The U-shaped combustion tube containing the extract concentrate is
placed in an electric heater, so that the temperature can be raised slowly
to vaporize the oil uniformly,

A stream of natural gas is passed through the tube over the extract
concentrate and burned. A small gas-pilot flame at the tip cf the com
bustion tube is found to be essential for good combustion, The combustion
gases are pulled by vacuum through a chimey and thence throvgh a porous
sintered glass disk in an absorption flask. The absorption flask contains
50 ml, of a solution approximately 0.2 N with respect to sodium carbonate and
0.1 N with respect to sodium arsenite. ‘The sodium arsenite is used to
reduce any free chlorine that may have formed in the combustion, so that all
of the chlorine from the tracer is present in the ionized state and is
quantitatively absorbed by the alkaline solution.

During the combustion of each oil sample, the temperature of the tube
is increased until the appearance of the flame indicates that all of the
oil has been vaporized. Almost a red heat was required to vaporize most
of the éils analyzed. Coke left in the tube is burned in place by re-
placing the stream of gas by a stream of air,

After combustion of the sample, the chloride in the absorption
solution is determined by an electrical method that eliminates observation
of color changes, ‘he absorption sclution is concentrated to ebout 25 ml.,
and then 1 ml. of concentrated nitric acid 1g added. The acid sclution
is titrated with 0.01 N silver nitrate solution, using the Beckman silver-
silver chloride electrode, The end point of the titration is determined
graphically by plotting the volume of silver nitrate against the observed
solution potential, expressed in millivolts, The end point is marked by a
sudden increase in potential, and this point is taken to be the inflection
point of the titration curve.

The quantity of chloride in a unit volume of érilling-fluid filtrate
is measured for each drilling-fluid sample. After the quantity of organic
chloride extracted from each core sample has been determined, the volure
of filtrate corresponding to that quantity of chloride is calculated.

The calculated volume cf filtrate found in each core sample is expressed in
terms of percent pore volume of the core sample.

Sample Calculation

The following sample calculation illustrates the method for determining
the volume of filtrate in the core samples:

Take, for example, that part of core 6 selected to represent the form-
tion in tho South Coles Levee field at a depth of 9338.5 feet. According
to table 1, the analysis of a unit volume of filtrate from the crilling
fluid in the hole while the core was being cut indicates that 6.03 m. of
chloride from the tracer was present in cach milliliter of filtrate. The
volume of filtrate per unit weight of chloride then was 1 = 165.8 m,.

‘ 0, 00605

3820 ~8-


FILTRATE (O1L), PERCENT PORE SPACE
$
4

FLUID

DRILLING

8

20)

Ol 1
EDGE OF CORE CENTER OF CORE EDGE OF GORE

Figure 7. - Relation of location of sample specimen within the cores from South Coles Levee
test to the volume of filtrate (oil) in the sample.


TABLE 4 - Core Analyses (Cont'd. )

Bureau of Mines - A.P.I. Cooperative Study

Field: South Coles Levee, Kern County, Calif.

i : i ] ' | Loca-
‘Liquid content, = : { ' j tion
ercent pore space 'Horizon- iChloride : ; of
Drill~ Poros- ital air jconc.,Cl | | sample
H ' i ing ity, jPermea- jgrains peri i din
H a | fluid ' percent! bility, lgation of | i core
Depth, Total! i ofl ‘of bulk;milli- jintersti- | ! (see
feet; ofl Mater filtrate! volume jdarcys |tial water’ Core description fig, 2)
9398. 5ef/20.2157.7, 12,0 | 21.8 | Th 702 | Fine soft sand | 14,18
9398.5 115.8'54.1: 8.20' 22.0 | 52 qOO °; do. 115,17
9398.5 ..11.2/52.7; 6.92 23.4 | 47 696 |. do, i 16
9399.5 10.3:51.6| 1.79 22.7 | 28 684 Fine med,scft ed
bow H : i | shale streaks , 1
9h00.5.: 6.3}75.5 4h 18K | 33 772 {Fino soft sand | 1
QhO1.5 | 20.6'50,1 9.19. 17.7 | 29 457 | Med. soft sand,
| 4 ' ; 1 I ' shale steaks 1
gh02.5 16, 4/56.3 | 6.71} 19.2 | 2h 70h Fine hard sand ‘ 13
9402.5 119.0)59.7; 17h! 17.2 1 35 BI | do. _ 2
9405.5 | 13.6161.9; 3.88! 15.7 1 32 | 721 [Fine med.soft sd 2
guol.5 |1220167.9| ~.86, 18.1 | 16 68 § do ioe
9405.5 | 12,9162.1 84) 19.7 : 18 | 658 -j do. : 2
9406.5 [18.01 47.2 | 8.64: 2h.6 1 25 618 do. 9
9406.5 1 17.5;58.7; JBL, 20.7 ! 3h 778 , de : 10
9406.5 |18.4;55.3, 6.34 ' 21.2 | 32 7332 | dow iu
9406.5 | 1716!50.0 8.35 , 22.9 | 26 686 i do. | 12
9407.5 1 14.8153.8| 6,15} 22.8 | 26 679 =; do. 1 2
9408.5 117.6:48.7; 10.9 ° 22.4 | 28 | 61 1 do. 2
9408.5 | 18.0:47.7} 9.68: 22.8 ; 38 1 OF 1 do. 3
9412.5 1 37.2'42.3| 32.0 | 23,6 '150 | 737 do. 3
gur2.5.|4aizin..5 | 31.2 | 25.2 imho =| 795) | aon 2
9423.5 141.8)39.5, 31.4 | 21.7 1170 | 830 {Fine crs.soft sdl 2
941.5 | 39.5:36.21 31.5 ' 21.7 |270 1 8 ! do. | 2
9415.5 154.1/52.1! 46.4 | 19.6 | 210 i ‘805° Fine soft sand i 2
9416.5 |45,7'39.2 | 37.5 | 21.9 270 | ghg | Fine-crs.seft adj 14
9416.5 {54.8/52.2| 45.6 | 18.4 | 280 ‘851 |. oy 16
9417.5 140,552.35} 31.9 18.2 | 210 » BL | do, 2
9418.5 130.6155.11! 24.3 | 18.6 | 16,0 80h "=! do. . 2
9419.5, }22.5'100 | 5.53, 5.90’ 1.5 j 633 {Fine bara sand 2
9420.5 125.5'52,6! 1.78 | 19.6 140 821 | Fine-crs.soft sd] 2
9421.5, 127.7'69.4: 4.80 | 10.9 2.3 922 |Fine med.soft sij 2
gk22.5, 125.8 154.2 | 19.3 , 17.6 | 77 "B68 i Fine-crs, soft sd| 14,18
9422.5 128.5 157.2 | 16.3 | 16.6 | 37 i 908 | do. 16
9423.5 125.7 38k | 1.1 | 17-7 | 52 + 763 =| do. ; 2
9uek.5 {24.5 (58.2 | 13.1 | 18.2 !120 ' 805 | do. 2
9425.5 j 16.1 65.2 6.53 | 18.7 60 i 706 [Fine md.soft ed) 2

3820 -17 -


















TABLE 3 - Relation of oil-base drilling-fluid
viscosity to temperature

Stormer viscosity (600.r.p.m. )
South Coles Levee (Calif.) test

Core | Temperature, ' ‘Driving weight,
No. } . | Centipoises grams
7! ee) pe ! 500
i 81 ' 102 i 400
i 100 | 72 i 300
| 135 i 43 200
i 201 H al 120

Rangely (Colo.) test

8 & i 66 | 280
| B 57 250
| 89 | 43 i 200
| 1: : 29 i 150
H 135 : a1 | 120
i 172 16 | 100
i 201 17 | 105

DISCUSSION OF RESULTS

The results of the analysis of the cores from the South Coles Levee
test listed in table 4 are shown graphically in figure 4, The average valucs
for the oil content, water content, drilling-fluid filtrate (oil) content,
permeability, porosity, and chloride concentration in the interstitial water
found in the cores recovered from the F-1 sand were determined, and the
average values With their corresponding statistical limits of uncertainty
are listed in table 5.

As is evident from figure 4, filtrate from the drilling fluid was found
in nearly ell of the samples from the South Coles Levee well, and an aver-
age of 21 percent of the pore space of the F-1 section of the Stevens sand
was filled by filtrate from the oil-base drilling fluid. It is interesting
to note in figure 5 that in general, although there are several oxceptions,
the cores that had the highest permeability have the largest percentage cof
pore space occupied by drilling-fluid filtrate.

Because most of the oil found in the cores from the South Coles Levee
well was filtrate, and because some oi1 was forced out of the cores by gas
expansion when the cores were brought to the surface, the total oi]. content
of the cores determined by customary core-analysis methods is not an accu-
rate measure of the oil actually present in the zone, However, tho water
content of the cores may be representative of the water content of the
formation if the following considerations are accepted, A formation test

3820 - le-






EFFECT OF OIL-BASE DRILLING FLUID FILTRATE ON ANALYSIS

OF CORES FROM SOUTH COLES LEVEE, CALIF.,
AND RANGELY, COLO., FIELDS

by

George L. Gates, 2/ Frank C. Morris,2/ and W. Hodge Caraway 2/

CONTENTS

Introduction..
Acknowledgments .....eeeeeie
Coring South Coles Levee well.
Coring Rangely well........eeeee
General considerations in coring.
Method of core analysis.......
Method of tracer analysis

Sample calculation.....ecccceccccconee
Properties of drilling fluids used in coring.....
Discussion Of TesultS...csecesscscsecceeecs
CONCLUSIONS cece eeerec cece eeeeseccereccsecerees

ILLUSTRATIONS

Lael
BBeowu ce worl

Follows

1. Schematic sketch showing probable changes of fluid
contents of cores during coring operation with
oil-base drilling fluid in the hole...........6.

2. Schematic diagram showing position of specimens
Within the COre..eeecceeeeeceeeeeeeceseccceceees

3. Apparatus for combustion analysis of organic
CHLOPId] oc ceeececececcccvercsccscecccsvevesssene

4. Coregraph South Coles Levee, Kern County, Calif.,
Test Welle sccsccerccccccsccccccacccccscsvvccces

5. Relation of the air permeability to the volume of
filtrate (oil) in the South Coles Levee cores...

12

12

1/ Petroleum engineer.
2/ Chemical engineer.

