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R E S P O N S E O F N E U T R O N

AND

FORMATION DENSITY LOGS

I N

HYDROCARBON BEA RIN G FORMATIONS

by

R. GAYMARD and A. POUPO N

SPE Sch lumberger - P a r i s

E DIT OR S NOT E : In t h i s i n t e r e s t i n g p ap e r t h e au t h o r s

co n s i d e r t h e e f f ec t o f r e s i d u a l h y d ro ca rb o n s o n t h e r e -

s p o n s e of t h e Neu t ro n an d F orm a t io n D en s i t y

logs

I t

s h o u l d b e o f i n t e r e s t t o t h o s e wo rki n g w i t h t h e s e po ro s-

i t y t o o ls . P o s s i b l e a p p l i c a t i o n s a r e i n c l u d e d fo r bo t h

s h a l y fo rm a t i o n s o r co m p l ex l i t h o l o g i e s .

ABSTRACT

I t is u s u a l l y a s s u m e d t h a t , i n o i l b ea r i n g fo rm a ti o n s,

t h e Neu t ro n a n d F o rm a t i o n Den s i t y l o g s are n o t s i g n i f i -

c a n t l y a f f ec t e d b y t h e r e s i d u a l o i l i n t h e i n v ad ed zo n e

an d t h a t t h ey r e s p o n d as if t h e v o l u m e i n v es t i g a t ed was

en t i re ly f i l l ed wi th mud f i l t ra te . However when poros i t i es

a r e f a i r l y h i g h, t h e e f f ec t of t h i s r e s i d u a l o i l

is

n o t

a l way s n eg l i g i b l e , p a r t i c u l a r l y if t h e oil is l igh t .

C o m p u t a t io n s h a v e b e e n m a d e t o e v a l u a t e t h i s e f f e c t.

F o rm u l ae h av e b een d ev e l o p ed fo r c l e an oil o r g a s

bear in g fo rmat ions . Th es e fo rmulae permi t a more acc u-

ra t e eva lua t io n of the poros i ty . Some of the fo rmulae may

a l s o h a v e a p p l i c a t i o n i n t h e cases of sha ly fo rmat ions

an d of co m p l ex l i t h o l o g i e s .

EFFECT OF HYDROCARBONS

ON TH E NEU TRON LOG

In Neutron logging, a s o u r c e e m i t s f a s t n e u t ro n s i n t o

t h e fo rm a t io n . T h es e n eu t ro n s a r e s l o wed d o wn th rou g h

co l l i s ions , mos t ly wi th hydrogen nucle i of t h e s u r ro u n d -

i n g m ed i u m , an d a f t e r r e ach i n g t h e s o -ca l l ed t h e rm a l

l e v e l of en e rgy , a r e ab s o rb ed b y n u c l e i of the fo rmat ion ,

u s u a l l y h y d ro g en o r ch l o r i n e n u c l e i; e a ch cap t u re o f a

thermal neu t ron

is

fo l lowed by th e emiss ion o f gamma-

ray s of c ap tu re . A d e t ec t o r , a t some d i s t an ce f ro m t h e

n eu tro n s o u rce , m eas u re s e i t h e r t h e g am m a-rays of

cap tu re (Neu t ron-Gamma type o f too l ) o r the thermal

neu t ron s (Neu t ron-Thermal Neu t ron too l ) o r the neu-

t ro n s b efo re t h ey h av e r each e d t h e th e rm a l l ev

(Neu t ron-Ep i thermal Neu t ron too l) . F or

all

t o o l s , t

s l o wi n g d o wn

of

t h e

fast

neu t rons by hydrogen nuc le i

th e p redominan t phenomenon , and th e read ing , fo r g iv

h o l e co n d i t i o n s , d ep en d s m o s t ly o n t h e h y dro gen i n d

of the fo rmat ion , p ropor t ional to the qu an t i ty

of

hydrog

per un i t vo lume of fo rmat ion near t he bor e ho le .

