TABLE 2, - Field data relating to well tests and subsurface sampling

Well No. L 2 3 5 6 7 8 9 11
Sampling point elevation,,..feet | 1/-666 ~768 -873 ~894 -899 -757| -930
Completion date........ ee eee eens 10-56 11-56 6-56 |10-56 4-56 6-57 12-56 10-57 1-57

Preconditioning period:
Flow rate, estimated

seeeceeeceeee. barrels per day 225 250 |250-550 - 400-800 225 - 100-150 -
Conditioning period:

Flow rate........64..d0, - 182 248 471 266 223 232 |150 (est.) 85

Length of flow test...,, hours - - 46] - - - - - 144

Gas-oil ratio test....... do. - 19 19 16 20 22 6 - -

Minimum flowing pressure at

sampling point...... p.s.i.a. - 1,349] 1,515] - |2/1,737|3/1,625 |4/1,554 1,878 -
Gas-oil ratio
..cu,ft,/bbl, stock tank oil5/ - 680 504 501 599 667 505 - -

Average separator conditions:

Pressure.......4. - 60 82 81 71 57 105 - -
ee - 60 61 70 59 70 67 - -
Specific gravity of separator
gas, air = 1.000........... a - 0,900} 0.960 j0,.852} 0.830] 0.860] 0.798 - -
Sampling conditions:
Duration of shut-in period.,..| 6 days| 76 hr, | 27 hr.| - 54 hr, |14 days - (6/) |46 hr.
Static pressure prior to
sampling..........2. p.S.i.a. |7/2,068 |7/1,763 | 1,681] = 1,836| 1,791 - 6/1, 894] 2,158
Date of sampling. -.| 11-56 4-57 4-57 | - 4-57 | 10-57 = 10-57 | 3-57
1/ Datum, sea level,
2/ Estimated from 15-hour drawdown, assumed stabilization in 45 hours,
3/ Flowing pressure during gas-oil ratio test following sampling.
4/ Measured at -953 ft, elevation,
5/ Gas volumes at 14,4 p.s.i.a. and 60° F,
6/ Well flowing.
7/ Calculated value,

as


TABLE 10, - Differential-gas-liberation and pressure-viscosity data,
subsurface oil sample from well 31/7

Differential gas liberation

Pressure-viscosity

Relative oil Specific
Gas in solution, volume, gravity
Expansion cu, ft,/bbl, residual oil Oil of gas in
Pressure, | of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.s.i.a, percent at 60° F,2/ 60° F. = 1.000 |g./ml. (air = 1,000) | p.s.i.a, | centipoises
2,500.... 0 615 1,327 0.7163 1.013 2,765 0.68
2,265.... 23 615 1,330 -7147 1.013 2,380 67
2,017.... 49 615 1.334 .7128 1.013 1,951 65
1,786.... 74 615 1.337 .7110 1.013 1,506 63
1,553.... 99 615 1.340 .7093 1,013 3/1420 62
1,4203/,, 1.13 615 1,342 . 7083 1,013 1,399 63
1,413.... 1,30 - - - - 1,310 65
1,407.... 1.72 - - - - 1,178 68
1, 384... 2,37 - - - - 985 74
1,348.... 3.68 - - - - 731 82
1,295.... 5.63 578 1,329 - 1.030 491 292
976.... - 475 1.287 27221 1.100 292 1.03
669.... - 378 1.247 «7334 1.200 142 1.20
397... - 287 1,209 «7444 1,313 il 1,95
171.... - 198 1,167 .7566 1.500 - -
51l.... - 103 1.114 .7708 1.770 - -
31... - 51 1,080 7785 1.880 - -
1l,... 0 1.041 «7884 - - -
i4/,, 0 1.000 8204 - - -

1/ Analysis temperature, 132° F,
2/ Gas volumes at 14.4 p.s.i.a, and 60° F,
3/ Saturation pressure,
vA

/ Temperature, 60° F,

ce


50 —"
LEGEND -1
1-2 MAXIMUM AND MINIMUM LIMITS FOR ITEMS a
1-7 INCLUSIVE, ANETH FIELD —_— 7
ao} —) #4 mpsa nea a t Tm P
5 ITEM I, RANGELY FIELD
2s 6 ITEM I2, ASHLEY VALLEY FIELD NX
=
= 30
2 Ss’
= Ss Sx Ly)
Ee ; D
= .
d 20 a 2, By Uy ma
a
: Gy, Lda
A “i, fo
10
0
0 2 4 6 8 10 12 14

ANALYSIS FRACTION NUMBER

FIGURE 13. - Correlation-Index Pattern Representing Oil From Pennsylvanian
Formations in Paradox and Unita Basins.

The sulfur content of the Pennsylvanian oils from the Aneth field (items
1-7, table 7) range from 0,02 to 0,13 percent and average 0.06 percent, Aneth
and Dove Creek oils, with sulfur percentages less than 0.50, are classified as
low-sulfur oils, The Rangely and Ashley Valley oils have sulfur contents of
0.85 and 0.83; these are classified as high-sulfur oils,

The pourpoints of the Aneth oils range from 10° to 15° F, Item 5, table
7, shows a pourpoint of 35° F,; however, this value is probably not represent-
ative and may have been caused by wax accumulating in the Rolo well checker at
the time the sample was taken, Items 8 and 9, table 7, have pourpoints of 30°
and 25° F,, possibly indicating the upper pourpoint limit expected in
Pennsylvanian oils within the Paradox and Uinta basins,

An average of the values of nitrogen content shown for the Aneth oils in
table 7 is 0,040 weight-percent, Because of its low nitrogen content, the
Aneth oil should be desirable refining stock.


38

TABLE 15, - Pressure-volume-temperature data, subsurface
oil sample from well 2

60° F. 90° F. 120° F, 134° F,1/
Specific Specific Specific Specific
Pressure,| volume, Pressure, |volume, Pressure, | volume, Pressure,| volume,
p.s.i.a, | ml/g. «S.i.a. | ml./g. |p.s.i.a. | ml./g. |p.s.i.a. | ml./g.
2,500 2/1357 2,500 | 2/1,382 2,500 | 2/1.409 2,500 | 2/1.423
2,187 1.360 2,200 1.386 2,256 1.413 2,231 1.427
1,872 1,363 1,905 1.389 1,928 1.418 1,965 1,431
1,519 1.367 1,614 1.394 | 3/1,671 1.422 1,768 1.434
1, 3683/ 1.369 | 3/1,520 1.395 1,631 1.431 | 3/1, 740 1.435
1,349 1.372 1,497 1.403 1,577 1.453 1,722 1.443
1, 336 1,382 1,449 1,422 1, 466 1,501 1,658 1.462
1, 303 1,396 1,369 1.457 1, 284 1.608 1,525 1.520
1,243 1.425 1,087 1.651 989 1,901 1,157 1.774
1,118 1,502 775 2.100 744 2,372 878 2.166
886 1.735 536 2,884 571 3,001 716 2,560
643 2,223 - - - - 570 3,150
495 2,809 - - - - - -
1/ Reservoir temperature,
2/ Determined with density pycnometer,
3/ Saturation pressure at indicated temperature,
TABLE 16, - Pressure-volume-temperature data, subsurface
oil sample from well 3
60° F. 90° F. 120° F. 134° F.L/
Specific Specific Specific Specific
Pressure, | volume, Pressure, |volume, Pressure, | volume, Pressure,| volume,
p.s.i.a. | ml./g, Sia. | ml./g. |p.s.i.a. | ml./g. |p.s.i.a. | ml./g.
2,500 2/1335 2,500 | 2/1,358 2,500 | 2/1,.384 2,500 | 2/1,396
2,193 1,338 2,150 1.362 2,149 1,388 2,201 1.400
1,885 1,341 1,858 1.366 1,819 1.393 1,881 1.404
1,578 1.344 1,472 1.371 1,487 1,397 1,569 1,409
1,194 1.348 | 3/1, 245 1.374 | 3/1, 366 1.399 | 3/1,420 1.411
1,1143/ 1.349 1, 238 1.377 1,346 1.407 1,405 1.417
1,088 1.360 1,196 1.394 1,308 1.423 1,367 1.434
1,060 1.376 1,154 1,418 1,262 1.448 1,326 1.454
1,022 1.401 1,091 1.459 1,217 1.477 1,255 1.495
947 1.462 975 1.558 1,136 1,532 1,156 1,563
844 1.627 808 1.764 896 1.780 948 1.770
705 1.771 569 2.322 680 2,194 643 2,392
525 2,245 - - 528 2.713 499 3.016

1/ Reservoir temperature,

2/ Determined with density pycnometer,

3/ Saturation pressure at indicated temperature,


member is mainly a sequence of varicolored limestones interbedded with minor
beds of green-gray shales and siltstones; this member has not proved produc-
tive within the field area, The Paradox member includes complex evaporitic
sequences with minor carbonate and black~-shale interbeds (6, p. 867). A zone
in this member, known as the Ismay or Bluff (preferably Ismay) has produced
only small amounts of oil from the Aneth field; however, in some of the sur~
rounding fields this zone is the most important producer. The Ismay zone,
composed of limestones and black-shale beds, overlies the Aneth zone and is
separated from it by the Paradox shale,

TABLE 1, - Stratigraphic classification chart of southeastern Utah (6)

SYSTEM SERIES FORMATION MEMBER
PERMIAN WOLFCAMP CUTLER
VIRGIL
MISSOURL UPPER HERMOSA
HERMOSA (Ismay or Bluff
zone
bes vos ae
PENNSYLVANIAN
PARADOX

LOWER HERMOSA

ATOKA MOLAS

MORROW

SPRINGER

CHESTER

MISSISSIPPLAN MERAMEC

OSAGE LEADVILLE



48

much of the gas liberated from solution during the flow period was redissolved
during the shut-in period before sampling. The production gas-oil ratio meas-
ured during the conditioning rate was 680 cubic feet per barrel of stock-tank
oil; this is approximately 85 cubic feet per barrel higher than the equivalent
production gas-oil ratio obtained from the flash-gas liberation analysis of
the sample, However, the value of these data is questionable for determining
the original saturation pressure of the reservoir oil, because a stabilized
flow was not obtained during conditioning. The unstable, heading-type flow
during conditioning is attributed to the 300-foot increase in elevation be-
tween the wellhead and the separator and to the small water production that
intermittently loaded the flow line causing varying back pressures on the
wellhead, Although the sample probably is not representative of original res-
ervoir oil in the vicinity of well 2, it is considered that the release and
re-solution of gas during flow of oil from the formation and accumulation in
the tubing, where sampled, caused little change in the composition of the
original reservoir oil, The sample is considered representative of the reser-
voir oil at its saturation and lower pressures reported for the laboratory
analyses, The saturation pressure of the original reservoir oil is believed
to be somewhat higher than that of the sample,

Well 3

Poor conditions existed in well 3 for sampling original reservoir oil.
These conditions were attributed to previous high rates of flow imposed on the
well, to a local pressure sink that had developed in the area, and to effects
of a paraffin-cutting operation a few days before the tests,

The first two conditions were not known at the time of the tests; the
high cleanout rate imposed during paraffin cutting was not considered serious
in itself because of its short duration, It was believed that the well could
be made suitable for sampling even though conditioning of the well would be
limited due to lack of tank space, The well was placed on a conditioning rate
of 248 bbl, a day for 46 hours; it was then shut in for 27 hours, and subsur-
face oil samples were taken, The minimum bottom-hole flowing pressure on the
conditioning rate was 1,515 p.s.i.a., the bottom-hole static pressure just
before sampling was 1,681 p.s,i.a., and the sample saturation pressure was
1,420 p.s.i.a. Thus, it might appear that the reservoir oil was undersatu-
rated, but this is contrary to results of previous and subsequent sampling,
Additional investigation of well conditions that may have affected sampling
disclosed that the well had been produced on rates as high as 550 bbl. a day
for 10 days just before sampling. Pressure data recorded in well 4, which had
been shut in approximately 5 months, substantiated the existence of a pressure
sink in this area of the field. Because of these conditions, it seems appa-
rent that gas was being liberated from solution within the reservoir formation
and that representative proportions of original oil and associated gas compo-
nents were not being produced into the well or the surface producing equip-
ment, The measured production gas-oil ratio on the conditioning rate is not
considered representative of the original oil or useful in determining its
saturation pressure, Therefore, the subsurface sample is not representative
of the original unmodified reservoir oil but may be considered representative
of the reservoir oil at the saturation and lower pressures reported in the
laboratory analyses.


53659-5.695,
3709-5, 715

Specific gravity, ons.
‘Sulfur, percent,
Saybolt Universal eer at 100° F,,.

Stace 1—Distillation at Qtmowrhgric pressure, ..
First drop, 23..°C. (. F)

_. Laboratory
Utah. nn
San_uan. nn County
Well 7 SHRI, 65 “Ma a
) 408. R. 24 E.
) GENERAL CHARACTERISTICS

ALP. I. gravity .41.5..° Pour point, ° F. -10_.

35... sec.jat Fy

Color, Been...
sec.Nitrogen, percent, 0.040

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

760. mm. He.

IDENTIFICATION

Well &

Specific gravity,

—_A.P. 1. gravity 421°
Sulfur, percent, Le! 10

Saybolt Universal viscosity at100_.° F., ...37... sec.; at

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

C..

45

250
26

1
8 225
9
0

‘STAGE 2—Distillation continued at 40 mm. Hg.
33.4. 0,840.
61:6 845.
(858,

Residoum . 2.
Carbon residue of residuum, Carbon residue of residaum,
Ramsbottom ‘Ramsbottom
APPROXIMATE SUMMARY
Light gasoline... 13.0. Light gasoline ws. rarer

ppyuoue

Spesysese

Pour point, ° F. 10_.

atmonpheric pressure, 160... mm. Hg.
F)

n/a. a
1. 408., R. 24 E.
GENERAL CHARACTERISTICS

oF, see. Nitrogen, percent, 0.040

‘Total gasoline and naphtha.
Kerosine distillate...