Report of Investigations 4716


Bureau of Mines-A.P.I,

TABLE 4 - Core Analyses (Cont'd, )

Cooperative Study

Field: South Coles Levee, Kern County, Calif.

T | H ' “fLoca=

‘Liquid content, ; H ' tion

percent pore space |Horizon-/| Chloride | of

| { Drill- | Poros- jtal air lconc. , cl sample

; ing | ity, |permea- jgrains per! in

; { fluid | percent; bility, |gallon of core
Depth, ‘Total oil of bulkimilli- /intersti- ‘(see
feet i oil Water (filtrate volume jdarcys tial water | Core description fig. 2)
9336.50/43.3 38.8, 34.3 | 19.2 | 78 948 do. [9
9336.5 43.7 41.2! 34.0 | 18.8 | 78 785 do. &
9336.5a/H0.5 42.41 32.1 | 18.5 | Th 683 do. 6
9337.5 55. 2 13 | 42.8 | 19.2 |190 165 Coarse soft sand| i
9338.5 | 43.2! 28.0 | 20,0 | 100 821 =-|Fine herd sand x
9339.5 55. h Wh.o] 45.3 ! 17.u | 62 784 do. | 3
9339.5 154.3 42.0! 41.6 | / ‘120 825 do. 1k
9340.5 40.6 47.7! 31.0 | ota | 58 762 do. 10
9340,50/43.8 45.9) 30.4 | 20,8 | 59 788 do, uu
9341.5 | 34,3! 514 21.9 | 19,0 |110 80h do. 2
9342.5 1 39.4:39.2) 25.2 | 21.0 | 6 985 do. | 3
9342.5 | 38.9.51.6| 28.9 | 18.0 | 100 843 do. ; 2
9343.5 48.7 47.7) 36.0 | 19.5 ; 160 | 917 Pine med.hrd,ed.; 3
9343.5 | 41.8 40.9 | 30.3 | 21.8 | 95 760 do. | 2
9544.5 124.7,40.6; 18.7 ; 14.9 | 79 847 Med.grain hrd.sd) 19
9344.5 129.4 46.8} 20,1 | 20.2 | 68 84h do. 1 22
934k. 5a/32. 6.44.6] 21.4 | 20.0 | 6% 883 do. | 22
934k.5 131.2 15.3 21,0 | 20,0 i 66 i 858 do. 20
9345.5 -38.4.37.3! 24.3 | 20.4 1160 i 957 Coarse hard sand! 1
9345. 5a/33.2. 45.5 | 22.9 | 20.7 | 99 | 858 do. 2
9346.5 (42.8; 36.2 | - | 21.1 1290 836 Crs,med.hrd,sand| 1
9347.5 | 6242) 39-2 | 48.9 | 16.9 1260 | 785 Med. soft fine- |

i H : crs. sand 3
9347.5 159.1/30.9} bed 18.5 ;210 884 do. 2
9348.5 173.5'41.4) 40.2 | 3,40 ell, Wh Fine hard sand 2
9349.5 i57.4'36.7] 45.6 | 19.4 1350 | 972 Fine med. soft ea 3
9349.5 (55.7:33.4) 48.3 | 20.8 |h20 ; 848 . do. 2
9350.5 53-8, 37.4! 46,2 | 18.5  |180 i 799 Fine to crs,med,

\ t | ; soft sand 2
9351.5 |45-9}39.8! 26.5 | 19.7 1140, | 785 |Fine-crs,med,

| ‘ i | { soft sand i o1
9352.5 151.6. 38.4) 40,1 | 21.7 {190 | 790 Fine soft sand | 3
9352.5 49.6 35.7; 40.9 | 22.5 |150 | 923 do. 2
9353.5 312 '32.0 | 41; 21.9 1330 | 890 Fine-crs.soft sd) 1
9354.5 “49.7 24.2! 35.7 | 22.3 {460 870 do. 2
9355-5 (35-3.14.K | 12,5 7.00 +23 | 928 Very hard sand 1
9356.5 8.5 (30.1) 37.3 | 22,0 |200 | 877 -|Fine-crs.soft ad; 1
3820 -15-


The toluene extract from the core taken at a depth of 9338.5 feet was found
to contain 0.008395 gm, of chloride. Therefore, the corresponding volume
of filtrate in the core was 165.8 times 0,008395, or 1.392 ml. As the
total pore volume of the core specimen was 4,976 ml., the filtrate occupied
1.392/4.976, or 28.0 percent of the total pore space as indicated in the
fourth colum of table 4.

Many of the core samples selected for analysis were taken from different
positions within the core, as listed in table 4 and shown in figure 2, to
determine whether the core was uniformly flushed or whether the contamina-
tion was concentrated in the outer portion of the core. By selecting
adjacent sections of the core, data have. been obtained that show not only
the tracer distribution in the core but the relationship between the
porosity, permeability, liquid content, and chloride concentration of ad-
jacent horizontal parts of the core specimens.

PROPERTIES OF DRILLING FLUIDS USED IN CORING

The oil-base drilling fluids used in both the South Coles Levee and
Rangoly wells were preparod by mixing a carefully selected Diesel oil,
air-blown asphalt, limestone, unslaked lime, and a small amount of water.
The properties of the drilling fluid used in the South Coles Lovee test are
given in table 1; those of the drilling fluid used in the Rangely test ‘are
given in table 2. ‘The volumes of filtrate from the drilling fluid samples
at 200%. were mch smaller than the filtrate volumes obtained from water-
pase drilling fluids in a l-hour filter test but were greater than those
obtained from many of the oil-base drilling fluids used in other wells.

The drilling fluid used in the tests has one unusual property. Or-
dinarily, the volume of filtrate obtained during a filter test is large
when the temperature of the drilling fluid is high, but several samples
of the drilling fluids used in the tests gave larger filtrate volumes at
140° F. than at 2000 F,

To indicate the degree of viscosity decrease of the fluids when the
temperature of the fluids is increased, the data obtained from the tests
made on two of the samples are given in table 3.

The results of laboratory tests listed in tables 1 and 2 show that the
water content of the filtrate from the drilling fluids contained less than
0.2 percent by weight of water. The filter tests were made with filter paper
as the porous medium; and because the filter paper may have absorbed water
from the filtrate, the tests were repeated at approximately 230° F. on two
samples of drilling fluid, using for the porous medium a silica disk that
contained a known volume of wator. After 2) hours filtration, the water
content of the filtrate and the silica disk were measured by toluene dis-
tillation and recovery of the water In a trap, It was found that no
measurable volume of water had filtered into the disk during the test. Ac-
cordingly, these tests indicate that the water content of the filtrate from
the drilling fluid was negligible. The tests indicate that the water found
in the coreg did not come from the drilling fluids.

3820 -9-


4 i
vi
vd
/ V
{ fi \
oa teed
+ f
is ee eee =
S Ss =
_ [4 = + Ro
BS
= 4 53 FP
Fe SO 8 kt
=Semme 1 ihe ai E et Fe]
SE bal Fs PP-
ia 5 ¢
> an
ie ra eeeee Kt
Pe ie: sansa ced
LH ; O06 ¥| Se
t 7 1 tr Yi Sec
f # # coer
fe
“PI 7
t 2 »
iF
+ { + me t
I % H Cee I
eee
: as
A ca 4
S| ist a
aos Kod
=sa08 ett ® i
5 : >
4 ¥ aa
"4
4 ry id Lb :
Ed k
i iad at SN
¢ E
+ HE
Ld -$ p, > 4
4
<a +
ras
am
fA Fai Lf
i i

Figure 4. - Coregraph South Coles Levee, Kern County, Calif., test well.



TABLE 2 - Properties of oil-base drilling fluid used in core-contaminetion test,
Rangely field, Rio Blanco County, Colo.

'Drilling-fluid

T
Hy
{ 7 Drilling fluid ! filtrate i
Hl Filtrate volume, [f T "Water Tracer
Core ‘milliliters per hour. Weight density | Water content | content yL/ concen-
Core. interval, Room , 140° 20007 7 Weight, Volume, weight ‘tration, |
No. feet temp., F. | F. | 1b./gal. 1b./et.3 percent! percent, percent mg. /ml. Remarks
i { H 7 * ~ + +
3 s9he-5992 10.8 12.2 ! 2.2 ! 9.0 67-7 + 3.30 3.58} 5675”
4 5992-60h2 1.1 (2.8 | 3.0 + 9.0 67.5 3.30} 3.58: 5 5 .bh
5 6042-6083 £1.1 12.3 ; 2.0 . 9.3 69.5 3-10 3.45) | 5.56
(a)6 6083-6134 1.2 11.3 P24 1} 9.2 68.2 2.96) 3.2h) , 5.99
(o)6 , ; 1.0 (1.4 j 20 : 9.0 67 4 3-36: 3-63. 0.1 | 5.95
8 6183-6233 i1.2 i144 | 2.0 { 9.1 + 68.0 3.26 3.55! i 5.88
9 6033-6283 2.0 11.8 | 2.0 | 9.0 | 68.9 3.51. 3.88; 5.42
11 6288-6338 {1.6 12.0 | 2.0 | 9.2 | 69.0 3-53: 3.901 5-55
12 6338-6357 1.1 12.4 | 3.1 | 9.1 | 68.0 3.57 | 3.89 5.23
4}- 6357-6407 1.1.7 j2.2 12.2 | 9.0 | 67.5 3.53 3.82 | 5.40

1/ The drilling-fluid filtrates from all of the mud samples were combined, and the water content of
the combined samples was less than 0.1 percent by weight.

3820 -u-


TABLE 4 - Core Analyses (Cont'd. )

Bureau of Mines - A.P.I. Cooperative Study

Field; South Coles Levee, Kern County, Calif.

