throughout the and the uranluni and t h o l ~ u m serles (Tables 2 and 3) a r e 111 rad~oactlve eclull~br~um

As defined, A U IS the total number of gamma rays e m ~ t t e d per second by a gram of ulanluni in e q u l l ~ b r ~ u m wlth ~ t s decay ploducts I t s value I S obtalned by summlng the contr~butlons from each of the radlonucl~des In the ulanium serles, the presence of the membels of the actlnluni serles belng ~gnored because the parent, Us" IS only 0 72 pelcent abundant. Uslng the most recent data available and summlng only photons h a v ~ n g energles greater than 100 kev,* we find

A U = 2.8 X 104 pl~otonslsec-grwn U .

I11 11ke manner, summat~on of the gamma rays e ~ n ~ t t e d by the elements of the thorlum serles ylelds

A,,, = 1.0 x lo4 pl~oto?zslsec-gra?~ T h

'Phohns of energ! lrss than 100 kev are strongly absorbed by the formal102 and the p..essure houslng of the logg~ng tool

A P I CALIBRATION FACILITY FOR NUCLEAR LOGS 291

Table 2. The Uranium Series3

Natural potassium has a much lower speclfic actlvlty, to date nearly 80 gamma-ray 11nes have been observed VIZ , ~n the spectiurn of the former, 60 in the latter.*

S e q u e n c e

Some idea of the complex~ty of the emisslon properties 'Theergrgles and lntens~t~es of the most Intense lines In the spectra

of the U23R and Th2" familles follows from the fac t t h a t of the naturally occurring rad~onuclldes are l~sted In Table 4 , p 293 Note that the gamma rays ernltted by K4O are monoenergetlc.

Half L l f e

9 4 . 5 1 x 1 0 y r

24.1 d

1 . 1 8 rnin

6 .66 h r

5 2 .48 x 1 0 y r

4 8 . 0 x 1 0 y r

1 6 2 0 y r

3 .82 d

3 .05 min

-2 s e c

1 . 3 s e c

26 .8 min

1 9 . 7 min

1 . 6 x 1 0 - ~ s e c

1 . 3 2 min

1 9 . 4 y r

5 . 0 1 d

1 3 8 . 4 d

4 . 2 min

- -

A , = 3.4 pJz.oto~tslsec-yrrtm K,

than elther uianiunl or thorium but is of approximately equal importance In its contiibution to the gamma-ray act~vl t ies of most sed~mentaly rocks because ~t is a much more abundant element.

Mode o f D i s i n t e g r a t i o n

a

p

B , IT,

B

a

, a

a

a

a, a

a, p

a

p

a, p

a

B

B

a, I3

a

I3

S t a b l e

U I

uX1

uX2

u z

U I I

I0

R a

Rn

Ra A

R ~ A '

RaA"

RaB

RaC

R ~ C '

RaCgL

RaD

RaE

Ra F

RaE'

RaG

Although the data on the energles and intensltles of many of these a re to some extent uncertain, ~t is belleved t h a t the aforementioned values of A , and ATh and the general features of the energy dlstrlbution curves a s outhned ln Table 5, p 294, a r e sufficiently accurate f o r present-day logging studies.

N u c l i d e

238 9 2'

gOTh234

234m 9 1 P a

234 9 lPa

2 34 9 2'

Th230 9 0

2 2 6 88Ra

222 86Em

218 84"

85AL218

218 86Em

82Pb214

8 3 B i 2 1 4

214 8 4"

81T1210

82Pb210

210 83Bi

210 84'O

81T1206

82Pb206

Table 3 The Thorium Series3

The baslc characteristics of the 3 naturally occurring source m a t e r ~ a l s a r e summar~zed In Table 6, 11. 294. The radium portion of the uranlum series 1s included a s a separate item for reference purposes.

It is impoltant to observe tha t the response of the natural gamma-ray log does not necessarily prov~de a d ~ r e c t measuie of formatlon ac t~v i ty In fact, the nature of the problem 1s such that , In some condltlons, the quant~ t ies recorded on a log cannot even be qual~tatively related to gamma-ray emission without the a ~ d of calibration data and departure curves T h ~ s happens, of course, because the magnitude of the signal produced a t the output of the detection system 1s ~nfluenced by a combination of factors. To elucidate, the response of a detect01 statlonary In a borehole oppos~te a glven formation will depend upon:

1. Specific activity of the formatlon (gamma rays/ sec-gm)

2 Speclfic a c t ~ v ~ t i e s of borehole f lu~d, caslng, and cement

3. Character~stlcs of the detector and the counting system

4. Posltion of the detector ~n the borehole 5 Density of the f lu~d 111 the borehole 6. Dlanletel of the borehole 7. Thickness and denslty of the caslng and cement 8 Bulk density of the foirnation

and in some cases 011

9. Relative concentrat~ons of the radioactive materials (U, Th, and K ) In the formatlon.

The physical pi lnclples nlvolved here can perhaps best be understood if the contrlbut~on to the response, d I ,

a

N u c l i d e

made by the radioactive materlal contamed In a n element of volunle dl', havlng denslty p, located a t a dlstance r

Mode o f D i s i n t e g r a t i o n

a

B

B

a!

a!

a

a

B

alp

a

B

S t a b l e

Th

MsThl

MsTh2

RdTh

ThX

Tn

ThA

ThB

ThC

ThCr

ThC"

.ThD

from the center of the sens~tlbe volume of the detector 1s expressed In the general form (for the case of a pulse-type detector) :

d l = .4~Pfi)O(r)pdIT ( 2 ) W l ~ e r e ~ n : . A 1s the number of gamma rays emitted per second per gram of rock; O(r), the fraction emltted in

2 3 2 goTh

88Ra 2 2 8

2 2 8 8 gAC

~ h ~ ~ ~

2 2 4 88Ra

2 2 0 86Em

2 1 6 84"

82Pb212

8 3 B i 2 1 2

2 1 2 8 4 P ~

8 1 ~ 1 2 0 8

82Pb208

Half L i f e --

1 . 4 2 x l 0 l 0 y r

6 . 7 yr

6 . 1 3 h r

1 . 9 1 yr

3 . 6 4 d

5 1 . 5 s e c

0 . 1 6 s e c

1 0 . 6 hr

6 0 . 5 m i n

0 . 3 0 p s e c

3 . 1 0 m i n

- -

S e q u e n c e

V

- a

A L

B

- I3

a A-SL

a A L

a JL

JL a

3 5 . 4 % a

B v

A L

6 4 . 6 %

a

API CALIBRATION FACILITY FOR NUCLEAR LOGS 233

Table 4 Gamma-ray Lines* in the Spectra of the Important

-- Naturally - - Occurring Radionuclides3 -- _I___- -- -_..A

* w ~ t n ~ntens~tres greater than 0 05 photons per dls~ntegrat~on and enera~es greater than 100 kev.

the d ~ r e c t ~ o n of the sensitive volume of the detector, p(r), a factor to corlect for the absorption and scat- terlng of gamma rays In the reglon between the element dV and the detector, and E, the effic~ency of the detector for gamma rays, I.e., the f r a c t ~ o n of those incldent on the detector t h a t actually contributes to the response.

The factor P accounts for the fact tha t the number of gamma rays w h ~ c h reach the detector (ApflpdV) IS strongly ~nfluenced by the environment In w h ~ c h the source and the detector a r e s~ tua ted I t s value vanes w ~ t h the types of mateilals present, the dlstance ,I., and the energy of the gamma rad~a t lon

Expressed alternat~vely, E 1s the plobabi l~ty t h a t a gamma ray ~ncldent on the detector w ~ l l produce a count The character is t~cs of the counting system (e.g , the setting of the d ~ s c i ~ m l n a t o r used wlth a sclntlllation counter) and the characteristics of the detector determine ~ t s value In any glven s l tua t~on The three p r ~ n c ~ p a l classes of gamma-ray detectors currently In use a re lon~zatlon chambers, Ge~ger-Muller tubes, and sclntllla- tlon counters Even w ~ t h n l a given class, ~ n d ~ v l d u a l ~ns t ruments can have wldely different energy-response characteristics unless they a re bullt to exactly the same specifications. Tha t IS, In addltion to b e ~ n g not equally

294 BELKNAP, DEWAN, KIRKPATKICK, MOTT, PEARSON, A N D RABSON

Table 5

Spectral Composition of the Gamma Rays* Emitted by the Uranium Series, the Thorium Series, and Potassium'

Energy Interval,

Mev

0.1 - 0.3 0.3 - 0 5 0 5 - 0.7 0.7 - 0 9 0 9 - 1 1 1 1 - 1.3 1 3 - 1 5 1 5 - 1.7 1.7 - 1.9 1.9 - 2.1 2.1 - 2 3 2 3 - 2.5 2 5 - 2 7

Ural i~uni Senes Thor~unl Series - Number of I 1 Number of I

Photons per D ~ s ~ n t e g r a t ~ o n 111 E q u ~ l l b r ~ u m

Mixture

Relative Intensity, Percent

Photons' per D ~ s i n t e g r a t ~ o n in E q u ~ l ~ b r ~ u m

Mixture

Relatlve Intensity, Percent

'Includ~ng l ~ n e s u ~ t h energles greater than 100 kev

Number of Photons per

Disintegration

Relat~ve Intensity, Percent

Table 6

Characteristics of the Three Naturally Occurring Source Materials'

Element

sens~t ive to gamma rays of all energles, the manner In whlch the efficiency vanes with energy can d ~ f f e r from one type of counter to the next

Thus, In p~lnciple P IS affected by v a r ~ a t ~ o n s in the last six items l ~ s t e d previously. The last seven Items w ~ l l affect the energy d ~ s t r ~ b u t i o n of gamma rays w h ~ c h , in tuin, affects the spec~fic value of E . The descript~on IS

s~mpl~f ied considerably however, ~f we assume, In accordance wlth a common practice whlch may or may not be well-grounded, that under the c o n d ~ t ~ o n s normally encounterecl In well logging

a. the energy spectrum of the gamma rays e m ~ t t e d by the formation does not vary much from one formatlon to another (l.e., w ~ t h the re la t~ve concentrations of U, Th, and K) , and

h. the spectrum ~ n c ~ d e n t on the detector is not shifted s~gn~f ican t ly by changes In horehole and formation condit~ons.

p will now vary only w ~ t h Items 4 to 8, E, with Item 3

Number of Dls~ntegraton I Sec-Gm

of Element

u ( + ) T h i + ) K R a ( + )

Calibration and Standardization of Gamma-ray Logs

*Photons w~:h energles less than 100 kev not Included ( + )Ser~es In equ~l~brlum.

1.23 x lo4 4 02 x 103

31.3 3.63 X 1010

2.24 I 2 8 x lo4 2.51 1 0 x 104

0 11 2.20 8 0 X loi0

I t IS apparent from the "Introduct~on" tha t many of the d~ff icul t~es mherent in gamma-ray logging would be e l ~ m ~ n a t e d if a common u n ~ t of measurement defined In telnls of a standard source of ganima radiation (which could be used to ca l~bra te each type of gamma-ray logging tool) were adopted by the servlce companies Once the Subcomm~ttee agreed tha t t111s approach to s tandardizat~on was practicable and would not 11lfr1ng.e upon the r ~ g h t s of ~ n d ~ v ~ d u a l companies, the h a w pioh- lems of namlng and defining the standard unit of log response were attacked E x ~ s t ~ n g units wele first con- s~dered. Some of these had desirable characteristics but none was completely d e s c r ~ p t ~ v e of the ganinla-ray enils- slon propert~es of the formatlon Because of t h ~ s and the d~fficulty of achlevnlg agreement between the competi-

Number of - P h o t o n s / D ~ s ~ n t e g r a t ~ o n

In E q u ~ l ~ b r l u m M~xture* -

0.80 0 93 1.46 0 81

Number of Photons1 Sec-Gm

of Element* Mean Photon Energy, Mev*

APT CALIBR.ATION FA( :ILITY FOR NUCLEAR LOGS 295

tive l o g ~ l n g companies on an e s ~ s t ~ n g u n ~ t , a new tenn "API Ganinla-Ray Unit" was apploved by the Sub- conlln~ttee

As ~ n d ~ c a t e d e a l l ~ e r , In gamma-lay logg~ng one IS

only co~icernetl w ~ t h the gamma lays eni~ttetl by K40, U"s, T h " h n d the decay products of the latter t\vo I t mas reasonable therefore tha t the Subcomm~ttee defined the API u n ~ t m terms of the magnitude of the response produced by a n estended med~um c o n t a ~ n ~ n g a m ~ s t u r e of the three naturally occurrmg r a d ~ o a c t ~ v e source materials For one thing, w ~ t h logs calibrated 111 such units, effects caused by energy-dependent factors (such a s the d~fferent energy-iesponse characterist~cs of the various logging systems) w ~ l l be n ~ ~ n ~ m i z e d , ~f in fact they do play an ~ m p o l t a n t part.

Although some co~isiderat~on \\,as glven to the poss~- h ~ l ~ t y of u s ~ n g a geolog~cal f o ~ m a t ~ o n as a primaly standard, the unanimous cho~ce of the Subconlm~ttee was a c a l ~ b i a t ~ o i i pit because ~t appeared to meet the b a s ~ c requirement of bemg both practical and pelma- nent The p ~ t (see Sect~on IV h e r e ~ n ) would he com- pr~set l of two zones - one of very low r a d ~ o a c t ~ v ~ t y , the othei h a v ~ n g essent~ally the same relative concen- t r a t ~ o n s of uranium, thor~um, and potasslum a s an average shale, but appros~niately twice the total a c t ~ v - ~ t y . Both zones would be used for c a h b r a t ~ n g purposes, w ~ t h one API Gamma-Ray U n ~ t defined a s 11200 of the d~fference 111 the lo? deflections produced by the two levels of radiation.

Espiessed in terms of equation (2), after ~ n t e - g r a t ~ n g , the c a l ~ b r a t ~ o n p ~ t w ~ l l glve:

I , - I2 = A l ~ , X l - A 2 ~ , k 2 = ,000 API Gn?n~nc~-Rny U7tzts (3)

Wllere l?~ k l and k, a r e constants of p ropor t~ona l~ ty Therefole, if energy-dependent effects a re ~gnored, the equat~on for the iesponse of every cal~hrated tool In a fo rmat~on of spec~fic ac t iv~ty .4 w11l be.