In c l e an wa t e r b ea r i n g fo rm a t io n s , t h e h y d rog e n

found in th e water on ly , and wi th a conce n t ra t ion whic

is prac t ica l ly independen t f rom the combined effects

t em p e ra t u re an d p r e s s u re ; in t h e s e c o n d i t i o n s t h e Neu tro

read i n g

is

d i rec t ly re la ted to poros i ty . Hydrocarbon

l i k e wa t e r , co n t a i n h y d ro g en, b u t a t v a r i a b l e co n cen t r

t i o n s wh i ch d ep en d m o s t l y o n t h e d en s i t y of the hydr

ca rb o n s i n s i t u . F o r some o i l s , t h e h y d ro g en co n cen t r

t i o n w i l l b e p r a c t ic a l l y t h e s a m e a s in w a t e r ; b ut g

an d l i g h t o i l h av e s u b s t an t i a l l y l o wer h y dro gen co n ce

t r a t i o n s . A s a r e s u l t th e p r e s en ce

of

g as , o r l i g h t

oil,

t h e v o l u m e of fo rmat ion near the bore ho le , is l i k e l y

h av e a s u b s t a n t i a l e f f e c t o n t h e Neu tro n r ead in g .

L e t S r h : res idua l hydrocarbon sa tu ra t ion in invade

z o n e

=

: h y d rog en i n d ex o f t h e h y d ro ca rb o n s

: hydrogen index o f th e mud f i l t ra te .

I f th e Neu tron log

is

ca l i b r a t ed i n f r e s h wa t e r b e a r i n

fo rmat ions , we can wri te :

Neut ron log

p o r o s i t y = +

[ a s , h +

p

(1-

S,,

) ]

(

T h e h y d rog en i n d e x o f f r e s h wa t e r is 1, by def in i t io

the hydrogen index o f a sod ium ch lo r ide so lu t ion

s o m ewh a t s m a l l e r t h an

1.

I t w i l l b e a s s u m e d i n t h e fo l l o wi n g t h a t t h e Neu tro

l o g

is

ca l ib ra te d through cross -p lo t s Neu t ron-Format io

Den s i t y o r Neu t ro n -R es i s t i v i ty i n wa t e r b ea r i n g i n t e

v a l s ; t h en o n e can wr it e :

a

4 N = $

[ p S h

+ 1

s,,]

(2

THE

L O G A N A L Y S T

3

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EFF ECT OF HYDROCARBONS

ON THE FORMATION DENSITY LOG

Compound

F o rmu la

f'b

________ -~

Quartz

510, 2.654

Ca lc 1 tc

CaCO,

2.710

Dolomi te CaCO, MgCO,

2.870

Anhydr i te CaSO, 2.960

F r e s h

water

H

1.000

Sa l t water

200,000 ppm 1.146

II(CH2) 0.8501I

I

G a m m a r a y s of f a i r l y h i g h e n e r g y l e v e l a r e e m i t t e d

by a r a d i o a c t i v e s o u r c e . T h e i r i n t e n s i t y i s g r a d u al l y

a t t e n u a t e d t hr ou g h c o l l i s i o n s w i t h t h e e l e c t r o n s i n t h e

f o rm a t i on ( C o m p to n e f f e c t ) . T h e S c h l u m b e r g e r F o r m a t i o n

D e n s i t y l o g i s b a s e d o n t h e m e a su r e m e n t of t h e g a m m a

r ay d e c a y e d i n t e n s i ty a t

a

g i v e n d i s t a n c e fro m t h e

s o u r c e .

C

0.9985

0.9991

0.9977

0.9990

1.1101

1.0797

1.1407

A s t h e C o m p t o n e f f e c t i s p r o p o r ti o n a l t o t h e n u m b e r of

e l e c t r o n s p e r u n i t v o l u m e a n d as, i n f i r s t a p p r o x i m a t i o n,

t h e n u m b e r of e l e c t r o n s p e r u n i t v o lu m e i s p ro p o r t io n a l t o

t h e d e n s i t y of t h e f o r m a t io n , i t i s c l e a r t h a t t h e l o g

r e s p o n d s t o t h e d e n s i t y

of

t h e f o r m a t i o n .