Loca~
' Liquid content, i : (tion

: 4
ercent pore space | i\Horizon-| Chloride | of
[Drill- | Poros- (tal air ‘conc.,Cl | isample

: 1 ing | ity, j|permea- grains per: in
A ‘ fluid ' percent} bility, | gallon of core
Depth, Total, oll of tulkimilli-” :intorsti- | (see

feet | oil Waterfiltrate! volume ‘darcys ‘tial water|Core description fig.2)
9356.5 49.1,55.7, 38.8 20.8 ' 490 * 875 Rine-cre.soft sd. 3

wrk seokeie soe | iy geo | ee aa a
9357 5 . 58. 12 . “Toa / : Oe

9358.0 '53.7:51.6 44.5 © 17.6 1 460 i 630 ‘Med.fine soft sdi 1
9358.5 1 48.9:32.3 , 39.7 | 22.9 130 | 1220 |Fine soft sand 1
Boas Grie363. ips | a8 go Bede 2
9359.5 (50.4'36.8 41.9 | 20.4 "170 819 ‘Fine-crs.soft sd 1
9359.5 1 46,3°32.5 35.3 | 21.2 500 + 875 - do. : 2
9360.5 49.8°33.9 39.7 , 22,2 480 i 189 Fine soft sand 2
9361.5 49.7;31.8 42.7 22.6 330 789 do. iu
9362. 5a/50.9, 25.0 | 36.6 | 2h. - / 72 Fine-crs, soft’ sa! 10
9362.5 52.5:27.1 40.0 | 21.8 - » 1020 |! do, 19
9362.5 51.3,28.5. 41.1 | 21.50/ - 88h | do, 12
se} etek tie wei mo | Bet SS iD
9363.5 5762.37.04, bb6 1 17.5 | : : O.

9364.5 ' 33.843... 2h.3 20,1 170 . 668 ‘Fine soft sand 1
9380.5 1 43,0:45.1! 35.8 :; 21.9 | 210 | 789 i ao, 1
938045 ‘yak ur. : 2.9 20.9 f 300 | je ' ao. i 3
9380.5 51.7 55. 3.0 | 18.5 | 210 | 3 : o. :
9381.5 23.7/63.1 ; 16.0 H 19.5; 61 751 'Fine mod.soft sd: 2
938h.5 28.3 es ! 16.7 11-3 ; 32 i Zz Pine ned.hard sd | 2
9385.5 (11.3:78.5 + 9.80; 17.0 © ; 9 oO. H
9386.5 14.1:70.9 | 7.47 | 20.6 | 50 » 902 Fine hard sand | 13
986.5 Pree : he? : £007 | 110 3 ! 23 i ac. | 2
9387.5 27. .2! 16.2 |! 11,2 2.8 | ' O. A
9388.5 26.5.58.5 , 11.5 | 18.1 | 79 : 626 IFine md.soft sd} 2
goood ingens | Saline | ee | de
9391.5 °38.0;51.0 ' 13.0 | 3.8 | ol 589 Fine hard sand | 2
9592.5 17.2'53.5 | hse. 22.4: hg 820 Fine med, soft sd 1
9595-5 “21,2°60.7 | 8-95 : 2h : io : pe vine hard sand | ;
9394.5 119.6 50.0; 8.52 , 21.0 > 15 } 50 O. |
9394.5a/20,0:51.8 6.54 20.1 | 13 \ 793  +|Fine hard sand | 2
9395.5 22.5:61.5 | 7.98 | 18.5 : 2 823 do. | 2
9396.5 149/819 { 658) 121 | 43 | 5k Hard sandy shale | 2
9397.5 | 9.2475.2 1 1,63} 17.6 | 32 . 768 ‘Fine soft send | 2

'

3820 - 16-






Stanolind engineers selected the core samples and sealed them in cans
at the well. One of the samples was sealed in a can, oil filling the space
not occupied by the core sample to prevent vaporization of fluids in the core.
In all, 22 samples selected from cores taken between depths of 5,943 and
6,429 feet were shipped to the San Francisco laboratory of the Bureau of Mines,
where they were analyzed in the same manner as were those from the South
Coles Levoe field.

GENERAL CONSIDERATIONS IN CORING

To better understand the probable movement of fluids into and out of
cores during coring operations, reference is made to figure 1, in which the
relative volumes occupied by the various constituents of cores in tho suc-
cessive stages of the coring operation are shown diagrammatically. In its
natural state, a reservoir sand consists of sand with the pore spaces filled
with oil (generally saturated with gas), interstitial water, and sometimes
free gas. At the loft of figure 1, case I-A represents a reservoir that
contains only oil and water in the poo space, and case II-A represents a
reservoir that contains not only oil and water but also free gas.

When the pressure at the bottom of the colum of drilling fluid exceeds
the reservoir pressure, filtrate from tho drilling fluid enters the cores
and displaces some gas and oil, as is shown diagrammatically for cases I-A
and II-A by cases I-B and II-B, respectively. In formations that do not
produce water with oil, very little if any interstitial water probably is
displaced by filtrate from oil-base drilling fluids. The reasons for this
pelief are discussed more fully in subsequent soctions of this report.

After the core is cut, and while it is being brought to the top of tho
bore hole, gas dissolved in the oil comes out of solution as the pressure
is reduced. The evolved gas probably displaces some oil, so that whon the
core is removed from the barrel it contains, in addition to crude oil and
interstitial water, some oil filtrate from the drilling fluid. The distri-
bution of fluids in the cores at the end of this step are illustrated
diagrammatically in casos I-C and II-C in figure 1.

The volume of the filtrate in the core may be measured by moans dis-
cussed later in this report, and if it is assumed that the interstitial water
content of the cores has not changed, the volume of crudo oil and free gas
remaining in the cores may be calculated. However, the volumes of oil and
free gas originally in the core specimen are not know, because the gas
coming from solution in the oil carries with it, from the core, som of the
crude oil originally present, together with an unknown volumo of filtrate
(oil) that entered the core during the coring operation.5/ If, however, it
is believe that no free gas exists in the cored zone and it is assumed that
no interstitial water left the core during the coring operations with oil-

5/ For simplicity, it is hero assumed that no water is moved from the core
by gas expansion. The possible movement of water by gas expansion is
discussed more fully in subsequent sections of this report,

3820 -he-


and subsequent production from the well showed that the fluid produced from
the formation contained virtually no water. Because no watér is produced
from this well, it follows that virtually none of the water in the sand is
moved into the well bore when crude oil flows through the formation. Con-
sequently, it is reasonable to believe that no water moved in the cores
when oil from the drilling fluid filtered into them. Water was found in all
of the cores, and because the oil-~base drilling-fluid filtrate was found ta
be essentially oil and not water (see tables 1 and 2), it follows that thé
water found in the cores was present in the formation when the cores were
cut.

The effect of the reduction of pressure that takes place when cores are
brought to the surface was investigated by Botset and Muskat ) who report
that the expansion of gas in the cores during the pressure reduction produced
oil with little or no loss of water unless the sands had abnormally high
water contents.

The filtrate volume curve in figure 5 follows the total oil-content
curve, indicating a uniform relationship between these two core properties.

A part of the electric log of the F-1 sand is shown in figure 6, and the
water content of the cores from the corresponding depths is plotted on the
same graph to show the relationship between these two. properties. Although
the relationship found was only fairly uniform, as would be expected, the
section of the zone found by analysis of the cores to contain the larger
quantity of interstitial water also was found to have the lower resistivity.
Specifically, the average water content of the cores from the section between
9,320 and 9,350 feet is about 4O percent of the pore space, and the resistiv~
ity of that section averages about 7 ohm n/m. In contrast, the average
water content between 9,380 and 9,400 feet is about 65 percent of the pore
space, and the resistivity of that section averages about 5 ohm n/m. Also,
considerable variation of the resistivity may be expected as @ result of
the variation found in the chloride concentration in the interstitial water.

Because both the quantity of interstitial water in a zone and the con-
centration of chloride in the interstitial water may affect the resistivity
of the zone, a graph showing the relationship of these factors is shown in
the lower portion of figure 6. An increase in either the interstitial-water
content or the concentration of salts in the water should cause a decrease
in the resistivity of the zone in which they occur. For that reason, the
resistivity of the zone was plotted with the reciprocal of the product of
the water content of the cores (expressed as a decimal) and the concentra-
tion of the chloride found in the core water. It may be seen in figure 6
that the curve obtained by plotting these deta roughly follows the trend of
the resistivity curve of the electric log. .

g/ Botset, H. G., and Muskat, M., Effect of Pressure Reduction upon Core
Saturation: Trans., Am. Inst. Min. Met. Eng., vol. 132, 1939, P. 182.

3820 - 13 -


It should be remembered that the chloride concentrations cannot be
reduced by dilution of the interstitial water by filtrate from the drilling
fluid when oil-base fluids are used, because the filtrate is o11 and not
water. However, the measurements of chloride concentration in the inter-
stitial water depend on both the chloride determination and the determina-
tion of the volume of the water in the core sample and as a result contain
the experimental errors inherent in both procedures.

As calculated, the limits of uncertainty are the result not only of the
number of observations and the natural variations in the properties of the
cores but hinge also upon experimental errors in the core analysis, The
variations in the limits of uncertainty resulting from experimental errors
are approximately the same for all cores except those of very low porosity,
such as most of the cores from the Rangely field, Because cores of low
porosity contain small quantities of water and oil, the experimental error
in measuring the small quantitics (0.40 ml. of water, for example) becomes
a large percentage of the total, and this accounts probably for the erratic
data obtained when Rangoly core samples were analyzed.

Variations noted in the results of the analyses of Rangely cores may be
the result, also, of the unusually large concentration of chloride in the
interstitial water. As stated in the section of this report describing tho
mothod of core analysis, the weight of chlorides found in the cores was
found to be important, and corrections to the oil and brine contents of the
cores were made on the assumption that all of tho salts present were sodium
chloride, as determined in the inorganic chloride analysis. Undoubtedly,
other salts also aro present, and no attompt was made to measure them be-
cause at the timo the core analysis was mado it was not realized that they
might be important.

The effect of a large quantity of water-soluble salts in cores is
illustrated by the Rangely samples, which contained large concentrations of
chloride. When corrections for the salts were made, the oil content was
decreased an average of about 7 percent of the pore space, and the water
(brine) content of the cores was increased about 2 percent of the pore space.

The relationship between water content of the cores from the South
Coles Levee well and air permeability of the cores is shown in figure 19.
The general trend of the data indicates that the water saturation increases
as the permeability decreases and is similar to that shown by Schilthuisl2/,
for the relationship between water content and permeability of cores from
the Anahuac and Tomball fields of Texas, and by Pyle and Sherborne13/ for the
cores from a California field.