I = [ZOO A k I ( k , A , - l i2A2)] (API Gamma-Ray U n ~ t s ) (4 )

I t IS ev~dent that the response I 1s independent of tool type and other instrumental factors and thus sat~sfies the cond~tlons fo r s t a n d a r d ~ z a t ~ o n

111. FACTORS INVOLVED IN NEUTRON LOG STANDARDIZATION

Neutron Logg~ng Principles

Altliougl~ t h ~ s papel 1s not intended a s a nuclear logging primer, ~t IS necessary to set forth c e r t a ~ n ~ u d ~ m e n t a i y p ~ ~ n c ~ p l e s 111 order to make clear the reasoning beh~nd some of the Subcommittee's dec~s~ons . Coiisequently, 111 t h ~ s sect~on, only those top~cs d~rec t ly related to the cal~blat lon and s tandaidizat~on of neutron logs w ~ l l be d~scussed and then a s br~efly a s poss~ble For a comprehens~ve treatment of the phvs~cal principles of neutlon logg~ng, the readel IS refelred to the work of T i t t n i a i ~ . ~

I n the sequence of events wh~cli character~zes the neutlon logg~ng method, energetic neutrons from a source 111 the logging sonde enter the format~on, lose energy (slow down) through elastic and inelast~c

nuclear co l l~s~ons u n t ~ l they a r e in thermal equ~l~bi iu in w ~ t h the moderating medluni, and then d~ffuse unt11 they a r e absorbed by the fo rmat~on nucle~. A steadp- state cond~tron is reached when the total number of n e u t ~ o n s absorbed per second equals the number em~t ted per second

In the steady state, neutrons of all energles a r e d ~ s t r ~ b u t e d 111 space about the source For any glven set of bolehole and f o l m a t ~ o n cond~t~ons , t h ~ s clout1 of neutrons will have the follow~ng general features:

a The total neutron density ( ~ . e , the n~ilnber of neutrons per unit volume), a s well a s the dens~ty of neutlons w h ~ c h have been slowed down to some energy E, \\rill be a decleasmg func t~on of the distance from the source.

h Neutrons of energy E will be distributed, on the average, closer to the source than those of some lower energy E'.

c. The spatial d~stribution of thelmal neutrons will always be broader than that of ileutrons of energles just above thermal

As for the effect of the in t r~ns ic propert~es of a fo rmat~on on the s ~ a t ~ a l d~stnbution. we observe tha t in slo\ving down to a low energy E, neutrons (on the aveiage) travel far ther f iom the source when the hydrogen concentrat~on is low than \vl~en ~t is high In other wolds, a s the hydrogen content of the rock Increases, the neutrons a r e slowed down more effectively and the distribut~on is narrowed. To ~ l lus t ra te t h ~ s we cons~der two formations, one having n, hydrogen atouis per u n ~ t volume, the other 1t2, where 7 7 1 > ?L?. If the neutron deiisit~es 111 these fo rmat~ons a r e q , ( E ) and q 2 ( E ) , respectively, and all other factors a r e constant, then, a t the source p o s ~ t ~ o n , q l ( E ) > q 2 ( E ) . However, upon movmg away from the source a p o ~ n t IS reached (the crossover p o ~ n t where q, = q2) beyond wh~cli q1(E) < rp(E).

A generalizat~on of the foregoing 1s that, regardless of the energy of the neutrons under considerat~on, a d~s tance can be found a f te r whrcli neutron density always dimin~shes w ~ t h hydrogen content. We need to remember, however, tha t the exact magn~tude of the reduct~on IS a func t~on of neutron energy; I e , a t a glven d~s tance from the source, the thermal and epl- thermal densities, for esample, w ~ l l not change a t the same rate with hydrogen The detectors of all presently ava~lable neutron logging devices a r e located In the reglon where the neutron density decreases a s the hydrogen concentrat~on increases

Of course, elements other than hydrogen can influence the manner in w h ~ c h neutrons of a glveil energy a r e d ~ s t r ~ b u t e d about a source. For example, a t low hydrogen concentrat~ons, where the s low~ng down of fas t neutrons IS no longer dom~nated by hydrogen, effects caused by d~fferences in the nlajor elements present 111

the rock matrices become significant. Further , the diffusion of thermal neutrons can be apprec~ahly affected by small concentrations of elements w h ~ c h a r e strong neutron absorbers (e.g., chlorine) and, under c e r t a ~ n cond~tions, by the major elenients in the formation.

, MOTT, PEARSON, A N D RABSON

Reference should also he made to the fact t h a t the energy, E,, of the source neutrons plays a role In determlnlng the character~stlcs of the spatial d ~ s t r l - h u t ~ o n of neutlons All other factors belng equal, changes In E, can alter the relationship hetween neutron d e n s ~ t y and hydiogen content.

The niechan~sms underly~ng the neutron l o g g ~ n g niethod a r e such t h a t the funct~onal r e l a t ~ o n s h ~ p hetween hydrogen concentrat~on and log response can vary from one type of logg~rlg system to another. I t is thls charac te r~s t~c , more than any other, which compll- cates the s tandard~zat ion problem

The h a s ~ c d~fferences In response vs poros~ty ~ n d e x of the various logging systems stem from the use of d~fferent types of detectors, source-detector spacings, and - probably to a niuch lesser extent - types of sources. Each of these factors is discussed briefly following

CL. Detector T y p e . Three generlc types of detector a r e used In commercially ava~lable neutron loggnlg tools. F o r our purposes, we w ~ l l refer to these s~nip ly a s thermal neutron, eplthernial neutron,* and gamma-ray detectors, ~t h e ~ n g understood -hut ~gnored - that not all thermal-neutron detectors a re completely insensl- tive to ep~thermal neutrons andlor gamma rays and converse1 y.

The responses of the first two detectors a r e propor- tional to the densities of thermal and epithermal neutrons, respect~vely, In t h e ~ r ~ m m e d ~ a t e v ic in~t~es . Hence, from the dlscuss~on In the prevlous sectloll, we see that for a . given set of horehole a d format1011 cond~t~ons , a neutron-thermal neutron and a neutron- ep~thermal neutron log can have q u ~ t e d~fferent hydrogen response curves

A t fiist s ~ g h t ~t appears a s though a sonde contaln~ng a gamma-ray detector wlll produce a log w ~ t h a response veiy s ~ m ~ l a r to that of the thermal neutron log. The reasonlng 1s as follows When thermal neutrons a r e absorbed by format1011 nucle~, the excess react~on energy 1s promptly released In the form of gamma rays Inasmuch a s the number of neutrons absorhed per u n ~ t volume a t any polnt In the formation IS propor- t ~ o n a l to the thermal neutron dens~ty a t tha t polnt, the gamma-ray-em~tt .~ng nucl~des a re seen to he d ~ s - t i ~ h u t e d about the source (of neutrons) 111 exactly the same manner a s the thei-mal neutrons Up011 assuming.

tha t few gamma rags con t i~hute to the courit~rig l a t e unless they a re produced very near the detector, one 1s led to the conclusion tha t the response of the gamma- r a y detector 1s proport~onal to the thermal neutron dens~ty In ~ t s v~clnity.

Of course, the fallacy 111 t ins reasonlng IS that In the neutron capture process the eneigy ant1 numbei of the gamma rays eni~t ted depend upon the particular rlucle~ mvolved. Tha t is, fo r the same thermal neutron density, the number of gamma rays e m ~ t t e d per unit time by a unlt volume of rock can he larger, smaller. o r the same a s t h a t from some other u n ~ t volume havlng

8.4 neutron w h ~ c h has not completed the slow~ng-down process

s l ~ g h t l y different elemental composlt~on Under some conditions, then, the response curves of the neutron- gamma and neutron-thermal neutron logs niay be the same, whereas under others there may he major d~fferences.

b. Soztrce-detector S p n c ~ n g . The a h l l ~ t y of a neutron log, regardless of type, to ieflect changes m the hydrogen content of a formatlon IS dlrectly related to the spaclng between the source and the detect01 As the spaclng is Increased (in the reglon above crossover), the fractional change in response f o r a u n ~ t change In hydrogen IS also ~ncreased. However - a s was mentioned prev~ously - neutron density always decreases w ~ t h the d~s tance from the source, so that the larger spaclngs a re char- acterlzed hy lower count~ng rates and greatei s ta t~s t lca l fluctuat~ons

Each sei-vlce company selects source-detector spaclngs In accordance w ~ t h its own ideas a s to what constitutes the best comproni~se between hydrogen resolut~on and count~ng rate fo r its 1)artlcular cond~tions The result 1s tha t even tools of the same type (e .g, neutron- gamma) a r e operated w ~ t h d~fferent spacings and, a s a consequence, can have d~fferent response cuives.

c. Nezctron Solirces. The neutrons em~t ted hy the sources In present-day neutron logglng tools a l e produced by a nuclear reaction w h ~ c h may be described symbolically a s follows:

.,Be9 + .He4 -+ ,nl + ,C1" 5.71 Mev.

A small par t of the react~on energy (5 71 Mev) is used up In the reco~l of the C 1 h n c l e u s , all o r p a i t of the remainder may be carrled off by the neution

Sources of t h ~ s type a r e made by mlxlng -or com- h l n ~ n g chemically - a n a l p h a - e m ~ t t ~ n g r a d ~ o n u c l ~ d e such a s Raze, Pox0, Ac227, or PuZSY, w ~ t h beryll~um, the t a ~ g e t matella1 The neutron y~eltl of a glverl source prlnclpally depends upon the r e l a t ~ v e proport~ons of alpha en11tte1 and target m a t e r ~ a l used, the manner of mlxlng or conihln~ng, and on the decay propert~es of the alpha e m ~ t t e r Approx~mate y~e lds obta~nahle In p rac t~ce from some of the commollly used alpha-neutron sources a r e listed in Tahle 7.

An (a,?~) source always e x l i ~ h ~ t s a wrde spectium of neutron energles even when the alpha pal-t~cles all have the same i m t ~ a l energy. T h ~ s IS hecause: (I., many alpha par t~c les lose par t of t h e ~ r energ!, 11g coll~slon processes hefore they ~ n t e r a c t with a heryl l~um nucleus; h , the product nucleus 1s somet~mes left In a n e s c ~ t e d s tate 111 which case the energy of e s c ~ t a t ~ o n 1s not ava~lahle to the emitted neutron, and c, the recoll energy of the product nucleus varies wlth the angle hetween the Incl- dent alpha par t~c le and the ou tgo~ng neutron In general, detalls 01.1 the neutron energy spectla of (a.7~) sources must he determined exper~mentally Short of expeil- merit, one can say wlth ce r ta~nty only tha t the nlaslmum energy of the neutions w ~ l l he less than the slim of the alpha-part~cle energy and the react1011 energy Data on the spectrum of neution energles emitted hv R3-Be, Po-Be, and Pu-Be sources a r e included In Table 7.


I t I S genelally thought that the energy spectra of ((X,7~) so~irces a r e not sufficiently dtfferent to cause pronounced effects In neutron loggtng However, a s f a r a s can be detenntned, there a r e no data ava~lable on the \7anous types of l o g g ~ n g tools In use w111ch can justify completely thls po111t of mew.

.-

Table 7

Pract~cal Yields from Alpha-Neutron Sources

Calibrat~on and Standardtzatton of Neutron Logs

The Subcomnltttee cons~dered a t length the cholce of a common neutron log u n ~ t None of the eslstlng neutron l o g g ~ n g unlts renewed In the "Introduction" had out- s t a n d ~ n g merit a s a n Industry standard. I11 p r ~ n c ~ p l e ~t would be ]deal to scale neutron logs dlrectly in total poroslty unlts (free water plus eclmvalent bound water) , slnce thls IS a physical unl t and IS the quantlty ultl- mately deslied from the log However, t h ~ s IS not feas~ble because the response of a neutron tool IS not llnearly related to poroslty and because ~t IS so affected hy borehole variables. The Subcotnm~ttee therefore adopted a new unlt, des~gnated a s the A P I Neutron Unlt, to be defined In terms of a p ~ l n i a r y standard ava~lable to all servlce compan~es

The follow~ng posslbllltles were consldered a s p i ~ m a l y standards fo r neutron tool c a l ~ b r a t ~ o n :

1 An actual well, probably a n abandoned cased hole. 2. A specla1 sleeve or tank slmllar to ex~s t lng

calibrators. 3. A test pit wlth predeterm~ned formatlon and bore-

hole condlt~ons. The first scheme mas rejected on the giouncls tha t the well mtght not remain usable ~ndefin~tely, tha t char- a c t e i ~ s t ~ c s of the formatron would not be known pre- c~sely, depth measu~etnents might be In error, c a s ~ n g would corrode, etc. The second scheme was also rejected on the basls tha t e s ~ s t l n g ca l~bra t lng sleeves were , tallored to particular types of tools, I e , they contamed air spaces, sliding mechanisn~s, neutron absorbers, etc., and tha t it mould be ~mposs~hle to choose or even des1g.11 a un~versal sleeve w h ~ c h mould appeal the same to all slzes and types of tools I11 other words, a p a r t ~ c u l a r callbrator, des~gned to glve the same response a s a 10-percent porosity format1011 w ~ t h one tool, m ~ g h t glve a 15-percent porostty response to another tool ,and a 5-percent response to a t h ~ r d .

The forego~ng object~ons do not apply to a properly deslgned test plt c o n t a ~ n ~ n g "subsurface" formations. Thls was therefore adopted a s a standard Because of the widespread utlhzatlon of neutron logs In Innestone reservoirs and the a v a ~ l a b ~ l l t y of clean Ilnlestone, the "standard format~on" was chosen to be I n d ~ a n a Ilme- stone of 19 percent average l~oroslty saturated w ~ t h fresh water and drllled wlth a 7%-ln. hole; the hole would he uncased and full of fresh water The A P I Neutron U n ~ t was defined a s 1/1,000 of the response of a tool In t h ~ s format~on.* By def in~t~on then, all neutron tools scaled In A P I u n ~ t s must show a deflection of 1,000 A P I unlts ~f the tool passes through a clean l~nlestone bed where poros~ty, sa tu ra t~on , and hole c o n d ~ t ~ o n s a l e ~ d e n t ~ c a l w ~ t h the A P I standaid Wlth t h ~ s system n e u t ~ o n logs wlll show deflect~ons fioni a few hundred to a few thousand API n n ~ t s fol the

Yleld, I Neutron Energy, Mev Neutrons/ Sec-Curte Average 1 Mas~tnum

usual range of borehole and format~on condttlons

Alpha Energ~es , Mev Source

In add~t lon t o the callbratloll formatlon, the Suh- . conimlttee consldered ~t advisable to provlde two add]-

Alpha Em~t t . e i

Ra:Be

Po. Be Ac : Be

Pu :Be

ttonal l~mestone f o r n ~ a t ~ o ~ ~ s , one of lower poros~ty and one of hlghel poros~ty than the standard The three formattons would serve to establ~sh one common departure curve and would pern~t t each servlce company to normalize ~ t s laboratory departure curves to the A P I standard. T h ~ s provision appeared necessary because there was considerable var la t~on ~n porosltles asslgned by servlce companles to the same type of limestone In thelr lndlv~dual test p ~ t s , par t~cular ly a t the lowest poros~tles. W ~ t h the f a c ~ l ~ t ~ e s a t thetr d~sposal, the R P 3.9 Subcomm~ttee could determine extremely accurately the poroslt~es of the stone installed ln the test D I ~ and these values would be nlutually acceptable to all.

