TABLE 5. = Oil-production statistics, Anahuac

field, Chambers County, Tex. (Cont'd)

Rl. 3579

, Monthiy Average. daily | Gumlative”
Wells, production, productich per |proCuction,
No. bol. well, bbl. bbl.
14g 283 aes! 62
155 OL 80
165 ies 636 85
174 49015730 7T
182 43632 vas
189 377 683 oT
. 197 387,563 &3
August.....+ 203 298,420 63
September... 207 351, 342 57
October..... 210 309; Ohe 51
November.... 217 278,583 43, .
December.... 222 276, uss i) 7,276,450
1938: .
January. 229 - 267,124 38
February.. 232 22,338 35
March... 234 255,303 35
April... 239 251 Whe 35
May... aka 225,020 30
June.. a8 215,672 > 30
July... 2 257,196 ay
August...... Paite) 264,662 uy
‘September... 253 218,658 29
October. 258 236 ,268 29
November. 262 227 ,968 +29
December... 265 ele uot 30 10,162,531
1939: .
Jamiary..... 270 246,678 29
February.... 272 217,49 29
March... 275 253,023 30
April. 276 abn G41 29
May... 281 245,732 28
June.. 28 217,867 26
July... 2¢) 229,832 26
August 287 lal, 878 1
September. 290 205,313 rat
October... 292 210,132 "23
November. . 296 203,193 23
December... 302 205,331 22 12,763, 6
190 . 2 5,33 763,645
January...., 306 197,212 21
February.. 308 182 290 21
March... 309 agt he 31
April... 312 277,980 30
May... 3 257 O44 26
June.. 316 200, 949 21
July... 319 179,704 18
August... 322 167,173 17
September... 324 162,992 17 14,683,482
8231 -19-


RI. 3579

Cross sections were drawn through each north-south and each east-west
line of wells in the Anahuac field, and the subsurface formations were plotted
on an exaggerated scale, The area of the formtions was measured by a
planimeter 22 and its recordings were converted into square feet. The volume
between any two cross sections then was calculated by multiplying one-half
the sum of the areas of the two cross sections by the distance between cross
sections. For example:

ve Arte) 24
2

’

where

volume between cross sections A, and Ao.

area of producing stratum on first cross section,

area of producing stratum on adjacent cross section, and
distance between first and adjacent cross section.

As stated, the producing formation in the Anahuac field contained a
large quantity of shale, and the volume of the shale bodies was deducted from
the total computed volume of the reservoir. In calculating the reservoir
volume by the average end=area method the area of shale based on the average
percentage of shale calculated for each cross section was deducted from the
total area of each cross section. The end areas used to compute the volume
therefore represented the net productive area of each cross section.

Isopachous map

Isopachous20/ naps are used frequently by geologists to show the regional
thickening and thinning of a subsurface stratum. The isopachous map (fig. 7)
is based upon the thickness of the Frio sandstone (25 to 135 feet) betwoen
the gas-oil and the oil-water contacts. The discontinuity of the gas-oil
contact caused by intensive faulting necessitated great care in preparing an
isopachous mp. The volume of the producing rock between any two isopachs
was computed by multiplying the area (as measured by planimeter) by the mean
thickness of the producing rock between the isopachs.

The calculated net reservoir volume of the Frio sandstone in the Anahuac
field between the gas-oil contact and the oil-water contact by average end
areas and by an isopachous map differed by only 1.39 percent. -

PRODUCTION OF OIL AND GAS
Initial Production

All wells in the Anahuac field were equipped with 2~inch—diameter tubing
at the time of completion, and tests to determine the initial production of ~
the wells were made while the wells were flowing through 1/l-inch-diameter
positive chokes. The hourly production was gaged in a tank, and the gas was
measured by an orifice meter. The duration of the tests was 1 to 2 hours,
but all tests were converted to initial production rates per 24 hours.

i9/ An. instrument for measuring the area of any plane figure by passing a
tracer around the bounding line.

20/ An isopach is a line drawn through points of equal thickness as projected
onto any particular plane.