12/ Schilthuis, Ralph J., Connate Water in Oil and Gas Sands: Trans., Am
Inst. Min. and Met, Eng., vol. 127, 1938, p. 209.

13/ Pyle, Howard C., and Sherborne, John E., Core Analysis: Trans., Am,
Inst. Min. and Met. Eng., vol. 132, 1939, p. 49.

3820 - 23-


Fair correlation was found between the electric-log resistivity of the
formation and (1) the water content of the cores, and (2) the reciprocal
of the product of the chloride concentration of the water content of the
cores at corresponding depths.

The tests indicate that filtrate from oil-base drilling fluids entered
virtually all of the cores from the South Coles Levee and Rangely fields. ~
Similar tests now being made in shallower wolls will reveal whether uncon-
taminated cores my be obtained from formations at lower pressures and
temperatures than were found in the wells described in this report.

The described method of tracer analysis has been found satisfactory,

and it should be helpful in all probloms that require a tracer soluble in
oil and insoluble in water.

3820 - 25 - “Interior - Bureau of itines, Pittsburgh, Pa."






INTRODUCTION

It is generally.recognized that if the loss and contamination of the
fluid content of a core specimen from an oil- and gas-bearing formation in a
well were eliminated, the value of core analysis would be increased greatly.

In an effort to obtain uncontaminated cores, extensive field tests were
carried out in the Mercy field, San Jacinto County, Tex., with the Carter
pressure core barrel and an oil-base drilling fluid having very low rates of
filtration as measured by the stanrdard A.P.I. drilling-fluid filter test. A
tracer was added to the drilling fluid filter test. A tracer was added to
the drilling fluid to provide a means for quantitatively measuring the volume
of oil filtrate in the cores,

Unfortunately, as stated in the report on the Mercy tosts,3/ very little
oil sand was penotrated in coring the well; and because the performance of
the pressure core barrel was not entirely satisfactory, the tests did not
provide a complete answer to the question: Does a filtrate from an oil-base
drilling fluid enter cores, and how does it affect the results of the core
analyses?

Consequently, it was docided to continue the cooperative study on the
contamination of cores with conventional core barrels instead of the
pressure core barrel to cut the cores in oil-base drilling fluids to which
a tracer is added,

Through arrengements made with the Ohio O11 Co. and the Stanolind Oil &
Gas Co. cores were obtained froma well in the South Coles Levee field, Kern
County, Calif., and from a well in the Rangely field, Rio Blanco County,
Colo., which were cut by conventional core barrels while oil-base drilling
fluid containing an organic tracer soluble in oil and insoluble in water was
being circulated.

A theoretical materiel balance was made for probable changes in the
fluid content of cores during coring operations. One hundred three samples
of cores from the South Coles Levee well and 22 samples from the Rangely
well were analyzed, and the volume of oil filtrate in the samples, the total

37 Pressure Coring: A Bureau of Mines Report to the Special Study Committee
on Well Spacing and Allocation of Production of the American Petroleum
Institute, on Tests Made With the Carter Pressure Core Barrel in the
Mercy Field, San Jacinto County, Tex., February 29-March 26, 194.
This report, dated November 1,.1945, was presented by D. B. Taliaferro,
of the Bureau of Mines, Bartlesville, Okla., at the Chicago meeting,
November 1945, American Petroleum Institute Production Bulletin 231,
vol. 25 (IV), 1945, pp. 78-86.

-1-


10.

ll.

1e.

13.

4.

15.

16.

17.
18.

19.

3820

ILLUSTRATIONS (Cont'd. )

Relationship between electric-log resistivity and
water content and reciprocal of the product of
the water content and the chloride concentration
in the interstitial water of South Coles Levee
COTES ceeserccccccccencscvesseesscesssesesecesees

Relation of location of sample specimen within
the cores from South Coles Levee test to the
volume of filtrate (oil) in the sample........6-

Relation of location of sample specimen within
the cores from South Coles Levee test to the
total volume of the oil (crude and filtrate) in
the sample....ecccsscccccevesseece

Relation of location of sample specimen within
the cores from South Coles Levee test to the
volume of the interstitial water in the sample..

Relation of location of sample specimen within
the cores from South Coles Levee test to its
POKOSIEY. ss eeeeececerevece

Relation of location of sample specimen within
the cores from South Coles Levee test to its
PETMCADILITY . cece rerccecceccccccaccseccsccseecer

Relation of location of sample specimen within
the cores from South Coles Levee test to its
interstitial water chloride concentration.......

Relation of location of sample specimen within
the Rangely cores to the volume of filtrate (oil)
in the sample..ccccccccccccccssccsecscescseseces

Relation of the location of sample specimen within
the Rangely cores to the total volume of the
oil (crude and filtrate) in the sample..........

Relation of the location of sample specimen within
the Rangely cores to the volume of water (brine)
in the sample ..ccccccecccecccececccesssscesecese

Relation of the location of sample specimen within
the Rangely cores to its PoroSity....secssscceee

Relation of the location of sample specimen within
the Rangely cores to its permeability.........06

Relation of the location of sample specimen within
the Rangely cores to the chloride concentration
in the interstitial water in the samples........

Relation of the air permeability to the water
content of the South Coles Levee cores....sseeee

-ii-

Follows

Page

12

20

20

20

20

20

20

20

20

20

20

20

22


METHOD OF TRACER ANALYSIS

The tracer used in the tests was suggested to the Bureau of Mines by
the Shell 011 Co., which has applied for a patent covering its use. As
the tracer and the general method of analysis wore found to be’ suitable
for tho study here reported and the tracer may have helpful application to
other problems, its use will be explained in some detail. "

The tracer used in the tests discussed in this report is known to the
trade as "Aroclor." This tracer material was selected primérily because it
is soluble in oil, insoluble in water, not affected by bacteria, and very
stable at high temperatures. Aroclor 1254 was used in the South Coles Levee
well and Aroclor 1270 in the Rangely well. Aroclor 1254 contains approx-
imately 54 percent and Aroclor 1270 approximately 70 percent chlorine by
weight. Aroclor has no harmful effect on the men handling it and does not
affect materially the customarily determined properties of oil-basé drilling
fluids, such as their viscosity and filter characteristics.

In the tests, the Aroclor was added to the drilling fluid in a minimm
concentration of about 3 pounds per barrel of drilling fluid. It is
important to mx the material with the drilling fluid uniformly; to do so
required knowing the volume of liquid in the hole and elsewhere in the
drilling-fluid circulating system, When the tracer is to bé dissolved in
the drilling fluid, it is necessary to know the time required for the
drilling fluid to circulate completely around the flow system so that the
tracer can be added uniformly while tho drilling fluid is pumped to the
pottom of the hole and back to the surface. Also, the solubility of the
tracer in the oil-base drilling fluid should be checked to be sure that all
of the tracer is dissolved in tho drilling fluid.

While a core is being cut, the drilling fluid in the hole is sampled;
the samples are taken so that they will be representative of the drilling
fluid opposite the formation cut by the bit. Later, these samples are
filtered in the laboratory at 200° to 230° F. to obtain enough drilling-
fluid filtrate so that the concentration of the tracer in a unit volume
of the filtrate can be measured,

The combustion analysis for the chlorine content of the tracer is
based largely on the method described by Winter8/, modified slightly to fit
the neods of the described work,

The tolvenc extract obtained during the core analysis contains the oil
and tracer that may have been in the core samples. To avoid loss of the
tracer, tho toluene extract is concentrated in a stream of air without
application of heat until the extracted ofl remains. The extract concen-
trate is washed into a combustion tube (fig. 3) with benzone, which thon is
evaporated from the tube with a small stream of air.

8/7 Winter, Paul K., Detormination of Halogen in Organic Compounds: Ind. and
Eng. Chem, anal. ed., vol. 15, 1943, pp. 571-574.

3820 -7-


of 9,284 feet, and the water-base drilling fluid was displaced by oil-base
drilling fluid. The next fow feet were cored before a tracer was added to
the drilling fluid, so that blank tests could be made on the cores obtained,
The cores were cut with a 6-inch bit with a conventional-type core barrel
capable of holding a core 15 feet long. The cut cores were approximately
2-1/4 inches in diameter.

Before the second core was cut, 1,500 pounds of froclory/ (an organic
chloride tracer material that is insoluble in water, soluble in oil, extrem-
ly stable et high temperatures, and is not affected by bacteria) was added
to the drilling fluid as uniformly as possible. It was calculated that
that quantity of Aroclor would yield a concentration of about 3 pounds per
parrel of drilling fluid in the well, tanks, and circulating system, Before
the tracer material was added to the drilling fluid, the “round-trip" time
required for the drilling fluid to pass down the drill pipe and up the
annulus between the drill pipe and the casing in the hole was determined by
adding approximately 1 quart of oats to the drill pipe and noting the time
required for the oats to appear on the screen of the shale "shaker" when the
circulating pump was operated at constant speed. After measuring the time
required for the drilling fluid to complete the round trip in the woll, the
Aroclor was added to the drilling fluid at a uniform rate upstream from
the shale shaker, so that the last pound of tracer was added to the drilling
fluid at the time the latter completed the circuit from the tanks to tho
bottom of the hole and back to the surface.

A nearly continuous section of the F-1 Stevens sand zone 150 feet thick
then was cored toa depth of 9,444 feet; samples wore taken from each foot
of tho 103 foet of core recovered. ‘Thirty additional samples wero selected
from small sand sections of the coros recovered from the F-2 Stevens zone
below the F-1 zone,

It is estimated that the bottom-hole temperature in the well is
approximately 250° ¥,

CORING RANGELY WELL

Engineers of the Stanolind Oil & Gas Co. cored the well in the Rangely
field and obtained cores from tho Weber sand while an oil-baso drilling
fluid containing dissolved Aroclor as a tracer was in the hole. The Rangely
test diffored from the South Coles Levee test in that the bottom-hole
temperature (approximately 160° F, ata depth of 6,000 feet) in the Rangoly
field is about 90° F. less than that. in the oil-bearing reservoir in the
South Coles Levee field. Also, the diameter of the cores cut in the Rangely
well was 5-3/4 inches, whereas those obtained in the South Coles Levee field
wore 2-1/4 inches in diameter. Furthermore, the cores cut in the Rangely
well were sbout 50 feet long, or slightly more than three times as long as
those cut in the South Coles Levee well, and the selected samples at Rangely
wero spaced mich farther apart than in tho South Coles Levee field,

A/ A product of the Monsanto Chemical Co., St. Louis, Mo.