-- Ra"26 plus 1 0 - 1.5 x 10' decay products 5.49, 5 99, 7.68 p0~lo 2 5 x l o6 Ac"" plus 4 94, 5.86,5.97, 1.7 - 2.0 x lo7 decay prodi~cts 6 03,6.62,6 82,

7 36, 7.43, and

pu?:3s 5.15, 5 13,5.10, 1 4 - 2.0 X lofi

To sumniarlze, ~t was declded to establ~sh -as a means for standardlzmg neutron logs - a standard for- m a t ~ o n ava~lable to all se~vlce companles. The response of any tool m t h ~ s standard format~on would be deslg- nated 1,000 A P I u n ~ t s Each servlce company woulcl compaie the response of a tool 1x1 ~ t s field callbrator to the response In the test formatlon and thereby asslgn to the field callbrator the appropr~a te number of A P I

'Although there are several advantages to a cal~hratron hased on two standard format~ons, the Subcommittee bel~eved that the two-po~nt method would Impose practical drficultles on the servlce companles In scaling thew logs whrch could lead to numerous scaling errors

3 8

4.3 . 5.0

4 2

12

10 8 1 3

10.5

ur i~ t s In acld~t~on, each servlce company could Issue depalture curves In API u n ~ t s w ~ t h these curves nor- mal~zed to the response of the tool In the API s tandaid format~ons

IV. DESIGN O F T H E API NUCLEAR LOG CALIBRATION FACILITY

Facility 1,ocation and F~nancing

As outl~netl in Sect~ons I1 and 111, the Subcomm~ttee on Revtew of A P I R P $3 dec~ded tha t the best method of ach~evlng s t a n d a r d ~ z a t ~ o n of gamma-ray and neutron logs was to construct prlmary standards In the form of c a l ~ b r a t ~ o n p ~ t s , one for gamma-ray and one for neutron logs These p ~ t s would conta~n f o r n ~ a t ~ o n s w ~ t h pie-

c~se ly known plopel ties, s ~ ~ n u l a t ~ n g s u b s u ~ face condl- t ~ o n s a s closely a s poss~ble. They would be set up a t some central locat1011 wheie each servlce company could l u n ~ t s logglng tools and lelate the response of each tool In the prlmary standard to the field ca l~hra tor normally used.

In January 1957, a panel' of the Subcomm~ttee was appointed to dlaw up a d e s ~ g n for the neutron and gamma-ray calihrat~on p ~ t s and to ~ n v e s t ~ g a t e posstble locat~ons and methods of f inanc~ng The first f a c ~ l ~ t y cles~gn dlawn up by the panel was plesented to the Suhcotnm~ttee In May 1957, and a final rev~sed des~gn , w ~ t h a firm cost es t~mate of $48,000 (exclud~ng land) , was approved In October, 1957

Poss~ble s ~ t e s for the f a c ~ l ~ t y , a s well a s alterriatlve methods of financing, were then ~nves t~ga ted by tlie panel A letter request~ng a n ~ n d ~ c a t ~ o n of ~n te res t and a proposal for construct~ng and operating the call- bratton fac~l i ty was sent to 14 commerctal research orgamzat~ons and u n ~ v e r s ~ t ~ e s The answers recelved establ~shed that ~t would not be feas~hle for a commerc~al organlzat~on to constluct the p ~ t s and amor t~ze the cost on a serv~ce-charge b a s s The p o s s ~ h ~ l ~ t y of the servlce compames alone h e a r ~ n g the major par t of the cost was d~scussed and rejected for several reasons, a major one b e ~ n g tha t objective and ~ m p a r t ~ a l control of the f a c l l ~ t y hy opera t~ng companles could not be exerased under these condit~ons. I t was concluded that the only feas~ble f inanc~ng method would he for the A P I to underwr~te the construct~on costs of the f a c ~ l ~ t y , w ~ t h ma~ntenance and opera t~ng costs to he covered by selvlce charges for t h e ~ r use Accord~ngly the A P I undertook a s o l ~ c ~ t a t ~ o n of approx~mately 50 opera t~ng and 10 servlce companles, w ~ t h the suggested c o n t l ~ b u t ~ o n of each company betng loughly proportlonate to the stze of the company By June 1958, tlie necessary $48,000 had been rece~vecl

I t was agreed that the f a c l l ~ t y should be located In the Houston area hecause of the fact that applox~mately '7.5 percent of the servlce compan~es' general offices and shops a r e In tha t area Accordtngly, the U n ~ v e r s ~ t y of Houston was approached and lece~ved the proposal favorably. The University agreed to ded~cate the land

*W E Mott. Clrart~~ta:)~. Gulf Research and Development Co . J T Dewan. Schlumberger Well Surveying Corp. A S McKay. Texaco Inc . G~lbert S w ~ f t , Well Survevs Inc . R H Wlnn, Welex, Inc . and W B Belknap, ex ofic&o, Phrlllps Petroleum Co.

necessary, to act a s the prlme cont~ac tor fo r the A P I In construct~ng the f a c ~ l ~ t y , and to p rov~de for the subsequent supelvlslon, operat~on and malntenance of the fac~l i ty , wlth the understand~ng tha t ~t could be used for educat~onal purposes. Constluct~on and opera- t ~ o n of the faclllty was placed undel the ~ u r ~ s d ~ c t ~ o n of the Petroleum Englneer~ng D e p a ~ t m e n t w ~ t h Professor C V. K~rkpatr lck, head of the D l v ~ s ~ o n of Chenilcal and Petroleum Eng~neerlng, d~rec t ly In charge.

In February 1958, a second panel of SIX memhers? was appo~nted to coord~nate the construct~on and opera- t ~ o n of the A P I f a c ~ l ~ t y w ~ t h Professor K ~ r k p a t r ~ c k T h ~ s comm~ttee worked very closely w ~ t h Profess01 Kirkpatrlck d u r ~ n g the detatled planning and construc- t ~ o n stages As the p l a n n ~ n g progressed, a numher of changes were made to lower the cost and Increase the f l ex~b~l t ty of the lnstallat~on, particularly In regard to the bu~ld lng and type of h o ~ s t spec~fied Consequently, the final des~gn , descr~bed follow~ng, embod~es the work of hoth of the panels, a s well a s some mod~ficat~ons suggested by the Unlverslty of Houston staff supervlslng the construct~on

Design of the Garnrna-ray Pi t

Spec~ficat~ons l a d down for the gamma-ray p ~ t wele threefold. Flrs t , the d ~ m e n s ~ o n s of the p ~ t had to be large enough that the syn the t~c format~ons appear to be ~ n f i n ~ t e to gamma-ray detectors. Second, the p ~ t had to have a rad~oactive zone c o n t a ~ n ~ n g the three naturally occurring r a d ~ o a c t ~ v e elements - uranium, thor~uni , and potasslum - In apploximately tlie same r e l a t ~ v e concentrat~ons a s occui In a n average shale, the total a c t ~ v ~ t y of the zone had to be approx~mately twtce the a c t ~ v ~ t y of the shale. T h ~ r d , the r a d ~ o a c t ~ v e elements had to be dispersed In a large mass of relat~vely ~ n e r t mate r~a l . Inasmuch a s such m a t e r ~ a l w ~ l l have some s l ~ g h t rad~oac t iv~ty , a second zone of t h ~ s m a t e r ~ a l had to be prov~ded. The d~fference In ~ n t e n s ~ t y between the two zones IS used to define the A P I Gamma-Ray U n ~ t

C o n s ~ d e r ~ n g first the slze, tt IS known tha t the 95-percent depth of lnves t~ga t~on of the gamma-ray log In a very porous formation IS appl-ox~mately 15 In. (from the borehole wall). Consequently, the r a d ~ u s of the test ptt was set a t 2 f t . Even w ~ t h the longest counter In present-day use, about 3 f t , a good "plateau" should be ob ta~ned In the center of the zone durlng logglng T h ~ s , coniblned w ~ t h a 1 5 - ~ n depth of pene- tration, led to a - c h o ~ c e of 8 f t fol the bed thtckness

Data on a n a l y s ~ s of over 200 shales ~ndlcated tha t the aveiage shale contained approx~mately 6 ppm uranlum, 1 2 ppm thor~um, and 2 percent potasslum S ~ n c e ~t was desired to o b t a ~ n twice the average shale levelf In the radioact~ve zone of the p ~ t , concentrat~ons for the la t ter were set a t 12 ppm uranlum, 21 ppm thorlum, and 4 percent potassium From the constants

TW B Belknap, Chazr71rnn. Phlllrps Petroleum Co . W E Mott. Gulf Research and Development Co . J. T Dewan, Schlumberger Well Surveying Corp . W R Rabson. Pan Geo Atlas C o w . A J Pearson. The Atlantrc Refining C o . and A B W~nter , Lane-Wells Company

$This level was chosen as berng h ~ g h enough to mrnlmlze statistrcal fluctuatrons but st111 w t h ~ n llnearrty capabrl~t~es of exrstrng logglng tools.


glven ti Table 6 ~t 1s seen that , w ~ t h the ielatlve con- centratlolls ~ n d ~ c a t e d , nranlum contributes approxl- mately 47 percent, thorlum 34 percent and potasslum 19 peicent to the effective gamma-ray a c t ~ v ~ t y of thk forniat~on.

Although ~t m ~ g h t have been poss~ble to find a shale wltli the des~red concentrat~oii of radloact~ve elements, the prospect of a long search for a su~tab le shale out- cropping was riot a t t r a c t ~ v e Consecluently, ~t was considered tha t the best way of achlev~llg the deslred ac t lv~ty was to obtain accurately assayed amouilts of uranlum, thorium, and potasslum in concentrated form, mlx these un~fornily In a slurry w ~ t h low-act~vity cement, pour the s luiry Into the p ~ t , and allow ~t to set up In the foim of concrete T h ~ s has the advantage that ~f the r a d ~ o a c t ~ v e ~ n a t e r ~ a l IS umforn~ly d ~ s t r ~ h u t e d to begln w ~ t h , ~t lenialrls so ~ndefin~tely

Fig. 1 shows the final d e s ~ g n of the gamma-ray p ~ t I t contams 3 sepalate concrete zones each 4 f t In d~anietel and 8 f t deep. The top zone, of low-act~vlty concrete, 1s prlniallly a cosmlc-ray s h ~ e l d ; the m~ddle fo rmat~on- IS the r a d ~ o a c t ~ v e zone (see Tables 9 and 10 for final concentrat~ons of r a d ~ o a c t ~ v e material) , and the bottom format1011 1s the low-ac t~v~ty "background" zone. A 5%-in 17-lb caslng extends thlough the three zones and 15 f t below them.

I' 4 ' D l ,*" STEEL DECK PLATE

LOW ACTIVITY CONCRETE

CORRUGATED P I P E

RADIOACTIVE CONCRETE

LOW ACTIVITY CONCRETE

CASING (5; 0 0.17 : ~ - 5 5 )

Fig. I-Gamma-ray Log Calibration Pit

T h e niaxlmum a m o u n t s of r a d ~ o a c t i v e m a t e r ~ a l reclu~red for a 4-ft clianieter by 8-ft deep racl~oactlve zone were est~mated prlor to construct~on to he approximately 75 g iams of uranlum, 150 grams of t h o r ~ u m , and 500 Ib of potasslum, although the actual compo- s ~ t ~ o n ul t~mately d~ffered f lom these est~mates . Sources of these l n a t e ~ ~ a l s in su~tah le form fpr n i ~ s i ~ i ~ m tlie slurry were sought This was not a s ~ m p l e task smce In each case there were certam cons~dera t~ons to be taken Into account. For uranium and thor~um, a knowledge of the rad~oactlve dlslntegrat~on schemes IS iieces- sa ry ; these a re shown In Tables 2 and 3 Both elements decay In a serles of steps, each w ~ t h its characteristic half-11fe and type of emlsslon (gamma, beta, alpha, o r comhlnat~on thereof), untll a stable lsotope of lead 1s reached The Important polnt 1s that when uranlunl or thor~unl a r e contamed In a glven volume for a suffic~ent length of t ~ m e , a condlt~on of equ~l rbr~um 1s reached whereln the numbers of atoms of each member of the serles N , , N2. N3. etc. become constant and a l e cliar- acter~zed by the relation N , I T I = N21T, = N,?IT,, etc., where TI, T,, T,, etc a r e the correspond~ng half-l~ves of each. (Thls condltlon of e q u ~ l ~ b r ~ u m 1s generally assumed to exlst In subsurface folmat~ons, although dev~atlons from equ~llbrlum have been reported, par- t~cu la r ly fo r outcropplngs or marine sed~ments where selective leach~ng or d~ssolution of specific Inemhers of the serles has occurred.)

In the case of uranlani, ~t 1s easler to ohtaln a pre- c~sely known quantity of the daughter product radlum than of uranluni Itself I t IS l e g ~ t ~ i n a t e t? use r a d ~ u m because approxiniatelv 98 pelcent of the gamma l a y s above 100 kev eni~t ted by uranlum In equl l~bnum m ~ t h ~ t s daughter products come from the products below r a d ~ u m . The requ~red amount of r a d ~ u n i In equ~llbrlum w ~ t h 75 grams of uranlum IS 25 nilcrograms In order to have some r a d ~ u m available for p ~ l o t nllxes, a cluantity of 40 micrograms of radlum in the form of a n insoluble compound (sulfate) ~ n t ~ m a t e l y m ~ x e d w ~ t h 40 Ib of 20130 Ottawa s ~ l l c a sand was obtalned. Nat~ona l Bureau of Standards cel-t~ficat~on on the amount of r a d ~ u m was prov~ded.