Medium lubricating di
‘Viscous lubricating distillate

Residuum .
‘Distillation Joes

‘Above 200

“Total gasoline and naphtha ~
Kerosine distillate... . te
Gasol... . tae

‘Nonviacou lubricating diatilate © |

‘Medium lubricating distillate...

‘Viaeous lubricating distillate

Residoum 2. . Lee ee
Distillationlow tee

9”




Bureau of Mines ...arante

IDENTIFICATION

Paredox. limestone i oo) Well 2

GENERAL CHARACTERISTICS

41.5.° Pour point, ° F. 15.
Color,
sec. Nitrogen, percent, 0.041

Specific gravity, 0.818.
Sulfur, percent, Les:
Saybolt Universal viscosity at .100° F.,

A. P. I. gravity
io

38. sec.zat °F,

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

STAGE 1—Distillation at atmospheric pressure, ..760.. mm. Hg.
First drop, --28..°C. (....82.° F.)

Bureau of Mines .
Sample...

IDENTIFICATION
Anesth... fek
“Paradox ‘Limestone. Gneth Fay) Well 3

15.496. feet

Ww SER sec. 17,
oR. 26 E

GENERAL CHARACTERISTICS

Specific gravity, 0.819... A.P.I. gravity 41.3..° Pour point, ° F. .
Sulfur, percent, less. than..0.10 ‘sine 4
Saybolt Universal viscosity at 100.° F., ...36..see.;at .....° Fy sec, Nitzgen, percent, 01035"

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

‘STAGE 1—Distillation at atmospheric pressure, ..760_ mm. Hg.
First drop, 28..°C. (....82.° F.)

Pye ree AR rn an nd rye im, any
1 60 1m 50 0.644.
2% 167 6 681.
3 1 na
4 ast
5 io ne
6 ie un
1 30
2 om ar.
8 oe
wom an
‘Stace 2—Distillation continued at 40 mm. Hg.
100m 49
12 225487 46.
Bom a 38
4 28% 527 86.
13 30a igo
Rasiduom ss.
Carbon residue of residuum, 3 percent. percent.
Ransbotton 10
APPROXIMATE SUMMARY APPROXIMATE SUMMARY
tec, APL Viel Pemnt nd, AR ity
Light gases . Ths. Ligh gusting : 14.8
‘Total gasoline and naphtha De 60... ‘Total gasoline and naphtha . we
Kerosine distillate. 2... | . _ 42.3. ‘Kerosine distillate 428.
36.6 Gas oil

878 °35,4229.7 so-r00
888 29.7:27-9 100-200
Above 200

36.6.

35.0230.0 50-100
26-4 100-200
-fibove 200

Nonwiecous ls distillate

Mediom lubricating distillate... .
Viscous lubricating datilate

Redo

Distillation love Fee eee



16

The viscosity-pressure relationship is the viscosity of the liquid phase
in the reservoir at various reservoir pressures, The viscosity is minimum at
saturation pressure and maximum at atmospheric pressure, where the oil is com-
pletely denuded of gas under the imposed conditions of the analysis,

Flash liberations were made on approximately 60 ml. of sample in a large-
volume cell, The method consists essentially of expanding a constant weight
of sample, at some pressure above saturation pressure and at formation tempera-
ture, to an average pressure and temperature as recorded at the field gas-oil
separator. At these separator conditions the liquid and gas phases are brought
into equilibrium, and the gas phase is transferred out of the cell at constant
pressure, The remaining liquid phase is then expanded to atmospheric (equiva-
lent stock-tank) conditions of 11 p.s.i.a, and 60° F., the liquid and gas
phases are again brought to equilibrium, and the gas is transferred out of the
cell,

Flash liberation approaches the vaporization process that occurs when oil
flows up the tubing, through the flowline, and into the oil and gas separator,
where the liquid and gas phases are separated under dynamic equilibrium condi-
tions, Flash of the separator liquid represents the vaporization process be-
tween the field separator and the stock tank, Comparison of field-production
gas-oil ratios with those obtained by flash liberation of a subsurface sample
provides a means of detecting excessive gas production at the well or indicat-
ing the amount a subsurface oil sample may have been conditioned by a drop in
bottom-hole pressure before sampling.

Flash-liberation data for samples from the seven wells are summarized in
table 4, page 21. The values for gas in solution at saturation pressure and
separator conditions equal the total volumes of gas liberated between these
and stock-tank conditions, Data are shown as ratios of volume of gas per bar-
rel of residual or equivalent stock-tank oil at 60° F, Differences between
gas in solution at the saturation pressures and at separator conditions give
production gas-oil ratios corresponding to field-production gas-oil ratios,

The formation volume factor is the relative volume of the oil at an in-
dicated pressure and temperature divided by the volume of residual or equiva-
lent stock-tank oil, It represents the volume that 1 barrel of stock-tank oil
occupied in the reservoir when saturated with gas at the indicated pressure
and temperature,

Pressure-volume relationships were determined on the samples from wells
2, 3, 6, 7, and 9 at several temperatures between 60° F, and formation tempera-
ture, The data are given in tables 15 to 19, appendix I, They were determined
by stepwise expansions of approximately 65 ml. of a sample at a given tempera-
ture in a large-volume cell, The specific volume of each sample, at 2,500
p.s.i.a,. and the indicated temperature, was determined separately in a pres-
sure-density pycnometer, which has been described by Eilerts and others (2).
These pressure-volume-temperature data are useful for determining the compres-
sibility and thermal contraction properties of a reservoir oil.


TABLE 11. - Differential-gas-liberation and pressure-viscosity data,
subsurface oil sample from well 61)

Differential

as liberation

Pressure-viscosity

Relative oil Specific
Gas in solution, volume , gravity
Expansion cu, ft./bbl, residual oil Oil of gas in
Pressure, | of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.s.i,a, percent at 60° F,2 60° F, = 1,000 | g,/ml, (air = 1,000) | p.s.i.a. centipoises
2,500.... 0 723 1,381 0.7018 0.964 2,682 0.59
2,195.... 034 723 1,385 6996 964 2,403 58
2,004,,.. 56 723 1,388 -6980 964 2,134 257
1,8203/,, 79 723 1,392 6963 964 1,860 56
1,27 - - - - 3/1, 820 56
1.79 - - - - 1,786 257
2.44 - - - - 1,719 358
1,704.... 3.75 - - - - 1,559 +60
1,649.... 5.05 679 1.370 - +983 1,330 +65
1,268.... - 558 1.320 .7142 1.041 1,039 73
- 453 1.276 .7261 1,114 705 83
- 358 1,236 .7376 1.216 424 98
- 267 1,197 .7490 1.353 226 1,11
- 178 1,156 7612 1.570 121 1,25
- 81 1.099 «7758 1.800 66 1,34
- 0 1.041 .7893 - il 1,94
- 0 1,000 .8215 - - -

1/ Analysis temperature, 133° F.

2/ Gas volumes at 14.4 p.s.i.a, and 60° F.

/ Saturation pressure.
/ Temperature, 60° F,

HE


CONTENTS

Introduction and summary......
Acknowledgment s....cccceccccccccccccccccseves
History and development of Aneth field.....
The Aneth reservoir.,
Structure....
Stratigraphy..
Well sampling and testing. ee eeccces
Original reservoir pressure and temperature,,,
Laboratory analySeS......cccccccccccceencs
Reservoir-oil characteristics... eee
Original saturation presSUTe,...ccscceseccecees
Characteristics from differential liberations..
Characteristics from flash liberations.........
Pressure-volume-temperature relationships............
Oil-gas viscosity ratio and gas-formation volume
factor. se eeeee
Composition of produced an
Crude-petroleum analyses of produced oil..
Conclusions........
Appendix I, emcee rere cece cesses cecvane
Appendix II, - Crude-petroleum analyses of produced-oil
samples.. eee e cee eee e erence een eee eeeeeeerees
Appendix III, - Well conditions attending sampling,
Bibliography. ....cccccccccccccccccccscvecccccceceane

ILLUSTRATIONS

1, Oil and gas fields, Four Corners area...........
2, Structure contour map of Aneth field............
3. Northwest-southeast correlation cross section of
Ameth field... ccece cece ence cece cece cecceees
Gamma ray-neutron log and core analysis correlation.
Subsurface temperature gradient in Aneth field..
Volume and specific gravity of gas in solution

aur

At 133° Focccccccccccccccceecrsccscessesscsscescees
. Relative oil volume and oil viscosity at 133° F, .
Oil density at 133° Fr... cece ee eee eee
Formation volume factor at 133° F.....

won


27

T T T T T T T T T

700+ 7
2
oO
a 600} 7
>
a
a
#2 5007 1
ai
a
a
~S £4005 4
rg
>
ok
=~ 300F 4
S
5 PENTANES+ AND
2 200 NITROGEN 4
mn
z ETHANE
7) PROPANE 1 +
Z 100

BUTANES s

J— it L i 1 1 a

0 200 400 600 800 1000 1,200 1400 1600 1800
PRESSURE, PS.LA.

FIGURE 12. - Composition of Gas in Solution, Well 6.

pressure (1,820 p.s.i.a.) to pressures near 300 p.s.i.a,, the change in total
gas in solution results almost entirely from the change in volume of methane
in solution, Thus, gas liberated from oil within the reservoir, with pres-
sures in this range, will be composed principally of methane gas, Effects of
the liberation of higher boiling components of the gas in solution become more
and more noticeable as the pressure is reduced further and approaches atmos-
pheric, Moreover, much of the propane and heavier gaseous components will re-
main in solution in the unrecoverable reservoir oil unless the reservoir pres-
sure at abandonment approaches a pressure near or less than 100 p.s.i.a.

CRUDE-PETROLEUM ANALYSES OF PRODUCED OIL

Table 7 summarizes Bureau of Mines crude-petroleum analyses of crude oils
from seven wells in the Aneth field, Similar analyses of crude oils from the
Ashley Valley, Dove Creek, and Rangely fields are included for comparisons,
All of these fields produce from formations of Pennsylvanian age in the Paradox
and Uinta basins of Colorado and Utah, A correlation-index pattern for these


13

Production gas-oil ratio tests of a few wells were included in the sam-
pling program of April 1957, It was intended that these tests would be used
to supplement the subsurface samples and permit a satisfactory coverage of the
field with a minimum number of subsurface samples, However, because market
outlets were limited, lease tanks of most of the wells sought for the gas-oil
ratio tests were full, and the wells were not available for testing. Only two
tests were made; these were on wells 5 and 8, The tests were made with a Rolo
well checker, and the results are shown in table 2. The effort to obtain gas-
oil ratio data was discontinued, and emphasis was placed on obtaining enough
samples to provide the field coverage considered necessary for proper evalua-
tion of the reservoir-oil characteristics of the large producing area,

During the April 1957 tests, static pressure and temperature measurements
were also made in two wells for data on the initial reservoir conditions,
Wells 1 and 4 had been shut in approximately 5 months before subsurface meas-
urements, Pressures and temperatures were recorded at several depths in the
tubing with Amerada~type gages lowered on a wire line, The effects of md
circulation during drilling of the wells and of limited production after com-
pletion probably were eliminated during this relatively long shut-in period,
The temperature data are believed to be representative of true subsurface tem-
peratures in the field area, The recorded pressures represent reservoir pres-
sures around the wells at the time of measurements, Additional pressure data
were obtained from measurements made in conjunction with preliminary testing
of well 10 for sampling, These pressure and temperature data are presented
and discussed in the following section on original reservoir conditions,

ORIGINAL RESERVOIR PRESSURE AND TEMPERATURE
Bottom-hole static pressures measured by other investigators in two wells

in 1956 and those measured by the Federal Bureau of Mines in April and October
1957 are as follows:

Pressure at
datum 900
feet below
Completion Shut-in]| sea level, Date
Well Location date time p.s.i.a, test made
(1/) [Center NW1/4 NE1/4 sec,
23, T, 40 S,, R. 24 E,| Feb, 20, 1956] 102 hr. 2,173 (2/)
6 |NW1/4 SW1/4 NE1/4 sec,
22, T. 40 S., R. 24 E,| Apr. 28, 1956]3/72 hr. 2,183 Q@/)
1 |Center NW1/4 NW1/4 sec,
18, T. 40 S,, R. 24 E.| Oct. 1, 1956 5 mo, 2,146 Apr. 3, 1957
10 |SE1/4 SE1/4 NW1/4 sec,
30, T. 40 S., R. 25 E. | May 23, 1957 do, 2,064 Oct. 16, 1957

1/ Texaco, Inc., Navajo C-I discovery well.

2/ At time of completion, These data obtained from the transcript of the
Aneth Spacing Hearing, Salt Lake City, Utah, Sept. 25, 1957.

3/ Exact shut-in time not known; company procedure required a minimum of 72
hours.