8231 -17-


RI. 3579

Production record during sampling period

Provxetion rate per 2k hours|Gas-oil |Separator pres—
Date Time Oil, Gas delivery, |ratio, ov.|sure 1bd./sq.in,
boi. M, cu. ft, | ft,/bol, | Gage
OS 1] Gi00 eat | 6042 312 51 n
19 2300 am “| 6097 29.9 4ge 721 87
ene 20,9 M65 80.3 95.3
bil aioo int | Stel 29, 53 82.4 97.6
bal eieo eens fol Go 31.4 491 79.0 94.0
Average uss 1% 92.0
L

Laboratory datas

Saturation pressure of subsurface oil sampled at
6,879 feet below sea level , , pounds per square inch absolute, 3,020

Reservoir pressure at 7,011 feet below sea level ,...... dO ..... 3,235

Differential Flash
liberation, liberation,
at 177° F. 177° F. to 30°F,

Gas in solution to 3,920 pounds per square
inch absolute of residual oil measured
at 60° F, .,.......cubic feet per barrel Bus 519

Gas in solution to 3,235 pounds per square
inch absolute of residual oil measured
at 60° F, ........,cubic feet per barrel 580 553

Volume increase of residual oil from atmos-
pheric pressure to 3,020 pounds per square
inch absolute .... seeeeeebercent 27.4 26.8

Volume increase of residual oil from atmos-
pheric pressure to 3,235 pounds per square
inch absolute .......e.seeeeeeeee percent 28.9 23.5

Volume decrease of reservoir oil from 3,020
pounds per square inch absolute to atmos-
pheric pressure ..,.-esseeee-+0--- percent 21.5 21.13

Volume decrease of reservoir oil from 3,235
pounds per square inch absolute to atmos- ;
pheric pressre ....e.eeeeeeee eee percent 22,4 22,2

8231 ~ abe


RI. 3579

Brought forward .......cceeeeeeeseeceeeeeees ct eeeeeeeee $3,873.13
Appurtenances:
One 9-5/3-inch float $4s, 65
One 9-5/8-inch guide 32,18
One 5-1/2-inch float « 32,79
One 5-1/2-inch guide 17.65
One 2 1/2 x 2-inch, 6,000-1o, g 8, 37
One 9-5/8 x 51 /2-inch, §, 000-1, bradenhead, hes, 63
One Christmas tree ........... . 480,00

One Packer, 3-1/2 x 5-1/2- inch voles.

Handling charge, 10 percent .......... ae eeeee 130, OL
1,430.12

Miscellaneous material:

25,900 voard feet of lumber .......e cece eeeee 929,00
Total material COSt ...eeeee cece see e cence vee $13, 311,98
Transportation, Company account .........ee ee eee 550,00 550,00
Rentals:
Water, 14 days at $20 a day ........ see eens 280,00
Gas, 500,000 cubic fect a day (1U days)... "357,00
Rig, 15 days at $165 ..... sesecece . ee
Derrick, $150 a month ....e.ceecesecceeeeeeee
3,262, 00
20,997.11
Contingencies, 10 percent ....c.ceceeeeeees seeeee 25 oan 7L
Grand total cost .... . « oereoes 3,096, 82

SUBSURFACE INTERPRETATION

Oil and gas were found in the Marginulina and Frio formations, Cross
sections of these two formations were prepared on an exaggerated scale to
permit a detailed study of the reservoir, The cross sections were drawn as
nearly as possible through each north-south and each east-west line of wells,
Two typical north-south cross sections are shown in figures 3 and UL and two
typical east-west cross sections in figures 5 and 6.

The top of the principal geologic strata above the Frio formation was
determined by identifying indigenous microfossils in the drill cuttings obd-
tained at regular intervals of depth during drilling, Formation cores and
electric logs gave more complete information about the Frio formation,

8231 -g-


RI. 3579
TABLE 10, ~ Laboratory data by flash liberation method from
171° to 80° F,

Subsurface oil sample No, 1 obtained August 20, 1937 from Humble 0i1
& Refining Co, Tyrrell well No, 8, Anmahuac field, Chambers
County, Tex,

Pressure, | Temp- Qil sample Gas in solution
lv. /sq.in.| erature | Volume,|Percent vol- |Density, | c.c. [ous “tt, fobl.
absolute |° F, c.c, |ume increase gms,/c.c.; cumu- jof residual
of residual oil) lative | oil
3,2354, | 177 - | Ves,5 Yor | - | ¥553
3, 0202/ 177 17.7 26,8 736 5.774 51g
2,287 UT TL 20,9 .758 | 4422 los
922) 177 66.7 88 ~813 | 1,894 174
91 80 62,2 1,5 2 849 224 20.50
145 80 61.8 ofS 2851 10) 0
14,5 60 61.3 0 «858 0 0

