Respond of neutron and formation density log in HC bearing

Hydrocarbon Correction for Neutrondensity log. Respond of neutron and formation density log in HC bearing. How to correct them? Hydrocarbon Correction for Neutrondensity log. Respond of neutron and formation density log in HC bearing. How to correct them? Hydrocarbon Correction for Neutrondensity log. Respond of neutron and formation density log in HC bearing. How to correct them?

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

HYDROCARBON B E A R I N G FORMATIONS

by

R GAYMARD and A POUPON SPE Schlumberger - P a r i s

EDITOR’S NOTE: In this interesting paper the authors

consider the effect of residual hydrocarbons on the re-

s p o n s e of the Neutron a n d Formation Density logs I t

should be o f interest to those working with t h e s e poros-

ity tools P o s s i b l e applications a r e included for both

shaly formations o r complex lithologies

ABSTRACT

I t is usually assumed that, in oil bearing formations,

the Neutron and Formation Density logs are not signifi-

cantly affected by the residual oil in the invaded zone

and that they respond a s if t h e volume investigated was

entirely filled with mud filtrate However when porosities

a r e fairly high, t h e effect of t h i s residual oil is not

always negligible, particularly if the oil is light

Computations have been made t o evaluate t h i s effect

Formulae have been developed for clean oil or g a s

bearing formations T h e s e formulae permit a more accu-

rate evaluation of the porosity Some of the formulae may

a l s o have application in the cases of shaly formations

and of complex lithologies

EFFECT OF HYDROCARBONS

ON THE NEUTRON LOG

In Neutron logging, a source emits fast neutrons into

the formation T h e s e neutrons a r e slowed down through

collisions, mostly with hydrogen nuclei of t h e surround-

ing medium, and after reaching the so-called thermal

level of energy, a r e absorbed by nuclei of the formation,

usually hydrogen or chlorine nuclei; e a c h capture of a

thermal neutron is followed by t h e emission of gamma-

rays of capture A detector, a t some distance from the

neutron source, measures either t h e gamma-rays of

capture (Neutron-Gamma type of tool) or the thermal

neutrons (Neutron-Thermal Neutron tool) or the neu-

trons before they have reached t h e thermal l e v e l (Neutron-Epithermal Neutron tool) For all tools, the slowing down of t h e fast neutrons by hydrogen nuclei is

t h e predominant phenomenon, and t h e reading, for given hole conditions, depends mostly on t h e hydrogen index

of the formation, proportional t o the quantity of hydrogen per unit volume of formation near the bore hole

In clean water bearing formations, the hydrogen is found in the water only, and with a concentration which

is practically independent from the combined effects of temperature and pressure; in t h e s e conditions t h e Neutron reading is directly related to porosity Hydrocarbons, like water, contain hydrogen, but a t variable concentra- tions which depend mostly on the density of the hydro- carbons in situ For some oils, the hydrogen concentra- tion will be practically the same a s in water; but g a s and light oil have substantially lower hydrogen concen- trations As a result the presence of gas, or light oil, in

t h e volume of formation near the bore hole, is likely to have a substantial effect on the Neutron reading

L e t Srh : residual hydrocarbon saturation in invaded

zone

= : hydrogen index of the hydrocarbons : hydrogen index of t h e mud filtrate

If the Neutron log is calibrated in fresh water bearing formations, we can write:

Neutron log porosity = + [ a s , h + p (1 - S,, ) ] (1)

T h e hydrogen index of fresh water is 1, by definition; the hydrogen index of a sodium chloride solution i s somewhat smaller than 1