Radlum decays initially to the element radon, whlch 1s a heavy gas and whlch has a 3 8-day half-l~fe. It was ant~clpated tha t some of the ladon, w h ~ c h would be 111 equilibrlunl w ~ t h the r a d ~ u m when rece~ved In sealed conta~ners , would escape to tlie all d u r ~ n g the process of mlxlng the cement s lulry for the test p ~ t . Thls necessitated waiting sevelal radon half-lives- about 3 weeks-after pourlng the conclete foi the activlty of p l o t nllres 01 of the gamma-ray p ~ t ltself to b u ~ l d up to e q u ~ l ~ b r ~ u n ~ before maklng measurements One advantage of niaklng the test p ~ t concrete 1s tha t ~t p o s ~ t ~ v e l y Insures that radon m ~ l l not escape The loss In an unconsol~dated format1011 would, In all proh- ab l l~ ty , be n e g l ~ g ~ b l e hut a small element of doubt would exist.

In the case of thor~um, a somewliat s~ml la i problem of e q u ~ l l b r ~ u m arlses The ~ n i n ~ e d ~ a t e daughter of

300 BELKNAP, L)EWAN, KIRKPATRIC

thorlum is Mesotholiuni I, whlcli has a 6 7-year half-life. T h ~ s nuchde 1s an lsotope of r a d ~ u m so t h a t when t h o r ~ u m 1s refined and the radlum w h ~ c h 1s nornially present IS lemoved, the Mesothor~um I IS also removed. A perlod of 20 to 30 yeais IS r equ~led for the Meso- thorlum I to agaln approach equillbnum Attenipts were therefore made to locate a supply of t h o r ~ u m refined 20 to 30 years ago; thls proved to be f r u ~ t l e s s I11 l ~ e u of the refined t h o r ~ u n ~ 2 k~lograms of finely ground ~ n o n a z ~ t e ore assaying 9 70 percent thonum o x ~ d e (approx~mately 180 grams of t l ior~um) and only 0 37 percent uranlum oxide was obtained from the New Brunswlck laboratory of the Unlted States Atomic Energy C o m n ~ ~ s s ~ o n A check by the AEC laboratory showed that the monaz~te mas w ~ t h l n 2 percent of equlllbr~um. An a d d ~ t ~ o n a l 2 2 k~lograms of monazite ore assaylng 5 5 to 6.0 pelcent T h o 2 and 0.1 peicent U,08 eras donated to the project but not used

In the case of potass~um, a d~fferent type of problem arose. Here a relatively large amount of the material is required. However, easily ava~lable potass~um com- pounds, such a s KCL and KISO,, a r e water-soluble Thls posed a d~fficult problem of assuring u n ~ f o r m d ~ s t r ~ b u t i o n of the potassium In the cement s lurry slnce any excess water would tend to collect on top a s the cement set up The solutlon adopted was to use mlca, whlch is insoluble, a s a source of potass~um. Potassium mlca (muscovite) has the theoretical formula H2KA13 (S1O4)3 w h ~ c h ind~cates it contains 9 8 percent potassium by weight A quantlty of magcomica assayed 9 percent K, 1 0 8 ppm U and 0 01 ppm Th T h ~ s was used In the test p ~ t However, slnce the p ~ t recluired a con- centratloll of 4 percent K, it was necessary to fill about half of the volume of the r a d ~ o a c t ~ v e zone with mlca. Slnce mlca has a ie lat~vely low bulk dens~ty, t h ~ s led to a n overall bulk denslty in the r a d ~ o a c t ~ v e zone considerably less than In the other two zones and less than nornially occurs In subsurface forniat~ons.

Details of the rnlslng procedure, pilot checks, laboratory analysis, etc f o r the gamma-ray pit a r e glven m Sect~on V.

Design of the Neutron Pit In designing the neutron p ~ t , there was conslderahle

expellence to draw upon slnce a number of the members of the afolenient~oned panels had already constructed somewhat s ~ m i l a r plts fo r thelr p a r t ~ c u l a r operating or service companies.

General spec~ficat~ons for the neutron p ~ t were tha t ~t hat1 to contaln three clean (shale-flee) l~mestone folmatlons - one of low poros~ty, one of Internledlate poros~ty, and one of h ~ g h porosity. The formations hat1 to be "infi~nte" in slze, conipletely saturated w ~ t h fresh water, and have accurately known poroslt~es. The ~ n t e r m e d ~ a t e poroslty zone would be used a s a standard for c a l ~ b r a t ~ o n ; whereas the other two, in conlunction w ~ t h the mtelmediate zone, would he used to establ~sh a standard departure curve

For the thiee types of stone there was little alterna- t ~ v e but to choose those already nl common use, v ~ z ,

Carthage marble, w h ~ c h has a poros~ty 111 the range 1 to 3 percent, I n d ~ a n a limestone w ~ t h a porosity of 17 to 20 percent, and Austln l~mestone w ~ t h a poroslty of 25 to 30 percent. These mate r~a ls wele obta~nable in Houston, 111 the form of closely mach~ned blocks.

The slze of the format~olls 111 the neutron p1t 1s

determined by the depth of inves t~ga t~on of the neutron log T h ~ s IS strongly dependent upon poros~ty, b e ~ n g greatest In low poros~t~es , where the 90-percent depth of penetrat~on with radium-beryllium neutrons, IS about 2 f t However, in the future the use of 14-mev neutrons (from the H"d,n)He4 reaction) may become w ~ d e - spread These neutrons should have a somewhat Increased depth of investigation although the exact amount IS difficult to assess. Consequently, a r a d ~ u s of 2% f t was chosen. The depth of the formation was set a t 6 f t , based on the exper~nlental fact tha t the vertlcal depth of lnves t~ga t~on of ileutron logs is approx~mately 2 f t The 6-ft depth should therefore give a good l o g g ~ n g "plateau."

Fig. 2 shows a-cross-sect~onal vlew of the neutron pit. I t IS 24 f t deep overall, with a 15-ft rathole ex tend~ng below the bottom zone. The 3 l~mestone folmations a r e each made up of 6 octagonal blocks, each 1 f t t h ~ c k , wlth carefully machined flats. A 7%-ln. hole extends through the center of the p ~ t The porosit~es ~ n d ~ c a t e d 011 Flg. 2 a l e the final values assigned to the formations (see Sect~on VI) On top of the stone IS a 6-ft layer of water w h ~ c h serves the dual purpose of provldlng a 100-percent poroslty reference p o ~ n t and s h ~ e l d ~ n g the gamma-ray detector In GRN tools from excessive rad~a t lon from the neutron source a s the tools log the upper lrnlestone

CARTHAGE LIMESTONE (POROSITY=I 9V.AVG)

INDIANA LIMESTONE (IS%AVG POROSITY)

AUSTIN LIMESTONE (26 % AVG POROSITY)

BLOCK DETAIL

Fig. 2-Neutron Log Calibration Pit

A P I CALIBRATION FACI

zone A %-In. thlck luclte pipe extends through the water zone to guide upper decentralizing dev~ces on logglng tools Thls pipe has no noticeable effect on the neutlon response In the water zone.

The method of saturating hhe blocks w ~ t h fresh water and of nieasuiing the amount of water in them af te r s a t u l a t ~ o n was given a great deal of attention. Industry experience on smaller blocks indicated tha t vacuum implegnatlon was absolutely necessary to achieve complete saturat~on. Furthermore, slnce the blocks would be too heavy for a n accurate measurement of d i y weight and wet welght, the only method of measuring directly the amount of water held by each block was to completely desaturate ~t first and then to measure the amount of water whlch ~t took up on saturation To thls end an elaborate s a t u l a t ~ o n system was deslgnetl, a s descllbetl In the follow~ng sect~on

V. CONSTRUCTION O F T H E CALIBRATION FACILITY

Contracts and General Layout

Upon ieceiving authorization f lom the API, the University of Houston, acting a s prime contractor, sol~clted tulrlkey blds for the major constluct~on Items of the fac~l i ty . All blds were considered too high, whe~eupon subcontracts wele let for varlous poltlons of the construction a s follows.

1 Fol major, slte wolk includ~ng ~nstallatlon of the two plts, all concrete slab wolk, g l a d ~ n g , and sliell~ng of adjacent areas.

2 . Fol ~nstallation of a 20-ft x 40-ft ~nsulated, heated, a l l -cond~t~oned, metal bul ld~ng ~ n c l u d ~ n g work benches, a small office, lavoratory, and dark room.

3. Fol installat~on of a specla1 jib-type crane wlth a 40-ft boom and electrlc controls

4. Fol qualrying and fabrication of the limestone blocks fo r the neutron pit

5 Foi necessaly carpentry, electrical, and plunlblng work

6 F o r ~ n s t a l l a t ~ o n of a steel fence sur round~ng the facillty.

All contract work was performed 111 accordance wlth speclficatlons furnished by the Umversity of Houston

I i GATE

1 , 7

GAMMA-RAY LOG

(ELECTRIC CONTROLS I CALIBRATION PIT

NEUTRON LOG CALIBRATION P I T

lz=='" - 108'-0. I

Fig. 3-Plan and Layout of API Nuclear Logging Calibration Facility

and approved by the API. Construction of the faclllty was begun In September 1958 on a %-acre slte on the University of Houston campus, and completed in June 1959. Genelal layout of the facillty IS shown In F ig 3

The Gamma-ray Plt

Follow~ng excavation and placement of the walls and centered caslng in the gamma-ray pit, the first low- activlty zone was poured using a neat portland-cement slur1 y. To insure uniforniity of cement used In all zones, a quantity suffic~ent fo r all three zones was pu~chased from the same source.

Table 8

Calculations of Material Quantit~es Required for Gamma-ray High-activity Zone

I T/olicme of Portron of P L ~ to be F ~ l l e d Volnme = .rrR" depth

=.rr(2 ft)" 8 f t = 100.5 c u f t 11. Spec~fic Gravity and Density of 11Izx

As deteim~netl from small pilot mixes Spec~fic gravlty = 1.47

Denslty = 91.6 lb pel cu f t 111. Totc~l TT7eig1rt of Mix Regutred

Weight = density x volume = 91 6 x 100.5 = 9,220 lb

01 4 18 x l o 6 gm IV. Qiu~nt i t y of IIf~ca Reqi~rred for 4-percet~t K

( b y weiglit) Concentrcctzo?~ r t ~ JItx (Mlca analyzed to contain 9.0 percent K ) Q u a n t ~ t y mica = ( K concentlat~on deslredl

K concentration In m ~ c a ) ( w e ~ g h t of mix)

= (0 04010.090) (9,220) = 4,097 lb mica

V. Qluitzt~ty of Wonaztte Sccnd ( T h o n ) Reqiiirecl fot: 24-ppn Tli ( b y we igh t ) Co?rce>rtrc~tto)~ In Jf lx (Sand analyzed to contaln 9.7 percent T h o o and 0 37 pelcent U308) Q u a n t ~ t y sand = [ ( T h ppm desired) ( w e ~ g h t of

mix)] / [ (Tho2 concentiatlon 111 sand) (molecular w e ~ g h t T h / molecular w e ~ g h t T h o 2 ) ]

= [(24) (9,220)]1 [ (0.097) (2321264)l

= 2.6 lb o r 1.177 Em sand , - V I. Qtic~nttty of R a d z z ~ n ~ Ottmrucc. Sand Regz~z.red for

12-ppm U ( b y w e z g l ~ t ) Concentrutzo,t~ z 7 ~ Mzz (Sand analyzed to contain 1 bg R a per Ib) From (Half llfe Rathalf llfe U )

= [(l.620) (lo3) l(4.51) ( lo9)] = (0.359) (10-6)

U = 0.359 ppm R a U cleslred x RaIU . .

= Ra necessary per gm mix 12 x 10-6 X 0 359 X 10-6

= 4 31 x 10-'"1 R a per gm n11x 4 31 X 10-1- 4 18 X lo6 gm

= 18 bgm Ra required = 18 Ib sand requlred

VII. Qziat~ttty of Neclt Portlc~nd Cement Required for M Z X

13 percent a s determined from pilot mlx 0 13 X 9,220 Ib = 1,200 lb cement

VIII Qriant~ty of W a t e r R e q l ~ i r e d f o r I l f t z 42.33 percent a s determined from pilot nlix 0.423 X 9,220 Ib = 3,903 lb o r 468 gal water

For the hlgh-act~vlty zones the materials selected for m ~ x t u r e wlth cement were r a d ~ u m - c o n t a ~ n ~ n g sand, t l ior~um-conta~ai i~g monazlte ole, and potasslum-con- t a ~ n l n g mica Three major 011-company laboratories analyzed the cement and mica by gamma-ray spec- t~oscopy to de te~mlne the U, Th, and K content. Content of the Ottawa sand and monaz~te ore was certified by the supphers. From this ~nformatlon the quantities of ina te~ la l s used 111 the n u s were calculated a s shown In Table 8

Extensrve pilot nlixung was then performed to confirm adequate nilsmg control and to check the calculations. The samples wele first mlxed 111 % and 1 cu f t batches, and finally a test barrel of applox~mately 25 cu f t was p o u ~ e d a ~ o u n d a plece of 5%-ln. caslng 011-company labo~ato i les ran spectral analysls on each pllot mlx a f te r each lnlx was allowed to set fo r 2 to 3 weeks for equ~l~bra t lon of the radon gas In a d d ~ t ~ o n the large test balrel was "logged" by two servlce compan~es for p~act lcal confilmatlon of over-all r ad~oac t~vl ty

F ~ o m thls wolk ~t was found tha t considerable d ry mlslng was necessaiy to obtain uinform dls tr~bution of the ratlloactlve n ia te l~a l throughout the mlx For actual poullng of the pit the inaterlal was dry n ~ ~ s e d In a cornme~c~al mlxel for 8 hours, then 2 hours more af ter the a d d ~ t ~ o n of water Conipos~t~on of the mlx wlth s u ~ n n i a t ~ o n of radloactlve components 1s presented In Table 9, from mhlch ~t 1s seen that total component act lv~t les a re very close to the spec~fied d e s ~ g n values A s l u ~ r y volume a p p ~ o x ~ m a t e l y 15 percent In excess of the calculated volume needed was mixed. Durlng the pourlng operation a sample was taken a s each foot of the sect1011 was poured for later laboratory analysis by gamma-ray spectroscopy

The upper low-actlvlty sectlon, s l m ~ l a r In compos~tlon to the bottom sect~on, was poured approx~lnately one month a f te r the h~gh-ac t~vl ty zone

The Neutron P i t

After selection of stone types to be ~nstalled In the neutron pit, quarrles throughout the U n ~ t e d States were contacted, and a Houston company was selected a s best ecluipped to furnlsh the stone blocks 111 conformance with the following approved speclficatlons

Slx octagonal blocks of each type stone (Carthage marble, Indlana limestone, and A u s t ~ n llniestone) to measure 5 f t across the flats and 1 f t thlck w ~ t h a 7%-ln dlameter hole bored through the center of each, all machlned surfaces to be w ~ t h ~ n 1132-ln. tolelance, each type of block to be cut from a slngle large piece of stone to provlde nlaxlmunl un1- formlty of poroslty, n i a t r ~ s character, and geo- logical environment.