1/ Reservoir pressure at 7,011 feet below sea level (oottom of casing),

2/ Determined saturation pressure of oil sample at 177° F, taken at 6,789
feet below sea level, : ‘ ‘

3/ Value obtained by extrapolating data to 3,235 pounds per square inch
absolute,

U/ Separator pressure,

The production record of the Humble Oil & Refining Co, Tyrreil well
No, 8, for six test periods between August 17 and August 22, 1937, showed
an average producing gas-oil ratio of 96 cubic feet a barrel (table 8),
These tests were conducted while the well was producing at daily rates rang-
ing from 44,9 to 64,0 barrels of oil a day, The gas was measured at an
average gas~oil. separator pressure of 92 pounds per square inch absolute,
The close agreement of the producing gas-oil ratio (496 cubic feet a barrel)
during these test periods with the gas-oil ratio (498.5 cubic feet a barrel)
as determined by the laboratory analysis (table 10) is further indication
that the reservoir oil in the vicinity of the Humble Oil & Refining Co,
Tyrrell well No, 8 was undersaturated when the sample was taken,

Optimum efficiency of oil recovery from a flowing well is achieved
when the gas~oil ratio of production does not exceed the volume of natural
gas in solution per unit volume of reservoir oil, As mentioned, the average
initial gas-oil ratio of 315 wells in the Anahuac field was 728 cubic feet
a barrel, or 230 cubic feet a barrel more than would be released at a gas-
oil separator pressure of 91 pounds per square inch absolute, An analysis

8231 - 28 ~






RI, 3579
The following equation is used:

R= LO8Vesy;

L+x

whore R = reeoverable residual o41, barrels;

Vv = net volume of reservoir, acre-feet:
‘ p = effective porosity, percent (expressed as a decimal);
s = oil saturation of effective pore space, percent

(expressed as a decimal);
x = volume increase of residual oil from 3,235 pounds
per square inch absolute to atmospheric pressure, percent
(expressed as a decimal);

y

estimated oil recovery, percent (expressed as a decimal); and
7,758 = volume of 1 acre~foot expressed in barrels (42 gallons),

Then the recoverable residual oil in the Frio reservoir of the Anahuac
field would be estimated as follows:

1.158 (¥) (0,30) (0,50) (y) .
1 + 0,268 ,

R

W

where 7,758 = number of barrels (42 gel.) per acre-foot;

V = net volume of oil-saturated Frio reservoir, acre-feet,
(computed by end areas or isopachous map);
0,30 = average porosity of Frio sandstone (table 3);
0,50 = crude-oil saturation factor

(allowance made for connate water 50 percent (table 4);

y = recovery factor, percent; and

u

0,268 = volume increase (by flash liberation) of residual
oil from 3,020 pounds per square inch absolute

to atmospheric pressure (table 8).

8231 ~ 32 ~






RI. 3579
TABLE 6.

Sample No. 35066

Middleton well No. 1 Anahuac field Texas
6,990~7 ,050 feet Frio sahd Chambers County
Humble 011 & Refining Co. Section 52
GENERAL CHARACTERISTICS
Specific gravity, 0.848 ALP. I. gravity, 35.4°
Sulfur, 0.15 percent Pour point, 20°F,
Saybolt Universal viscosity at TTF. ie SeCe. Color, greenish~black
Saybolt Universal viscosity at 100°F., sec.

