determination of residual oil saturation with pulsed neutron logs- field experiment

8/10/2019 Determination of Residual Oil Saturation With Pulsed Neutron Logs- Field Experiment

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SOCIETY OF PETROLEUM ENGINEERS OF AIME

6200 North Central Expressway

Dallas, Texas 75206

THIS IS A PREPRINT --- SUBJECT

Determnati on of Resi dua

: ; P; sSPE 512

TO CORRECTION;:

Oi I Saturat i on

w th Pul sed Neutron Logs

A Fi el d Experi ment

By

T.J. Smith and S.J. Stieber*j Members SPE-AIME, Shell Oil CO.

*Currently with Butler, Miller and Lents, Ltd.

@Copyright 1974

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

This paper was prepared for the 49th Annual Fall Meeting of the Society of Petroleum

Engineers of AIME, to be held in Houston,

Texas, Oct.

6-9, 1974.

Permission to copy is

restricted t~ an abstract of n~t more than 300 words.

Illustrations may not be capied.

The abstract should c~ntain conspicuousacknowledgmentof where and by whom the paper is

presented.

Publication elsewhere after publication in the JOURNAL OF PETROLEU JTECHNOLOGY

or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Edit~r

of the appr~priate journal provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of’any discussion should be sent to

the Society of Petroleum Engineers ~ffice.

Such discussions may be presented at the above

meeting and, with the paper, nay be considered f’orpublication in one of the two SPE ragazines.

ABsTRACT

accuracy and overall applicability, this method

compares favorably with the other available open

The Log-Inject-Logmethod of determining

and cased hole techniques for Sor determination.

residual saturation in cased holes, previously

documentedby Richardson, et al., was recently

Robinson’s2

studv of the amlication of the

field tested with the SchlumbergerDual Spacing I pNC device to Snw

det&nination’;evealed as di;-

Thermal Neutron Decay Time Log (TDT-K). A

companion test with a Pulsed Neutron Capture

(PNC) device speciallymodified by Schlumberger

resulted in saturationuncertainties of three

percent pore volume or less. Measurement with

the standard TDT-K resulted in residunl oil

saturationuncertainties of 10 to

20

percent

pore volume. The test procedures for both

devices are presented with recommendations

designed to minimize potential errors in future

applications.

INTRODUCTION

The number of new domestic oil discoveries

is declining, thus, it is increasinglyimportant

to produce the maximum quantity of oil from

known reservoirs. ~is, coupled with current

higher crude prices, makes tertiary recwery

much more attractive. The most elemental datum

for evaluation of the potential of these proj-

ects is the swept zone residual oil saturation

(Sor).

One of the most promising of the avail-

able Sor determination techniques i~ the Log-

Inject-Log (LIL)methodl utilizing Pulsed

Neutron Capture Logs (PNC).

In vieu of cost,

a previous revi& by Richardson, et al.,~ that

existing standard devices do not achieve the

required accuracy.

However, he showed that with

certain modifications the PNC device could ob-

tain the desired accuracy without the use of

departure curves to correct for borehole and

diffusion effects.

After Schlumbergerconducted laboratory

tests which confirmedRobinson’s findings, Shell

conducted a field test using both the modified

and conventional tools and compared results.

THEORY

The theory associatedwith this test is

explained in detail by Robinson2.

Basically,

the method involves the stepwise injection of

fluids with contrasting capture cross-sections

into the formation and logging after each

injection. Assuming that no free gaa exists,

and that oil is present at residual (immobile)

saturations,the following equations hold:

‘Za,A= Xma

(l-0) +

Xw,[email protected]+

8hy.0.(1-Sw) + Correction.,..(l)



DETERMINATIONOF RESIDUAL OIL SATURATIONWITH PULSED NEUTRON LOGS -

A FIELD E

Ea,B =

Xma

(l-0) + ZW 13afb SW

Ehy”O~(l-Sw) + Correction ....(2)

Solving these two equations simultaneouslywe

obtain, if the correction terms are equal,

(O”sw) =ti=;:’::::’j . . ..*.*.....(3)

s 9

In order to evaluate Sor, porosity must be

known from an independent source. It follows

that:

P

or=l- O*c...””*i**””*””””””*(4)4)

PROCEDbRE

The test well, located in southern

Louisiana,was recompleted to the “N” sand

interval for the purpose of performing a single

well tracer test3 to evaluate residual oil

saturation. During this workover, a TDT-K base

log was run to insure the interval had been

flushed at this location. Open hole surveys

and mechanical configuration of the well are

shown in Figure 1.