800

PRESSURE, PSA.
FIGURE 7. - Relative Oil Volume and Oil Viscosity at 133° F.

iN T T T T T
ay \
Fs | 2
. o
met 600 4S
“s | r=)
> >
oS sowuTion | SATURATION -
5 = 400- [ pressure, 135
5° | 850 SIA. S
os SPECIFIC | re)
2 B 200 GRAVITY 4%
2a | "oO
now | a
pes a
0 | ! | l | 0.9
0 400 800 1200 1600 2000 2400
v J
PRESSURE, PSA.
FIGURE 6. - Volume and Specific Gravity of Gas in Solution at 133° F.
2.0 T T T fj T 1.4 a
RELATIVE OIL =
us VOLUME 2
wn
216 13 =
& Ss
=
& le — SATURATION Zu
o 1.2 PRESSURE, 1.2 S'5
a 1850 PSA. ae
S Nt
B 8 sul =
>
s
Hi °
ala 60°F. l ! a 10s
0 400 800 \200 600 2000 2400

19

1.0)

IN SOLUTION (AIR


15

About 170 ml. of sample is used for the differential liberation, In this
method a small gas space forms in the upper sight-gage section of the appara-
tus while the sample is being expanded at a constant weight to determine ex-
pansion properties and saturation pressure, Thereafter, the volume of the
cell (and sample) is maintained constant, and gas is liberated from the top of
the sight gage in several nearby equal pressure decrements until atmospheric
pressure is reached, Shrinkage of the liquid and a corresponding increase in
the gas volume are observed directly by measurement of the gas-oil interface
in the sight gage. At each step in the process the saturation pressure of the
oil is the cell pressure reached after a period of agitating the cell. The
gas liberated during each step is collected and measured, and samples are
taken for determining specific gravity and composition.

The differential-liberation analysis is made at the average formation
temperature, determined from measurements in the well sampled, The changes in
characteristics with pressure determined by this method are believed to ap-
proach those of the vaporization process that occurs in a reservoir with de-
cline of reservoir pressure.

Differential-liberation data are given with pressure-viscosity data in
tables 8 to 14, appendix I, for the seven sampled wells, Gas in solution, in
cubic feet per barrel, is the ratio of the volume of gas liberated between an
indicated pressure and atmospheric pressure to the volume of residual oil at
60° F,. Specific gravity of gas in solution (with air = 1,000) is the specific
gravity of the gas that is differentially liberated between an indicated pres-
sure and atmospheric pressure, Relative oil volume is the volume of the liquid
phase at an indicated pressure divided by the volume of the residual oil at
60° F, All gas volumes are corrected to 14.4 p.s.i.a, and 60° F,

Data for wells 1 and 11 were analyzed by other investigators and furnished
by the operating companies for use in this study, It is recognized that data
for wells 1 and 11 might have differed slightly if the samples had been ana-
lyzed in the Laramie laboratory because of differences in procedures and equip-
ment and the low atmospheric pressure, 11 p.s.i.a., at Laramie, Where the
data appeared to deviate appreciably from averages of data for the five sam-
ples analyzed by the Bureau of Mines, they were not included in estimating the
original average reservoir-oil characteristics.

Pressuré-viscosity relations were determined in a rolling-ball-type visco-
simeter described by Hocott and Buckley (7). The determinations are made at
formation temperature and various pressures from several hundred pounds per
square inch above saturation pressure to atmospheric pressure, The values are
in centistokes and are converted to centipoises by multiplying the values by
the density at corresponding pressure, as determined with the differential
liberation, Pressure is reduced in approximately equal decrements by bleeding
part of the sample from the viscosimeter; thus, the method of vaporization of
the sample in the cell corresponds closely to that during differential libera-
tion, The viscosity of the residual oil at atmospheric pressure is determined
with a modified Ostwald viscometer,


28

oils is shown in figure 13, Tables 25 through 30, appendix II give individual
analysis results for items 2 through 7, table 7; these items correspond to
wells 2, 3, 5, 6, 8, and 7, respectively,

TABLE 7, - Summary of crude-petroleum analyses of Pennsylvanian
oils from Paradox and Uinta basins

Viscosity
Grav- at Pour- | Sulfur, |Nitrogen,
ity,| 100° F.,|point,|weight-| weight-
Item Company Well Field | °API| S.U. Ss.| ° F. |percent| percent
1..| Texaco, Inc, |Navajo C-1]Aneth 41.3 37 10 0.11 0.042
2..|Gulf-Aztec Ismay~ do. 41.5 38 15 02 2041
. Federal-2
3..| Texaco, Inc, | Navajo do, 41.3 36 10 03 2035
G-1-X
4. do, Navajo D-4] do, 41.5 37 10 .03 035
5. do. Navajo D-1] do, 41.7 37 35 05 2044
6.. do, Navajo C-5] do, 42.1 37 10 02 .040
7..|The Superior |Navajo A-6| do, 41.5 35 10 13 040
Oil Co,
8../H.B.R. Eva Marr |Dove 48,5 34 30 <.10 .003
Drilling Co.} No. 1 Creek
9,.|Byrd-Frost- |White do, 43.8 36 25 <.10 -010
Western Federal
Natural Gas | No. 1
& PLB.
English
10.. Western Driscoll do, 63.4 32 <5 <.10 000
Natural Gas | No. 1
11..|Pan American |M.C. Rangely | 33.2 47 5 85 -
Petroleum Hagood
Co, A-2
12.. do, U.S.A. Ashley 30.8 61 15 83 134
Stanolind| Valley
well 4

The correlation index (C.I.) is a number whose magnitude indicates types
of hydrocarbons present in a crude-oil distillation fraction, The index sys-
tem is based upon the average boiling points and specific gravities of the
analysis fractions, It is so arranged that benzene has an index of 100, and
the straight-chain paraffins have a C.I, value of 0,

The Aneth oils, bounded by lines 1 and 2 (fig. 13), are more naphthenic
than the Dove Creek oils and slightly more paraffinic than the Ashley Valley
or Rangely oils in the high-boiling range, The Aneth oils have a higher aro-
matic content in the gasoline range (analysis fractions 1-7) than Ashley Valley
and Rangely oils, Although not evident from figure 13, the Aneth oils have
gasoline contents ranging from 33 to 35 percent, whereas those of the Rangely
and Ashley Valley oils range from 22 to 23 percent.


10

done in the Aneth field; however, the restricted production also posed problems
in the selection of wells for the sampling program, Tank storage at many wells
was inadequate to permit conditioning the wells for sampling at low flow rates,
and production facilities were lacking at many wells that were in a shut-in
status,

The first subsurface oil samples were obtained in April 1957 from wells
2, 3, and 6, about 13 months after discovery of the field, These and other
sampled wells are identified by symbols in figure 2. Additional sampling was
conducted October 1957 in wells 7 and 9 to provide data from the eastern end
of the field. Attempts to obtain subsurface samples from wells 8, 10, and an
additional well southeast of well 9 were prevented because of mechanical ob-
structions that prevented lowering the sampler in the tubing, Although samples
were not obtained from this area, knowledge of the oil characteristics is pro-
vided by sample data from well 11 furnished by the operating company, This
well is not within the producing area of the Aneth field, as defined by the
Oil and Gas Conservation Commission of Utah; however, recent extensions to the
proved productive area have indicated that well 11 produces from the same res-
ervoir and that the sample data are representative of the oil in the southeast-
ern part of the Aneth field, Sample data from well 1 were also furnished by
the operating company and have been included in the study,

Procedures used in selecting and preparing wells for subsurface sampling
and for obtaining samples were basically the same as those developed and re-
ported in 1941 by Bureau of Mines engineers (4). Generally, when preparing a
well for subsurface sampling, it is desirable to condition the well at the
lowest rate of stable flow for enough time to remove oil near the well bore
that had been affected by lower bottom-hole pressures on previous higher rates
of flow, The minimum stable flow rates were not known for wells in the Aneth
field, and their determination before the sampling tests was impractical.
Thus, an arbitrary rate of 100 to 200 barrels a day for a period up to 2 weeks,
depending on available tank space, was used in conditioning most of the wells.
Where the conditioning flow period was necessarily short, the procedure was to
bleed 100 to 150 p.s.i. off both the tubing and casing just before sampling to
cause additional oil to flow into the well bore with only a slight drop in
bottom-hole pressure, No attempt was made to evaluate the effectiveness of
this means of improving sampling conditions. During the final 24 hours of the
conditioning period, oil and gas production rates were measured with a Rolo
well checker or with existing lease facilities. The bottom-hole flowing pres-
sure and temperature were also measured,

After the conditioning period, each well was shut in approximately 24
hours; then the bleeding procedure was applied, and subsurface samples were
taken, Shut-in pressure-depth traverses were used to locate fluid levels and
select the lowest possible sampling point.

The subsurface sampler used in this study is self-closing for clock opera-
tion, The valves are locked in the open position, and the clock is set for a
predetermined time before the sampler is lowered into the well, The volume of
the sampler between valves is approximately 620 ml,




of the processes of origin, migration, and accumulation of oils, The study
will also provide fundamental data on the reservoirs studied for present and
future use in the engineering control of production to promote efficient re-
coveries of the Paradox-basin oils.

The data reported have been obtained from laboratory analyses of subsur-
face oil samples taken from five wells in the Aneth field, To these have been
added analyses furnished by operating companies from two other wells, On the
basis of these data, the average characteristics of the original reservoir oil
have been determined. Data are also given on the composition of the associated
gas and the crude-petroleum analyses of the produced oil, General information
on the history, development, and geology of the area are given as background
information,

Sampling programs also have been completed in the McElmo Creek, White
Mesa, and Ratherford fields, which lie a few miles southeast and south of the
Aneth field, The characteristics of the reservoir oils in these fields will
be described in future reports,

ACKNOWLEDGMENTS

The Bureau of Mines wishes to acknowledge the wholehearted cooperation of
the managements of Gulf Oil Corp., Shell Oil Co., The Superior Oil Company,
and Texaco, Inc., for making wells available for this study and to their field
personnel for assistance in arranging the field tests,

HISTORY AND DEVELOPMENT OF ANETH FIELD

The Aneth field, San Juan County, is in the southeastern corner of Utah,
about 40 miles west of Cortez, Colo,, and 20 miles southeast of Blanding, Utah,
Aneth, the largest of several oilfields discovered in the Paradox basin in re-
cent years, lies in the southern part of the Paradox basin, northwest of the
San Juan basin, and northeast of the Black Mesa basin, All but a small sec-
tion of the Aneth field lies within the Navajo Indian Reservation, as shown by
the broken boundary line in figure 1, From the junction point of the Montezuma
Creek and the San Juan River to the west, the river becomes the boundary line.
Figure 1 locates the Aneth field in relation to the geographic Four Corners
intersection and surrounding oilfields within the area,

Oil production in the Four Corners area of the Paradox basin dates back to
1908, when a well, Crossing No, 1, in sec, 7, T. 42 S., R. 19 E,, was completed
in the Rico formation near the settlement of Mexican Hat, Utah (3, p. 156).
During the next 43 years sporadic wildcatting in the area resulted in a few
discoveries of little significance, In 1951, Shell Oil Co. reported shows of
oil in a wildcat well near Bluff, Utah, These reports helped to stimulate in-
terest in the area and led to discovery of the Desert Creek field in November
1954,

The discovery well in the Aneth field was Texaco, Inc,, Navajo C-1
in Center NW1/4 NE1/4, sec, 23, T, 40S., R. 24 E. This well was completed
February 1956, for a natural flowing potential of 1,704 barrels of 43° API


25

ethane 20.4, propane 8.5, butanes 2.8, and pentanes and heavier components
0.7. Table 2 shows the production gas-oil ratios of these wells when the gas
samples were collected; the ratios ranged from 501 to 680 cubic feet per bar-~
rel, The average of these ratios is 576 cubic feet per barrel. The liquefied-
petroleum-gas (LPG) content for this average composition and average gas-oil
ratio is 2,0 gallons per barrel of stock-tank oil, equivalent to 3,5 gallons
per thousand cubic feet of separator gas,

Table 6 gives data comparable to produced-gas composition, These data
were obtained from analysis of gas flash-liberated from subsurface oil sam-
ples, The average composition of gas liberated at separator conditions from
the subsurface oil samples from wells 2, 6, 7, and 9 is 76.5 percent methane,
15,2 percent ethane, 5.9 percent propane, 1,8 percent butanes, and 0.6 percent
pentanes and heavier components, The average equivalent production gas-oil
ratio (table 4) for these four samples is 611 cubic feet per barrel. By con-
verting the LPG components to liquid equivalents on the basis of the average
composition and average equivalent production gas-oil ratio, the LPG content
of separator gas is 1,49 gallons per barrel of stock-tank oil, or 2.4 gallons
per thousand cubic feet of separator gas, Data for the subsurface sample from
well 3 (table 6) were not included in the averages, owing to the low satura-
tion pressure of the sample,

TABLE 6. - Summary of compositional analyses of flash-liberated gas

Well NOvsscccsseesesecseccceceeeeteseseeessesees | 2 3 6 [77 [oi7
Average separator conditions
Pressure, p.s.i.a. seeee wees 60 82 71 57 80
Temperature, ° F...eseceeee sec e cece ec cees 60 61 59 70 60
Composition of separator gas, mole-percent:
Methane........ eeeeee scecceceee | 78.1 (82.4 | 81.9 | 70.4 | 75.4
Ethane. eooee | 15.1 112.6 | 11.9 | 17.9 | 16.0
Propane weeee | 4.6 | 3.5] 4.1] 8.4] 6.6
Butanes,, eeeee | 1.5 |] 1.0] 1.4] 2.7 | 1.7
Pentanes,, esees 4 4 6 +6 +3
Hexanes plus se eeeee 3 el al - -
Composition of stock-tank gas,2/ mole-percent:
Methane. ...ccccccccesccccecccescecceeceseseses | 15,4 114,1 | 10.6 | 20.8 | 22.2
Ethane. ..ccescccccsccccccsssccsecseeee 30.5 |32.8 | 22.4 | 29.8 | 30.2
34,2 |33.6 | 34.7 | 32.0 | 31.5
BUCANES 6. eee ee eee eseeeneeee 15.8 {14.8 | 22.7 | 14.1 | 13.2
Pentan€S...cesseeesesessceees 3.7 | 3.8] 7.9] 3.0} 3.0
Hexanes plus. ...ssesreresessseresesrsrerereees +6 .8| 1.6 4 -