DISTILLATION, BUREAU OF MINES HEMPEL METHOD

Dry distillation Barometer, 73% mm. First drop, 36°C. (97°F.)
Temperature, Cut, Sun, Sp. er. oa, P. I. Viscosity Cloud Pomperature,
°C. per- per~ of cut of cut at 100° F. test,
cent cent or.
Up to 50 . Up to 122
50 - 75 : 122 = 167
75 - 100 3.4 3.4 0,720 65.0 167 - 212
100 ~ 125 Yo Te 750 57.2 212 - 257
125 - 150 2.7 10.1 770 52.3 257 = 302
150 - 175 B 14.0 +785 4g.8 302 - 347
175 - 200 +2 (18.2 “197 46.0 3U7 = 392
200 = 225 5.7 23.9 810 43,2 392 ~ 137
225 — 250 8.3 32.2 282k 40.2 437 - uge
250 - 275 10.5 42.7 . 2839 37.2 482 = 527
Vacuum distillation at 40 m,
Up to 200 91 9.1 2856 33.8 tM 10 Up to 322
200 ~ 225 8.5 17-6 858 3344 1g 35 392 ~ 437
225 = 250 8.2 25-8 +868 31.5 65 60 «(137 = bee
250 - 275 566 31. 878 29-7 105 75 82 ~ 507
275 =- 300 8.0 39.4 887 28.0 180 90 «4527 = 572
Carbon residue of residuum, 4.8 percent. Carbon residue of crude, 0.8 percent
APPROXIMATE SUMMARY
Percent Sp. gre, °ay P. I. Viscosity

Light gasoline 3.4 0.720 65.0 .

Total gasoline and naphtha 18.2 9.766 53.2

Kerosene distillate 14.0 #818 W5

Gas oil aug 2849 35.2
Nonviscous lubricating distillate 13.9 3342-29. 9 50-100
Medium lubricating distillate 9.5 29 9-27.67 100-200
Viscous lubricating distillate 2.2 2767-2721 Above 200
Residuum 15.9 19.7

Distillation loss 2.0 ~- -

8231 -20-


RI, 3579

The large productive area of the field (7,300 acres) and the great
thickness of the producing zone ranked the Anahuac field fourth among the
Gulf Coast fields of Texas as regards estimated ultimate recovery of crude
oil, The Conroe field in Montgomery Comty, the Hastings field in Brazoria
County, and the Friendswood field in Harris County in this region exceed the
Anahuac field in estimated ultimate recovery,

This field is especially interesting because about 75 percent of the
productive area in it is owmmed dy one company, and the remaining 25 percent
is held by four other companies, With the small number of operators, develop-
ment has progressed in a logical and orderly manner, A systematic drilling
program has been followed, and the well-spacing patterns were chosen for
optimum recovery and greatest economic return,

ACKNOWLEDGMENTS

This investigation was conducted under the direction of R, A, Cattell,
chief engineer, Petroleum and Natural Gas Division, Bureau of Mines, and
H, B, Hill, supervising engineer, Dallas field office, Dallas, Tex,

The writers gratefully acknowledge the generous cooperation of executives,
engineers, and geologists of the Humble Oil & Refining Co,, Gulf 0i1 Corpora~
tion, and Sm Oil Co, for making available for use in preparing this report
their information and data on the Anahuac field and granting permission to
conduct experimental tests in the field, Special acknowledgment is made to
W. W. Scott (deceased), former chief petroleum engineer, and C, BE, Reistle,
Jv,, chief petroleum engineer, Humble Oil & Refining Co., for the interest
they took in the study, Thanks for valuable assistance, eriticisn, and sug-
gestions are due L, T, Barrow, Olin Bell, Paul Clark, G, R. Gray, EB. A. Green,
Ed Hammer, T, V, Miller, T, v Moore, F i, O'Banzon, Perry Oleott, R. J,
Schilthuis, 3 U Teague (resigned), L, P, Teas, and Fred N, Wise, all of the
Humble 0i1 & Refining Co.; to John Cooper and Paul Weaver, of the Gulf Oil
Corporation; and to C, B, Claypool, Harold Denton, Philip S, Justice, and
R, W. Pack, of the Sun Oi1-Co., Beaumont, Tex.; to W. E, Wrather, Dallas, Tex,;
to R, A, Stehr, Seaboard Oil Co,, Dalias, Tex,; and to F, B, Heath, R, EB,
Rettger, and D, M, Collingwood, Sun Oil Co,, Dallas, Tex,

The writers acknowledge further the assistance of the following members
of the Bureau of Mines, who offered suggestions and criticisms or otherwise
assisted in the investigation: H, 0, Fowler, Washington, D. C.; W. H, Barlow,
A. B, Cook, Peter Grandone, R, E, Heithecker, M, A, Schellhardt, George 3B
Spencer, D, B, Taliaferro, and C, J, Wilhelm, Bartlesville, Okla, } Relph H,
Espach, Laranie, Wyo.; H.C. Miller, San Francisco, Calif.; and R, K, Guthrie,
James M, Seward, and Gustav Wade, Dallas, Tex,

GEOLOGY OF ANAHUAC FIELD

The Anahuac oil field is in an area formerly occupied by rice fields,
The surface elevation is approximately 25 feet above sea level, and a low
mount with a relief of 4 to 5 feet is evident over the main part of the field,
An irrigation canal that diverts water from Trinity River intersects the
southwest part of the field,
8231 -3-


Note: Shale secre i lenses throughout

in'stown graphically has ne significance.