3820 -3-


aie
topes

saeasenens ~ = { 7 T x : $T@i4 tee fe

+-

SEaeunes
ty

thet

rf

=O |

oess

Ss Saseeesers &
footed tee

Petts pet
Seseeeenen|

PERMEABILITY '

ORICLING] FLUTO
PORGSITW |



Bureau of Mines SS Brreay
Report of Investigations 4716 “EPARTENY

“EF U~ 1959

EFFECT OF OIL-BASE DRILLING FLUID FILTRATE ON ANALYSIS
OF CORES FROM SOUTH COLES LEVEE, CALIF.,
AND RANGELY, COLO., FIELDS

By GEORGE L. GATES, FRANK C. MORRIS, AND W. HODGE CARAWAY

United States Department of the Interior — August 1950

| metadc38546






a
>
wn
m

z
ws SAND SAND SAND
5 og LA 8 Wad
oad
> z3
a>
x
su DRILLING FLUID [GAS _
2 w| CRUDE OIL FILTRATE (OIL) ORILLIN Ff ail
«a CRUDE OIL GRUDE OIL
a> WATER WATER WATER
OIL RESERVOIR CONTAMINATED AT SURFACE AFTER
NO FREE GAS CORE AT BOTTOM GAS EXPANSION AND
OF HOLE COOLING ON THE
CASE 11 WAY TO” THE SURFACE
SAND SAND SAND
LA 8 Pa
a TCaae On| ppp
IL IL DRI Ur
cRupe oi. | | —-—— {oiL) rath rE, IL
GRUDE OIL GRUDE OIL
WATER WATER WATER

OIL RESERVOIR
FREE GAS PRESENT

CONTAMINATED
CORE AT BOTTOM
OF HOLE

AT SURFACE AFTER
GAS EXPANSION AND
COOLING ON THE
WAY TO THE SURFACE

Figure I. - Schematic sketch showing probable changes of fluid contents of cores
during coring operation with oil-base drilling-fluid in the hole.


MILLIDARCYS

PERMEABILITY,

AIR

100)

80)

8

co

20)

ol a> ——— a) "

ll

2__]
EDGE OF CORE CENTER OF GORE EDGE OF CORE

Figure 17. - Relation of the location of sample specimen within the Rangely cores
to its permeability.






w 500}
Ba
°o
4
<
2
i
=
= 300
=] b- °
a So
<
w
2 200 °
w
2 ———————————
s=
[4 —o
+ 100
2 ——— _
———— ——— ————— = ——
p—-— >

~O

EDGE OF GORE

CENTER OF GORE

EDGE OF CORE

Figure Il. - Relation of location of sample specimen within the cores from South Coles Levee

test to its permeability.



ELECTRIC LOG

SOUTH GOLES LEVEE FIELD
KERN COUNTY, CALIFORNIA
SELF POTENTIAL RESISTIVITY
= 100 _MILLIVOLTS t O OHMS Mo/u_!0
z 3 =
x o _
=. 2 = =
—}
= z =
= ——
<< 2
S a
a
a f=)
OL
—— w| $ —
‘= 2
= a a
°o

[)
WATER PERCENT

100
PORE SPACE

1000 / GL x HOH

: : £___4 6.
z g —
= —
al 2 = = —_
= 3
by a = ————
Sz © —————
= 2
z = =
= a! =
ee ° a
——_ FS Ss
- SELF POTENTIAL 4 lo RESISTIVITY 10
100_ MILLIVOLTS OHMS M°/M

Figure 6. - Relation between electric-log resistivity and water content and
reciprocal of the product of the water content and the chloride
concentration in the interstitial water of South Coles Levee cores.




oil and water content, the chloride concentration in the interstitial water,
the porosity, and the dry air permeability of the core samples were deter-
mined. Some of the samples were selected horizontally along the diameter of
the cylindrical cores to find how the core properties vary from the outside
to the interior of the cores,

The results of the core analyses from the South Coles Levee field
indicated that oil filtrate from the oil-base drilling fluid was present
in virtually all of the permeable cores. An average of about 20 percent of
the pore space of the cores was occupied by filtrate. Of particular
significance is the indication that the water content of these cores may be
considered representative of the interstitial water content of the sand cored
in this well,

It was found, also, that the resistivity curve of the electric log for
the South Coles Levee well correlates in some degree with the following
core properties: (1) the interstitial water content and (2) the reciprocal
of the product of the water content and the chloride concentration.

ACKNOWLEDGMENTS

The study was conducted under the general supervision of R. A. Cattell,
chief, Petroleum and Natural Gas Branch, Bureau of Mines, and H, C. Miller,
formerly supervising engineer, San Francisco Office, now Bureau of Mines
Regional Director, Region III, in cooperation with the American Petroleum
Institute Special Study Committee on Well Spacing and Allocation of Pro-
duction.

Grateful acknowledgment is made to Robert N. “Ayars, Ross Bolton,
Clayton W. Stephens, and W. H. Barlow, of the Ohio Oil Co., for their co-
operation and assistance in supplying cores and well facilities in the
South Coles Levee field test. Grateful acknowledgment algo is made to
Robert W. Stuart, J. B. Clark, and J. M. Roe, of the Stanolind 011 & Gas
Co., for their cooperation and assistance in supplying cores and field date

in the Rangely field test.

Special thanks are due to Jan Law, consulting engineer, Los Angeles,
Calif., for his helpful advice and suggestions regarding the statistical
methods used in analyzing the core data.

The writers express their thanks for review and constructive criticism
of the manuscript to Robert N. Ayars of the Ohio 011 Co., J. B. Clark of the
Stanolind Oil & Gas Co., and to H. C, Miller, G. B. Shea, Frank G, Miller,
Arthur N. Fried, and Donald C. Crowell, of the Bureau of Mines.

CORING SOUTH COLES LEVEE WELL
The first of the two tests discussed in this report was made in a well

in the South Coles Levee field, Kern County, Calif. After 7-inch casing
was cemented at a depth of 9,250 feet, the hole was cleaned out to a depth

3820 -2-


TABLE 1 - Properties of oil-base drilling fluid used _in core-contamination test,
South Coles Levee field, Kern County, Calif.

Drilling-fluid

: Drilling fluid filtrate
i l Piltrate volume , i ; Water ‘Tracer :
| Core milliliters per hour | Yletier content content ,1/iconcen- :
Core! interval, Room | 1ho" i 200 Weight density. |Weight, Volume; weight tration,!
No.! feet itemp. E- | oF. —lb-/ml. 1b. /tt .3 , percent percent: percent img. /ml. | Remarks
1 39204-92961 0.7 ) 1-2 12.6 ! 9.5 + 71.0 | 3.77) 4.30! / 0.00 | Before tracer added.
2 |9296-9300} .0; 1.3 2.0 | 9.3 | 70.0 ° 3.56; 4.00; | 4.85 | After tracer added.
3 ;9300-9315! A} 1.2 1.6 { 9.5 \ 71.4 2.88; 3.30! ' 6.22 |}
A j93i5-9321' 2} 1b | 7} 9.38 5 69.5 = 3.15, 3.51) 15.56 |
5 19321-9335! 4) 6 | 66 | 9.5 | 7165 2.70} 3.10; 15.65 |
6 19335- 79318 | 311.0 | 5 | 9.5 | 71.0 2.79! 3.18! £6.03;
7 | 9348-9358 0; 6 | 66 1 965 | 721.0 2.54: 2.90, <0.2 +5076 i
8 19358-9365; 011.2 } 8 | 9.8 | 73.5 2.96) 3.49: jd72 |
9 19366-9380! .5! .7 i Ll | 9.9 | 74.3 3.35! 4.00! 15.86 |
10 (9360-9361! 1} .6 | 1.6 ' 10.0 | 75.5 : 3.51; 4.25) 15. |
11 19381-9398 | oj} .5 | .6 : 10.0 | 75.0 | 3.18 | 3.83: 16.03}
12 19398-9412; 60; 8 | 8 | 10.1 | 75.9 , 3-72} 4.53 15.72 |
13 \9h2-9he8; 66 ]1.1 | 1.6 | 10.4 | 77.8 | 3.7h 467 jab |
14 9428-9437} .0; .1 | 6 | 10.2 76.8 3.54! 436 | | 5.24 i
15 (9437-9hk! 0; 66 | 8 | 10.7 | 79.9 : 3.49] 4.48! '5.01 | priviea 186 ft. ana
21 (9630-9638; --11.0 | 1.2 ; 11.9 i 89.0 2.86, 4.ogi <1 1 1.87 | fishing job during
23 964k-9647 : 3} «9 | Lek | 11.9 | 89.3 2.991 h.29: /1.92 | time between cores
au |96k7-9658; 1, 2, «7 | 11.9 | 69.3 : 2.89) dik '1.79 | 15 ana 21.
26 19669- 9678 | Oj} 5 11.5 | 12.1 1 91.0 2.971 ¥.3hi £1.75 !
29 19692-9697: -Ljl.l i 14 2.1 | ol. 3.41) 5.00; / 1.87 |
30 19697-9702! .0; .8 | 2.0 12.3 | 92.0 3-43, 5.07 : 61.830 j
31 ‘9702-9711: .0 .6 | 1.4 1.8 ! 88.8 3-13 46 11.56
33 '9718-9723 «2: 8 2.0 12.1 | 91.0 3.17 ' 4.63 1.64
3h 19723-5728 OO: 5-1. 11.9 ) 89.5 3.26: 4.68 1.58

1/ The drilling-fluid filtrates from samples 1 to 11, inclusive, were combined, end the water content of the
combined samples was less than 0.2 percent by weight. The drilling-fluid filtrates from remaining
samples were combined, and the water content of the combined samples was less than 0.1 percent by weight.