1/ Analyses by gas chromatograph; all others by mass spectrometer,
2/ Gas liberated between separator pressure and atmospheric pressure
(11 p.s.i.a.) at separator temperature,

The LPG content of gas produced from field separators during the early
life of the field should average 2,4 to 3.5 gallons per thousand cubic feet of
gas. The difference in LPG content obtained from produced-gas samples and


22

The average production gas-oil ratio in the field should be about 624
cubic feet per barrel, as determined from the flash-liberation data in table
4, From a plot of total gas in solution against the sample saturation pres-
sure, the volume of gas in solution in the average reservoir oil may be ob-
tained; this amounts to 690 cubic feet per barrel, The average of the gas in
solution at separator conditions is 66 cubic feet per barrel, The difference
(690 - 66) gives the equivalent production gas-oil ratio of 624 cubic feet per
barrel,

Pressure-Volume-Temperature Relationships

Pressure-volume data at 60°, 90°, 120°, and 133° F, for the average res-
ervoir oil are given in table 5 and shown graphically in figure 10. These
data were obtained by averaging data obtained from analysis of the subsurface
samples from wells 6 and 9, These two samples were used because their satura-
tion pressures were almost equal to the average original saturation pressure.
The use of data obtained from the other samples would have involved consider-
able extrapolation to obtain data corresponding to the average original satu-
ration pressure and higher pressures; this was considered unwarranted,

TABLE 5, - Pressure-volume-temperature relationships
of the reservoir oil

60° F. 90° F. 120° F. 133° F./

Specific Specific Specific Specific
Pressure,| volume, |Pressure,| volume, | Pressure,| volume, |Pressure,| volume,
p.s.i.a.| ml./g. |p.s.i,a. | mi./g. |p.s.i.a. | mi./g. |p.s.i.a. | ml./g.
2,500 1.362 2,500 1,389 2,500 1.416 2,500 1.428
2,200 1,366 2,200 1,392 2,200 1.421 2,200 1.432
1,900 1,370 1,900 1,396 1,900 1,425 1,900 1.437
1,600 1.374 2/1, 625 1,400 2/1,782 1.426 2/1, 850 1,438
1,4752/ 1,375 1,500 1,449 1,600 1.494 1,700 1.488
1,400 1.404 1,400 1.496 1,500 1.540 1,600 1,529
1,300 1.450 1, 300 1,553 1,400 1,594 1,500 1.577
1, 200 1,506 1, 200 1.620 1,300 1.656 1,400 1.634
1,100 1,580 1,100 1,696 1, 200 1,728 1, 300 1.703

1/ Reservoir temperature,
2/ Saturation pressure at indicated temperature,

Average values of coefficients of isothermal expansibility and isobaric
thermal contraction may be determined from the data given in table 5 and figure
10. The coefficient of isothermal expansibility is 12.0 x 1076 barrel per
barrel of reservoir oil at 2,200 p.s.i.a. and 133° F, per pound-per-square-
inch decrease in pressure from 2,200 to 1,850 p.s.i.a. The coefficient of
isobaric thermal contraction is 6.3 x 10-4 barrel per barrel of reservoir oil
at 2,200 p.s.i.a. and 133° F, per degree decrease in temperature between 133°
and 60° F,


20.
21,
22,

24,

Composition
Composition
Composition
Composition
Composition

of gas
of gas
of gas
of gas
of gas

TABLES (Con.)

in
in
in
in
in

solution,
solution,
solution,
solution,
solution,

Well Q...ccccseeeeees

iii


ii

13,

12.
13,
14,
15.
16,
17.
18.

19,

ILLUSTRATIONS (Con.)

Pressure-volume-temperature relationships,
Gas-formation volume factor and oil-gas viscosity

TACLOL cece eee ec cece c cee renenecnencresceseeeeeeenee

Composition of gas in solution, well 6........ee0005
Correlation-index pattern representing oils from
Pennsylvanian formations in Paradox and Uinta

DASLNS ...ee eee cece eect cence cent ee eeceeenseneens

TABLES

Stratigraphic classification chart of southeastern
US) eee eee eee eres
Field data relating to well tests and subsurface
Sampling. .....cececeeeesceeeees
Characteristics of reservoir oil..
Summary of flash-liberation analyses......
Pressure-volume-temperature relationships of the
FESCTVOLT OL. ccc s cece er ees cerecescceeescceneeeere
Summary of compositional analyses of flash-liberated

BOS rece cece ence cece cence eee e eee eee sees eeeeeseeeees

Summary of crude-petroleum analyses of Pennsylvanian
oils from Paradox and Uinta basins............0.08%
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 1.. wee
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 2.. eee
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 3............
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 6............
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 7............
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 9. sees
Differential-gas-liberation and pressure-viscosity
data, subsurface oil sample from well 11l...........
Pressure-volume-temperature data, subsurface oil
sample from well 2.......ccscceeecececees bea eececee
Pressure-volume-temperature data, subsurface oil
sample from well 3. ...ccccccccccccccccccccccseveese
Pressure-volume-temperature data, subsurface oil
sample from well 6.,. sees
Pressure-volume-temperature data, subsurface o:
sample from Well 7. .cccccccccccccccsccccssceccevees
Pressure-volume-temperature data, subsurface oil
sample from well Q...icccccccccccccccccceccevscneee

25

28

31

32

33

34

35

36

37

38

38

39

39

40


11

Duplicate samples were obtained from each well, as determined by tests of
each sample before transfer to a storage cylinder. These tests, at atmos-
pheric temperature, are for valve-opening pressure, saturation pressure, and
volume compression of the sample during resaturation of evolved gas and com-
pression of the sample to several hundred pounds per square inch above the
saturation pressure, Where values of saturation pressure and volume compres-
sion for two samples from the same well agreed within 1 percent the samples
were considered duplicates, and both were transferred to separate storage cyl-
inders and brought to the laboratory for analysis. The tests were also useful
in determining that each of the samples obtained was a single-phase liquid
when the sampler valves closed and that leakage did not occur while the sampler
was being brought to the surface, Thus, no samples can be considered faulty
owing to sampler malfunction or trapping of extraneous gas or water during
sampling,

Table 2 gives data on sampling and testing seven wells and on gas-oil
ratio tests of two wells, By appraising these and other data relating to well
conditions before sampling, during the conditioning period, and at time of
sampling, the writers have concluded that the four samples from wells 1, 6, 9,
and 11 are representative of the original reservoir oil, Three of these wells
were sampled relatively soon after they were completed when well conditions
were almost ideal for representative sampling of the reservoir fluid. Condi-
tions were good in the fourth well because of its high productivity index,
although considerable production had been taken from the area of this well.
Samples from the other three wells represented reservoir oil that had under-
gone some degree of modification during flow from the formation into the well.
The modification resulted from an evolution of solution gas from the oil while
the bottom-hole flowing pressure was less than the original saturation pressure,
followed by at least a partial re-solution of the evolved gas with buildup of
pressure after the well was shut in, More detailed discussions of well condi-
tions and the effects on quality of the samples are given in appendix III for
each of the seven wells sampled,

The subsurface sample from well 7 is believed to represent only slightly
modified reservoir oil, whereas the samples from wells 2 and 3 represent oil
that had been modified to a greater degree. Therefore, none of the three
samples indicates the saturation pressure of the original reservoir oil in the
vicinity of the wells. The gas liberated from solution in the samples shows
little change in composition in the first few hundred pounds per square inch
below saturation pressure, Thus, the modification of the reservoir oil that
occurred in sampling these wells may be thought of merely as a decrease in the
saturation pressure of the oil, Moreover, these samples are considered to be
representative of the physical properties and behavior of the reservoir oil at
pressures equal to and below the sample saturation pressure,


RESERVOIR-OIL CHARACTERISTICS, ANETH FIELD,
SAN JUAN COUNTY, UTAH-Y

by

R. F. Zaffarano,2/ C. Q. Cupps,2/ and J. Fry2/

INTRODUCTION AND SUMMARY

Data presented on the physical characteristics of the reservoir oil, Aneth
field, represent the initial phase of a study of Paradox basin reservoir oils
being made by petroleum engineers of the Federal Bureau of Mines, Petroleum
Research Center, Laramie, Wyo, The purpose of the study is to determine the
original reservoir-oil characteristics in several important new discoveries in
the Paradox Basin and to investigate correlations between the properties of
these reservoir oils and structural or areal position within the basin,

If migration is limited and oil accumulates near its source, then it
seems probable that few differences should be found in the composition and
characteristics of reservoir oils occurring in given age rocks and within the
confines of a geological basin, Within these limits, conditions that effected
the formation of oil and its transfer from parent to storage rocks probably
were rather uniform in an area the size of most of our geological basins.
However, if oils should migrate considerable distances, it seems logical to
expect that the relative quantities of gas and liquid components may differ in
traps occurring along the direction of migration, corresponding with the
theory of differential entrapment (8) .4/

In the first instance, no consistent or uniform variation in the relative
quantities of gas and liquid, and thus the pressure-volume-temperature (P-V-T)
characteristics, should occur, as would be expected if oils were entrapped
following a directional distant migration, On the other hand, a radial varia-
tion in composition and characteristics of reservoir oils from the central to
boundary parts of a basin may occur as a result of changing character and
thickness of source beds and limited migration of the oil.

It is hoped that the study of the physical characteristics of several
reservoir oils in the Paradox basin may provide a better understanding

1/ Work on manuscript completed February 1959,

2/ Petroleum engineer, Bureau of Mines, Region III, Laramie, Wyo,

3/ Supervising petroleum engineer, Bureau of Mines, Region III, Laramie, Wyo.

4/ Underlined numbers in parentheses refer to citations in the bibliography
at the end of this report. Page references refer to pages in the cita-
tion and not in this report,


17

The composition of the gas in solution was calculated from analyses of
the gas liberated between each pressure step of the differential-liberation
analysis. The liberated gas was analyzed by either a mass spectrometer or gas
chromatograph, Tables 20 to 24, appendix I, show the data for composition of
gas in solution, The value at a given pressure represents the composition of
the gas that would be liberated, by the differential-liberation method, between
that pressure and atmospheric pressure,

Composition of the gas in solution was derived similarly from the flash-
liberation analyses, and the data are given in table 6, page 25,

RESERVOIR-OIL CHARACTERISTICS

Original Saturation Pressure

The average saturation pressure of the original reservoir oil in the
Aneth zone is estimated to be 1,850 p.s.i.a, This estimate is based upon sat-
uration pressures obtained from the sampling of wells 1, 6, 9, and 11. These
samples, which are believed to be representative of the original reservoir oil
in the vicinity of the wells, had saturation pressures of 1,816, 1,820, 1,875,
and 1,745 p.s.i.a, Because of their locations, the first three wells represent
the major part of the Aneth field. The saturation pressures of samples from
wells 1 and 6 may be slightly less than that of the reservoir oil due to well
conditions attending sampling, as discussed in appendix III. Thus, 1,850
p.s.i.a, is considered a realistic value for the average saturation pressure
of the original reservoir oil in most of the Aneth field, The relatively low
value of saturation pressure, 1,745 p.s.i.a., found for well 11 is believed to
be due to a local condition existing in the extreme southeast extension of the
field, It is also recognized that this lower saturation pressure may represent
a change in characteristics of the reservoir oil between the major part of the
Aneth field and the McElmo Creek field to the southeast, but additional data
will be required to determine if this is so,

The available production gas-oil ratio data for wells in the Aneth field
vary so widely that they are unreliable for quantitative evaluation of average
solution gas-oil ratio of the reservoir oil, Reliable gas-oil ratio data
should provide a check on the estimate made of the average saturation pressure,
However, these production gas-oil ratios have not indicated any areas of low
or high gas-oil ratio, which would be expected if the reservoir oil differed
significantly in various parts of the field, That the reservoir oil is essen-
tially the same throughout the field is illustrated more precisely by comparing
the characteristics of the subsurface samples obtained from the field, Because
of the similarity of the subsurface samples, values for the characteristics of

the original reservoir oil have been obtained by averaging the data derived
from the samples,

Characteristics From Differential Liberations

Several graphs were prepared showing the variation of oil characteristics
with pressure, Tables 8 to 14, in appendix I, give the basic data used to
prepare these graphs, Smoothed curves were drawn through the data points and


Bureau of Mines
Report of Investigations 5511

CALIFORNIA
INSTITUTE oF
Nov 2 1959
TECHNOLOGY

RESERVOIR-OIL CHARACTERISTICS
ANETH FIELD, SAN JUAN COUNTY. UTAH

By R. F. ZAFFARANO, C. Q. CUPPS, AND J. FRY

United States Department of the Interior — 1959
metadc38678


40

TABLE 19, - Pressure-volume-temperature data, subsurface
oil sample from well 9

60° F. 90° F 120° F. 134° F.L/

Specific Specific Specific Specific

Pressure, | volume, Pressure, |volume, Pressure,| volume, Pressure, | volume,
p.s.i.a. | ml./g. «s.i.a. | ml./g. |p.s.i.a, | ml./g. |p.s.i.a. | ml/g.
2, 500 2/1364 2,500 | 2/1391 2,500 | 2/1,419 2,500 | 2/1.430
2,256 1,368 2,241 1,395 - - 2,290 1.434
2,006 1.371 2,009 1.398 - - 2,100 1.437
1,761 1.374 1,815 1,400 - - 1,923 1.440
1,5053/ 1.378 | 3/1,653 1.402 3/1,875 1.441
1,499 1.378 1,642 1.406 1,849 1.448
1,439 1.400 1,581 1.425 - - 1,789 1,467
1, 366 1.432 1,494 1.460 - - 1,684 1,504
1,174 1.540 1, 201 1.636 - - 1,407 1.641
945 1.764 875 2.029 - - 1,038 1.996
714 2.174 701 2.424 - - 784 2.490
518 2,866 569 2.917 - - 598 3.184

1/ Reservoir temper

2/ Determined with density pycnometer,

ature,

3/ Saturation pressure at indicated temperature,

TABLE 20. - Composition of gas in solution, well 2

Pressure, Volume percentl/
p.s.i.a. Methane | Ethane | Propane | Butanes | Pentanes | Hexanes + | Nitrogen
56.6 21.1 13.3 6.3 2.0 0.6 0.1
54,3 21.9 14.2 6.7 2.1 7 al
49.9 23.5 15.8 7.6 2.4 7 al
43.1 26.0 18.3 9.0 2.8 8 0
34.3 28.7 21.7 11.0 3.4 ) i)
23.0 31.6 26.4 13.6 4.3 1 te)
9.0 32.1 33.2 18,2 6.1 1.4 0
6 20.1 41.7 25.9 9.7 2.1 0
(0) 9.3 40.3 33.8 12.8 3.7 i)

1/ Calculated composition based on mass spectrometer analysis of gas differ-

entially liberated at a temperature of 134° F.