Wi

UN PENETRATED ZONE

FIGURE ©, - COMPOSITE SUBSURFACE INTERPRETATION, ANAHUAC FIELD, CHAMBERS



OEPTH BELOW SEA LEVEL. FEET

Sun Oi, Sul )Icore, Gulf 3» core
PU wh te Wo Xromies WT Aromles
Nos we? NaS

ro ne
5900
3900 |
bt 600%
6000 |-
[ €'00
6100 | a]
| e 6200
zoe | i
L { ]
| 6305
6300 |- |
{ i
if fe 6400
6490 c
L L 65C0
6500 -— =
(

rt
een te Nap ncn

egoc
[ 6709
€700 |-
L 6829
6800 |- i
r i
6929
6900 | {
>
L a 7609

7000 F-
S

7100
: =! sagt coe ay Ge way ns oA cone wag Spr sgnye cere va we be) Gn ere 4 1200
~200 SE RE Se mm gre tga ope nate eee tay See ag ort Pag oer ae PUR IP aR Phe
. be. - ous , 12: 18 ts 0 9 Palo opd PF blaned oF be. i ‘ fe OR a

18, Tipitel on production, veie par day —
GOR Gen on ratio. est per bor Moritontal seale feet,

FIGURE 6.- EAST - WEST CROSS SECTION 14-14, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.

FEET

OFPTH BELOW SFA LEVEL,


DeptH BeLow Surrace,FeeT

Porosity Los,

Pesigrvity Los, Ohms
>| 408 12

wintwalts. ie" 20
100 W040
| icone
i Los |
ty
Top of Frio.
| | Saattene ~
E b
| |
D
f pe=
r aa
c ||
E
Feas0 |

6900 :

Tt

T
Fy
&
3

consnoson-} Zone of gas-seturated sandstone}

T
3
8
8

Top of oil-

Tt

‘saturated s:
Leseno

[EEE] Send

Note: To find approx-
imate elevations

below sea level, sub-
tract 30 feet from
£ indicated depths.

-water contact|

FIGURE 8.- TYPICAL ELECTRIC LOG OF WELL
WITH CORRELATIVE CORING INFORMA -
TION, ANAHUAG FIELD, CHAMBERS COUNTY,
TI :