It will be assumed in t h e following that the Neutron log is calibrated through cross-plots Neutron-Formation Density or Neutron-Resistivity in water bearing inter- vals; then one can write:

a

E F F E C T O F HYDROCARBONS

ON THE FORMATION DENSITY LOG

D o l o m i t e CaCO,!MgCO, 2.870

Gamma r a y s of fairly high energy l e v e l a r e emitted

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 gradually

a t t e n u a t e d through c o l l i s i o n s with t h e e l e c t r o n s in t h e

formation (Compton effect) T h e Schlumberger Formation

D e n s i t y l o g i s b a s e d on t h e measurement of t h e gamma

ray d e c a y e d i n t e n s i t y a t a given d i s t a n c e from t h e

s o u r c e

C 0.9985 0.9991 0.9977 0.9990 1.1101 1.0797 1.1407

As t h e Compton e f f e c t i s proportional t o t h e number of

e l e c t r o n s per unit volume and as, i n f i r s t approximation,

t h e number of e l e c t r o n s per unit volume i s proportional t o

t h e d e n s i t y of t h e formation, i t i s c l e a r t h a t t h e log

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

Actually t h e number of e l e c t r o n s per unit volume i s

not e x a c t l y proportional t o t h e d e n s i t y In 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 true d e n s i t y and p e t h e ((elec-

tronic density)) t o which t h e s o n d e r e s p o n d s , i t c a n b e

shown that:

Z Z = atomic number

A A = a t o m i c weight

with C = 2- w h e r e

Similar r e l a t i o n s h i p s e x i s t for compounds p e d i f f e r s

from pb only t o t h e e x t e n t t h a t C i s not e x a c t l y e q u a l

t o 1

For most of t h e e l e m e n t s and compounds found in

sedimentary formations C i s very c l o s e t o o n e , a s shown

in t a b l e s herebelow:

Element

H

C

0

N O

S I

C I

Ca

A

1.008 12.011 16.000 22.99 28.09 35.46 40.08

1

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 from 1 which a p p e a r s for

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

of which is almost e q u a l to 2

To c o p e with t h i s problem t h e tool 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 bearing l i m e s t o n e s with

p o r o s i t i e s ranging from z e r o t o 40%; in fresh w a t e r bearing dolomites or s a n d s t o n e s , t h e error r e s u l t i n g from

t h i s c a l i b r a t i o n is very s m a l l , and e n t i r e l y negligidle

F o r fresh w a t e r bearing l i m e s t o n e t h e s t a n d a r d formula

r e l a t i n g d e n s i t y t o porosity c a n b e written both for true

d e n s i t y and for e l e c t r o n i c d e n s i t y :

(5)

By virtue of formula (3), equation (5) writes:

By eliminating 4 between (4) and (6), then r e p l a c i n g

p L s , p w , C I S and C w by their numerical v a l u e s , we g e t :

pb = 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 way t h a t p

p b in fresh water bearing l i m e s t o n e s , it r e s u l t s that:

=

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 , between t h e e l e c t r o n i c

d e n s i t y and t h e reading of t h e Formation D e n s i t y log

L e t u s now revert to our problem and c o n s i d e r a c l e a n hydrocarbon bearing formation T h e fluid in t h e i n v e s -

t i g a t e d z o n e i s made up of Srh % of hydrocarbon and (1 - S r h ) % of mud filtrate E q u a t i o n s (7) and (6) write:

where C m a p,,, C m , p m f and 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 of t h e matrix, of t h e mud f i l t r a t e and

of t h e hydrocarbons

By eliminating p , between (7) and (8), and noticing

t h a t :

= 1.07 Cma p,, - 0.188

pmo

w e get the relationship between pb (log reading) and 4

(true porosity):

T h e apparent porosity $,, derived from the Density

log, by definition:

p m a - p b

4 D = , is given by:

p m a - P m f

INFLUENCE O F MUD SALINITY

T h e following parameters, used in formulae (2) and

(lo), depend on mud filtrate salinity:

P mud filtrate hydrogen index,

p m f mud filtrate density,

C, pd mud filtrate electronic density

We s h a l l l i m i t our study t o the case where the mud

filtrate c a n be considered as a pure N a C l solution

In the formulae the filtrate salinity will b e given a s

a function of the parameter:

N a C l Concentration (ppm)