To permlt checking the unlform~ty of poroslty between stones of a glven type, and fol late1 cornpailson wlth final saturation values, ~t was agieed tha t extens~ve core analysls should be performed on samples of each block To f a c ~ h t a t e thls, the suppl~er first mach~ned the blocks ~ n t o 5-ft squares, 1 f t t h ~ c k Under the supel-vlslon of the student project englneel, the colners were sawed off, formlng the fin~shed octagon, and then a 4-111. d~anleter core was taken from the center before lathe turnliig to the 774-1n finished d ~ a n ~ e t e r Each removed corner sectlon and center core was carefully tagged and coded to the block from whlch ~t was taken These sectlons were then cut ~ n t o SIX pleces (each tagged) , and samples from all sectlons dlv~ded Into SIX sets, each set thus h a v ~ n g a sample from every corner and centei core removed from every block.

Table 9

Composition of Flnal Mix for H~gh-activity Zone, Gamma-ray Pit

Radlat~on Component

1 86 x lo6

R a d ~ u m Ottawa Sand1' 1 8,180 18 1 I 1 . 1 . 5 0 3 1 0111

Water

3,903 1 1 1 1 . 1 . 1

-

aAEC cert~ficat~on of 9 7 percent Tho, and 0 37 percent Us08 b18 lb Ra sand = 18 ugm Ra = 50 3 g m U equ~valent cSame type cement poured neat for lower and upper low-act~v~ty zones Concentration determ~ned by gamma-lay spectroscopy

I

102 49 ( 0 2256 54 545 0 12035 Total

Conipos~t~on By Weight

- Actual

Deslred

4.07 percent K

4 percent K

24 4 ppm Th

24 ppm Th

13 1 ppm U

12 ppm U


I Arrangements were made with four major oil-company

laboratories to run small core analyslson a complete set of these samples The Unive ls~ ty of Houston Petroleum Engineering Depaitment analyzed the fifth set and the slxth set 1s being retamed a t the facihty foi possible future use Results of this analysis a r e tabulated and discussed In Sectlon VII Samples of dust obtalned duling the cuttlng of the blocks were also collected and

Table 10

Semi-quantitative Spectrographical Analysis of Carthage Marble, Indiana and Austin Limestones

Performed in 1951

(Percent by weight; oxygen omitted)

Carthage I n d ~ a n a Austln Marble L~mestone Limestone

C a l c ~ u n ~ 37. 35.6 38.07 Slllcon 1 8 0.8 0.39 Ilon 0.1 0.14 0.19 Magnesium 1.3 2 4 1.06 Chl o n ~ ~ u n ~ 0 002 0 0008 0 0001 Aluminum 0 2 0 03 0.03 Copper 0 001 0.0074 0.0034 Manganese 0.06 0.0033 0.0016 T ~ t a n ~ u m 0.009 S t ron t~um Trace

SIGHT GLASS

I SATURATION CHAMBER

are b e ~ n g retamed for chem~cal analysls a t a later date. For general information, semi-quant~tative spec- trographlc analysis made by a testlng laboratory In 1951 on samples of slmllar stones IS given In Table 10

F o r processing the blocks, many methods were con- s~dered and d~scussed with particular regard to the accuracy w ~ t h which final saturation could be measured The method selected prov~ded for accomplishment of the following objectives

1 Effective dehydrat~on and satuiation of all blocks wlth accurate measuiement of all expelled and absorbed water

2. Means of measuring bulk volumes of the blocks fol comparison wlth geometi~cally computed values.

Arrangement of the eclu~p~nent necessaiy fol the process selected IS shown in F lg 1. One of the principal components 111 the system 1s the s a t u r a t ~ o n cham be^ which was leased from a company who had constructed ~t for a slmllar purpose I t s dimensions weie sufficient to permlt piocesslng three blocks a t a time, and 0-r lng s e a l ~ n g of the lid piovided foi m a ~ n t a i n m g the necessaiy high vacuum durlng phases of the piocess. To ald in dehydratlon of the blocks, eight 1,500-watt 220-volt s t r lp heaters were Installed withln the chambei uslng spark plugs for electrical connect~ons through the chamber wall.

3 - P I N TEMPERATURE

START CALIBRATION A-A'

E N D CALIBRATION 8 - 0 '

VOLUME SOURCE TANK 3 ' ~ 30'

Fig. &Flow Schematic for Limestone-block Saturation

304 BELKNAP, DEWAN, KIRKPATRICK, MOTT, PEARSON, A N D RABSON

Connected to t h ~ s chamber was a h~gh-capacity Klnney Model KS-13 vacuum pump which performed very satis- factorily In obtaining good vacuum in the chamber. Installed In the llne com~ecting the chamber and pump was a large spherical-type watei t rap equlpped wlth a compiessol-diiven refr~gerat ion uni t assembled by t h e project engineers Thls vei y effectively removed water vapor befoie reach~ng the pump

Also connected to the s a t u r a t ~ o n chambe] was the large-volume tank which was fabricated a t the faclllty fioni a 25-ft section of 3-ft dlameter pipe Henilsphencal heads were used to cap the plpe ends to form the tank, w h ~ c h was sand-blasted and painted before using. A slght glass was fitted to the s ~ d e of the tank to permlt water-level measurements necessary d u r ~ n g the satura- t ~ o n process

A beam-type scale with a capac~ty of 69 5 kilograms, accurate to 12 5 grams, was purchased to enable accurate measurement of water quant~ t les added to the volume tank fol block s a t u l a t ~ o n M~scellaneous equipment included a multl-pen recordei and thermocouples for continuous monltor~ng of watel temperatuies 111 the system, and a DuBrovln ahsolute pressule gage for vacuum measurements.

In practlce tlie s a t u r a t ~ o n procedure was repeated s e v e ~ a l tlmes confi lni~~ig accuiate placement of all marks on the glasses In addltlon, tempeiatures of the watel In the saturation chamber and volume tank were cont~nuously iecorded t o provlde means for volume corrections due to denslty changes of the water from temperatule vai latlons The system y a s then drained and made leady for dehydiatlon of the first group of blocks.

The 18 blocks were d~vlded into 6 i roups, 2 each of similar type, and the geonietrlcal volume of each block calefully measured and added w ~ t h others 111 ~ t s group f o ~ asslgnnient of each group's bulk volume. In all cases this amounted to shghtly over 60 cu f t

The fiist gioup of three stones was sealed Into the saturatlon chamber and, wlth Valve A closed, a vacuum was pulled on the chamber. After about 200 hours of evacuation of the first group, it was dec~ded t h a t heaters should be mstallecl within the chamber to accelerate and Insure better dehydrat~on. T h ~ s was done, enabling a chamber temperature of 200 to 220 F. to be malnta~ned during evacuation. W ~ t h thls temperatule, e v a c u a t ~ o i ~ time ranged from 175 hours for one group of Austlii blocks to 77 hours fo r one group of Indlana blocks Water collected 111 the t r a p was measured to deterln~ne or~glna l water saturatlon of the blocks Values vaned s~gn~f ican t ly even between s ~ m ~ l a r groups because of some blocks belng exposed to cons~derable ram whlle a w a ~ t l n g processlng

Satisfactory dehydration of the blocks was deteimlned with the a ~ d of a DuBrovin ahsolute pressure mercury gage Reacllngs of 2 mm H g for Carthage, and 6 to 7 mni H g for Indlana and A u s t ~ n were found to be acceptable.

P r ~ o r to commencing sa tura t~on of the limestone blocks, the equ~pment was carefully checked for proper

leak-free connections. Through simulated runs, specific procedural instructions were worked out and hsted t o avold errors occurrii~g during actual processlng. Thls piocedure is described in Table 11.

Table 11 Procedure for Calibrating

L~mestone-block Saturation System (Refer to Fig 4 for System Layout)

h1(1,rk A' was aibltrarily etched on the s ~ g h t glass of the volume tank near its top and the tank then filled with water to t h ~ s level, also f i l l~ng the lower line to valve or illark A.

Illark B was a r h ~ t r a r l l y etched on the s ~ g h t glass extending above the sa tura t~on chamber. IJc~lve A was opened allowlng water to flow fioni the volume tank to the satuiatlon chamber reachlng Mr~rk B. (No blocks In chamber )

dlark.B' was etched on the volume chalnbei's glass a t the level to which water had fallen. (The volume of the tank between Mc~rks A' and B' represents the total volume of the saturatlon chamber from f i lwks A to B.)

Using the we~ghing scale (installed on the supporting scaffold), and w ~ t h Value A closed, exactly 60 cu f t of watei weie welghed and added to the volume tank and Illcwk Bm etched on the glass a t t h ~ s level. The volume of the cliambe~ between I lIc~~ks B' and Bq~t is 60 cu f t whlch represents the nommal bulk volulne of a group of three blocks. The volume between Il.lo+ks A' and Bnz ~epresen ts the volume of the s a t u r a t ~ o n chamber from Illark A to B minus tlie nonilnal bulk volume of three blocks

Wlth a group of dehydrated hlocks in the saturatlon chamber under full vacuum, the follow~ng s a t u r a t ~ o n process was begun.

1. Volume tank and line (to valve Illnrk A ) was filled wlth water to Illark A' on the s~gllt-glass.

2. llc~lve A was opened allowing water from the volume tank to be drawn ~ n t o the saturatlon chambef up to nlr~rk B, completely floodlng the blocks The water level In the volume tank Imme- diately clropped to Mark Bm, representing the volume of the vold space 111 the saturatlon chamber However, a s the blocks absorb watei the level dropped below dlark Bw1 and Vcthre -4 was a d ~ u s t e d to mali i ta~n the watei level 111 the saturation chamber a t I\lark B. Perlod~c adjustment of Vc~ltre 4 was necessary to ma~nta i i i the level a t Ilrlr~rk B untd a b s o ~ p t l o l ~ ceased (38 hours for Carthage blocks).

3. Upon l e a c h ~ n g statlc equlllbr~um, and w ~ t h the water level a t jlln+li B, ( w ~ t h Valve A closed), the watel level In the volunie tank was a t some point below Mark BWL.

4. A volume of water was carefully welghed ~ n t o the volume tank to brlng the level hack up to filnrk B?n. After necessary temperature correct~ons, t h ~ s volume of water represented the quantlty absorhed by the group of three blocks for complete saturatlon.

Tab

le

12

Neu

tron

Pi

t - B

lock

P

roce

ssin

g D

ata

Blo

ck N

um

bel

sa

I16

,17

,18

1

13

,14

,15

1 1

0,1

1,1

2

17

,8,9

1 2

,4,6

1

,2,3

Car

thag

e M

arbl

e A

ust

ln

1;;;:; 1 In

d~

an

a

Ty

pe

L~

mes

ton

e L

inie

ston

e L

imes

tone

Sta

rted

Pro

cess

, D

ate

4-18

-59

5-17

-59

Ind

lan

a L

lmes

tone

5-23

-59

Car

thag

e M

arb

le

5-29

-59

6-6-

59

10

2.1

0

99 0

Pla

ced

~n

Pit

, D

ate

5-7-

58

5-17

-59

Ev

acu

atio

n T

lme,

Hr :

Mln

1

21

.30

Ref

rlg

erat

~o

n Tlm

e, H

r :M

in

154.

30

174.

10

Wat

er R

emov

ed,

Cc

153,

995

Com

pute

d S

atu

rati

on

Bef

ore

Pro

cess

ing,

Per

cen

t 3

3 0

Tlm

e H

eate

rs O

n, H

r : M

ln

1 176:

50

12

1 :3

0

Satu

rati

on

Tim

e. H

r : M

ln

133:

45

139.

30

Sa

tura

t~o

n Tem

per

atu

re o

f V

olum

e T

ank

, Deg

F., S

tart

Fin

~sh

174

178

1-18

2 17

6178

18

0185

17

7186

To

tal

Wat

er f

rom

Pu

mp

an

d S

ph

ere

du

rln

g S

atu

rati

on

, C

c 12

50

6,70

2

IWe~

eht.

Gra

ms =

Vol

ume.

Cc

/ 445

.430

13

4.19

5 -

,

To

tal

Wat

er W

e~

gh

ed

In IW

ater

Tem

per

atu

re,

Deg

k.

I '

1 72

1 79'

Bul

k V

olum

e W

eigh

t, G

ram

s 32

,263

W

eig

ht

In

Tem

per

atu

re,

Deg

F.,

Tan

kJW

ater

6

6/6

5

Geo

met

r~c B

ulk

Vol

ume,

Cu

Ft.

6

1 1

37

61

.394

Po

rosi

tv I

ndex

. P

erce

nt

125

55

1 26

37

Dev

iati

on

Mea

sure

d f

rom

Geo

met

ric

Bul

k V

olum

e, C

c 10

10

I +5

00

10

1 +

6,80

0h

aBlo

cks

1,

3. 6

. 4,

7

9,

an

d 1

0 a

re h

arlr

ne-

crac

ked

b

Sh

ou

ld h

av

e b

een

7%

h

ou

rs l

ess

du

e to

pu

llin

g I

n w

ate

r v

ap

or

ov

ern

igh

t cI

lsed

dw

Ice

an

d a

ceto

ne.

~

ce

-cre

am

salt

d

va

lie

sh

ou

ld b

e 8.

