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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) +
8hy.0.(1-Sw) + Correction.,..(l)
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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
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: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.
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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
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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 .
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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”.
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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.
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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.
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Fig. 3 -
Comparison of (XB -ZA) values calculated from the
modifiedPNC and the TDT-K shawing uncertainty bars.
(One standard deviation.)
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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).