The test well was then produced for two anc

one-halfmonths at 50 to 70 BTF/D, and during

this time zero to eight percent oil cuts were

observed. The small amount of produced oil may

have fed in from the tighter rock from 7525 to

7527.

Tl?isinterval was probably not totally

flushed prior to the test. Formation water was

collected as the well was produced and its

capture cross-sectionwas determined later.

The following sequence of operations was

then performed:

1.

Log

2. Inject - Fluid A (low salinity)

Log A

;:

Inject

- Fluid B (high salinity)

5.

Log B

Each logging step above includesmore than

one operation. First, the TDT-K was run across

the zone of interest and was repeated five or

more times to achieve better statisticsand

define precision.

Second, the speciallymodi-

fied PNC log was run with stationary readings

made every one to two feet.

The time spent at

each station was dependent on the net count

rate.

Fluids A and B were designed to obtain a

large contrast 5etween the capture cross-

sectionsmeasured by Logs A and B.

The minimum

practical salinity that could be used for Fluid

A was 30,000 mg/1 NaC1. T is concentrationwas

indicatedby previously observed adverse effect

of low salinity fluids on “N” sand clays. Flui

,was used even though its capture cross-section

rasvery near that of the formation water

Iecauseany free gas present before injection

Ould severely complicate the

WdUatiO

AlSO

he capture cross-section of the formation water

;ouldnot be calculated as accurately as that of

‘luidA because the composition of Fluid A was

ure salt and fresh water. The maximum feasible

talinityfor Fluid B was 150,000 mg/1 NaC1.

his concentrationwas governed by the solubil-

.tyof the salt at surface mixing conditions

md the extended time required to obtain a

mfficient number of net counts for r’ sired

measurementprecision.

The capture cross-section of each fluid was

measuredin a large test tank with the specially

nodifiedPNC tool, however, the values actually

lsedwere calculated.2 The Calculated capture

:ross-sectionswere:

Fluid A

ZW,A = 3?.53* 0.31 C.U.

Fluid B

XW,B =

73,382 0,71 C.U.

Formation

Water

ZW,F

=

4i.36i 1.61

C.U.

ITEMS OF CONCERN

We had anticipateda number of potential

problems associatedwith the test: (1) strippin[

of the residual oil, (2) shrinkage of the

residual oil, (3) the effect of the screen and

liher, gravel pack and perforations, (4) a

temperature change between log runs due to

ir.jectionof liquids”at ambient surface temper-

ature, and (5) a nonuniform injection prcfile

and incompletedisplacement of fluids.

Stripping,as it is used herein, connotes

the physical removal of trapped, immobile oil

from the vicinity of the borehole, primarily by

viscous forces.

To prevent this from being a

problem, the fluids were injected at very low

rates, 10 to 15 BPH, into the 20 feet of perfo-

rations. Maximum allowable rates can be

estimated.4 Removal of oil from within the

radius of investigationof the logging device

would result in a calculatedvalue of residual

oil saturation that would be lower than the tru(

in-situ value. Oil stripping between log runs

would also result in a negatfve error, i.e.,

ignorance of the removal of five percent pore

volume oil would cause a six percent reduction

in calculated Ser.

Shrinkage, as it IS used here, is the

reduction in volume of the trapped oil due to

volubilityof dissolved gases and the lighter

hyd--ocarboncomponents. Oil is in contact with

un- :uratedwater in this test, thus, some

shi.nkagepossibly occurs.

This effect is

enhanced if free gas is present.

Free gas can



:Pl?5120 TT cwrm bton c T c T rvn 3D Q

. ---—-

W

-. J

be generated by a drop in pressure near the

through the maximum radius of investigationof

wellbore during production.

Shrinkage was the logging devices.

initially expected to be very minor. Volubility

data, however, have indicated that potential

RESULTS

reduction in oil volumes may be appreciable if

the trapped oil has a significant solutionGOR

This test yielded two primary determina-

as it perhaps had when encroachmentoccurred.

tiona. Foremost is the satisfactoryresolution

For example,removal of all the solutionmethane

of Sor using the modified PNC log. Additionally

from the “N” sand crude would result in a rela-

calculationswere made based on the TDT-K

tive oil volume reduction of 14 percent based on response and were compared with modified PNC

data from Standing5 and PVT analysis. The

results. Figure 2 shows a typical TDT-K re-

potential error due to shrinkage is negative

sponse for each of the three log runs.

and may be major, depending on the properties of

the crude, the amount of water exposure and the Robinson2 determined that the “N” sand Sor

rock properties of the system.

was 27.4, 28.6 and 12.0 percent respectively fol

the three intervals analyzed. Each of these

The effect of the boreh(,leenvironment (see zones has a porosity of 32.5 percent (see Table

Figure 1) on log response was initially suspect-

land2). A summary of the average values of

ed to be major.