552

du

e t

o o

ull

lne

In

wa

ter

vao

or

ov

ern

~c

ht

ev

%&

sh

ould

be

~~

0d

ue

hp

;l

l~

~~

-1

~~

ka

te

~~

va

p~

~~

o~

em

~~

6t

~

fEx

ce

ss b

ulk

v

olu

me

Wa

ter

wer

uh

ed

In a

fte

r sa

tura

tio

n d

~d

n

ot

Incl

ud

e th

e v

olu

me

abo

ve

the

60

cu f

t m

ark

on

vo

lum

e ta

nk

. o

r 1 3

26 c

u f

t E

Ex

cess

bu

lk v

olu

me

Wa

ter

wel

gh

ed I

n a

fte

r satu

rati

on

d~

d

no

t ~

nc

lud

e the

v

olu

me

abo

ve

the

60

cu

f

t m

ark

o

n

vo

lum

e ta

nk

, o

r 1

25

2

cu

ft

hD

ue

to 1

.200

cc

celo

tex

vo

lum

e

111 establishing the bulk volume illark Bm, a nominal value of 60 cu f t was utilized, whereas actual geomet- rlcally computed bulk volumes of the groups of three blocks varied from 61.1 to 61.4 cu f t . Consequently, the level of Ilfark BWL was adjusted for each group to account for thls difference. To check the accuracy of the geometrically calculated bulk volume of the blocks, the following procedure was used.

1. With the blocks completely saturated, the saturation chamber was drained back to Valve A, and volume tank filled to Mark A'.

2. Valve A was then opened, b r l n g ~ n g the water level to Illark B with the aid of a slight vacuum, and then closed.

3 The water level in the volume tank should then have retuined to Il.lmrk B ~ I . . Any deviation represented the difference between ge~metr ical ly calculated and expe~iineiltally detern~med bulk volume. Deviations were found to be qulte small, thus confirming the accuracy of the computed bulk volumes.

Flnal operation involved transferring the saturated blocks from the chamber into t h e n ploper positlon in the neutron p ~ t . Thls was accoinplished through use of a specially constiuctecl sling des~gned to clamp tightly around the octagonal periphery of the blocks. Processing and pit-stacking order fo r the blocks was prearranged (using porosity values from core ana lys~s) , to provlde maxlmunl unlfo~nll ty of poros~ty near the mlddle of each zone.

Principal data accumulated durlng the processlng of the 6 groups of blocks a r e given in Table 12. Porosity- index values for each group of blocks were computed from the measured data a s the ratio of absorbehzuater volumelblock bz~lk volzsnae. Of particular ~ n t e r e s t is the very close agreement of these values between groups of s l m ~ l a r stone types. These values also compare favorably with those obtalnecl from core ana lys~s of the small samples, fur ther confirming the adequacy of the selected processlng technique ~n fulfill~ng the com- mlttee's objectives.

VI. DETERMINATION O F FINAL STANDARDIZATION VALUES

I11 considering the role of the two plts a s industry standards, i t 1s ~ m p o r t a n t to understand their dual nature in this capacity. I n one sense the plts a re "arbitrary" standards established by A P I assignment of arbitrarily selected numbers of A P I units to the response of logging instruments exposed to certain zones in the pits. As "arbltrary" standards, the pits completely satisfy the b a s ~ c objective sought In p r o v ~ d ~ n g means of standardizing all log responses 111 common units related to a standard environment In another sense, the plts may be consiclered "absolute" standards, provid- Ing certain basic and useful properties quantitatively established 111 absolute u ~ u t s Snch knowledge furthe1 enhances the value of the plts to the industry by accurately interrelating these properties to A P I u n ~ t s and hence to log response.

The many measurements made durlng construct~on of the plts provide for deterininat~on of certaln of these properties 111 absolute units. F o r the gamma-ray pit, concentration of each of the three radioactive components (U, Th, and li) may be established, a s may be porosity-index* values fo r each of the three zones In the neutron p ~ t A discussion of the cleteimlnation of these absolute properties from the measured da ta follows. The Gamma-rag Plt

I n the determination of the U, Th, and K concentrations in the high-activity zone of the gamma-ray pit, two sets of measured data were available-one from the small samples taken during pourlng of the zone which were laboratory-analyzed by gamma-ray spectroscopy, and the second from measurement of quantities of these matenals actually introduced into the mlx. The latter measurements have been presented 111 Table 9 from which i t is seen that d e s ~ g n values a r e closely met fo r each of the three elements. The results of the sample

*Poroslty ~ndex IS defined as the product of fractional water saturatlon tlmes poroslty

Table 13 Spectral Analysis of Gamma-ray Pit Samples as Performed by Three Laboratories

7 Depths, Ft

Uranium, Ppm Potassium, Percent

A

Thorium, Ppm

TOP 1 2 3 4 5 6 7 7%

Bottom

a M 2 $ -4

A

---- % I <

5 9 7.5 4.4

12.6 11.8 13.6 14.7 12 9

9.5 6 0

4.27 4.31 3 97 4 06 4 21 4 06 3 7 0 3 99 4 4 6 4.47

B - C B A

4.5 3 6

11 1 12 2 13.2 9 5

15 1 3 6 3 9

25.7 24 7 24.2 23 6 23.6 21 9 21.4 25.1 2 4 5 28 0

C B

4.24 3 97 4.27 4.28 3 91 3.92 3.57 3 99 4.41 4.51

-- C

25 2 24 0 23 8 21.4 20 7 22.3 25.8 23.4 27 6

5 18 6 34 4.28

11 83 11 43 12.63 11.02 13 28

6 2 6 4.56

Q)

bL 2 : 4

4 00 4.00 3 90 4 00 3 90 4.10 3.70 4.00 4 20

22.4 24.5 22 1 21 3 24.0 21.4 19.0 22.1 21.6 26 6

29.0 24 3 26 6 25 7 25.4 23 7 22.9 27 4 28.4 29.8

4 2 6 4 09 4.08 4.08 4.04 3 96 3 7 0 3.89 4.29 4.39

4 4 6 7.03 4.83

11.80 10 30 11 10 8 8 6

11.85 5.67 3 79


I

Fig. 5-U, Th, and K Content of Samples Taken from the Gamma-ray Pit as Determined by

Gamma-ray Spectroscopy

analysis p e i f o ~ m e d by three oil-company laboratories a r e shown In Table 13. These measurements a l e also plotted ln Flg. 5 Comparison of these measurements a t 1-ft Intervals with antlclpated average measurements fo r the whole zone (froin Table 9 ) indicates close agreement f o r I( and Th with u n ~ f o r n ~ concentratlon of these elements throughout the depth of the zone. This IS not t rue for the U component whlch, from the sample analys~s, IS found to be substantially lower than des~red In the upper 2 f t and lower 1 f t . This is not well- understood, but is belleved to be partially caused by a mechanical fallure of the cement-mls~ng equlpnlent which allowed an undetermined amount of escess water to be added durlng the pourlng of the upper few feet Some of the Ra was probably washed from the sand and removed wlth the excess water. Fortunately, a rela- t ~ v e l y umform ac t lv~ty for each element is indicated In the center region of the sectlon, and was subsequently verlfied by "loggmg" checks. Thls region (I e., the 4-ft

I I I I I I I I I I I 4 5 6 7 8 9 10 1.1 12 13

PHOTONS/SEC-GM OF M I X ( W I T H ENERGY >IOOKEV

TOP

I

w 2 - Z

2 LL 3 - 0 a 0 + 4 - I 0

5 . I +

6 .

7 .

8

Fig. 6-Radioactivity of Gamma-ray Pit (Calculated Using Specific Activities Given in Table 6)

-

-

level) was therefore selected a s the officlal calibration polnt fo r the zone. Shown a s F ig 6 IS a computed curve of total radloactivlty obtalned by weighting the U, Th, and K concentrat~ons according to t h e ~ r speclfic activity a s determined froin Table 6 . Thls approslmates the ielatlve response expected to be obtained froin a n average logglng instrument.

After careful consideration of these data, the Sub- committee felt justified in assigning the offic~al callbratlon point 111 the zone the following absolute values of concentratlon of the three elements:

U - 13 ppm; Th - 24 ppm; K- 4 percent. To confirm the adequacy of the zone In fulfilling the

baslc objective, three servlce compames logged the well measuring radlatlon levels with their respective u n ~ t s a s piesently used The results a r e the following:

Average Low-activity Hlgh-actlvlty Shale from

Company Zone Zone Field Logs

The activity in the cal lbrat~on zone IS thus approximately twice tha t designated by the servlce companies to be obtained in a n average shale.

Slnce, by A P I definition, the response of a logging instrument to the h~gh-ac t lv~ty zone mlnus the response In the low-actlvlty zone, is assigned the value of 200 API Gamma-Ray Units, ~t may be expected tha t field logs w ~ l l show shales, on a n average, to be about one-half thls ~ntensl ty , o r 100 A P I Gamma-Ray Units

Thus the gamma-ray pit, by vlrtue of determination of its baslc radiation properties and assignment of stand- a rd unlts to the response of instruments to these properties, truly just~fies its classification a s a n "absolute" a s well a s "arbitrary" Industry standard.

The Neutron P i t In determination of porosity-index values to be

assigned to the three zones In the neutron pit, two sets of measurements were available to the Subcom- mittee - one from data acqulred during saturat!on of the large blocks and the other from core analysis of the small samples removed from the comers and centers of the large blocks It had been previously agreed by the Subcoinmlttee that, In case of discrepancy between the two measurements, the large-block data should be considered more applicable. The former have been presented 111 Table 12.

For analysis of the small samples submitted to the varlous 011-company laboratories, each was requested to:

1 Measure poroslty of each of the 90 samples ut i l iz~ng their best technique.

2. Measure poroslty Index, using t a p water, on a t least 40 percent of the samples.

These measurements a re glven In Table 14. A description of the analysls techniclue used by each laboratory IS

inclitdcd In Appendlx B, p 317.

- ---

* + 22 m

0 A

m

0 * N N N

a m w N 0.1

0 N * 01 0.1

0 rl 5 4 NO.l

C U

0 w s2 0 N m

* (3 m m 0.1

0 * w m m 0.1

0 0 0 3 -- 0 t : - 0 N C \ I

t- 0 C- a N7-4 - 4

A 2 0.1

t- 0 . 1 0 - m w o q h l N A

m O N w N

A O C - m w C - 0 . 1 N 4 ,

r l N 0.1 m

0 C- :: -

0 C: 0.1

0 Q, m m

0 rl X

E Z W

. " .

03

w N

N m * 4 0 . 1

01

0.1c

N c , \

* w 4

CO

m N E

a

C-

m

2

m

& 2

-7 4 u)

?<

I I

I m V 'R C(

I I a

0

c CO s E $ 4

T V

4 U

9 Q

Car

thag

e M

arbl

e (c

onti

nu

ed)

T

Lay

er

1 A

vera

ges

Blo

ck

9 A

verc

~g

es P

oro

s~ty

= 1

8.85

- P

.I. =

18.

55

Pos

ltlo

n

Blo

ck

5

2

Por

osit

y P

.I.

LI

RL

IR

1

Por

osit

y P

.I.

LR

lL

IR

1.

67

1.29

1

70

1

50

1 1

54

Blo

ck

9 T 1;

Ave

rage

s

3

Por

oslt

y P

.I.

LR

lL

lR

1.

71 1 1.90

1 8

4

5 E

m

P

>

2

o z '4 * 2 E 2 z z 5 P

m * z t'

0

R

cn

Blo

ck 5

Av

e~

ag

es

Po

rosz

ty =

1.7

74 - P

.Z. =

1 8

49

18

1

18.7

18

.5

18

5 1

18

45

18.9

"5

17

18.5

1

18.1

1

8 9

1

8 1

5 1

18

4

18

39

18.8

::!71

88

19.0

7

18.4

3

211,187

19

21

4

Por

oslt

y P

.I.

LI

Rl

LR

1.68

2.

54

15

0 1 1.69 18

5

Blo

ck

6 T

Lay

er

1 g A

vera

ges

18.6

5 1

9 0

,

18.7

8

5

Por

oslt

y

LI

R

1.80

1 4

85

1.

50

16

0

18.6

5

:::I

188

18

41

1.60

1

84

5

1.70

1 162

1.69

Blo

ck 6

Ave

rag

es:

Por

osat

y =

2.0

13 - P

.Z. =

1.7

12

6 C

ar

thg

e Marble

Blo

cks,

Ave

rag

es:

Po

rosi

ty =

1.88

4 - P.

Z. =

1.8

29

Indi

ana

Lim

esto

ne

2 3

6

1 50

1

25

1 5 20 2.58

1.61

1 ?1: 1.66

18

0

18.4

2

17

91

18

.7

18

26

1.67

1

20

1.83

1 1415

15

3

1 5

0 1

90

1 176 1 7

2

15

0

17

5

1.56

1.

45

1 1.

57

1 5

2 2

10

1

63

2 0

9 1

1 8

4

17

0

16

2 1 170

1 6

7 19.9

1

8 1

1 18.2

18.6

18.7

0

Blo

ck

7 T

Lay

er

1 A

vera

ges

18

2

::::

I94

19.1

5

1.40

1 178 1.

74

1.40

3.

08 1 1.66

2.05

18

9

19

0 lo

0

19.3

0

19

8

20

0 1 18 6

18

8

18

85

19.2

1

Blo

ck

7 A

vera

ges

P

oros

ztg

= 1

9.1-

7 --

P.Z

. =

18.

75

19.0

1

8 1

5

"'1

89

18

81

1.73

5 2

21

1

70

1 1.83 18

7

19.0

1

19

3 19

37

1.72

2

01

2

50

I 2

08

2.30

1

96

5

19

0 1 2.06

19

6

" :'i 1 184

18

80

19

42

19." 1

8 2

1

9 2

19

.0

18

96

Blo

ck

8 T

Lay

er

1 18

.1

19.3

1 27 0

18.4

5

20

5

23

0

2 50

1

2.28

::: 1

17

5 1 18 3 1

8 8

18.2

0

19

2

19.4

:i:i 1

19

93

::.: 1 23.6

1 6

9 1

81

1

60

1 2 74 1

96

18.1

1

9 3

1

9.4

3 18

.9

18

93

18

2

19

1

18

55

1 18.4 18

.56

1 7

1 1 8

0 1

90

1 180

1 8

0

19

1

19.9

1 18 75

19

2 19

24

18.9

1

8 6

, 1 18

56

18

9

18.6

1

8 2

1 17 3

18.9

2

1 4 19

6

17

17

18

9 bg 5

18

.8

17.5

5

18.4

5 1

8 9

19

.4

1 19 5 1

9 1

5

19

2

1 19

0

Tabl

e 14

(co

nti

nu

ed)

1 E In

dian

a L

imes

to~l

e (co

nti

nu

ed)

Pos

itio

n

I 1

2

I I

4 I

5 I

Blo

ck 1

0

- L

ayer

F Al

v1

0

2

> 5 ' 2

Blo

ck 1

2 A

ve

rag

es.