A c t u a l l y t h e n u m b e r o f e l e c t r o n s p e r u n i t v o l u m e i s

n o t e x a c t l y p r o p o r t io n a l t o t h e d e n s i t y . I n t h e case of a n

e l e m e n t , if w e c a l l p b t h e t r ue d e n s i t y a n d

pe

t h e ( ( e l e c -

t r o ni c d e n s it y ) ) t o w h i c h t h e s o n d e r e s p o n d s , i t c a n b e

s h o w n t h a t :

Z

Z

=

a t o m i c n u m b e r

A

A = a t o m i c w e i g h t

wi t h C = 2- w h e r e

S i m i l a r r e l a t i o n s h i p s e x i s t f o r c o m p o u n d s . pe d i f f e r s

f r o m

p b o n ly t o t h e e x t e n t t h a t C i s n o t e x a c t l y e q u a l

t o 1.

For m o s t of t h e e l e m e n t s a n d c o m p o u n d s f o u nd i n

s e d i m e n t a r y f o r m a t i o n s C i s v e r y c l o s e t o o n e , a s s h o w n

i n t a b l e s h e r e b el o w :

Element

H

C

N O

S I

C I

Ca

A

L

~

5 - 2 1

A

1.008

12.011

16.000

22.99

28.09

35.46

40.08

6

R

1 1

1 4

1.9841

,9991

1 oooo

,9569

,9968

,9588

,9980

T h e . r a t h e r l a r g e d e p a r t u r e f r o m 1 w h i c h a p p e a r s for

o i l a n d w a t e r i s d u e t o t h e p r e s e n c e of h y d r o g e n , t h e C

of

whi ch is a l m o s t e q u a l t o 2.

To c o p e w i t h t h i s p r o b le m t h e t o o l r e s p o n s e is

c a l i b r a t e d i n c l e a n f r e s h w a t e r b e a r i n g l i m e s t o n e s w i th

p o r o s i t i e s r a n g i n g f ro m z e r o t o 40%; i n f r e s h w a t e r

b e a r i n g d o l o m i t e s

or

s a n d s t o n e s , t h e e r r or r e s u l t i n g f rom

t h i s c a l i b r a t i o n is v e r y s m a l l , a n d e n t i r e l y n e g li g i d l e .

F o r f r e s h w a t e r b e a r in g l i m e s t o n e t h e s t a n d a r d f o r m ul a

r e l a t i n g d e n s i t y t o p o r o s i ty c a n b e w r i t te n b o t h f or t r u e

d e n s i t y a n d f o r e l e c t r o n i c d e n s i t y :

(5)

By vi r tue of formula

(3),

e q u a t i o n (5) wr i t es :

By e l i mi na t i ng 4 b e t w e e n

(4)

a n d (6), t h e n r e p l a c i n g

p L s ,

w ,

C I S a nd Cw b y t h e i r n u m e r i c a l v a l u e s , w e g e t :

p b =

1.07

p -

0.188

S i n c e t h e l o g i s c a l i b r a t e d in s u c h a w a y t h a t p

p b i n f r e s h w a t e r b e a r i n g l i m e s t o n e s , i t r e s u l t s t h a t :

=

l o g

P l o g = 1.07

p -

0.188 (7)

T h i s , t h e n , i s t h e r e l a t i o n s h i p , be t w e e n t h e e l e c t r o n i c

d e n s i t y a n d t h e r e a d in g

of

t h e F o r m a t i o n D e n s i t y l o g .

L e t u s n o w r e v e r t t o o u r p ro b l em a n d c o n s i d e r a c l e a n

h y d r o c a rb o n b e a r i n g f o r m a ti o n . T h e f l u id i n t h e i n v e s -

t i g at e d z o n e i s m a de u p of Srh

of

hydrocarbon and

(1 - S r h ) of mud f i l t r a t e . Equat i ons (7) a n d

(6)

w r i t e :

f l o g

1.07

p

- 0.188 (7)

w h e r e C m a p,,, Cm, m f a n d C h

p h

a r e r e s p e c t i v e l y t h e

e l e c t r o n i c d e n s i t i e s o f t h e m a t r i x , of t h e m ud f i l t r a t e a n d

of t h e h y d r o c a r b o n s .