The casing, the cement sheath, Sor for tliiesand from other available sources

the wellbore fluids and the gravel pack, com- is shown in Table 3.

posed of a screen and liner and .017 x .033

gravel, could adversely affect log measurements.

The 12 percent Sor in the interval from

While this is the case with the TDT-K, full-

7532 to 7533 appears anomolous. This interval

scale laboratory tests by Schlumberger indicated

is thought to be different from the remainder

that these conditionshave minimum sffect on the

of the section because it alone did not appear

r,odifiedPNC response.2 Perfora?ims, however,

to initially have free gas present when results

slightlychange the apparent geometry around from the base log and Log A are compared with

the borehole and possibly the capillary

the results from Logs A and B.

properties of the nearby rock.

These effects

were believed minimal and were not investigated A comparisonbetween the responses of the

furt5er.

modified PNC and the TDT-K is shown in Tables 1

and 2, and Figures 3 and 4. Tables 1 and 2

XW’S were corrected to in-situ temperature

depict the calculated values for (ZA - zB) and

2 Some cooling of the

nd pressure conditions,

(O*Sw), respectively. Not included is the

fmmation due to injectionwas expected, but

value for Sor because an independentmeasure of

accurate relative corrections of Zw would not be

porosity is required to calculate its value.

possible if a significantdifference in forma-

These tab?es also present the values of (ZA -ZBI

tion temperatureocc~rred between log runs. Due and (@*Sw) from the TDT-K which are corrected

to the relatively small volume of fluids in-

fer the effect of diffusion using Schlumberger’

jetted and the time required for injection, no curves for seven inch casing in a ten inch bore,

significanterrors were introduced.

Maximum

hole and for 17 and 36 percent porosity. These

recording thermometersrun with the logs indi- corrections are not sufficient to reconcile the

cated a temperaturechange between log runs of

results of the two tools, and because a large

only two or three degrees.

scatter resulted due to these corrections,no

attempt was made to interpolate to the actual

A uniform injection profile was desirable,

porosity of 32,5 percent. These curves Go not,

but if for some reason this was not obtained,

however, include a diffusion correction for the

knowledge of the actual profile would allow this

screen and liner and gravel pack, and this

data to be screened. The very low injection could be the source of the observed difference

rates precluded the use of conventional tech-

between the results. Figure 3 is a comparison

niques for defining the irijectionprofiles, and

between (ZA -

XB) for the TDT-K and the modi-

a precise profile could not be obtained although

fied PNC with statisticaluncertainties indi-

this information is desirable.

Contrasting

cated.

Figure 4 compares the T readings for th(

salinitiesexisted between the two fluids, thus,

tools. This last figure suggestsa systematic

the TDT-K response represented the best injec-

error exists in the TDT-K response in compac-

tion monitor even though it would provide only a

ison with the modified PNC. Analysis of these

qualitativeassessment of the profile. In

data indicate that the standard tool shouldnot

retrospect, for this purpose it would have been

be used in Sor determination,unless the

better to inject the high salinity fluid first.

necessary departure curves for the actual

Approximately

85

barrels of each fluid were conditions encountered can be made available.

injected into the formation to insure total dis- However, the TDT-K is used extensively in the

placement of the water around the wellbore.

S.P. Block

27

Field to evaluate drainage

This volume of brine passes several pore volumes

conditions in oil and gas reservoirs.



DETERMINATION OF RESIDUAL OIL SATURATIONWITH PULSED NEUTRON LOGS -

~ VrWllll?YDUQTMWNT

cm

c 9n

n J.i.lu” A .. U*. ULU.. A

.3 LJ 2J. ~

CONCLUSIONS an equivalent device should be used

pending development of departure curves

This test verifies that Sor can be deter-

which correct the response of the stan-

mined with good accuracy using the LIL-PNC dard tools for the actual conditions

technique. The following items shouldbe care-

encountered. However, due to the large

fully consideredby those planning a similar number of possible borehole configura-

tes:

tions and the current availabilityof

the modified PNC which is not affected

1. Fluids utilized for injection should by the borehole, developmentof these

be mixed in single batches.

departure curves may not be practical.