South Gulf Of '
_— - Humbaciteretca “2 | | Tidewater | Humble Ot Ref.
suite {
b
H ve enters cmcons
wiles | tote) 16 Blake ete! nat Carthy at
barrow 70 7.
not
i dum eI Guif Oil Corp. Humble Oil » Ref. Co.
laymen ete Humble Oi Reco et
JAMES Mc GAHEY Magnolia Pet. Co.
B.F. BARROW Sen ii Co.
Humble Oo
rw Seafield
arres Gen Ot Go
Cw
O.W.CARTER a
Great State
J wasn ~~
Mes EA EH Browning
F Browning °7|
: BC Thampson
Ore 01 Prop. te.
Anchor Ranch Co
yo mite Mi Podront
-
A Guy Oil Corp. - Humble Oil w Ref. Co,
H I, 7 * \ e ¢ T.BNIO.R.R.
jGirae * Humble 0.8
Pot “ ES Pa eu aan ee
ot >) Bite Tome onegerce |, Seat \ yee ae | Br,
tof ae —— ol (ER Sa See WA. e Ser
aunt we LEN et,
por G.cmitoer
= Tumble Ole Rete Humble 0.88
Humble Oil Ref. Co, oo ol ((% ONT, . oo lp
oe Hubs gi wo my
——} 7 ~ ~~ 3 "ne Marsbei |
‘Humble Oit « Ref. Co, " Tapoe ML Garth etal
A LKiemine Me coe EE
Ai Srver Pa ow Be
aoe 7
on
x sno, . Humble 0
8 Tiny Rice Disoosel Co
a AW Mershall Cy
e nts A Bhat
Fd Humble 0: #Ref.Co, Humble i ef. ca,
S | mpwincer om wher Humble | Navarro
ove tinal — | Sr Breuzord » -
cy ” TM ~ u
AD.Mialeton ecu N
Le Themes wo a om | See StBok| 16 Cet
| fal we i -
+ . * H.T.§B.R.R.
LE iad abort ADMiddieton Humbe OM eRe. Co.
Jomple Oil » Ref. Co, * “cr
SONGS @ .
wo Humble Oil Ref, Sun co,
q
me Tyeret Te
do >a °
i 25.
Humble Oil eRet Co mar
JE Bepusserd
ADAddleton Tees Metl Bank
7 Pid
- AD Middleton
Shat Pet Corp. Humble OM eRe Co, Gut Oi Corp. ADMisdletn LE Broussard — | TM Young Subd.
a ditebert
aes 7
T AD date .0.Meubovies Fred & Morrison Humble Oil #Ref. Ca, Humble 01 Ref Co,
ANSON TAYLOR
SYMBOLS 100 0 too0 00090808000 tm LEGEND FOR weLts
Out wet producing from Frio sandatene
Sitwell producing from “luarginuline’ send? ‘Scale: foot Dupin th mater wes found
Ges welt producing from Frio sandstone PB. * Depth below ea level $0 which wall wan. plugged beck below the tt nt
& Gee all reducing from uargineine Sand? "Ea Poet ihe pees Becta es balou the pap-cll contact the \nopachs represent the |
+ Ory and abandoned tiiginans Oe

FIGURE 7.- ISOPACHOUS MAP, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.











RI. 3579

trapped in a graduated tube or burette, in which its volume may Se read. The
percentage of saturation of connate water is determined as follows:

Saturation, percent = Volume of water
Volume of core,

The porosity of the sample wag determined by a method described by
Taliaferro, Johnson, and Dewees.18/ Arter the sample has been dried in a
constant-temperature oven its volume is obtained by immersing it in water
until absorption ceases, then dropping it in a 100=cc. graduate of water, and
noting the displacement.

Determinations of Shale

Soon after development of the Anahuac field was begun it became evident
from examinations of drill cuttings, coring data, and electric logs that the
Frio sandstone in the Anahuac field was interspersed with numerous shale
lenses. These lenses had no regularity or uniformity, and their extent and
thickness could be determined only by drilling.

The Humble Oil & Refining Co. odtained samples of cuttings of the Oligocene
formations at 30-foot intervals during the drilling of each well. ‘he cut tings
were examined by the company geologist for microfossils and for determination
of the depth at which coring should begin. Because the reservoir rocks were
highly faulted it was necessary to be on the alert when drilling continues below
6,000 feet to determine accurately the top of the Frio formation and the gas-oil
contact. .

Coring with conventional or wire-line core barrels was begun in most wells
near the top of the Frio, sandstone and continued until the desired depth was
reached. The number of feet of shale cored in each well was recorded carefully
by the company geologist. Shale has low porosity and ordinarily contains no
oil; it must be excluded from all calculations used to estimate the reservoir
content. As the shale bodies in the Frio sandstone reservoir in the Anahuac
field were very lenticular, uniform shale beds could not be correlated from
well to well, and hence the area of the shale lenses could not be measured
directly on the plotted cross sections. As the coring data and electric logging
records for most of the wells showed close agreement in thickness of the shale
penetrated, the writers averaged the shale reported in the wells on each cross
section. Most of the wells were drilled only 75 feet below the gas-oil contact
and hehce were not deep enough to yield information on the character of the
lower 60 feet of reservoir rock between the bottom of the drilled wells and
the oil-water contact. Accordingly, the ratio of shale to sand in the un~
explored 60 feet was assumed to be the same as in the 75 feet penetrated. In
the absence of more complete data estimates of shale on this basis are thought
to be justifiable.

In estimating the shale content of the Anahuac reservoir the percentage
of shale recorded in each well was listed on all east-west cross sections.