3820 - 10 -


Wd

PERCENT
CJ

POROSITY ,
Nx

—o
6
5 ee
4
3
1
EDGE OF CORE CENTER OF CORE EDGE OF CORE

Figure 16. - Relation of the location of sample specimen within the Rangely cores to its porosity.


limits of uncertainty are calculated for Ps of 0.9545, the objective aver-
age may be expected to be included within tho calculated limits 95.45 percent
of the time, These limits become mch more narrow when the number of ob=
servations is increased.

The calculation of limits of uncertainty of an average 1s based on tho
assumption that the data are distributed "normlly" about a central average.
It has been pointed out by LawL2/ that by means of a geometric progression a
normal distribution of permeabilities generally can be obtained and that,
although permeability generally is not normally distributed when analyzed
by arithmetic means, the arithmetic average associated limits of uncertainty
calculated by geometric means is probably a better represontation of the
permeability of the reservoir sand than the value obtainod by arithmetic
methods.

It is evident from figure 4 that the permeability of the samples from
the South Coles Levee well is not uniform, but that, in the whole interval,
there are two separate sections that have higher permeability than the rest
of the interval. In spite of this lack of uniformity, which is not shown
in the porosity, the limits of uncertainty of the average permeability are
given, even though they are not as accurate as would be expected for a
normal distribution, to show the degree of nonuniformity as indicated by
the wide limits of uncertainty. ‘The average permeability alone does not
show this lack of uniformity of the reservoir rock,

The average of tho 103 determinations of the South Coles Levee core
samples indicates calculated arithmetic average permeability of 172 milli-
darcys with limits of uncertainty ranging from 122 to 225 millidarcys.

The chloride concentration based upon 102 determinations made on cores
from the F-1 zone in the South Coles Levee field averaged 778.4 Cl grains
per gallon with plus or minus 23.90 Cl grains per gallon limits of un-
certainty for Ps = 0,9545. Figure 4 shows that variations between chloride
concentrations found for adjacent feet were cometimes wide (for example, 630
to 1,220 grains por gallon), and these variations may be important in any
system of analysis in which it is assumed that the chloride concentration
of the formation water is constant.

While making the tests, it was observed that the chloride concentration
was abnormally low in the cores that contained large quantities of silt or
shale. This fact is illustrated by the last two samples listed in table 4,
particulerly by the shale’ sample, which had only 108 Cl grains per gallon,
in contrast to the average of 778.4 Cl grains per gallon for the wholo sono.
It may be that adsorption of the salts on the immense surface exposed in the
tiny pores of the shale becomes important and is the cause for the wide
variations observed in the chloride concentrations.

il/ Law, Jan, Limits of Uncertainty of Permeability Averages: Paper presented
before the Am, Inst. Min. and Met. Eng. meeting in Los Angeles, Calif.,
Oct. 1947.

3820 - 22-


It is significant. that the resistivity of the zone, as shown by the
electric log, is related to the properties of the liquids found in the cores.
Apparently, the liquids in the cores are similar to the formation liquids.
thet affect the electric-log resistivity. It also follows that the contact
type of electrode used in the oil-base arilling fluid was making électrical
contact with the formation.

No apparent relationship between the electric-log self-potential and
any of the core analysis results was found.

The data presented in tables 4 and 6 and figures 7 and 13 indicate that
the filtrate completely penetrated the cores, because the concentration of
tracer found in the specimens selected from the center of the cores was
virtually the same as the concentration of tracer in the core specimens
from the edge of the same core.

These data supply evidence of the mechanism by which cores are flushed
by dil-base drilling fluids at temperatures, pressure, and length of time
filtration takes place as compared with those in tho South Coles Levee and
Rangely tests. Some petroleum engineers consider that the filtration of
drilling fluid into a core takes place radially after the core is cut. How-
ever, others believe that filtration takes place downward ahead of the bit
before the core is cut. If filtration takes place ahead of the bit, the
recovered cores should be flushed uniformly. and the tracer concentration
should be the same in the center as it is at the edge of the core, which is
what the tests made on the South Coles Levee and Rangely wells indicate
took place.

TABLE 6. - Core analyses
Bureau of Mines - A. Pp. I. Cooperative Study

Field: Rangely, Rio Blanco County, Colo.

Drilling fluid: Oil-~base drilling fluid with tracer added.

Core samples: Sealed 3-3/k-inch/diameter samples from Weber sand received
January 17, 1947. :

Horizon~ ‘Chloride ‘Location
Liquid content, Poros- jtal air |conc.,NaCl of
percent pore space ity, jpermea- jgrains per sample
-|Drilling-| percent |bility,: | gal. of in core
Depth, |Total fluid oilj of bulk lmilli- intersti- Core (see
feet oil |Brine|filtrate | volume |laarcys [tial water | description|fig. 2)
5043" 749.5 [15.7 eh 10.3 Laie) 7,550 Wery fine 9
hard sand
5943 {51.4 112.9 9.51 12.0 | 11 8,460 do. 10.
5943 119.2 115.9 9 8h 9.9 | 28 8,380 do. 1
5943 143.2 115.8 19.3 a/l2.6 13 6,740 do. 12
5986 119.6 118.8 -64 | 12.4 | 1.5 12,300 do. | 2
6010 127.7 {17.8 1.35 | 11.3 1 9 12,200 do. 2
6016 |17.0 14.9 | 38.9 o3 | "8h | 20,800 ao.e/ 1
6055 {14.3 jek. 374 2.3 | b/.22 | 13,700 do. 2
6065 |15.1 21.6 47.7 GB 2 1.6 11,900 dos 1 2
i

3820 -19 -


TABLE 6. - Core analyses (Cont'd.)

Horizon-[Chloride | Location
Liquid content, Poros- |tal air conc «NaCl. of
recent pore space ity, | permea- {grains per; sample
Drilling-| percent; bility, | gal. of j in core
Depth,| Total fluid oil) of bulk|/milli- j|intersti- | Core | (see
feet | oil |Brine|filtrate | volume |darcys jtial vater | description] fig. 2)
@115 [12.6 12h.7 38.8 6.7 12 13 ,800 [Very fine _ ||
; hard sa a/ 2
6130 |26.8 |32.1 | 72.3 3.8) .71 | 18,000 | foal {4
6158 | 20.5 |19.2 30.2 11.5) 2.3 14,900 | do. i 2
6179 |55.7 | 5.6 43.6 13.3 | 20 28,000 | do. | 26
6179 |57.1 |12.0 49.7 12.1 | 51 13,900 i do. { 2
6193 | 18.0 |45.3 59 8.8} 3.1 8,960 | do. i; 2
6212 78 193.1 128.0 3.8 16 11,100 Hl do. 2
62ho [11.2 |37.2 86.4 TA] 2.0 12,600 | do. i 9
62ho | 15.4 |34.8 87.7 6.9 «94 =| 15,700 do. oe)
62h0 |17.1 [34.0 87.5 6.9 81 16,300 do. lh
62ho |18.9 [33.7 83.2 6.5 ‘72 16,700 do. |
6280 |19.3 [20.9 57.9 11.1 9.5 17,400 do. | 26
6280 |20.5 |19.8 4o.7 11.0 | 23 19,000 do. 2
6290 |71.2 [13.4 42.8 12.5} 69 12,300 do. i 26
6290 | 64.6 {13.1 34.2 12.2 | 90 12,000 do. { 2
6295 |28.5 |hh.6 he .2 11.6 | 2.9 8,700 . do.e/ 26
6295 {28.0 |ko.4 88.8 1.8 2.0 9,200 do. 2
6340 - - - 4 16 | 26,100 Crystalline |
‘ . quartzite | 2
6377 | 19.0 |23.8 59.8 15.0} 2.3 | 10,700 {Very fine !
i heard sanac/| 25
6377 [17-1 [34.5 83.1 11.3 2.6 10,300 do. 1 23
6377 | 20.4 |29.9 55.7 2.1] 2.9 10,200 do. ! 10
6377 | 18.0 [31.2 65.0 11.9] 3.2 10,100 do. i.
6377 117-3 {32.0 58.2 11.6 |. 2.2 9,670 do. ak
6389 | 23.1 (31.8 39.2 15.6 | 12 7,810 do.c/ 2
6407 «8 {81.7 138 4.6 08 8,350 do. 2
629 |2h.0 152.8 | 126 3.9 | 4.2 12,100 do. 26
6429 | 10.9 136.0 105 5.2 +09 14,000 | do. 5 2
al Sand-grain density is taken as the average of the densities of the adjacent

samples.
v/ Permeability along bedding plane. The bedding plane is at an angle of
about 30° to the horizontal.
cf Cans not completely sealed. ,
@/ Cores broken and sealed in small ‘cans.
e/ Sample packed in oil (apparently lubricating oil).

3820

20 -


SPACE

PORE

PERCENT

OIL CONTENT,

70

60)

50

30)

20

i)
EDGE OF CORE CENTER OF CORE EDGE OF CORE

Figure 14. - Relation of the location of sample specimen within the Rangely cores to the total
volume of the oil (crude and filtrate) in the sample.






PERCENT PORE SPACE

WATER,

100

80
e
o
ary . $ q
4 ° bel] ¢
60 - ‘. : rT} —
° e ry
e el 4 el el | le
+ © 4e aa % 2 ge ° ° e
a a t+.
e e e
e Boe Peers
40 le ° 7 “=
? o see od * I
° e e
e eo | @
— -
.
20
te)
2 4 6 8 10 20 40 60 100 200 400

AIR PERMEABILITY, MILLIDARCYS

Figure 19. - Relation of the air permeability to the water content of the South Coles Levee cores.





PERCENT

POROSITY,

24)

EDGE OF CORE CENTER OF GORE EDGE OF CORE

Figure 10. - Relation of location of sample specimen within the cores from South Coles Levee
test to its porosity.


PORE SPACE

PERCENT

WATER CONTENT,

70

60)

50

40

30

20)

S=
——————

—S= OO

—oO
—_
> —o
—"
—— =
——
oe
fl
EDGE OF CORE CENTER OF CORE EDGE OF CORE

Figure 9. - Relation of location of sample specimen within the cores from South Coles Levee

test to the volume of the interstitial water in the sample.


The spread or width of the band of points on figure 19 indicates the
uncertainty of the average values. For oxample, the averago permeability of
the sand in the F-1 section in the South Coles Levee field was found to be
about 170 millidarcys, and from figure 19 the average water content corre-
sponding to that permeability could vary from about 30 to about 60 percent.
For that reason it should be recognized that the relationship is not uniform
but is subject to wide variations.