P =

1,000,000

T h e following approximate formulae have been es-

blished:

(11)

p = 1 - 4 P

INFLUENCE O F HYDROCARBON DENSITY

a On Neutron log

In formula (2) above (Neutron equation) D( is the

hydrogen index of the hydrocarbons or, more precisely,

t h e ratio of the hydrogen content per unit volume of

hydrocarbons over that of water It c a n b e shown that D(

is given by the following formula:

where nH is the proportion of hydrogen by weight in the

hydrocarbons

By plotting nH v s ph for a number of saturated hydro-

carbons (C,H,, + ,) under average reservoir conditions,

and assuming the heaviest components to have a density

of 0.9, we have obtained the following empirical formula:

For methane which h a s a density of 0.2 f 25% under most of reservoir conditions, t h e formula gives nH =

0.25, the correct figure, i f one t a k e s ph = 0.2

For heavy oil, i.e p h = 0.9, the formula gives nH =

0.15, which is a good approximation s i n c e the lower

l i m i t for heavy saturated hydrocarbons is:

1

7

- 0,143

By eliminating nH between (14) and (15), we obtain

= as a function of p,only:

(16)

2

a = 9 p, f0.15 + 0.2 (0.9- p,) 1 Graphical study of t h i s third degree function shows that i t can reasonably b e assimilated to two different linear functions, depending on t h e range of P, values ( s e e fig 1):

- if 0.25 p h < 0.9, i.e practically for oil, we can write:

- if p,< 0.25, i.e practically for gas, we can write:

b On Formation Density log

In formula (lo), C, p h is the electronic density of the hydrocarbons C, is related t o nH through the fol- lowing simple formula:

Taking (14) into account, we get:

c, p, - (1 + rlH) p, = x +

9 p h

i f 0.25 4 p h < 0.9 using the expression of O( given by (17), we find:

c h p h = 1.11 ph + 0.03

- if ph < 0.25 with 0: = 2 2 p h , we find:

C, ph = 1.24 p ,

d RELATIONSHIP BETWEEN DENSITY OF HYDROCARBONS A N D THEIR HYDROGEN INDEX

( H index)

G E N E R A L F O R M U L A E EXPRESSING T H E E F F E C T

OF R E S I D U A L H Y D R O C A R B O N S A T U R A T I O N O N

N E U T R O N A N D F O R M A T I O N D E N S I T Y LOGS

a Neutron log

I t is a s s u m e d i n t h e following t h a t t h e Neutron l o g

i s c a l i b r a t e d t o g i v e t h e t r u e porosity in c l e a n i n t e r v a l s

w h e r e t h e r e is mud f i l t r a t e only, with n o r e s i d u a l hydro-

c a r b o n s If a n i n t e r v a l c o n t a i n s r e s i d u a l hydrocarbons,

t h e a p p a r e n t Neutron porosity + N will n o longer b e e q u a l

t o t h e t r u e porosity 4 :

T h e v a l u e of A& i s readily found by combining

r e l a t i o n s h i p s (2), i l l ) , (17) or (18), and (23):

for oil:

for g a s :

b Formation Density l o g

T h e p r e s e n c e of r e s i d u a l h y d r o c a r b o n s i n s t e a d of mud f i l t r a t e c a u s e s a variation Ap, of t h e log reading,

a n d a s a r e s u l t t h e apparent porosity q5D differs from

t h e true porosity 4

T h e e x p r e s s i o n s of Apb and ApD are found by com- bining e q u a t i o n s (9) or (19), (12), (13), (21) or (22), and (26)

for o i l :

(28)

+ 1.07 4 Srh [1.11 (1 - ph) + 65 P - .031

A + D =

p , - 1 - .7 P for g a s :

A p b = - 1.07 4 S r h [1.11 + 65 P - 1.24 p h l (29)

(30)

+ 1.07 4 S , h [ l l l + 65 P - 1.24 p h I

A 4 D =

p , - 1 - .7 P

F i g u r e s 2 and 3 are a graphical representation of t h e and Ap, r e l a t i o n s h i p s