137 Taliaferro, D. B., Johnson, T. W., and Dewees, BE. J., Work cited in
footnote 8.

8231 -1-


RI, 3579

On October i, 1940, the cumulative oil production for the Anahuac
field was 14, 683,482 barrels, all of which had been produced under res~
tricted flow as aresu.t of proration, The average daily production per’
well of all wells in the field had not exceeded 119 barrels by October 1,
1940, Proration was based upon acreage (50 percent) and well potentials
(50 percent), Most of the wells in the Anelwac field were connected to
centrally located gas~oil separators and tank batteries; each unit was
equipped with a pipe manifold, auxiliary gas~oil separator, and orifice

meter, which facilitated routine operation and frequent testing of indivi-
dual wells,

Depth—pressure measurements in the Frio reservoir were made periodically
in 30 key wells, and the data thus obtained were used to construct isobaric
maps of the reservoir pressure, The estimated original static pressure in
the Frio reservoir at a datum of 7,015 feet below sea level in March 1935
was 3,265 pounds per square inch, whereas the estimated static reservoir
pressure on February 1, 1940, was 3,178 pounds per square inch, indicating
a loss of pressure of 87 pounds per square inch, or 6,7 pounds per square
inch per million barrels of cwnulative oil production,

The methods and formulas used to calculate the ultimate oil and gas
recovery have been described in detail in the body of the report, As a
reault of the refinement in estimates of reserves made possible by modern
engineering technique, the so~called porosity-saturation method is best for
estimating the ultimate recovery of crude oil and natural gas from a
reservoir of the Anahuac type, In this procedure the net volume of the
producing sandstone in the reservoir is computed from subsurface records of
the wells, and porosity, saturation, and shrinkage of the oil are determined
by analysis or experiment, Recovery is tne only factor that cannot be
determined by analysis or experiment and should be based as nearly as pos—
sible upon the records of depleted or nearly depleted lds that have
similar characteristics or upon theoretical considerations.

Slow, uniform withdrawal of oil and gas from the Frio oil-producing
reservoir in the Anahuac field has resulted in an exceedingly well-controlled
upward movement of mobile connate water, On December 1, 1940, almost 6
years after the field was discovered, only 14 wells (1 producing from the
Marginulina sand and 13 from the Frio sandstone) were producing 2 to 90 percent
water,

8231 -3T-


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RI, 3579 °

that the results of the porosity determinations are in close agreement and
average about 30 percent, whereas the results of ‘the permeability determina-
tions range from 9,6 to 590 millidarcys, The erratic permeavility values
may be explained by the lack of consolidation of the cores.
TABIE 3, - Results of porosity and permeability tests of Frio
sandstone from Humble Oil & Refining Co. wells,
Anahuac field, Chambers County, Tex.’

Lease Well Depth at which Permeability, Porosity,
No. core was taken, feet nillidarcys percent
-1 |@ripon 5 7, O48=7, 058 15.4 31.7
2 do. 5 7, 058-7, 068 3709 30,0
3 do. 5 7, 068-7, 078 - 31.1
& |marstcl. 3 7, 032-7, O42 9.65 30,6
5 do. 3 7, 042-7, 052 41,0 37.1
6 do, 3 7, 042-7, 052 - 36,0
7 do, 3 7, 052-7, 062 - 29,4
8 do, 3 7, 052-7, 062 og 2569
9 do, 3 7, 062-7, 072 254,0 30.6
10 |Middleton nl 7, 032-7, 42 - 31,1
11 do, 1 7, 052-7,.062 370,0 31.1
12  |Zappe 2 7, 015-7, 025 590,0 30,6
1 do. 2 7, 025-7, 035 371,0 32,7
uv do, 2 7 045-7, 055 - 25,2
15 do, 2 7, O45~7, 055 vy; 98.9 2304
16 do, 2 7,055-7, 665 2h 31.7
17 do, 2 7, 055-7, 065 | - 29.8
18 do. 2 7, 065-7, 075 i 222 25.8

i Direction of flow was at right angles to bedding plane.
Direction of flow in all other tests was parallel to bedding plane,

Connate Water

Experimental work performed in the Anahuac field disclosed that so-called
fixed connate water occupies as much as 50 percent of the interstices or pore
.space in the reservoir rocks,

Connate waterta! has been defined vy Meinzer as "water which was deposited
simultaneously with the deposition of solid sediments and which has not since
its deposition existed as surface water or atmospheric moisture." As the
rocks of petroleum reservoirs were laid dowm in the presence of water it is
believed ‘that this water has been an important factor ‘in the ‘accumulation of
petroleum and natural gas, Connate water, however, may exist as so-called
mobile or fixed water, Mobile connate water is fouad in and below petroleum
deposits and is produced at some time in the Life.of most oil wells, Several

10/ Fay, Albert H., Glossary of Mining and Mineral Industry: Bureau of
Mines Bull, 95, 1920, p. 177.