Po

ros~

ty = 1

9.45

- P

.I. =

19.

0.5

6 In

dia

na

hm

est

on

e B

lock

s, A

verc

cges

: P

oro

stty

= 1

9.23

-P

.Z.

= 1

8.78

Au

st~

n Lim

esto

ne

Po

ros~

ty

T

M

B

E' g 2 "d S 5 n

X 2 cj

5 E a j

cj

M

19

6

19.8

Ave

rage

s

Blo

ck 1

0 A

ve

rag

es.

Po

rosz

ty =

19.

31 - P

.Z. =

18.

98

Blo

ck 1

3 T 1 b:

Ave

rage

s

19

2

18.4

5 20

2

P.I

.

23

8

24.3

28.9

1 .

27.4

9

19.4

4

18

1

18.3

:ai8

.41;

;::5

18

.04

'

24 1

21

.7

26.7

1 31.2, 25

93

Blo

ck 1

3 A

vera

ges

. P

oro

stty

= 2

6.49

- P

.I. =

25.

81

19.1

1

8 9

5

19.0

26 7

2

5 0

24

.1 1 30.9

26.6

8

18

79

Por

osit

y

18.7

0

Blo

ck 1

1 T 1;

Ave

rage

s

Blo

ck 1

1 A

wer

ag

es.

Po

rosz

ty =

18.

93 - P

.Z. =

18.

57

24 9

2

3 8

30

.1

1 29 6 27

.10

Blo

ck 1

4 T

ye

1 :

Ave

rage

s

19

8

P.I

.

17.1

19

.1

17

15

:8:18.1~~-,,),,,,,,,!,,,191"'E

18

24

18.3

.

18.8

5

Blo

ck 1

4 A

vera

ges

. P

oro

s~ty

= 8

8.82

- P

.Z. =

28.

73

27 8

26

2

1 27 3 34

.8

29.0

3

18

0

20.0

18

.0

Por

oslt

y

19

2

19.8

20

0

19

67

19.6

7

19

4

19

57

19

2

1 19

.39

Blo

ck 1

2 T

Lay

er

( 1: A

vera

ges

28

2

24 9

1 I::

26.2

0

27 6

24

.0 1 24.3

25

26

2: 1 27.3 29 0

28.9

0

19.2

7

17

8

19

7

19

52

20.0

P.I

. 18

9

17.8

17

.3

18.2

8

26 G

27.3

1 ::if 2

6 3

8

27.6

24

0

23 9

22

.1 1

24.4

0

26

5

27.4

24.6

1 27'":::

26

58

26.9

1 29

3

28

8

28

75

Por

osit

y

18.5

0

18.5

1

9 2

ppppp-

LR

LR

LR

LR

LR

LR

LR

LR

LR

LR

--------------------

18

6

19

9

18.8

7

18

3

18.8

7

20 5

19.1

1

19.3

6

27

2 1 26'0

25 2

5

25.6

2

5 9

30 0

L

1

29.4

27 7

3

Por

osit

y P

I.

20 6

17

.75

19.3

1

19.5

19.1

28

4

25.9

1 27 33

.2

28

83

19

6

19

2

20 3

1

82

7

19

9

18.5

19.2

8

18

8

19.1

8

19

ti

19.5

19

1 18.7

19.2

3

28

4

26.8

1 31 2 29

40

.

P.I

.

19.6

18

8

19

1

18.6

19.1

9

19

1

18

25

19.1

6

28.3

" 27.7 30 8

32

0

29.7

0

26.8

28

1 29 2-

28.1

3

19.5

19.4

5

19.3

0 19

.07

18.7

29.2

26

.6

27 4

1 29 1

28.0

8

19.2

2

19.9

1

7 3

5 1 18.1

18.4

5

19

0

20.1

21

.3

18

25

1 1

9 6

6

l8.6

19

.8

19'"

19

1

19.3

3

27.9

27

.2

1 32.3

29.1

3

18

7

18

3

l9

"9.1

18.8

5

19

85

:::: 1 19.1

19.2

9

19.4

19

.4

19

2

19.1

(

19.2

8

20.8

1

9 2

1

9 4

1 19.1

19.6

3

CARTRACE W R l L E AUSTIN L I U L S m N E

5

POROSITY - PERCENT P W O S 1 T I - PERCENT POROSITY-PERCENT

Fig. 7-Porosity Distributions from Analyses of Small Samples Taken from the Corners and

Centers of Large Blocks

I t IS interesting to note the geneially good agreement of measured poroslty values a s submitted by the d~fferent laboratories using varlous techniques, and the very unlform poroslty character~stics of the I n d ~ a n a limestone w h ~ c h was chosen a s the offic~al cahbratlon environment Close agreement between measured poros- i t y a n d p o r o s ~ t y - i n d e x va lues indicates n e a r l y 100-percent water s a t u r a t ~ o n of the small samples I n F ig 7, these porosity data a r e plotted In graphical form vs. the number of samples fo r determinat~on of mean poroslty values fo r each stone type.

For easy comparison of porosity-index values determined from the small samples and the large blocks, Table 16 IS presented Blocks a r e listed in Col. 1, 111

the order 111 which they a r e stacked in the pit. Of particular importance to the Subcomm~ttee In assign-

ment of poros~ty-index values to the zones were the values determined from s a t u r a t ~ o n measurements of the groups of large blocks, a s tabulated following.

Poroslty Index, Percent

Group 1 Gronn 2 Carthage marble 1.829 1 881 Indiana l~mestone 19.10 19 07 Austin limestone 26 37 25 55

The very small d~fferences between measurements of the Carthage and I n d ~ a n a blocks, respect~vely, confirms the adequacy of the s a t u r a t ~ o n techmque used, and fur ther Indicates good unlfoimity of porosity-~ndex values between individual blocks of a type The sllghtly h ~ g h e r d~fference between measurements of the A u s t ~ n blocks 1s believed caused 1)y var~a t ions In porosity character- l s t ~ c s of the ~ndividual blocks. T h ~ s is substantlatea by "sample" measurements of the ~ndlvldual blocks in which values of porosity lndes vary from 2.5.08 to 28.73.

From these measurements and with cons~deration to their llmits of accuracy, the three zones mere ass~gned the following values of poroslty Index by the Subcommittee:

Carthage marble - 1.9 percent Indlana limestone - 19 percent = 1,000 API

Neutron Units Austin limestone - 26 percent

I t IS ~n te res t ing to compare these carefully determined values fo r the three types of stones with values currently assigned similar type stones 111 servlce-company test plts For ~nstance, Carthage marble, here determined to be 1.9 pelcent has carried values from 1 percent to 3.5 percent in such privately owned plts, wlth s i m ~ l a r differences for the other stone types Now, through use of logglng ~ns t ruments callhrated in the A P I pit, ~t wlll he possible f o r servlce compames to more accurately establish porosity-nldex values f o r slmllar zones in their own pits. This w ~ l l serve the whole industry In providing means f o r clerlv~ng more accurate charts relat!ng log response to format1011 poroslty index o r porosity.

VII. OPERATION O F T H E FACILITY

Operating Agreement

Operation of the cal lhrat~on f a c ~ l ~ t y was begun Imme- dlately following formal opening on June 24, 1950, under contractual agreement mlth the University of Houston Under tlils agreement, the Unlvers~ ty provides personnel and servlces as necessary to the operation of the faclhty. This Includes techn~cal consultation and d~rectlon, a quahfied observer to witness and certlfy calihrat~ons, maintenance to assure full serviceability of f a c ~ l ~ t g , and secretarlal and account~ng servlces

I n ~ t i a l fees for use of the c a l ~ b r a t ~ o n f a c i l ~ t y were set at $15 per hour o r major por t~on thereof, and $7.50 per minor port1011 The fee is u n ~ f o r ~ n f o r all callbra- t ~ o n s , but is subject to period~c adlustment by agreement between the Unive~s l ty and Institute.

Subject to prior approval 1)y the A P I Suhcomm~ttee, the University is autl~orlzed to p e r m ~ t usage of the f a c i l ~ t y fo r purposes other than those whlch a r e pursuant to APZ RP 35, and to charge such fees a s a r e agreed upon by the Unlverslty and the Subcommittee. The University is fur ther authorized to use the f a c l l ~ t y for purposes of ~nstruction, experimentation, o r research. All such usage IS subord~nate to requirements of ind~viduals o r companies desiring service pursuant to APZ R P 33.

Llmitatlons a re placed on usage of the callbration prts to prevent any posslble damage or dev~atlon of the standard calibration No mod~ficat~on of the plts wlll be allowed tha t IS not completely and ~mmedlately

, removable The University IS guaranteed a mlnlnium compensa-

tlon for opera t~ng the cal~hratlon facllltp durlng the first year of opera t~on The guarantee is $6,780. The API oligmally made the guarant.ee; ~t was subsequentlv underwritten by the logg~ng companies usmg the call- h r a t ~ o n facl l~ty.

An API Subcommittee of five members and a repre- sen ta t~ve from the Univers~ty of Houston was estah- llshed to admlnlster the operation of the f a c ~ l ~ t y The contractual agreement w ~ t h the Un~vers l ty IS suhlect to revlew semi-annually or a t any t ~ m e on request of , either party. P n o r t o each revlew, the Un~veis l ty IS

required t o submit a financ~al report to serve a s a hasls

1 for revlslon of cahbra t~on fees and alteratioll of such

APl CALIBRATION FACILITY FOR NUCLEAR LOGS 313

Table 15

Final Tabulated Data - Neutron Pit C M = Carthage Marble; IL Indiana Limestone, AL = Austin Limestone

I Block No., Type, Core A;al y sis,,

and Relat~ve Numerical Average Posl t~on in fo r Each Block,

Neutron Plt Results of 5 Surface Down Laboratories

P.I., Percent Percent

Core Analysis, Numerlcal Average for

Each 3 Blocks a s Saturated, Results of 5 Laboratories

Final Saturation Results of Large Pl t

Blocks

Graphical Mean, Each 6-block Group,

Results of 5 Laboratories Plotted on D l s t r ~ b u t ~ o n

Graph

P.I., Percent Percent

P.I., Percent Percent Percent

Average All 6 Bloclts

Average All 6 Blocks 1 I 1 25 96 1 26 196

NOTE P I = poros~ty Index defined as the product of the water saturation and the poros~ty. & = poroelty

Average Graln Density. Carthage Marble (CM) = 2 694 g m per cc: Indlana Llrnestone (IL) = 2 688 g m per cc; Austrn Llrnestone (AL) = 2 707 g m per cc

other telms, cond~tions, and procedures a s heco111e necessary Procedure for Logging Calibration Pi ts

The purpose of the A P I c a l ~ b r a t ~ o n 1s to assign to a logging company's field calibrator a specific number of A P I units a s determined by the relative responses of the company's logg~ng tool in tha t cahbrator and In the c a l ~ b r a t ~ o n plts

Before lowering the loggmg tool in either c a l ~ b r a t ~ o n p ~ t , the tool should be cahbrated using the logglng company's field cahhrator. The cahbra t~on w ~ t h the field callbrator fo r a gamma-ray tool includes a n i n ~ t i a l nlstrument zero, gamma-ray background 111 alr, gamma- ray callbrator reading, gamma-ray background in a i r

af ter cahbrator reading, and a final instrument zero. After calibration on the surface, two decentralized runs of the gamma-ray logging tool- a r e made 111 the gamma- ray callbration pit from the bottom to the top of the plt. Each r u n ~ncludes a continuous recording of the instrument zero, log deflect~on for the low-rad~oact iv~ty concrete, the radioactive concrete, the upper section of low-rad~oactivity concrete, and a final instrument zero. The logging tool 1s stopped for a readlng oppos~te the center of the radioactive-concrete section If deslred, the reading made while the tool is stopped a t the center of the radioactive-concrete sect~on may be made a f te r the continuous and repeat runs, thus al low~ng the two logs of the plt to be continuous Fol low~ng the logging runs

314 BELKNAP, DEWAN, KIRKPATRICK,

Fig. 8-Typical API Gamma-ray Log Calibration

MOTT, PEARSON, A N D RABSON

in the plt, the gamma-ray tool response with the field callbrator is again recorded. Fig. 8 shows a portion of a typical gamma-ray log callbration.

A P I callbratlon of neutron logging tools includes a recording of tool response using the logging company's field calibrator before and af ter logging. This consists of initla1 ~ n s t r u m e n t zero, neutron background 111 a i r , neutron calibrator deflection, and final ~ns t rument zero. Some calibrators have been designed for two callbration readings. I n this case, both callbration readlngs may be recorded instead of the one. Two runs a r e made In the neutron log calibration pit with the logging tool decentralized. Both runs a r e made from the bottom to top of the plt and ~nclude a recording of lnstrunlent zero, the three llmestone zones, the water shield, and a final instrument zero. The logglng tool is stopped opposlte the center of the Indiana llmestone for the calibration reading. Thls r e a d ~ n g may be made a f te r the logging runs, thus allowing the callbration runs to be contunuous Fig. 9 shows a portion of a typlcal neutron log calibratlon.

Llmitatlons a r e placed on the speed for running tools in o r out of the neutron pit. It 1s preferred t h a t the crane be used Instead of loggng-truck wlnches. A maximum speed of 16 f t per min is specified No logging tool o r other device is allowed in either the gamma-ray or neutron callbration p ~ t w h ~ c h would impair their usefulness a s a prnnary cal ibrat~on standard.

Relating of Field Calibrations t o API Units

The foregoing procedure calibrates both gamma-ray and neutron field callbrators in A P I unlts fo r speclfic tool types Experience Indicates t h a t gamma-ray cali- bratois in present use may readily be converted to A P I Gamma-Ray Unlts. Conversion f o r neutron callbrators is also straight-forward Once the values In A P I units for the respective gamma-ray and neutron field call-

, brators have been determined, each callbrator can be used to calibrate the logging tools directly In A P I Gamma-Ray o r Neutron Units. I n the changeover period from existing log units of measure to A P I units, some l o g g ~ n g companies may use conversion factors to convert their old units to A P I units. However, most companies have indicated they will swltch directly to calibratlon In A P I units, because they anticipate fewer errors in callbration by doing so

No serious problems should be encountered if ~t IS

desired to convert a n old log to A P I unlts, provided the old log was reliably cal~brated to some other u n ~ t s and the equivalent A P I units a re known.