By e l i mi na t i ng p , b e t w e e n

(7)

a n d (8), and no t i c i ng

t h a t :

= 1.07

Cma p,, -

0.188

p m o

4

S E P T E M B E R

-

OC T OB E R , 196

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3/18

w e g e t t h e r e l a t i o n s h i p b e t w e e n pb ( log read ing) and 4

( t rue poros i ty ):

T h e ap p a ren t p o ro s i t y

$,,

d e r i v ed f ro m t h e Den s i t y

log , by def in i t ion :

p m a - pb

D

=

,

is

given by:

p m a

-

P m f

INFLUENCE OF MUD SAL INITY

T h e fo l lo wi n g p a ram e t e r s , u s ed i n fo rm u l ae (2 ) an d

( lo ) , depend on mud f i l t ra te sa l in i ty :

mud f i l t ra te hydrogen inde x ,

p m f m ud f i l t r a t e d en s i t y ,

C, pd

m ud f i l t r a t e e l e c t ro n i c d en s i t y .

We

s h a l l l i m i t o u r s t u d y t o t h e

case

where the mud

f i l t r a te c a n b e c o n s id e r e d as a p u re NaC l s o l u t i o n .

In t h e fo rm u l ae t h e f i l t r a t e s a l i n i t y w i l l b e g i v en a s

a funct ion of the parameter:

NaC l C o n cen t r a t i o n (p p m )

P =

1 ,000 ,000

T h e fo l l o wi n g ap p ro x i m a t e fo rm u l ae h av e b een

es-

b l i s h ed :

(11)

=

1 - . 4 P

pm f =

1 + . 7 P

(12)

Cmfpmf =

1.11

+ .65

P

(13)

INFLUENCE OF HYDROCARBON DENSITY

a.

On N e u t r o n log

In fo rmula (2 ) above (Neu t ron equat ion) D

is

t h e

hydrogen index of the hydrocar bons o r, more p re c i se l y ,

t h e r a t i o of the hydrogen con ten t per un i t vo lume of

hydrocarbons over tha t of water . I t can be shown tha t

D

is given by the fol lowing formula:

wh e re n H

is

the p roport ion of hydrogen by weight in the

hydrocarbons .

B y p l o tt i n g n H v s p h

for

a number of saturated hydro-

carbons (C,H,, ,) u n d e r av e ra g e r e s e rv o i r co n d i t i o n s ,

a n d a s s u m i ng t h e h e a v i e s t c o m p o n e n ts t o h a v e a d en s i

o f 0.9, we h a v e o b t a i n ed t h e fo l l owi n g em p i r i c a l f o rm u l

F o r m e t h a n e w h ic h h a s

a

dens i ty o f 0 .2

f 25%

u n d

m o s t of r e s e rv o i r co n d i t i o n s , t h e fo rm u la g i v e s n H =

0 .25 , the co rre c t f igu re ,

i f

o n e t a k e s

ph

= 0.2.

For

h eav y o i l ,

i.e. p h

=

0 .9 , t h e fo rm u l a g i v e s n H

0.15, which

is

a good approx imat ion s in ce the low

l i m i t .

fo r h eav y s a t u ra t ed h y d ro ca rb o n s

is:

1

7

--

- 0 , 1 4 3

B y e l i m i n a t i n g n H b e t ween (1 4 ) a n d (15), we o b t a

=

as a

funct ion of p ,only:

(16

2

a

= 9

p, f0.15

+

0.2

0.9-

p,)

1

Grap h i ca l s t u d y of t h i s t h ird d eg re e fu n c t io n s h o w

t h a t i t c a n r e a s o n ab l y b e a s s i m i l a t e d t o t w o d i f fe r e

l i n ea r fu n c t i o n s , d ep en d i n g o n t h e r a n g e of

P,

v a l u e

(s e e f ig . 1 ) :

-

i f

0 .2 5

p h