2.

The two fluids injected should have the

NOMENCLAW

maximum possible contrast in salinity

or capture cross-section,

- Porosity

k

- Bulk volume of the formation

3.

Injection rates should be kept low to

occupied by water

avoid stripping by viscous forces. Sw - Water Saturation

Sor

- Residual Oil Saturation

4. Sufficient fluid must be injected to

Za,A -

Apparent capture cross-section

insure complete displacementwithin the following injection of Fluid A

radius of investigation of the logging

Xa,B -

Apparent capture cross-section

device. following injection of Fluid B

Xma

- Matrix capture cross-section

5.

A large excess of injection fluids Zhy

- Hydrocarbon capture cross-section

shouldnot be used because of possible

ZW,A -

Capture cross-sectfon of Fluid A

volubilityeffects and for the actual

XW,B

- Capture cross-sectionof4;&oid B

volume of fluid used the shrinkage

T

-

Neutron decay time, ‘r.—

from volubilityeffects shouldbe z

assessed.

REFERENCES

6

Injection profiles should be deter-

1. Richardson, J.E., Wyman, R.E., Jorden, J.R.

mined independently of the PNC TDT-K

and Mitchell, F.R.: “Methods for Deter-

measurements if possible.

mining Residual Oil Saturationwith Pulsed

Neutron Logs,” J. Petroleum Tech. (May, 197:

7*

The effect of temperature changes

593-606.

caused by injections should be analyzed

. 2*

Robinson, J.D.:

“Neutron Decay Time in the

Subsurface:

Theory, Experiment and an

8.

The likelihoodof the presence of free

Application to Residual Oil Determination,”

gas shouldbe considered and steps PapeY SPE 5119 presented at SPE 49th Annual

should be taken to remove any free gas

Fall Meeting, Houston, October 6 - 9, 1974.

that is initially present.

3.

Dalton, R.L., Deans, H.A., Shallenberger,

L.K. and Tomich, J.F.: “Single-WellMethod

9. Recognition should be given to the fact

to Measure Residual Oil Saturation,”

that for certain reservoir crudes a

J. Petroleum Tech. (February,1973) 211-218,

free gas phase can be created by the

4. Abrams, Albert: “The Influence of Fluid

preesure drop associated with produc-

Viscosity, Interracial Tension, and Flow

ing the well. Considerations should be

Velocity on Residual Oil SaturationLeft by

given to not producing the well prior

Waterflood,” Paper SPE 5050 presented at SPI

to the test.

49th Annual Fall Meeting, Houston,

October 6 - 9, 1974.

10.

Porosity

must be determined a~czrately.

5*

Standing, M.B.: Volumetric and Phase

Behavior of Cil Field Hydrocarbon Systems,

11. The test should only be performed in a

Reinhold Publishing Corp., New York (1952)

newly perforated intervalbecause the

33-42.

effects of stripping and changes in

6, Pickell, J.J.,

Swanson, B.F. and Hickman,

rock characteristicsaround the bore-

W.B.:

“The Application of Air-Meccury and

hole caused by sand production and

Oil-Air Capillary Pressure Data in the

formation slumpingmay cause measured

Study of Pore Structure and Fluid Distribu-

tor values not to agree with the actual

tion,” Society PetroleumEn ar..J.

Sor values. If a new completion is not

(March, 1966) 55-63.

available, the test should not be run

in or near an injector. —

.

12.

only the speciallymodified FNC tool or



TABLE 1 -

COMPARISON (xA - XB) VALUES

CALCULATED FROM THE MODIFIED PNC AND THE TDT-K

(ZA - zB)

INTERVAL

MODIFIED PNC

_ TDT-K1

TDT-K2 TDT-K3

7519

- 23

-9.91t 0.28

-11.60t 1.51 -9.575 1.18 -12.21? 1,?0

7525 - 27

Tighter Rock, Complete InjectionDoubtful

7529 - 31

-9.73i 0.30

-18.81* 1.78

-11.601 1.36

-19.56? 1.50

7532

- 33

-12.O@ 0.32

-18.32? 2.44

-19.34t 2.00 -20.13t 2.0~

1.

Uncorrected for diffusion effects.

2.