EFFECT OF HYDROCARBON ON NEUTRON LOG

A@,,, CHART

I

TRUE POROSITY 4 = 28%

EXAMPLE RESIDUAL HYDROCARBON

HYDROCARBON DENSITY Qh = 0-2

SATURATION srh = 60 % "N -s*5%

FILTRATE S A L I N I T Y 200.000 P P M THE NEUTRON LOG READS = 28 - 8.5 = 19.5 %

I

0 0 0

FIGURE 2

PRACTICAL APPLICATIONS

The final Neutron and Formation Density formulae

(24), (25), (27), (28), (29) and (30) look somewhat com-

plicated, but charts based on these formulae are easy

to use, a s will be illustrated on a few typical cases

a Clean sands ( p g = 2.65) containing hydrocarbons with

known p h

Let u s assume, for example, that the density of the

mud filtrate pf = 1, and that ph = .114 (this is the c a s e

of gas with a hydrogen index of 25)

From (25) w e derive:

From (29):

p b = 2.65- 1.65 4-1.04 4 S,, (32)

On a cross-plot of r,bN versus pb, the lines for con- stant values of + and for constant values of Srh are straight line The chart (fig 4) is practically the same

a s chart 40-1 in the Schlumberger Paris Chart Book of

1966

Similar charts can be made for other values of ph ( s e e fig 5 for ph = S )

Similar charts can also be made for clean limestones

(pma = 2.71) or dolomites (pma = 2.87)

EFBECU OF HYDROCARBON OM FORMATOOM DENSITY LOG

AQb CHART

RESIDUAL HYDROCARBON SATURATION S,h= 75%

TRUE POROSITY 6 = 21%

I

EXAMPLE HYDROCARBON DENSITY ph ~ 0 6

I FILTRATE SALINITY 150.000 P P M

b Clean Sands containing hydrocarbons of unknown ph

FIGURE 3

I t c a n b e shown that, t o a good approximation, regard-

l e s s of t h e v a l u e of ph:

Therefore, t o e v a l u a t e 4, we n e e d t o make an assump-

about Srh F o r t u n a t e l y t h i s assumption i s not c r i t i c a l

We c a n then c a l c u l a t e a v a l u e of ph which is e x p r e s s e d

a s a function of Srh and 0 = C , S ~ / + , in t h e following

e q u a t i o n s :

for oil:

for g a s :

T h e whole p r o c e s s i s s o l v e d graphically by figure 6

T h e more a c c u r a t e v a l u e of S r h which is n e e d e d t o

d e r i v e p h from t h e s e c o n d part of t h e c h a r t is c a l c u l a t e d from:

.62 Rm,

6 2 1 5 ( 1 - SrJ2

R x o =

c Hydrocarbon bearing shaly sands

Formulae (24), (25), (27) and (29) remain valid in

s h a l y formations T h e s e formulae s h o w t h a t t h e often

u s e d method of deriving a v a l u e of s h a l e c o n t e n t V s h

30%

20%

10%

a

I

60;

40%,

\

'

2

FIGURE 4

EFFECT O F GAS O N N-FDC C O M B I N A T I O N

1.65

1.25 1.114

I o

30%

20%

10%

A

I

z

7

FIGURE 5

EFFECT O F LIGHT OIL ON N - F D C COMBINATION

t

9

SANDSTOb

HYDROCAfi FRESH M U

\

2 6 5

m (X - a e -

P f = 1 0

D N D E N S I l Y f h = 0 5

I

L

f

0

a

SYMBOLS USED ABOUT T H E AUTHORS

T h i s paper h a s been written using a new set of

symbols which follows as closely as possible the

standard API list We apologize for t h e inconvenience

t h i s will certainly c a u s e those who are accustomed to

t h e old system, but we felt that the change had to b e

made:

Physical Parameters:

R resistivity

p density

q5 porosity

S fluid saturation

V proportion by volume

a hydrogen index of hydrocarbons

hydrogen index of t h e mud filtrate

ppm N a C l

P =

1,000,000

Subscripts:

s h s h a l e

Under t h i s system we have t h e following equivalence

t o the old list:

ROS Sro Residual oil saturation

RGS Srg Residual g a s saturation

Srh Residual hydrocarbon sat

Matrix (Grain) density

p, p m a

Mr R Gaymard graduated in 1947 from Ecole Polytechnique, Paris, and in 1948 from Ecole Nationale Supgrieure du P&- role, near Paris, with a Petroleum Engineering degree During

1948-1951 he was drilling and Production Engineer for SN REPAL, Algeria He joined Schlumberger in 1951 and was assigned successively to SWSC (U.S.), Surenco (South Ameri- ca), SPES (Europe and Africa) and Schlumberger Overseas (Libya)

He is presently working a t the Sales Interpretation Department

of Schlumberger, Paris, headquarters where he is i n charge of Computer Processed Interpretation

i

Andre Poupon graduated in 1933 from Ecole Polytechnique in Paris After one year of military service and three years with P

the French Railroad, he joined Schlumberger at the end of

1937 He worked in the field in Trinidad, France, and Vene- <

zuela and was transferred to Schlumberger’s research center I

I

Mr Poupon moved to Paris in September, 1955, to work in the

J Tittman and J S Wahl: “The physical Foundations of Interpretation Department of S.P.E Schlumberger He is at

P

of formation

1

REFERENCES

I

I

through a cross-plot of q5N versus pb gives Vsh values

that are too low in formations containing light oil In

gas beaiing formation the error on Vsh could be quite

large On the other hand, i f a reliable value of shale

content can be estimated independently of the Neutron

and Density logs (Gamma Ray, or SP, or Resistivity),

q5N and C$D can be corrected Then:

- if ph is known the clean sand charts (such a s those of

fig 4 and 5) can be used t o derive q5 and Srh;

- i f ph is not known the chart of fig 6 can be used to

estimate q5 and p h

chart indicates a porosity of 25% and the probable lithology is 50% h e s t o n e + 50% dolomite, with a grain density of 2.79

If that same formation contains hydrocarbons, both the Neutron and Density readings will be affected Let

us assume p = .55 and Srh = 40%

h

From (24) we find, for P = 0 (fresh mud):

d Complex lithologies: determination of q5 and p,,

In wate'r bearing mixtures of sand, limestone and

dolomite, with no shale, the porosity and approximate

grain density can be derived from a cross-plot of pb

versus q5N, such a s that of fig 7 which corresponds to

pf = 1 (chart 42 of the Schlumberger Paris Chart Book of

1966) For example, for C$N = 26% and pb = 2.34, the

the apparent Neutron porosity then is 26 - .015 = .245

or 24.5%

From (27)

A p b = - 1.07 X .25 X .40 (.S- .033) =-.05

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3 0

COMPLEX LITHOLOGY FIGURE 7

f

porosity )

the bulk density reading is 05 gr/cc too low, and is equal to:

2.34 - .05 = 2.29

The point for q5N = 24.5% and pb = 2.29 on the chart

of fig 7 falls exactly on the line corresponding to pure limestone, and indicates an apparent porosity of 24.5%

Due to the effect of the residual hydrocarbons, then, the estimated porosity is very slightly in error, and the estimated grain density is appreciably off, 2.71 instead

of 2.79 At low porosities the hydrocarbon effect would

be negligible

From this example, it can be concluded that in for- mations containing light oil, when the porosities range from medium to high, the residual hydrocarbons are not likely to have much effect on the porosity determination, but lithology cross-plots cannot be satisfactorily in- terpreted without taking into account the effect of the residual hydrocarbons The correction for residual hydrocarbons, needed for a correct determination of the

lithology can be made only if ph is known and i f an R x o

log is available to give an approximate value of Sr,