Diffusion of the tracer into the center of these cores was not measured
or calculated in this study. An attempt to evaluate the importance of
diffusion is planned as a part of the core-contamination study that is being
continued in several oil fields that have a wide range of permoabilities,
pottomhole temperatures, and pressures.

CONCLUSIONS

The tests conducted on cores cut in wells with oil-base drilling fluid
in the South Coles Levee, Calif., and Rangoly, Colo,, fields call attention
to now methods of obtaining cores and modified mothods of analyzing tho
cores and the resulting core data.

The primary purpose of the study was to determine whether uncontaminated
cores could be obtained with a conventional-type core barrel using an oil-
base drilling fluid having very low rates of filtration as the circulating
medium in the bore hole, The results of the tests indicate that nearly all
of the cores from the South Coles Leveo and Rangoly fields contained fil-
trate from the drilling fluid that apparently altered the oil content of the
cores but apparently did not affect their water content. As tho eil content
of the cores cut from zones known to contain no free gas is equal to the
total pore volume less the pore volume filled with interstitial water, tho
analysis of cores cut with an oil-base drilling fluid in the hole ponstrat-
ing a zone containing no free gas and from which no water is produced may
yield reliable data regarding the oil and water content of the producing
formation,

Cores having the highest permeability were found to contain the largest
volume of drilling-fluid filtrate, and the cores of lowort permeability
contained the largest volume of interstitial water. When the volumes of
interstitial water found in the cores wore plotted against the corresponding
core permeabilities, the data were found to be spread within wide limits.

Both the chloride concentration in the interstitial water and the dry-
air permeability were found to vary considerably within the sand sections.

The cores were found to contain the same concentration cf tracer in the
center as at their edges, regardless of whether they were 2-1/4 or 3-3/4
inches in diameter and of different permeabilities.

Only small variation was found in the properties (porosity, air-permo-
ability, oil content, water content, filtrate content, and chloride con-
centration in the interstitial water) of samples selected along the diameter
of the circular cross section of the cores.

3820 ~ 2h -


base drilling fluid in the hole, then it follows that the volume of pore
space occupied by o11 in the formation is the total pore volume less the
volume occupied by interstitial water.

If the cores had been cut with a water-base drilling fluid in the hole,
the filtrate that entered the cores would have been water and not oil.
Consequently, as the water filtrate may displace an unknown volume of water,
oil, and gas from the core, the fluid contents of the cores, as determined
by ordinary core-analysis methods, are of doubtful value. Furthermore, it
has been shown by Miller, Morgan, and Muskat6/ that water, and especially
fresh water such as that filtered from water-base drilling fluids, frequent-
ly causes swelling of clays in reservoir rocks, so that the permeability
of the formation to oil may be mich less than would be expected from the
laboratory dry-air permeability measuroments.

It seoms, therefore, inasmich as filtrate from water-base drilling
fluids enters cores, that (1). an unknown volume of the of1 and water con-
tents of the cores may be displaced by the filtrate, (2) the chloride content
of the interstitial water is diluted to an unknown amount by the filtrate,
and (3) the original permeability of the core may be altered by the action
of the filtrate on the clay particles in the formation. Consequently, in
view of the foregoing considerations, the porosity of the cores, as deter-
minod in the laboratory, is the only core property, as usually determined,
that may be considered representative of the formation, and special methods
utilizing special drilling fluids and tracers generally are necessary to
obtain a bettor undorstanding than is now had of the fluid characteristics
of actual reservoirs,

METHOD OF CORE ANALYSIS

The mothod of analyzing the cores from the South Coles Levee and
Rangely ficlds differs somewhat from that commonly used when analyzing cores
from wells; and, es the procedure followed in analyzing cores may affect
the results of the tests, a brief description of the method used by the
writers of this report follows:

Immedistely after the core is removed from the core barrel, samples
are taken at 1-foot or other convenient intervals, wiped or scraped clean
of drilling fluid, and sealed in cans. If possible, enough sample should
be taken to fill the cans, which theroafter should be maintained at a fairly
uniform temperature.

In the laboratory, the core sample is cut dry with a diamond saw, and
a@ specimen about 4 square centimeters in cross-sectional area and about 5
centimoters long is selected from'the positions listed in table 4 and shown
in figure 2. Tho bulk volume of the selected sample is determined in a

6/7 Miller, K. T., Morgan, F., and Muskat, M,, Some Permeability Experizonts
on Cores from the Stevens Sand, Paloma Field, Californias Producers
Monthly, vol, XI, No. 1, Nov. 1946, pp. 31-34.

3820 -5-


PORE SPACE

PERCENT

CONTENT,

WATER

60

50

40
ee °

_—o- ad

30

20 o— —o
oe °

to oo

O°

EDGE OF CORE CENTER OF GORE EDGE OF CORE

Figure 15. - Relation of the location of sample specimen within the Rangely cores to the volume

of water (brine) in the sample.



GALLON
fo
a

GRAINS PER
rm nn ro
co) Ld >

THOUSAND NACL
8
T
}

CHLORIDE ,
a

EDGE OF CORE CENTER OF CORE EDGE OF CORE

Figure 18. - Relation of the location of sample specimen within the Rangely cores to the chloride
concentration in the interstitial water in the samples.


FUEL GAS THERMOMETER

=F

CRUCIBLE PS

To
VACUUM

APPARATUS FOR COMBUSTION ANALYSIS OF ORGANIC
CHLORIDE. (AFTER PAUL K. WINTER, )

Figure 3. - Apparatus for combustion analysis of organic chloride.


EFFECT OF OIL-BASE DRILLING FLUID FILTRATE ON ANALYSIS
OF CORES FROM SOUTH COLES LEVEE, CALIF.,
AND RANGELY, COLO., FIELDS

By GEORGE L. GATES, FRANK C. MORRIS, AND W. HODGE CARAWAY

* # «© « « « « « «° Report of Investigations 4716

UNITED STATES DEPARTMENT OF THE INTERIOR
Oscar L. Chapman, Secretary
BUREAU OF MINES
James Boyd, Director

Work on manuscript completed May 1950. The Bureau of Mines will welcome reprinting
of this paper, provided the following footnote acknowledgment is made: “Reprinted from
Bureau of Mines Report of Investigations 4716.”

August 1950


TABLE 4 - Core Analyses (Cont'd.)

Bureau of Mines - A.P.I. Cooperative Study

Field: South Coles Levee, Kern County, Calif.

i i ;Loca~

Liquid content, | | | tion
percent pore space Horizon- | Chloride | of

i ; Drill- | Poros- | tal air| conc.,Cl }sample
i ing | ity, permea- |grains per in

fluid | percent; bility, | gallon of core

Depth, /Total oil of bulk} milli- | intersti- (see

feet _| oi] 'Water lPiltrate| volume| darcys |tial water|Core description) fig.2)
9426.5 =| 22.1] 53.1) 16.0 19.2 90 733 Fine med.soft sd] 2
9h27 5 18.1] 50.8 8.96 22.2 37 790 do. 9
9427.5 15.9| 63.2 8.04 19.5 | 32 816 do. 10
ghe7 5 16.1; 60.6] 7.33 20.1 45 810 do. 11
ghe7 5 17.2} 52.1 9.6L | 21.7 7 879 do. 12
9428.5 | 19.6) G.5| 9.01] 19.7 4S 726 do. 2
9429.5 31.2] 51.1] 15.5 22.1 59 829 do. 13
9429.5 | 22.7| 59.2] 13.1 20.1 51 728 do. 2
9430.5 | 25.0] 59.0) 16.0 20.4 56 860 Fine-crs.soft sdj/ 3
9430.5 | 23.0] 60.1] 13.2 20.2 B TeT do. 2
9430.5 | 23.2] 54.3] 16.0 21.5 66 TT do. 4

9431.5a/| 16.3! 52.0! 6.57] 2h.9 36 761 '|Fine med.soft sd 14,18
9431.5 111.8! 63.1); 3.08] 22,1 AL Tl} do. 16

9437-39 | 16.3 | ob 4 | +38 8.0 - i 108 2-1/2 ft. recov-

| ery shale 1
9437-39 (274 | bk! 4.26 | 12.9 | sli 343 | Fine hard sand 1

a/ Send grain density assumed to be the same as that of the adjacent sample. All
samples were consolidated. b/ Determined on separate sample. e/ Average of
three adjacent samples. a/ Assuming bulk volume off by 10 ml.

TABLE 5 - Limits of uncertainty of averages of core properties

Core interval, 9294 to 9438 feet. South Coles Levee field,
Kern County, Calif.

j Limits of un-!
: certainty of | Number
Javerage, for of

Ps = 0.9545 |samples, ‘Standard

Ageriies Lower :_ Upper n deviation

percent Pore SPACE eeesceeecseseves! 49,71 146.98 | 52.44! 102 13 56
Oil content,
Percent Pore SPACE cececeveveccveee, 32.11 / 28.81] 35.41} 102 16.43
Drilling-fluid oil filtrate, percent
POLE SPACE sescececscereceeccnseces
Porosity, percent of bulk volume ...
Chloride concentration in inter- i |
stitial water, Cl, grains per gal .| 778.4 (754.5 | 802.3 102 118.9
Air permeability, millidarcys .....2/l72 122 225 103 2/03
1/ The average air permeability is the arithmetic average calculated from the
geometric average which was determined by the method described by Law (see
footnote reference 12, p. 21).
2/ This value is the standard deviation calculated as a part of the statistical
analysis used to obtain the geometric average permeability.