Each value shown for an

indicated pressure is equivalent to composition of all gas liberated
between that pressure and atmospheric pressure,
2/ Saturation pressure of sample.


producing formation and perforate, Only a few open-hole completions have been
made, and these are in the central part of the field, Oil strings are usually
5-1/2-inch or 7-inch casing,

Mud programs have varied; however, the use of water-base mud to 2,500
feet, then gyp-base, oil-emulsion md to a total depth is the practice used by
most operators, The use of oil-emulsion md decreases mud cost and increases
penetration rate and bit life. The cost of drilling and completing a develop-
ment well in the Aneth field is about $90,000,

In April 1958 the Aneth field contained over 120 producing wells within
17,280 proved acres and had produced 2,380,000 barrels of oil, The reservoir
is being developed on 80-acre spacing,

During the 25 months after discovery of the Aneth field, 10 other fields
were discovered in the surrounding area to the east, south, and west. From
1956 to 1958 most of the crude-oil output was moved from the Aneth field by
truck to a railroad terminal at Thompson, Utah, and by rail to refineries in
Salt Lake City, Utah, In May 1958 the Four Corners Pipeline Co, completed its
16-inch line to the Los Angeles area and began moving crude oil from the
Paradox and San Juan basins at near capacity of 70,000 barrels a day. In late
June 1958 the Texas-New Mexico Pipeline Co, completed its 16-inch crude-oil
line, which connects with existing facilities near Jal, N. Mex. Initial capac-
ity of the line is 50,000 barrels a day, The construction of these two pipe-
lines has accelerated development and exploration activity in the basin, and
this increased activity should continue over the next few years,

THE ANETH RESERVOIR
Structure

The Aneth-field structure (shown in fig. 2) is an elongated anticline
with the axis trending west-northwest to east-southeast and with an indicated
closure of less than 50 feet, The maximum closure in the productive area is
about 150 feet, although this is not evident in figure 2, because individual
well data used to construct the contour map are not shown, There is no evi-
dence of faulting in the Aneth field. Dry holes have shown the probable pro-
ductive limits of the field on all but the south flank,

Stratigraphy

Oil production in the Aneth field is obtained from a vuggy, fossiliferous
limestone in the Paradox member of the Hermosa formation, One or two wells
have also been completed in the upper Hermosa member of the Hermosa formation,
but production is limited, The relative position of these members within the
Hermosa formation is shown on the stratigraphic classification chart in table
1 which was adapted from Herman and Barkell (6, p. 864). Oil production is
attributed to both structure and stratigraphy but may be effected more by
porosity development in the carbonate rocks. The Hermosa formation is of
Pennsylvanian age and is divided into three members: (1) The lower Hermosa
member, (2) Paradox member, and (3) upper Hermosa member, The lower Hermosa


le
R21E. R22E. RE R24E. fo R25 E. R.26E.

AS.
| |
BLUFF s
t
RECAPTURE CREEK. Y NETH -
TL ISMAY
40 TURNER eo
S alurr @| © Pevione
. aT “~, MESA
ly GOTHIC a
i MESA | RATHERFORD
Ss.
TOHONADLA Z
DESERT
L CREEK
42
AKAH
Ss. e !
a
a CHINLE
WASH
Ss. BOUNDARY
UTAH yur COLO.
fay - - q- -r -4 -- -
ARIZONA DP eEast BOUNDARY BUTTE N. MEX.
® @ huNav
NORTH BITA PEAK
TAH-A-TIN

FIGURE 1. - Oil and Gas Fields, Four Corners Area.

gravity oil a day, through a l-inch choke, Production is from the Paradox
member of the Hermosa formation,

Terrain in this area of Utah is rough, strewn with rocks, and cut by can-
yons and washes, Drainage is toward the south to the San Juan River, Most
field roads have been cut with bulldozers; they are extremely rough, and
quickly turn into quagmires during wet weather, yet they must be depended upon
to carry the large volume of traffic required to transport personnel and equip-
ment to and from the field area,

Drilling in the Pennsylvanian formation in the Aneth field is relatively
easy if a well-planned drilling program is used, Field transportation is a
major problem because of poor road conditions, Water for drilling is either
trucked from the San Juan River or supplied from junked holes that produce
“gyp" water from 550 to 1,250 feet in depth (8, p. 154).

Most wells in the Aneth field have been treated with varying proportions
of acid during completion. Normal field procedure is to set pipe through the




TABLE

13. - Differential-gas-liberation and pressure-viscosity data,

subsurface oil sample from well 91/7

Differential gas liberation Pressure-viscosity
Relative oil Specific
Gas in solution, volume, gravity
Expansion cu, £t,/bbl, residual oil Oil of gas in
Pressure, | of sample, residual ght volume at density, solution Pressure, | Viscosity,
p.s.i.a, percent at 60° F,& 60° F, = 1,000 | g./ml, (air = 1,000) | p.s,i.a, centipoises
2,500.... 0 755 1.386 0.6991 0.939 2,770 0.58
. 19 755 1,388 -6977 939 2,517 257
. 44 755 1.392 -6960 +939 2,227 56
. 69 755 1,395 6943 939 1,909 55
. 74 755 1,396 -6940 939 3/1,875 55
. 1,07 - - - - 1, 846 255
. 1.50 - - - - 1,635 58
. 2.03 - - - 1,231 67
. 2.92 - - - 977 75
. 5.04 699 1.373 - 959 640 86
. - 575 1.323 «7112 1,008 267 1.05
. - 467 1.279 «7232 1,070 92 1.25
671.... - 368 1.237 27354 1.164 ll 1.87
390.... - 276 1,198 7465 1.294 - -
160.... - 184 1.158 7577 1,462 - -
. - 84 1.104 -7712 1.692 - -
. - t¢) 1,039 . 7898 - -
- 0 1.000 8209 - -

1/ Analysis temperature, 132° F,
2/ Gas volumes at 14.4 p.s.i.a, and 60° F,
3/ Saturation pressure,
q

/ Temperature, 60° F,

9€


24

cof T T T T 7
L' a
404 4
“o30F 4
2 20- 5 200
oO
we
2. °
DIOL 4100 5
>
o BF VISCOSITY RATIO 7 80 =
5 6P 1 60 =
a fo)
a4 1 408
wo 3p 4 30>
we a
oO
ger + 20 ©
oO

0 400 800 1200 1600 2000 2400
PRESSURE, PS.LA.

FIGURE 11. - Gas-Formation Volume Factor and Oil-Gas Viscosity Ratio.

1l. The factor was determined from differential-liberation data for wells 2,
3, 6, 7, and 9, Data for wells 1 and 11 were not included in the determina-
tions because of the appreciable differences between specific gravities of gas
reported for these subsurface samples and the specific gravities obtained from
the subsurface samples analyzed by the Bureau of Mines. The gas~-formation-
volume factor at the average original saturation pressure (1,850 p.s.i.a.)
and reservoir temperature (133° F,) is 1.24 x 10-3 barrel per standard cubic
foot of gas.

Composition of Produced and Solution Gas

The mole-percentages of hydrocarbon components in samples of produced gas
from wells 2, 3, 5, 6, 7, and 8 ranged as follows: Methane 63,1 to 74.7,
ethane 18,1 to 22.4, propane 5.1 to 10.7, butanes 1.6 to 4,4, and pentanes and
heavier components 0,3 to 1.4, The compositions averaged methane 67.3,


41

TABLE 21, - Composition of gas in solution, well 3

Pressure, Volume _percenti/
p.s.i.a. Methane | Ethane | Propane | Butanes | Pentanes | Hexanes +
1, 4202/ seve] 5204 | 23.2 14.8 6.8 2.3 0.5
1,295.. o- 50.4 23.9 15.5 7.2 2.4 5
976.. sees 43.9 26.4 18.0 8.4 2.7 6
669.....46. 35.0 29.3 21.4 10.2 3.2 8
B97 a ceeeee 24.0 32.4 25.8 12.7 4.1 1.0
171..... . 10.5 33.6 32.7 16.6 5.3 1.3
51.. . 4 22.2 41.0 26.0 8.2 2.1
Bliccccecceeee (0) 15.1 48.3 21.1 12.1 3.4

17 Calculated composition based on mass spectrometer analysis of gas differ-
entially liberated at a temperature of 132° F, Each value shown for an
indicated pressure is equivalent to composition of all gas liberated
between that pressure and atmospheric pressure,

2/ Saturation pressure of sample.

TABLE 22, - Composition of gas in solution, well 6

Pressure, Volume percent
p.s.i.a. Methane | Ethane |Propane |Butanes |Pentanes | Hexanes + | Nitrogen
1,8202/ 56.3 20.1 13.6 6.1 2.4 1.0 0.5

54.3 20.7 14.4 6.5 2.6 1.0 )
48.6 22,7 16.5 7.6 2.9 1.2 )
41.6 25.0 19.3 9.0 3.4 1.4 3
32.4 27.8 23.0 11.0 4.0 1.6 2
20.3 | 30.5 28.1 13.9 5.1 2.0 al
7.5 28.6 35.7 18.7 6.8 2.7 0

4O.....06 3 16.8 41.7 27.4 9.8 4.0 0

1/ Calculated composition based on mass spectrometer analysis of gas differ-
entially liberated at a temperature of 133° F, Each value shown for an
indicated pressure is equivalent to composition of all gas liberated
between that pressure and atmospheric pressure,

2/ Saturation pressure of sample,


23

2300. 1
60F 9OF I20F 133 F.

2100

LOCUS OF SATURATION PRESSURES

1900 F

PS.LA

1700

PRESSURE

T

500

4,300

1100 1 1 1 1 1 i | | !
134 1.38 142 1.46 1.50 154 1.58 1.62 1.66 1.70 74

SPECIFIC VOLUME, ML./G.

FIGURE 10. - Pressure-Volume-Temperature Relationships.

Oil-Gas Viscosity Ratio and Gas-Formation-Volume Factor

The oil-gas viscosity ratio and gas-formation-volume factor are two char-
acteristics of a reservoir fluid system that have important effects on reser-
voir performance, Values of these characteristics at various pressures were
calculated for the Aneth reservoir oil and are shown graphically in figure 11.

To determine the oil-gas viscosity ratio, the oil-viscosity data in table
3 and shown graphically in figure 7 were used. The gas-viscosity data were
averages at several pressures of the viscosity of gas differentially liberated
from solution in five subsurface oil samples, The viscosities of these liber-
ated gases were determined from their specific gravities or composition, using
correlations of Carr, Kobayashi, and Burrows (1). Specific gravity was used
for samples from wells 6, 7, and 9 and composition for samples from wells 2
and 3,

The change in gas viscosity at reservoir temperature with decrease in
pressure is considerably less than the change in oil viscosity; thus, the var-
iation in the oil-gas viscosity ratio from 34,5 at 1,850 p.s.i.a. to 93,0 at
300 p.s.i.a. results principally from the increase in oil viscosity,

The gas-formation-volume factor is the volume that 1 cubic foot of gas at
standard conditions, 14,4 p.s.,i.a. and 60° F., will occupy at reservoir condi-
tions; it is expressed in barrels and varies with pressure, as shown in figure




14

The pressure measurements in the first two wells were made about 2 months
after the field was discovered and when less than 500 barrels of oil had been
produced from the wells, Except for the effect of well completion and the
limited production before the measurements were made, these pressures are ap-
proximately representative of the original reservoir pressure, which is esti-
mated to be 2,200 p.s,i.a, at a datum of 900 feet below sea level, The meas-
urements in wells 1 and 10 reflect the decline in reservoir pressure with
production,

The subsurface-temperature curve (fig. 5) was constructed from tempera-
ture-elevation measurements taken from wells 1 and 4 with a subsurface-temper-
ature-recording instrument, The curve is representative of subsurface temper-
atures in the Aneth field area, The temperature gradient is 1,4° F, per

hundred feet of depth,

4,000 T T —- T T and the formation tem-
perature 900 feet below
sea level is 133° F,

o WELL NO. |

3,000 -- 7
,' s WELL NO. 4 Temperatures applied

in the analysis of the
subsurface samples were
- derived from maximum
thermometer measurements
in the sampled wells,
These temperatures were
corrected to the midpoint
elevations of the perfo-
rated intervals and agree
+ with the temperature-ele-
vation curve (fig. 5).