Maintenance of Calibration Records

R P 33 provides t h a t the Univers~ ty of Houston shall keep records of all callbrations on a prescr~bed foinm In order for a calibration to be considered official, the form must be complete and the calibration log attached. Logglng companies a r e privileged to withhold record- ings w h ~ c h they d e s ~ r e to designate a s unofficial and confidential.

The University is required to maintaln a file of all logs designated a s official calibrations. This file is not

Fig. 9-Typical API Neutron Log Calibration

open for ~nspection. Requests for callbration data should be made d~rect ly to the logglng company involved, not to the Unlverslty. The University wlll verify calibration logs, but will not f u r n ~ s h coples of logs

Firs t Six Months of Operation

Durlng the first six months of operation of the callbratlon facility - from July 1, 1959 to January 1, 1960 - 10 logg~ng coml~anies completed and filed official A P I callbrations of nuclear logglng tools a t the University. It is anticipated these companies wlll convert thew nuclear logs to A P I standards by mld-1960.

The financlng of operat1011 of the f a c ~ l l t y has worked out a s est~mated by the Subcommittee. I t appears tha t the opera t~on wlll break even for the first year a t the rates estabhshed.

VIII. CONCLUDING REMARKS

The A P I Nuclear Calibrat~on Facihty was des~gned and b u ~ l t fo r one purpose: To prov~de the petroleum Industry wlth a means for obtalnlng standardized gamma-ray and neutron logs.

The first and one of the most Important benefits of standardization will be the d ~ r e c t comparison of logs run by various logglng companies. The engineer *or geologist (working on the well slte) can quantitatively compare the log on his well with the log on a n offset well. W ~ t h proper cons~dera t~on of well-bore cond~tions and tool response, he will be able to tell from the nuclear logs if and how formation cond~tions have changed between wells.

A standard u n ~ t IS essential before nuclear logging can t ruly progress from a qualitative to a quantitative a r t Its use should promote many s t u d ~ e s tha t w ~ l l Increase the.knowledge and quantitative use of nuclear logs. F o r example, only prehminary work has been done to determine porosity of a shaly forlnatlon from a combination of the gamma-ray and neutron curves. A standard uni t wlll provlde a b a s s for such studles Wlth standardized nuclear logs, research s t u d ~ e s of depositional changes in a f o ~ m a t l o n over a geoloac basin may be possible.

One by-product of standard~zation will be improved quality in nuclear logs. Because of the varlation in u n ~ t s , logs have been compared by appearance - a s a t ~ s - factory log was one' t h a t "looked" all r lght Wlth a standard uni t those logs tha t have not been properly cahbrated, scaled, etc wlll be r e a d ~ l y apparent and they will not be accepted by the well operator In addition, standardizat~on wlll p e r m ~ t quant i ta t~ve comparison of various types and models of gamma-ray and neutron Instruments.

Although the Cal ibrat~on Facllity was not primarily designed to provide departure curves, the neutron pit provldes some data for one response curve f o r each tool calibrated. This curve will provide the logging companies with a means of ver~fy ing the values assigned to their own test formations. F o r those companies t h a t do not have test pits, the Calibration Facihty will provide valuable response curves Also, wlth certain restric- tions, the pits may be used for research and development.

In summary, the A P I Nuclear Callbration Facjllty should lead to better nuclear logs and increased knowledge of nuclear l o g g ~ n g Most ~rnpol-tant, ~t w ~ l l provide the basis fo r t ruly quant i t a t~ve nuclear logs

At present the Callbration Fac~l i ty does not provide the range of conditions t h a t a r e needed to prepare a complete set of neutron response curves. For example, ~t u~oulcl be desirable to have pits: 1, saturated with salt water , 2 , with d~fferent hole sizes; 5, tha t could be cased; 4, tha t could be filled with mud of varying density, etc Many logging companies have already bulk such p ~ t s The A P I h a s no present plans t o hulld addl- t ~ o n a l p ~ t s , but they could be added to the Facility qulte econonucally Perhalis, through the cooperation of several companles, ~t w ~ l l be poss~ble to work out a n agreement whereby additional p ~ t s can be built. I t is not d~fficult to v~suallze a complete set of plts a t the Facillty a t some fu ture date. Complete response curves based on common test plts w ~ t h carefully determined properties would be valuable to all

I11 addttion, the Callbratlon Facility may be used 111

the future for calibration of other types of logs The present pits may be adequate for some aclclit~onal types of logs - In other cases, new test plts may be needed For example, the present neutron p ~ t appears appl~cable fo r evaluation of acoust~c veloc~ty logs and density logs The gamma-ray pit can conce~vably be used for a gamma-ray spectroscopy standard

Nuclear logging IS stiil a relatively new sclence. Many improvements in present logs and many new ,types of logs a r e anticipated. I t is hoped tha t the F a c i l ~ t y wlll a ~ d in the developme~lt of such logs.

The Callbratioil Facllity has already p1.ovecI its value to the University of Houston a s a t each~ng aid Student help was used wherever poss~ble In construction of the Facllity. Soine phases required research and wlll be the subject of theses prepared for graduate degrees. Student work on the Facihty during construct~on and operat~on has had a n addlttonal benefit. Several students have been provided the f inanc~al means for con tmu~ng t h e ~ r eclucat~on and for obtalnlng advanced degrees There a r e many ~)ossibilit~es for research using the F a c i l ~ t y

The Umverslty has the r ~ g h t to use the pits fo r t h ~ s purpose when they a r e not in use fo r calibrat~on. Use of the Fac~l i ty fo r Unlvers~ty research 1s expected t o be of value In the coming years.

ACKNOWLEDGMENT

So many have contrlbuted to the A P I Nucleai Log Callbration Facillty tha t any acknowledgment must be ~ncomplete. The men (and t h e ~ r 1-espect~ve companies) who served on the Subcommittee to Rev~ew APZ R P 33, the compantes which contr~buted funds to bulld the pits, plus many others who freely f u r n ~ s h e d expert a d v ~ c e on c e r t a ~ n phases of the project have all contributed ~mmeasurably.

Those organizat~ons which contrlbuted special services were Gulf 011 Corporation, Shell 011 Company, Humble 011 & Refining Company, The Atlantic Refin~ng Com- pany, Schlumberger Well Survey~ng Corporation, Pan Geo Atlas Corporation, Lane-Wells Company, and The Univers~ty of Houston

REFERENCES

'Matt, W. E and E d ~ g e r , N M: Nuclear Well Logg~ilg In Petroleum Exploration and Production, Proc Fif th World Petroleum Congress, X, 195 (1959).

ZBelknap, W. B: A P I Calibration and Standardizat~oii of Nuclear Logs, presentecl a t the sprlng meetings of the Mlcl-Continent and Rocky Mountain Districts, April 22-24, 1959, and May 6-8, 1959, respectively. P e t ~ o l e l ~ , ) ~ E n y z ~ t e w (adaptation, t ~ t l e . New A P I Factlity foi Standardization and Callbration of Nuclear Logs) 31 [I31 B-24, Dec. (1959).

3Strominger, D; Hollander, J. M ; and Seaborg, G. T Table of Isotopes, Rev. mod en^ PILYs., 30, 585 (1958)

Yhttman, Jay : Neutron Logglng, Proc. U n i v e ~ s t t y of Kunsus Petrolezcm Engzneering Conference, A p r ~ l 2-3, 1956.

APPENDIX A

L ~ s t e d follow~ng IS the membership of the API Nat~ona l Subcommittee on Rev~ew of R P 73 a t the t ~ m e of puhl~cation of the revls~on.

W. B Belknap, C ~ / L ~ / I ) I I * L ) I , Phllllps Petroleum Company

J. M Bird, Seismograph Serv~ce Col p. J T Dewan, Schlumberger Well Survey~ng Col p T. A. Johnson, Jr , Southern C a l ~ f o r n ~ a Gas Co A. S McKay, Texaco, Inc. W E Mott, Gulf Resealch & Development Co. A J Pearson, The A t l a n t ~ c Refining Co. A A Pereb~nossoff, Mob11 011 Company W R. Rabson, Pan Geo Atlas Corp G. T. N. Roberts, Shell 011 Company

H E. Schaller, McCullough Tool Co. J. A Stinson, Pan Amencan Petroleum Corp G11 Swift, Well Surveys, Inc Tei r y Walker, Welex, Inc. W. 0 Winsauer, Humble 011 & Refin~ng Co. A. B. Winters, Lane-Wells Company

The representation from spec~fic companles changed d u l ~ n g the progress of the Subcommittee's work because of chahges In indlvldual assignments w ~ t h ~ n those companles Recognition is also made of the contr~butions of the following persons to the work of the Subcommittee. C L Doyle, General Petroleum Corp ; Thomas G11- martin, P a n American Petroleum Corp.; Ben H Goocle, Lane-Wells Company; and R H Wlnn, Halliburton 011 Well Cen~entlng Company.


poios~ty percent fo r core plugs of the size used. .

COMPANY B The large triangular blocks suppl~ed were cut into

APPENDIX B LABORATORY CORE-ANALYSIS TECHNIQUES USED FOR CORE ANALYSIS OF LIMESTONE BLOCKS IN NELTTRON

PIT AS SUBMITTED BY COMPANY LABORATORIES

iectangular blocks, using a tap-water lubr~cated, dia- mond cutting wheel; the smaller, split cyhndrlcal samples were used 111 the condition furnlslied. P n o r to porosity measurement, all specimens were a~r -dr led a t 100-105 deg C

The best method of poroslty measurement conslsts of the use of whole core equipment fo r gram- o r solids- volume determlnatlon, followed by a water-d~splacement test to ascertain bulk volume A mod~ficatlon of the gas-expansion technique described by Coberly and Stevens was employed to measure g r a m volume. The mod~ficat~on conslsts p r ~ m a r ~ l y of the respectlve use of dry a l r and mercury manometers 111 lieu of hydrogen arid pressure gages. Following solids measurement, the samples were allowed to ~ m b ~ b e fully In t a p water and subsequently were immersed while suspended ~ n t o a counterbalanced, water-filled' vessel resting on a t o r s ~ o n balance. The volumetric d~splaceinent and bulk volume of the core was then calculated from the resulting welght changes The to rs~on balance used has a sensl- tlvlty of 0 1 gram.

For porosity-index pore-volume determinations, specimens were evacuated for approximately 6 hours and

COMPANY A

Core plugs (l-ln. long by %-in diameter) were drilled flom the limestone blocks, with t ap water used a s coolant The plugs were oven-drled for 24 hours a t 100 deg C. Aftei coollng, grain volumes weie determ~ned w1t.11 a pressure-type Boyles' Law a i r poroslmeter Fol- l o w ~ n g a d ry welglit, the plugs were evacuated a t less than 1 nlin mercury for 30 mln and then saturated with tap water The plugs were we~ghed w h ~ l e saturatecl wlth t ap water and agam while suspended in t a p water.

The bulk volume was obtained by subtracting the susl)eiided weight from the saturated weight and cllvld- Ing the difference by the tap-water dens~ty. The reported p3ioslty was calculated by subtractlng the grain volume f ~ o n i bulk volume and div~ding the diffe~ence by bulk v o l ~ ~ m e The volume of water In the coie plug was found by subtractlrig dry weight from saturated welght and t l ~ v ~ d l n g the difference by the density of t ap water The 1 atlo of this volume to bulk volume 1s the poros~ty index

A measuremint of porosity index is par t of our nnimal plocedure so tha t we will know how well the coles a re saturatecl. We l ~ a v e therefole llsted thls value f o ~ all of the core plugs ~ns tead of only those ~ n d ~ c a t e d on the list Because of experimental error, the porosity ~n t les 1s hlgher than the poros~ty in some cases. We belleve our pore-volume measurements a r e wlthin k0.02 cc of the t rue value. The maxlmum error f o r either of the reported values is therefore about 0.2 . "

the samples. 8 The poroslty nldex was calculated by dividlng the

pore volume by the bulk volume

submerged in de-aerated tap water a t atmospheric PleSsure fo r a t least 14 hours. The ~nduced welght changes also were measured on the torslon balance All allalysls of the Houston city t ap watel 1s not available, ho\vever, the specific gravity was 1 0026

COMPANY C

The porosity Index for each of the 90 samples was determined 111 the following manner

1. Cyllndrlcal core plugs, 1 in. In diameter and 2 to 4 In. In length, were clnllecl from the parent core samples using tap water a s the circulating fluid.

2. The core plugs were drled 111 a n oven a t 120 cleg C. fo r 14 l ~ o u r s This temperature was above the specified maslmum of 105 deg C. Therefore, the cores were saturated w ~ t h t a p water and then drled again a t 90 deg C.

3 The dried core plugs weie cooled to room teinpera- ture and the dry weight of each sample was measured

4. The core plugs were then saturated with t a p water by flushing de-aerated tap water mto the cores and cont inu~ng to maintain a vacuum for 12 hours before restoring atmospherlc pressure.

5 The we~gll t of each sample when water-saturated was then measured.

G The pore volume for each core plug was calculated by divid~ng the difference between the water- saturated and dry weights by the d e n s ~ t y of the tap water

7 The bulk volume for each core plug was calculated from measurements of the ~ h v s l c a l dlrnens~ons of

COMPANY D Three cyhndrlcal core samples were drllled from each

of the 90 large samples whlch were prov~ded The samples weie c111lled with t a p water and dried a t 100 deg C.

The core plugs representing those samples on which a porosity analysls with t a p water was requested were evacuated to 20 microns mercury pressure and saturated with de-aerated tap water. The grain volumes were determined by the buoyant-force method. A mercury- dlsplacement bulk-volume meter was used for the bulk- volume deternilnatlons.

These samples were again dned a t 100 (leg C. and included with the remaining core plugs fo r grain- volume determinations uslng toluene a s the s a t u r a t ~ n g fluid. The same buoyant-force method was used for these determinations.

UNIVERSITY O F HOUSTON

The University of Houston laboratory used the gas- expansion method uslng the RUSKA mercury pump for poroslty determ~natlons. Bulk-volun~e measurements were made by mercury dlsplacement. Checks by g r a m density were made period~cally.

API Calibration Facility for Nuclear...

Documents

Transcript of API Calibration Facility for Nuclear...