Corrected for diffusion effects using Schlumbe~ger’sdeparture curves for

1 11/16” TDT-K in i“ casing and 10” borehole (Chart Tcor-4 Schlumberger log

interpretationcharts) for 0 = 17 .

3. Corrected as above for 0 = 36 .

TABLE

2 -

COMPARISONOF (l?i”Sw)ALUES

CALCULATEDFROM THE MODIFIED PNC AND THE TDT-K

(O”sw)

INTERVAL

MODIFIED PNC

TDT-K1 TDT-K2 TDT-K3

7519 - 23

0.236t 0.008

0.277k 0.036

0.229t 0.031 0.292k 0.028

7525 - 27

Tighter Rock, Complete InjectionDoubtful

7529 - 31

0.232k .0085 0.449f 0.125

0.277f 0.036

0.467? 0.033

7532 - 33

0.286t 0.009 0.437k 0.0 58

0.462f 0.049

0.481A 0.048

1.

Uncorrected for diffusion effects.

2.

Corrected using Schlumberger’sdeparture curves for 1 11/16” TDT-K in 7“ casing

and 10” borehole (Chart Tcor-4 Schlumbergerlog interpretat~oncharts) for

0=

17 .

3.

Corrected as above for 0 = 36 .



TABLE 3 -

SUMMARY OF INDICATED Sor VALUES

Source

&

1. Log-Inject-Log

287A*

2. Single Well Tracer Test

31

3.

Resistivity Logs

4.

ileservoirPerformance

32 (assumes 95 sweep)

(Volumetric)

5.

Core

a. Measured So

31

b.

Countercurrent Imbibition

6 29

c. End-point relative perm.

30

M E 2

12-28.6

29.4-33.4 *

25-40

16.6-34.877**

27.5-30.5

28-30.5

*Thi~ value has not been corrected for shrinkage; if applied, this correction

could raise the value of Sor to approximately 33 .

**Formal statisticaluncertainty on curve fit only. A shrinkage correctionwas

assumed and the partitioning coefficient for the weathered prcduccd crude

was assumed applicable to the trapped crude.

***Thesevalues are from rubber sleeve cores and are corrected for “blow-down”.



RESISTIVITY

1: k

OHMS Mz/M

-H+

o

—

- -~

-.

--x

,.

+

----

‘.

b

<:-

_.

-.,

j-= W 5+

w

3)6 4.7 lb~./ft.

J -55 TUBING

7400

—

— 7408 ;~; ;ERMATRIEVE

h’

7“ 23 lbs. /ft.

CASING

:1

..

“”..,.$.

.....

.. . .....

...... .. .

,... “..:

.... .“..

:.. .

. .

. ...

::. .

. ..:.

.”

....

,..,.,

..

..-.

....

.:..,.

‘....

“.” ..

. . . .. ‘ . .“.

. ..... . .“ .“.”

.. ... .“ : .

..... . .

. .““

. “.

. .. .

.

.“.’..

5-

017 x .033

GRAVEL PACK

. . ..

”

.<...

7500

,’”.;.. ..: . .

. . . . . . .

... .

. . . .

. .

.. :.”. .

~ 2~a SCREEN LINER

/

::{.

,.. .

“N’’PERFS

. . . .

. . . .

7517-7537

. . .

,.. .

W/8 J.l? F

. . . .

u ii

Fig. 1 - Open hole surveys and mechanical configura~ionof the test well.



SP

10 MV

-H+

/

sp&

—

INTERVAL I

INTERVAL 2

INTERVAL 3

INTERVAL 4

II-CAPTURE UNITS (CU= IO-3 CM2/CM3)

60

30

0

“7450’-

—

.-.

7500’ -—

FR

—— —

TOT N

. ,,,od__

I

Fig. 2 - Typical TDT-K response for eaci,of the logging steps.



Fig. 3 -

Comparison of (XB -ZA) values calculated from the

modifiedPNC and the TDT-K shawing uncertainty bars.

(One standard deviation.)



30C

2s0

15G

/

b.

.0

/

*X

1 I

1

.

x

.

BASE /?u/V

AF7E. iAtd CT/NG FLUID A

AFT4’/? //VL/ECTIIVG FL U/D L9

{

/00

00

150 200

250

300

L, MO IFIE

W

Fig. 4 - Comparison of the log readings from the m~dified PIW

and the TDT-K (not all points are from the

“N” sand).

determination of residual oil saturation with pulsed neutron logs- field experiment

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