T

2,000

T

1,000

ELEVATION, FEET

SEA LEVEL

LABORATORY ANALYSES

-1,000 +

The two samples from
each well were reexamined
-2,000 ; ! ! : : in the laboratory for

40 60 80 100 [20 140 160 saturation pressure at
TEMPERATURE, F the field transfer tem-
FIGURE 5. - Subsurface Temperature Gradient perature and, if found
. + unchanged, were combined
in Aneth Field. ?
to avoid the minor dif-
ferences in values that often result when duplicate samples are analyzed sep-
arately, Combining the samples also provided enough sample for the extensive
laboratory analyses,

The combined sample from each well was analyzed by several methods to
determine its physical characteristics, These methods are differential lib-
eration, flash liberation, pressure-volume-temperature relations, and pressure-
viscosity relations, Procedures and equipment used for the first three are
basically the same as those described in Federal Bureau of Mines Technical
Paper 629 (4).


51

BIBLIOGRAPHY

CARR, N. L., KOBAYASHI, RIKI, AND BURROWS, D. Application of Correlation to
the Prediction of Viscosities of Natural Gas Mixtures. Jour, Petrol.
Technol,, vol. 6, No, 10, October 1954, pp. 50-55,

EILERTS, C, K., SMITH, R. V., AND COOK, A. B. Properties of a Petroleum-
Reservoir Liquid and Its Residua With Applications of the Data to Pro-
duction Problems, Bureau of Mines Rept. of Investigations 3474, 1939,
64 pp.

FOUR CORNERS GEOLOGICAL SOCIETY. Geology of Parts of Paradox, Black Mesa,
and San Juan Basins: Four Corners Field Conf., 1955, 217 pp.

GRANDONE, P., AND COOK, A. Collecting and Examining Subsurface Samples of
Petroleum, Bureau of Mines Tech, Paper 629, 1941, 68 pp.

GUSSOW, W. C, Differential Entrapment of Oil and Gas: A Fundamental Prin-
ciple, Bull, Am, Assoc, Petrol, Geol., vol. 38, No, 5, 1954, pp. 816-853,

HERMAN, G,, AND BARKELL, C, A, Pennsylvanian Stratigraphy and Productive
Zones, Paradox Salt Basins, Bull. Am, Assoc, Petrol. Geol., vol. 41, No.
5, 1957, pp. 861-881.

HOCOTT, C. R., AND BUCKLEY, S. E. Measurement of the Viscosities of Oils
Under Reservoir Conditions, Petrol, Technol,, vol, 3, August 1940,
pp. 1-6.

McGHEE, E, Are You Planning to Drill in the Aneth Area? Oil and Gas Jour.,
vol. 55, No, 25, June 24, 1957, pp. 152-159,

INT. ©BULOFMINES.PGH..PA. S18


APPENDIX I.

- LABORATORY DATA

TABLE 8, - Differential-gas-liberation and pressure~viscosity data,

subsurface oil sample from well 1+!

Differential gas liberation Pressure-viscosity
Relative oil Specific
Gas in solution, volume, gravity
Expansion cu. ft,/bbl. residual oil Oil of gas in
Pressure, | of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.s.i.a, percent at 60° F,2/ 60° F, = 1,000 ./ml. (air = 1,000) | p.s.i.a. | centipoises
1,914,... @/) 726 1,388 G/) 0,954 2,124 0,414
1381647 °° 726 1,390 2954 1,924 «409
1,614,... 660 1.362 978 4/1,816 405
1,401.... 593 1,334 1.009 1,674 441
1,167.... 521 1.304 1,050 1,564 -463
922.... 443 1,272 1.107 1,334 505
683.... 369 1,241 1,179 1,039 574
449, 292 1,208 - 644 702
248, 217 1,175 - 349 847
136, 165 1.149 - 14 1,863
14,,.. 0 1.037 - - -
143!) Cy) 1,000 - -
1/ Analysis temperature, 134° F,

Gas volumes at 14.4 p,s.i.a, and 60° F,

Values were not available from the analysis provided by the operating company,

Saturation pressure,

Temperature, 60° F,

Te


TABLE 9, - Differential-gas-liberation and pressure-viscosity data,
subsurface oil sample from well 24/

Differential gas liberation Pressure-viscosity
Relative oil Specific
Gas in solution, volume, gravity
Expansion cu, ft,/bbl. residual oil Oil of gas in
Pressure, | of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.s.i.a, | percent at 60° F,2/ 60° F, = 1,000 | g./ml, (air = 1,000) | p.s.i.a. centipoises
2,500.... 0 715 1.374 0.7029 0.978 2,577 0.60
2,265.... 27 715 1.378 .7010 +978 2,339 59
2,054... 52 715 1,381 +6993 978 2,067 258
1,837.... 77 715 1,385 .6975 978 3/1,740 56
1,7403/.. 88 715 1,386 6968 .978 1,501 61
1,729.... 1,10 - - - - 1,449 62
1,719. 1.43 - - - - 1,355 264
1,693, 1.96 - - - - 1, 226 67
1,665, 2.70 - - - - 1,065 «72
1,624, 4.04 - - - - 937 76
1,557.... 5.71 660 1,365 - 1.002 768 82
- 565 1,325 7118 1.050 588 «90
- 461 1,282 «7234 1.126 348 1.04
- 370 1.244 -7342 1,224 168 1.20
- 280 1,206 «7452 1,352 61 1,44
- 192 1.166 «7568 1.550 11 2.01
- 97 1,114 7691 1.815 - -
- 30 1,068 . 7800 1.970 - -
- 0 1.044 7867 - - -
- 0 1.000 .8210 - - -

7 Analysis temperature, 134° F,

/ Gas volumes at 14.4 p.s.i.a. and 60° F,
/ Saturation pressure.

/ Temperature, 60° F,

ve


47

APPENDIX III. - WELL CONDITIONS ATTENDING SAMPLING

A representative sample of a reservoir oil should contain the same pro-
portions of liquid and associated solution gas as the reservoir oil, Depend-
ing on the bottom-hole pressure drawdown, producing a well may cause these
proportions to be changed owing to release of gas from solution as the oil
moves through the formation and into the well bore, Thus conditioning a well
just before sampling, by flowing it at a reduced rate to eliminate the effects
of previous higher rates, is an important part of sampling, However, in a
saturated or near~saturated reservoir oil, release of gas from solution may
accompany even very low rates of production and prevent sampling oil that has
not been modified to some degree, Knowledge of well conditions attending sam-
pling often provides the only reliable means of interpreting results of sample
analyses, The following discussions of well conditions that preceded and
existed during sampling of wells in the Aneth field are given to show the
evaluations that were made of the quality of the subsurface samples and their
limitations for determining the original reservoir-oil characteristics,

Well 1

The sample from well 1 was obtained and analyzed by other investigators.
The sample data were furnished by the operating company for use in this study,
Data reported with the sample analysis showed that the well had been shut in 6
days when the subsurface sample was taken and that the bottom-hole static
pressure was 2,068 p.s,i.a, No data were reported on the pressure~production
history of the well; however, as far as could be determined, less than 500
barrels of oil had been produced from the well during its completion and ini-
tial flow test before sampling. During the initial flow test oil was produced
at a rate of 547 barrels a day, Considering the probable producing character-
istics of the well relative to those of other wells in the field, it seems
almost certain that at this rate the bottom-hole flowing pressure would have
been less than the saturation pressure of the sample, Unfortunately, the de-
gree of modification of the original reservoir oil with evolution and re-solu-
tion of gas during the imposed flow and shut-in conditions cannot be deter-
mined owing to lack of production gas-oil ratio data, The saturation pressure
of the sample (1,816 p.s.i.a.) compares favorably with values obtained from
sampling other wells in the field where conditions were known to favor repre-
sentative sampling. Thus, for all practical purposes, the sample is consid-
ered representative of the original reservoir oil in the vicinity of well 1.

Well 2

Well 2 had been producing at a rate of 250 barrels of oil a day for ap-
proximately 22 days when it was put on a conditioning rate of 182 barrels a
day, The conditioning rate was maintained for a period of 44 hours, during
which the minimum bottom-hole flowing pressure was 1,349 p.s.i.a. After the
well had been shut in 67 hours for a pressure build-up test, gas was bled from
the tubing and casing heads, and subsurface oil samples were taken, The
static pressure before sampling was 1,763 p.s.i.a. Comparing the saturation
pressure of the sample (1,740 p.s.i.a.) with the minimum bottom-hole flowing
pressure and the static pressure existing just before sampling indicated that


20

0.84

OIL DENSITY, GM./ ML.

72

|__ SATURATION
PRESSURE, 7
| 850 PSIA.

ee

68
0 400 800 200 1600 2000 2400
PRESSURE, PSIA
FIGURE 8. - Oil Density at 133° F.
a (‘'|4 T a
S RELATIVE OIL
= VOLUME
a 13h |
Bye
fo}
oS ESTIMATED
er ok FORMATION AVERAGE
w 1.2 VOLUME
Sy FACTOR r+ SATURATION
22 PRESSURE,
=) / 1850 PSIA.
SZ | “4
= Aw 66 PSIA |
= 70°F.
= iol l ae |
0 400 800 1200 1600 2000 2400

PRESSURE, PSIA.
FIGURE 9. - Formation Volume Factor at 133° F.


50

modified, It is concluded that the sample is representative of the reservoir
oil at the saturation and lower pressures reported for the laboratory analyses
and that the saturation pressure of the original reservoir oil was probably
somewhat higher than that of the sample,

Well 9

Well 9 had been producing oil on a conditioning rate estimated at 100 to
150 barrels a day for about 1 week when the flowline was accidentally broken,
causing the well to be shut in, Tests were begun 2 days later, when the well
was still shut in, The bottom-hole static pressure was 1,908 p.s.i.a, when
the well was put on a 6/64-inch choke, After 1-1/2 hours, the bottom-hole
pressure had dropped to only 1,878 p.s.i.a. The subsurface oil samples were
taken immediately with the well flowing. Saturation pressure of the sample is
1,875 p.s.i.a. Accordingly, saturation of the oil is indicated at the bottom-
hole flowing pressure during sampling, Pressure-volume data, determined dur~
ing testing of the sample before transferring it to a storage cylinder, show
that the sample was a single-phase liquid when the sampler valves closed,
Thus, if free gas existed in the flow stream at the time of sampling it was
not trapped with the sample on two sampling runs, It is believed that the
flow stream coming into the bottom of the well was a single-phase liquid, and
it is concluded that the sample is representative of the original reservoir
oil near well 9,

Well 11

Well 11 was sampled and analyzed by others; the sample data were fur-
nished by the operating company for use in this study, Data furnished with
the sample report show that the well had been produced at a rate of 85 barrels
of oil a day for 6 days and had been shut in 46 hours at the time the subsur-
face sample was taken, The bottom-hole static pressure at the time of sam-
pling was 2,158 p.s.i.a. The saturation pressure of the sample was 1,745
p.S.i.a. No information was reported on the bottom-hole-flowing-pressure or
gas-oil-ratio tests, It is doubtful if flow at 85 barrels a day would have
caused the bottom-hole pressure to drop as low as the saturation pressure of
the sample; therefore, it is concluded that the sample is representative of
the original reservoir oil near well 11.


This publication has been cataloged as follows:

Zaffarano, Richard F

Reservoir-oil characteristics, Aneth field, San Juan County,
Utah, by R. F. Zaffarono, C.Q. Cupps and J. Fry [Washington}
U. S. Dept. of the Interior, Bureau of Mines, 1959.

iii, 51p. illus., maps, tables. 27cm, (U. S. Bureau of Mines, Re-
port of investigations, 5511)
Bibliography: p. 51.