3820 - 18 -

j
|
Water content, |

21.10 118.05 | 24.15! 100 | 15.00

18.86 | 18.05 | 19.67 103 | 4.07









TABLE 4 - Core analysis
Bureau of Mines-A,P.I. Cooperative Study

Field: South Coles Levee, Kern County, Calif.
Drilling fluid: Oil-base drilling fluid with tracer added.
Core samples obtained October 1946.
Samples from the F-1 section of the Stevens sand at depths indicated below:

J H i Loca-

! Liquid content, : H | tion
percent pore space ! ‘Horizon-Phloriae ; of

H ; ; Drill- 'Poros- tal air conc.,Cl | sample
. i ing ity, ,permea- grains per: ‘in

; fluid percent: bility, gallon of ' ‘core

Depth, | Total, oil of bulk! milli- Hntersti- | (see

feet | oil Water filtrate: volume |darcys tial wator|Core description ifig.2)
9294-6 1,301 63.5 0.00 17.0 4.9) 752 Fine soft sand 1
929h-6]27.2 (36.6 ; 00 11.9 : 3.1 ' 862 ‘Fine med,hard sd. 1
9e9h-6|32.1 |39.1 | - 118.6 | 6 ' 787 iFine soft sand 1
9296.0] 6.5058.5 )  - 19.2 }200 i 734 | Soft fine~crs.sd. 1
9296.5140.5 '37.8 | 20.7 18.9 1 15 i 858 bdo. io
9297-51335 ‘37.7 | 10.2 14.8 81 i 1040 | do. coL
9298.5] 9.92;68.0 | .55 15.9 | 30 / 76h (Fine med.hrd.sd.: 1
9300.5}22.7 ;46.0 ; 94h | 23.0 35 : 198 'Fine med, soft sd, 1

Ol.5;- 1 = - oe 1 : ‘Fine soft sand | 1
331. - to - b e- — nt \ ee do. yy
9301.5/12.6 151.0 | 1.92 | 22.9 | 16 | 871 do, 16
9302.5) 15.8 j57-3 + 5.85 : 16.2 | 12 tf 81h Fine med.eoft sd, 1
9303.5} 36.5 ;41.5 | 20.8 | 17.5 |190 ' 896 ‘Fine soft sand {| 1
9304.5] 47.1 136.1 | 28.0 | 18.7 | 4hO + 631 'Fine-crs.med,hra!

{ H | | sand 1
9315.5118.8 '43.7 . 11.8 ! 18.9 | 3h0 : 886 ‘Fine med.hrd.sd.! 1
9316.5129.7 {k2.3 : 19.3 | 22.4 | 100 | 712 (Fine soft sand i 1
9317.5| 21.8 :37.0 ; 14.0 | 23.8 ie) 1 838 i do. iol
9321.5] 88.9 139.6 , 25.5 | 20.8 | &5 : 929 iFine hard sand | 1
9322,5}19.8 i46.7 | 26.0 ! 19,0 |270 ! 677 'Fine-crs.hd,sd. ; 1
9323.5, 32.4 139.9 | 21.4 | 20.5 45 | 987 'Fine med.hd. sd. 1
9324.51 35.3 {46.1 ' 2h.9 | 20.9 | he | 841 = [Fino soft sand | 1
9325.5} 24.7 j4b.7 | 15.3 ; 19.0 |1ho | 835 do. io
9326.5|-- | - | 348 § 17.4 | 87 ios Fine mod.hrd.sd. i 1
9327.5) 49.5 49.5 | Wg 20.3 10 ' 619 ‘Fino soft sand ; 1
9328.5) 38.2 {46,1 j 22.8 17.7 4d 1 766 » do. Jo.
9329.5} 24.2 135.9 | 18.9 | 22,3 |1ho | 855 do. , 3
9329.5, 27.6 38.5 | 20.5 20.4 | db ; 876 i do. i 2
9330.5; 18.1 “47.8 | 12.2 20.7 | 57 + 81k ‘Fine to med.sd. {| 1
9331.51 58.7 (30.1 | 46.7  : 21,0 | #2O i heh Fine soft sand 1
9335.5120.6 '53.7 : 13.2 19.5 | 46 | 769 ‘Fine hard sand | 1
9336.5: 41.8 ike.3 | 31.2 18.6 |110 ' 816 Fine-crs,med.hrd.i

i ! sand a)

3820 i. ake


PERCENT PORE SPACE

CONTENT,

OIL

60

50

40

30

20

Le) 1
EDGE OF CORE CENTER OF GORE EDGE OF CORE

o— ZS
| ———
——— o
—
—=3 ©
Pee =8
— =8

Figure 8. - Relation of location of sample specimen within the cores from South Coles Levee
test to the total volume of the oil (crude and filtrate) in the sample.


FLUID FILTRATE (OIL), PERCENT

DRILLING

100
80
60
w
°
= . e
0 ry ele . °
eo] |e
w, Ld ° 0 o® e
tr o_°o e
° e . - °
: | “
|_| ¢ Ie e ° e
e
e *
20 —
° e ¢ .
7 Les ° °
|
. e t °
0] e
2 4 6 8 10 60 100 200 400

AIR PERMEABILITY, MILLIDARGYS

Figure 5. - Relation of the air permeability to the volume of filtrate (oil) in the South Coles Levee cores.


! CTT

13

————,
£

15]16]17 3) 23}to] 1 2

Figure 2. - Schematic diagram showing position of specimens within the core.


The data presented in figures 7 to 12 and 13 to 18 indicate that
the porosity, permeability, water content, oil content, filtrate content,
and chloride concentration in the cores tested showed very little variation
between adjacent horizontal samples taken along a core diameter. The samo
uniformity of properties horizontally within the cores was found even in
the Rangely cores, which were nearly twice as large in diameter as the cores
from the South Coles Levee well. No great error would have been introduced
in the work discussed in this report had it been assumed that adjacent
horizontal samples had identical properties; it should not be inferred, how-
ever, that this is true for all formtions. Law and Cerini10/ have found
wide variation in the properties of adjacent vertical samples of cores.

Although small variations were found between the properties of adjacent
horizontal sections of cores, when comparisons between samples taken from
adjacent vertical feet of zone are made, relatively wide limits of un-
certainty were found in the average chloride concentration and the average
permeability of tho cores. (See table 5.)

In analyzing the data, use Was made of statistical methods to establish
the limits of uncertainty of the determined averages. This method was
suggested by Jan Law, and it is based primarily on the methods described
extensively in the A.S.T.M. Manual on Presentation of Data, 1946 reprint.

The statistical analysis to establish the limits of uncertainty has been
used in this report, because a single reported average tells nothing of the
accuracy of the average and nothing of the variations found between measured
values for the samples that were taken at 1-foot intervals from a continuous
sand section. In many field coring programs, only parts of the producing
zone in a few selected wells are cored, and the average values obtained are
used in reserve calculations for the ontire field without giving thought to
the limits of the uncertainty of the data obtained,

In an extreme case, for example, the average porosity for a zone in a
field may be reported as X percent, which by itself does not indicate
whether it is the average porosity obtained from observations made on two
samples or on several hundred samples. The desired average, called the
“objective average" by statisticians, is the value that would be approached
as a limit as more observations are obtained under the same essential
conditions, and the cumlative average computed. The probability. that the
average. value obtained from only two observations will be near the objective
average is very small, whereas the probability that the average value obtained
from several hundred observations will be near the objective average is mich
greater. However, the limits of uncertainty of an average of any known
number of observations may be calculated and stated.

In table 5 the limits of the calculated avorages have been calculated
for the value of "statistical probability", Ps, equal to 0.9545. When the

10/ Law, Jan, and Cerini, WilliamF., A Statistical Approach to Core-Analysis
Interpretation: Drilling and Production Practice, Am, Petrol. Inst.,
1946, p. 385.

3820 - 21-


GALLON

GRAINS PER

1000

_ 800

cL

CHLORIDE ,
4
8

600'

EDGE OF CORE CENTER OF CORE EDGE OF GORE

Figure 12. - Relation of location of sample specimen within the cores from South Coles Levee
test to its interstitial water chloride concentration.


PERCENT PORE SPACE

DRILLING FLUID FILTRATE (OIL),

120

80)

60

20

0}
EDGE OF CORE CENTER OF GORE

EDGE OF CORE

Figure 13. - Relation of location of sample specimen within the Rangely cores to the volume

of filtrate (oil) in the sample


displacement-type porosimeter. All of the sample except that part chosen
for the permeability determination is crushed quickly. ‘The crushed sample
and the specimen used for permeability determinations then ere place in a
porous thimble, weighted, and extracted with toluene.7/ The volume of water
distilled from the sample is read directly in the trap, When the toluene
extraction is completed, the sample is extracted with distilled water to
remove all of the water-soluble salts. After the water extraction, the
thimble containing the extracted sample is dried in an oven, cooled in a
desiccator, and weighed.

The loss in weight of the sample represents the combined weights of the
oil, water, and water-soluble salts romoved from the sample, Whon large
quantities of water-soluble chloride salts are present, as in the Rangely
samples, the weight of the salts (assumed to be NaCl) is subtracted from
the lous in weight of the extracted samples to obtain the total weight of
the oil and water in the sample. The volume of water distilled from the
sample is corrected to the vclume occupied by a brino of the concentration
found for the particular sample. If the weight of the salts is negligible
compared to the loss in weight of the extracted sample, the total weight
of the oil and water in the sample is taken to be the same as the loss in
weight of the extracted sample. The weight of the measured volume of water
is subtracted from the total liquid weight to determine the weight of the
oil extracted from the sample. The volume of oil is calculated by using
the density of the oil produced from the formation cored. When there are
large quantities of chloride salts in the core samples, as in the samples
from the Rangely field, the volumes of water and oil are corrected accord-

ingly.

The water extract from the cores is titrated for chlorides with w/10
silver nitrate, using potassium dichromate as an indicator, Care is taken
to make the water oxtract slightly acid with nitric acid, because the yH
of the solution ie a very important factor in the described method of
chloride determination.

To obtain the porosity of the sample, the sand-grain density is deter-
mined for all of the sample except that part selected for the permeability
determination. The volume of the sand grains is subtracted from the bulk
volumes of the core sample to obtain the total pore volume.

The dry-air permeability is measured, using the part of the core
selected but not crushed in preparing the sample for extraction. Tho sample,
which is gencrally 1 by 1 by 2 centimeters long, is cut, when possible,
with the long dimension parallel to the bedding plane.

By this method of analysis the porosity, liquid contents, and chloride
concentration are all made on one sample, and it is unnecessary to assum
that adjacent samples have 1dentical propertics.

‘7/ The extraction apparatus is described in Bureau of Mines Report of
Investigations 4004, entitled "A Method for Detormining Simltaneously
the Oi1 and Water Saturations of O11 Sands", by Cleo Griffith Rall
and D, B, Taliaferro, Dec. 1946.

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