1. Oil fields—Utah—San Juan Co, 2. Petroleum—Analysis. I. Title.
(Series)

[TN23.U7 no. 5511] 622.06173

U. S. Dept. of the Int. Library



TABLE 17, - Pressure-volume-temperature data, subsurface
oil sample from well 6

39

60° F, 90° F. 120° F. 134° FL/
Specific Specific Specific Specific
Pressure, | volume, Pressure, | volume, Pressure, | volume, Pressure, | volume,
p.s.i.a. | ml./g. |p.s.i.a. | ml./g. |p.s.i.a. | ml./g. |p.s.i.a. | ml./g.
2,500 2/1 360 2,500 | 2/1,386 2,500 | 2/1.413 2,500 | 2/1.425
2,229 1.363 2,194 1,390 2,233 1.417 2,280 1.428
2,014 1,366 1,899 1,394 2,021 1.421 2,058 1.432
1,855 1,368 1,607 1.398 1,803 1.424 1,856 1,435
1,511 1.372 | 3/1,598 1.398 | 3/1, 757 1.425 | 3/1,820 1.436
1, 4483/ 1.373 1,577 1.406 1,730 1.432 1,790 1.446
1,430 1,380 1,534 1.421 1,698 1.445 1,750 1.458
1, 388 1.392 1,469 1,446 1,642 1.463 1,692 1.477
1,338 1.415 1,390 1.487 1,540 1,505 1,597 1.514
1, 283 1.442 1,242 1.575 1,349 1,609 1,399 1,619
1,155 1.519 1,069 1,722 1,104 1,819 1, 209 1.766
935 1,728 853 2,016 852 2.199 953 2.082
700 2,148 677 2.438 691 2,622 759 2.505
545 2.674 566 2,860 585 3.045 571 3,247
1/ Reservoir temperature,
2/ Determined with density pycnometer,
3/ Saturation pressure at indicated temperature,
TABLE 18, - Pressure-volume-temperature data, subsurface
oil sample from well 7
60° F, 90° F. 120° F. 134° F.L7
Specific Specific Specific Specific
Pressure, |volume, Pressure, |volume, Pressure,| volume, Pressure,| volume,
p.s.i.a. | ml./g, |p.s.i.a, | ml./g. |p.s.i.a. | ml./g. |p.s.i.a. | mb./g.
2,500 2/1 ,360 2,500 | 2/1,376 2,500 | 2/1,403 2,500 | 2/1,417
2,235 1.364 2,226 1.379 - - 2,280 1.420
2,007 1,367 1,987 1.382 - - 2,059 1.423
1,745, , 1.370 1,697 1,386 - - 1,810 1.427
1,475 1.374 | 3/1,565 1.388 - - 3/1, 785 1.427
1,469 1.376 1,547 1.394 - - 1,757 1.437
1,421 1,395 1,497 1,413 - - 1,684 1.458
1,346 1.427 1,392 1.459 - - 1,564 1,506
1,234 1,480 1,197 1.576 - - 1,243 1.700
1,135 1,522 918 1,867 - - 896 2,132
974 1,701 717 2.257 - - 697 2.619
814 1,927 531 2.940 - - 556 3.208
624 2.384 - - - - -
469 3.075 - - - -

1/ Reservoir temperature,

2/ Determined with density pycnometer,
3/ Saturation pressure at indicated temperature,


TABLE 14, - Differential-gas liberation and pressure-viscosity data,
sample from well iL/

subsurface oil

Differential gas liberation

Pressure-viscosity

Relative oil Specific
Gas in solution, volume, gravity
Expansion cu, ft./bbl. residual oil O11 of gas in
Pressure, |of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.s.i.a. _percent at 60° F,2/ 60° F, = 1,000 | g./ml. (air = 1,000) | p.s.i.va. centipoises
/) 675 1,364 0.6997 0.933 2,214 0.470
612 1,339 7059 2955 1,914 0454
552 1,313 +7128 ~982 3/1, 745 442
494 1,289 7193 1.013 1,614 463
428 1,262 7265 1.057 1,414 .500
364 1,235 «7343 1,117 1,114 570
299 1,208 7419 1,193 714 692
230 1,178 -7508 1.302 414 826
167 1,147 27595 1.426 14 1,932
131 1,128 «7646 1,508 - -
0 1,037 «7896 - - -
0 1.000 .8190 - - -

1/ Analysis temperature, 135° F,

/ Gas volumes at 14,4 p.s,i.a. and 60° F,
/ Saturation pressure,
/ Values not available from the analysis provided by the
/ Temperature, 60° F,

operating company,

Le


DEPTH BELOW SEA LEVEL, FEET

CARTER OIL CO. GULF-AZTEC THE TEXAS CO. THE TEXAS CO. THE TEXAS CO. THE TEXAS CO. THE TEXAS CO. THE TEXAS CO.
NO.1 Hickman-Federal NO. 2 Ismoy-Federol NO. 5 Novojo G NO. 1-X Navojo G NO. 4 Novajo D NO. 1 Novajo D NO. | Navajo C NO. | Novojo F
sec. 1, T.40S.,R.23E. sec. 7, 1.40 S.,R.24E. $e¢. 18, 7.40S.,R.24€. sec. 17, T.40S.,R.24E, Sec.21,7.40S.,R.24 E. sec. 22,T. 40 $.,R. 24 E. sec, 23, 1.40 S.,R. 24 E. Sec. 19, T.40 SR. 25 E.
ELEV. §266' K.8. ELEV. 4735 K.8. ELEV. 4649' K.B. ELEV. 4585' K.8, ELEV, 4894' K.B. ELEV. 4951' K.B. ELEV. 4923' K.B. ELEV. 4963'K.8,
5 - =
[100 : 100 +
[- 200 f ~ 200 —
+ 300 5 300 +
L 4 >
t+ 400 400 4
>
5 Le >
L500 500 —
L600 600
o . ISMAY
+ 700 = e 1 700 +
= BLUFF ZONE -
x=
= > PARADOX SHALE Z
[+ 800 & 800 +
> ANETH
oR ay
L900 i DESERT CREEK ZONE | - > 900 4
s =>" =
- 1000 2 —— ---- = p00 4
= eee
a LL ge-o 0 3
L Woo TMILE 00 —

FIGURE 3. - Northwest-Southeast Correlation Cross Section of Aneth Field.


49

Well 6

Well 6 was conditioned 64 hours on a relatively high rate of 266 barrels
a day because of the much higher rates at which the well had been produced
before conditioning, The well was not available for a longer period of condi-
tioning, which would have been required had a lower rate of flow been used;
it was shut in to allow the pressure to build up before sampling. The bottom-
hole flowing pressure and static buildup were not reported because the pres-
sure-gage clock stopped, On two occasions during the shut-in period and be-
fore the first of two subsurface oil samples was taken, gas was bled from the
wellhead at a slow rate to move fresh oil from the formation into the tubing
for sampling. This procedure was repeated just before taking the second sam-
ple. The second sample had the highest saturation pressure, and it was used
for the laboratory analyses, After sampling, the well was flowed again and
then shut in, so as to duplicate and measure, as nearly as possible, the pres-
sure conditions that existed before sampling, for which period pressure data
were lacking. This measurement and a static-pressure measurement made just
before taking the second sample indicated that the minimum bottom-hole flowing
pressure on the conditioning rate was about 1,740 p.s.i.a, and the static
pressure at the time of sampling was 1,840 p.s.i.a, Saturation pressure of
the sample was 1,820 p.s.i.a. Liberation of gas from solution in the reser-
voir oil is indicated during flow on the conditioning rate, The measured
production gas-oil ratio, 599 cubic feet per barrel of stock-tank oil agrees
well with the equivalent gas-oil ratio obtained by flash gas-liberation of the
sample, 614 cubic feet per barrel, Thus, the gas evolved from the reservoir
oil was essentially redissolved when the sample was obtained. The sample is
considered representative of the original reservoir oil near well 6.

Well 7

Well 7 had been shut in 14 days at the time of sampling, Previously, it
was produced for several weeks at a normal rate of 225 barrels of oil a day.
To condition the well by flowing it at a lower rate would have delayed sam-
pling scheduled for other wells and was not considered warranted, However, an
attempt to improve the existing well conditions was made by bleeding gas
slowly from the wellhead to cause fresh oil to move into the bottom of the
tubing. Then, the subsurface samples were taken, The bottom-hole static
pressure at time of sampling was 1,791 p.s.i.a. The saturation pressure of
the sample was 1,785 p.s.i.a, After the well was sampled, it was placed on
the normal choke setting and produced at a rate of 223 barrels a day, From
this test, bottom-hole-pressure and production-rate data were obtained for
interpreting the sample analyses, The minimum bottom-hole flowing pressure
recorded on this rate was 1,625 p.s.i.a, As the flowing pressure is 160
p.s.i. less than the sample saturation pressure, obviously gas was being
evolved from solution in the oil during the producing period, Regardless,
the sample was approximately saturated at the bottom-hole static pressure
existing at the time of sampling. The production gas-oil ratio was about 56
cubic feet per barrel higher than the equivalent production gas-oil ratio
determined by flash-gas liberation of the sample, Although this amount indi-
cates that the sample should have had more gas in solution it seems excessive
and not representative of the degree to which the reservoir oil had been


This study has been confined to oil from the Aneth zone of the Paradox
member, A northwest-southeast cross section of the Aneth field (fig. 3) shows
relative positions of the Ismay and Aneth zones, their thicknesses, and com-
pleted intervals as developed with gamma ray-neutron logs from eight wells
across 8 miles of the structure, The cross section is identified in figure 2
by A-A', The Aneth zone occurs at depths ranging from 5,600 to 5,800 feet and
comprises both porous and nonporous carbonates with an average thickness of
about 170 feet. The upper part is crystalline and vuggy; the lower part is a
fossiliferous limestone. Developed porosity may be found at any level from the
top to the bottom of the Aneth zone, but few of these porous zones can be fol-
lowed for any appreciable distance across the field, Most initial completions
were made near the lower part of the zone.

The detection and correlation of porous intervals in the Aneth zone are
facilitated by the gamma ray-neutron log, A comparison of a gamma ray-neutron
log and a porosity log, constructed from plug-core-analyses data, is shown in
figure 4, These data were obtained from well 7, figure 2, A good correlation
between values of porosity and values on the neutron curve at corresponding
depths is evident. The calibration of neutron curves with porosity data per-
mits use of the neutron curve to determine the porosity of uncored intervals.
In well 7 porosity ranged from 1 to 26 percent and averaged 13,9 percent in
the 110 feet of perforated interval, This interval is shown by crosshatching
on the depth scale of the well log,

The Paradox shale, a consistent black-shale bed and a dependable geologi-
cal marker separating the Aneth zone from the overlying Ismay zone, is charac-
teristically shown by the well log between depths of approximately 5,560 and
5,580 feet, The thickness of the Paradox shale ranges from 6 to 17 feet in
the cross section previously discussed, (See fig. 3.)

Little attention has been given to exploration for production below the
Pennsylvanian formations, However, prospects for future development of produc~
tion from the Mississippian system are considered good in view of successful
completion of The Pure Oil Co,'s discovery well in the Big Flat field near
Moab, Utah, in the northern part of the Paradox basin, This well was completed
for an initial production of 319 barrels a day of 43° API gravity oil from the
Mississippian,

WELL SAMPLING AND TESTING

The subsurface sampling program in the Aneth field was planned to provide
samples from various parts of the reservoir along the major axis of the struc-
ture and known producing area, Specific wells were selected on the basis of
location, availability for testing and sampling, performance, and production
of water-free oil,

Sampling early in the life of a field provides the best opportunity for
obtaining the most representative samples of original reservoir oil, as the
decline in reservoir pressure is slight and has little effect on the oil, The
period after discovery, when optimum sampling conditions may exist, can be
extended where market outlets for the oil are slow in developing, This was


RESERVOIR-OIL CHARACTERISTICS
ANETH FIELD, SAN JUAN COUNTY, UTAH

By R. F. ZAFFARANO, C. Q. CUPPS, AND J. FRY

so» « « © »  » + + Report of Investigations 5511

UNITED STATES DEPARTMENT OF THE INTERIOR
Fred A. Seaton, Secretary
BUREAU OF MINES
Marling J. Ankeny, Director

1959


30

CONCLUSIONS

Comparisons of the results of laboratory analyses of subsurface oil sam-
ples from seven wells in the Aneth field show that the physical characteris-
tics of the oil in the Paradox-member oil reservoir have no significant varia-
tions either vertically or horizontally. Averages of the characteristics of
these subsurface oil samples give representative values of the physical char-
acteristics of the reservoir oil and show the changes in characteristics with
pressure,

Indications are that the oil has an original saturation pressure of 1,850
p.s.i.a, at the reservoir temperature, 133° F,, and is undersaturated by 350
p.s.i, at the initial reservoir pressure approximately 2,200 p.s.i.a,

At the saturation pressure, the oil contains 726 cubic feet of gas in
solution per barrel of residual oil. It has a relative volume of 1,388 bar-
rels per barrel of residual oil, and a viscosity of 0.55 centipoise. These
properties relate to a differential liberation of gas,

From flash-liberation data, average production gas-oil ratio is indicated
to be 624 cubic feet per barrel of stock-tank oil at average separator condi-
tions of 66 p.s.i.a, and 70° F, The gravity of the stock-tank oil is about
42° API, The oil has a formation-volume factor at saturation pressure of 1,365
barrels per barrel of stock-tank oil.

Gas produced from separators in the field may contain about 2.9 gallons of
LPG per thousand cubic feet of gas. Equivalent stock-tank vapors, flash-liber-
ated from subsurface oil samples, indicate that the field stock-tank vapors
will contain as much as 16 gallons of LPG per thousand cubic feet of gas,

The produced crude oil has a total gasoline and naphtha content of 33 to
35 percent, a relatively low pourpoint, and low sulfur and nitrogen contents,
The crude oil should be desirable refining stock,


18

extrapolated to the estimated average saturation pressure of the original res-
ervoir oil, 1,850 p.s,i.a, From these curves, values were obtained for each
oil characteristic at six pressures between atmospheric and saturation pres-
sure, Averages of these values are given in table 3 and were used to construct
the pressure-characteristic relations of the reservoir oil in figures 6, 7,

and 8,

TABLE 3, - Characteristics of reservoir out!
Gas in Relative
solution, oil volume, Specific gravity
cu, ft,/bbl.| residual oil Oil of gas in
Pressure, residual of1] volume at density, solution, Viscosity,
p.s.i.a. at 60° F.2/ |60° F, = 1,000/ g./ml, | (air = 1,000) |centipoises
2,200...... - 1.383 0.6983 - 0.56
1,8503/ , 726 1.388 6955 0.940 55
1,600, 654 1,358 - 7034 2975 259
1,300... 564 1,321 -7130 1.018 +66
1,000...... 469 1,280 27231 1.076 74
700... 374 1.241 «7343 1,165 84
400, 277 1,199 +7464 1,306 98
100, 143 1,139 -7643 1.594 1.30
1l,.. - 1,039 «7892 - 1.93
nl" ses - - .8204 = -

1/7 Analysis temperature, 133° F.

2/ Gas volumes at 14,4 p.s.i.a. and 60° F,
/ Saturation pressure,

/ Temperature, 60° F.

Figure 6 shows the variations in the volume and specific gravity of the
gas in solution with pressure from the average original saturation pressure to
atmospheric pressure, The average gas in solution at 1,850 p.s.i.a. is 726
cubic feet per barrel of residual oil at 60° F,; the specific gravity of this
gas averages 0,940,

Figure 7 shows variations in relative oil volume and viscosity of the
original oil with pressure, At the saturation pressure, relative oil volume
is 1,388 barrels per barrel of residual oil at 60° F,, and oil viscosity is
0.55 centipoise.

Figure 8 shows variations in oil density with pressure, At saturation
pressure the oil density is 0.696 gram per milliliter, The shape and slope of
the curves for density, relative oil volume, and viscosity above saturation
pressure were drawn to correspond to averages of curves for the samples from
wells 6 and 9, whose saturation pressures were close to the average original
saturation pressure, 1,850 p.s.i.a.

Characteristics From Flash Liberations

Table 4 summarizes flash-liberation data for the seven wells. The aver-
age formation-volume factor of the original reservoir oil and its variation




42

TABLE 23, - Composition of gas in solution, well 7

Pressure, Volume percent2/
p.s.i.a. Methane Ethane | Propane | Butanes Pentanes Hexanes +
—
1,7852/.......044 65.1 17.9 11.2 4.4 1.3 0.2
1,617... see 63.2 18.7 11.8 4.7 1.4 2
1,237. oe 58.2 20.7 13.7 5.5 1.6 2
898, 51.7 23,2 16.3 6.6 1.9 3
590. 40.8 26.6 20.0 9.8 2.4 4
325. 26.0 32.6 26.4 11.2 3.3 oo)
122, 10.4 33.6 35.3 33.6 4.3 7
39... 10) 25.1 44.0 3.4 6.7 9

1/ Calculated composition based on gas chromatograph analysis of gas differ-
entially liberated at a temperature of 133° F, Each value shown for an
indicated pressure is equivalent to composition of all gas liberated
between that pressure and atmospheric pressure,

2/ Saturation pressure of sample,

TABLE 24, - Composition of gas in solution, well 9

Pressure, Volume ercent2/
p.s.i.a. Methane Ethane Propane Butanes Pentanes Hexanes +
1,8752/,.........| 63.2 | 18.9 11.6 4.8 1.3 0.2
1,697. 61.3 19.7 12.3 5.1 1.4 2
1,317. 56.2 21.8 14.2 5.9 1.6 2
977, 49.6 24.4 16.7 7.0 2.0 3
671.... 40.9 27.4 20.2 8.7 2.4 4
390. 28.4 31.3 25.4 11.3 3.1 oo)
160. 12.8 33.2 33.3 15.6 4.3 7
46.. 0 24.4 41.9 25.0 7.4 1.3

17 Calculated composition based on gas chromatograph analysis of gas differ-
entially liberated at a temperature of 132° F, Each value shown for an
indicated pressure is equivalent to composition of all gas liberated
between that pressure and atmospheric pressure,

2/ Saturation pressure of sample.


26

from similar gas flash-liberated from subsurface oil samples may have been
caused by inaccuracies resulting from the gas sampling, All produced-gas sam-
ples were taken from the sight gage on the separator of the Rolo well checker,
The gas port of the sight gage is only a few inches above the normal liquid
level in the separator, where vapors rich in the heavier hydrocarbon gases
probably would be present, It is suspected that these gas samples contain a
higher content of the LPG components than they would if taken from a more re-
mote point on the gas-outlet line, Later sampling has been done from a valve
on the meter run.

The gas flash-liberated at separator conditions from the subsurface oil
samples from wells 2, 3, and 6 was displaced from the cell and measured with
awet test meter, Samples of the gas were taken just ahead of the meter, im-
mediately after the gas displacement was begun, If the gas in the flash-lib-
eration cell was not completely mixed before displacement, the samples might
contain too many light hydrocarbons and too few heavier hydrocarbons, It is
indicated that this may have occurred by comparison of the separator gas com-
positions for wells 2, 3, and 6 (table 6) with those for wells 7 and 9, The
gas liberated from the last two samples was collected and measured in a cylin-
der by mercury displacement, then sampled as a batch, The average LPG content
of the gas flash-liberated from the subsurface oil samples from wells 7 and 9
is 2.9 gallons per thousand cubic feet of gas, Because the characteristics of
these subsurface samples are very close to those of the average reservoir oil,
their LPG content should be representative of average LPG content of separator
gas produced in the field,

Samples of stock-tank vapors were not collected, However, the probable
composition of stock-tank vapors may be determined from the composition of gas
flash-liberated from subsurface oil samples between separator and stock-tank
conditions, Table 6 gives these compositions for wells 2, 3, 6, 7, and 9,
The average of these compositions is 16.6 percent methane, 29,1 percent ethane,
33,2 percent propane, 16,1 percent butanes, and 5,0 percent pentanes and
heavier components, Corresponding stock-tank gas-oil ratios for these wells
ranged from 47 to 96 cubic feet per barrel and averaged 73 cubic feet per bar-
rel, The LPG content for this average composition and gas-oil ratio amounts
to about 16 gallons per thousand cubic feet of gas; this is equivalent to
about 1,2 gallons per barrel of stock=tank oil produced,

The composition of gas differentially liberated from the subsurface oil
samples differs noticeably from that of gas flash-liberated, owing somewhat to
inherent differences in the two processes but mostly to the large difference
in temperature of the two analyses, Differential liberations are made at res-
ervoir temperature, whereas flash liberations are made at separator tempera-
ture, The probable variation in the composition of gas in solution in the
reservoir oil with pressure is shown in figure 12. The data were obtained
from the differential liberation of the sample from well 6, This relationship
between gas composition and pressure may be considered representative of that
of the average reservoir oil in the Aneth field, because the sample from well
6 closely approaches the average reservoir oil in most of its characteristics
and properties, Figure 10, page 23, shows quantities of the various components
of the gas in solution relative to the total gas in solution, From saturation


TABLE

12, - Differential-gas-liberation and pressure-viscosity data,
subsurface oil sample from well 71

Differential gas liberation

Pressure-viscosity

Relative oil Specific
Gas in solution, volume, gravity
Expansion cu, ft,/bbl, residual oil Oil of gas in
Pressure, |of sample, residual oil volume at density, solution Pressure, | Viscosity,
p.8.i.a, percent at 60° F.2/ 60° F, = 1,000] ¢,/ml. (air = 1,000) | p.s.i.a, centipoises
2,500.... 0 686 1,356 0.7058 0,934 2,691 0.60
2,275.... +23 686 1,359 «7042 934 2,353 259
2,056.... «47 686 1,362 +7025 +934 2,016 58
1,872,,.. 69 686 1,365 7010 +934 1,791 57
1, 73537; . 81 686 1,367 .7002 2934 3/1,785 57
1,774.... 1,01 - - - - 1,705 58
1.42 - - - - 1,465 63
1,90 - - - - 1,177 70
2.68 - - - - 892 78
4,93 637 1,348 - 951 588 90
- 520 1.300 .7166 1.002 318 1.03
- 417 1.258 .7281 1,072 122 1.23
- 323 1,219 7397 1.177 11 1.86
- 234 1,181 «7511 1,322 - -
- 151 1,141 ~7630 1,518 - -
- 68 1,092 «7760 1.700 - -
- 0 1.039 ~ 7892 - - -
- 0 1.000 ~8202 - - -
1/ Analysis temperature, 133° F.
2/ Gas volumes at 14,4 p.s.i.a, and 60° F,
3/ Saturation pressure,
4/ Temperature, 60° F.

it


21

with pressure is shown by the lower curve in figure 9, The upper curve in
figure 9 gives relative oil volumes obtained from figure 7, The solid part of
the formation-volume-factor curve was drawn so that the ratio between values
on the curve to values at the same pressures on the relative-oil-volume curve
equals 0,983, This ratio is the average value at saturation pressure as de-
termined from flash and differential liberations of six subsurface samples,

The average formation-volume factor of the original reservoir oil is
1.365 barrels at 1,850 p.s.i.a. and 133° F, per barrel of stock-tank oil at
60° F, This applies to a two-stage separation of oil and gas at average sepa-
rator conditions of 66 p.s.i.a, and 70° F, and stock-tank conditions of 11
p.s.i.a. and 60° F,

The dashed part of the formation-volume-factor curve (fig. 9) at pres-
sures from 400 to 66 p.s.i.a. has little significance, except to indicate the
effect of decreasing the temperature from 133° F, to the average separator
temperature of 70° F, The remaining part of the curve, from 66 to ll p.s.i.a.,
indicates the decrease in formation-volume factor resulting from the second-
stage flash to stock-tank conditions,

TABLE 4, - Summary of flash-liberation analyses

Well No...
Analysis temperature

L 2 3 6 7 9 11
134] 134] 132] 133] 133] 132) 135

Average separator conditions:

PYESSUTC. . oc cece cess eseee PoSeled. 64 60 83 71 57 80 54
Temperature.........e.006 °F 77 60 60 60 70 60} 100

seeee PeSei.a.| 1,816 {1,740 |1, 420 |1, 820 |1, 785 |1, 875 |1, 745

Gas in solution, cu, ft,/bbl.
stock-tank oil:L
At saturation pressure,,
At separator conditions

Saturation pressure,

693] 661] 566] 684} 658] 709) 654
52 66 96 70 47 86 46

Equivalent production gas-oil ratio
seeeees cu, ft,/bbl, stock-tank oil] 641] 595] 470} 614] 611] 623] 608

Formation volume factor, bbl,/bb1.
stock-tank oil:
At 2,500 p.s.i.a.....
At saturation pressure,
At separator conditions.......

1.342 1.297 |1.351 |1.343 |1.363
1.363 |1.354 |1.311 |1,361 |1.353 J1.373 |1.355
1,054 |1.081 |1,052 |1.052 |1.091

Gravity of equivalent stock-tank
OLL. ccc ee eee ececeecceeceeceees APT] 42.1] 41.7] 41.6] 41.8] 41.7] 42.1] 41.5

Specific gravity of gas in solution
(air = 1,000):
At saturation pressure....... 0.877 |0.742 10.794 ]0.749 |0.873 |0.877 |0.935
At separator conditions.... ++ { 1.389 [1.397 [1.351 [1.342 [1.351 (1.389 (1.386
1/ Gas volumes are at 14.4 p.s.i.a. and 60° F.; stock-tank oil is residual
oil at 11 p.s.i.a. and 60° F.



R, 23 E. R. 24 E. R.25E.

flies

r N

((\*

x

=s,

e8o4

LEGEND

@ SUBSURFACE SAMPLE WELLS
STRUCTURAL CONTOURS DRAWN ON TOP OF THE ANETH ZONE FOR WELL STATUS 4/1/58
DATUM: SEA LEVEL CONTOUR INTERVAL: 25 FEET

FIGURE 2. - Structure Contour Map of Aneth Field.


43

APPENDIX IL. - CRUDE-PETROLEUM ANALYSES OF PRODUCED-OIL SAMPLES

Crude-petroleum analyses are given for six samples of produced crude oil,
Four of the samples were obtained from wells 2, 3, 6, and 7, from which sub-
surface oil samples were taken, The other two samples were obtained in con-
nection with gas-oil ratio tests made on wells 5 and 8, Figure 2, page 5
shows the location of these wells, and table 2, page 12 gives data pertinent
to the tests made on the wells.


. Laboratory

IDENTIFICATION

Bureau of Mines .....
Sample

IDENTIFICATION

Well 5

GENERAL CHARACTERISTICS

Specific gravity, 0.818.
Sulfur, percent,
Saybolt Universal viscosity at 1¢

Pour point, ° F. ..

A. P.I. gravity —
210 Color, ren.

°F., 37.

sec. jat a. Fy

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

see, Nitrogen, percent, 0.035

Well 6

sons» fied
@. (aneth Pay)

‘tT. 40 8., Ro 24 Be
GENERAL CHARACTERISTICS

Pour point, °F, 35...
Color, Breen. -
sec. Nitrogen, percent, 0.

Specific gravity, 0.817.

A. PL. gravity
Sulfur, pereent, 9-10
Saybolt Universal viscosity at 100. F., 37...

see. at nn? By

DISTILLATION, BUREAU OF MINES ROUTINE METHOD

StAce Distillation at tmosphericpresoure, 760... mm. Hg. SrAGe 1—Disillatin at atmospheric presoure, 160. mm. Hg.
First drop, 26..°C. (.29.° F.) 8.2 C. (_...82.° F.)
en ae en
86. 1 50
on dD8 1 - 2 6
65. 3 100
39. 4 Bs
35 5 150
30. 6 115
a7: 7-200
col 8 5
42 9 250
6 onli 10 276 826.
‘Stace 2—Distillation continued at 40 mm. Hg. ‘STAGE 2—Distillation continued at 40 mm. Hg.
no” 8 4. 0.84) u1 200 92 D7
2 2 25 4
20 13250 aan
ume 25 ser
15 900 15-00 572
Resduom Residvom . =.
Carton residue of residuum, Carton residue of residuum,
Ramsbottom Ramsbottom
APPROXIMATE SUMMARY
eet s.r, sar Vieoty Peet
Light gueing . Light gasoline. ss
‘otal gasoline and naphtha ‘otal gasoline and naphtha
Keroeine dietilate . Kerosne datilate .
Gasol... Gaol...
Nonviacous lbricting distillate Nonviacous lubricating ditillate
Meiura lubricating dstilate Medium lubvieatng ditilte
Wison nreting itiate « fe
Residoom
Distillation

SY


WELL LOG CORE ANALYSIS

GAMMA RAY NEUTRON POROSITY, PERCENT
5550 30 ! O
L 7
[ i
ee
- = |
(So
5650 : te |
F a
‘§750 L 4

FIGURE 4. - Gamma Ray-Neutron Log and Core Analysis Correlation.