8231 -12-



AGE

Name oF
FORMATION

CHARACTER OF
FORMATION

THICKNESS
(Approximate)

Beaumont
Pleistocene, Beau Clay and sand 700+.
Uv
2
2
AND bagarto § Sand and shale 5050 Ft.t
Oaxviire — B 4
Miocene CataHoura &
cs
3
2
5
“ Discorsis Gray shale - one sand member 350Ft.t
Mipove Hererosresina | Gray shale -streaks of lime 350 ft
OLIGOCENE
: Gray shale containing fine
MARGINULINA textural shaly sands in lower 375 ftt
half -+wo sand members
Coarse textural quart3
Frio ‘sand and greenish- 1500 t+
Lower gray shale
Ouisocene
Viexsaure Brown to blackish -gray

FIGURE |.- GENERALIZED GEOLOGIC COLUMN

lignitic shale

ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.



RI. 3579

The formations penetrated by the wells begin with the Beaumont clay
of Fleistocene age (which occurs on the surface) and extend downwerd into
the Frio sandstone of Oligocene age, Figure lis 2 graphic log of the sub-
surface formations,

The Anahuac field is on a domal structure, as shown by a contour map
(fig. 2), and is considered by many to be a decp-seated or nonpiercement~type
salt dome, Contours draw on top of the Frio sandstone denote a closure of
460 feet within the productive limits of the field, On March 1, 191, the
productive limits of the field enclosed an area of approximately 7,300 acres,

DEVELOPMENT

The discovery well in the Anahvac field was the Humble Oil & Refining
Co, A, D, Middleton well No, 1 in sec, 58, Houston & Texas Central Railroad
survey, Chanbers County, Tex, Drilling was begun December 8, 1934, and the
well was completed March 2, 1935, in the Frio sandstone at a total depth of
7,050 fect, with an initial daily production of 120 barrels of oil and a gas-
oil ratio of 12,500 cubic feet a barrel through a 3/8-inch choke, : Later’ the
well was deepened to 7,088 feet in an effort. to reduce the gas-oil ratio,
and when recompleted on March 17, 1935, it produced 57 barrels of oil daily
through a 1/u-inch-diameter choke with a gas-oil ratio of 35 eubic feet a
barrel,

Little was mown of the limits of the structure, and the operators con-
fined the immediate drilling to direct offsets to the discovery well, Four
months after the field was discovered the proved area was extended 2-1/2 miles
northward from the discovery well by the completion, on July 21, 1935, of the
Glenn H, McCarthy and R, A. Mason, White well No, 1 with an initial daily
production of 588 barrels of oil through a 1/4-inch choke and with a gas-oil
ratio of 742 cubic feet a barrel, With this encouragenen+, two wells (C. B,
Bunte, Simon No, 1, 41/2 miles northeast, and Woodley, Blanke No, 1, 1-3/4
miles northeast of the Glenn H, McCarthy and R, A, Mason, White well No, 1)
were drilled into the Frio sandstone and abandoned as dry holes,

Following these two failures drilling was confined to the proved or
semiproved area, The early so-called proved area, however, was greatly re-
duced when the Gulf Oil Corporation, W, T., Knowles well No, 1 in sec, 8, a
direct east offset to a producing well, was completed as a dry hole at a total
depth of 8, 749 feet, The record of the well indicated the existence of a
north-south fault in the Frio sendstone, with a displacement of 340 fect,
and presaged the irregular structural features of the Anahuac reservoir that
were disclosed by subsequent drilling, Table 1 gives the status of the wells
in the Anahuac ficld on December 1, 1940,

8231 whe


——westr—E£A Ss T——e

Mumble OR, HumbleO.aR, Humble 0.0R, Humble OR, Humble sR. Humble 06R. Humbie oan, HumbleO.ar., tumble O.8R., Humble 0.0, Humble 0.4, Humble 0.8R., Humble 0.98,
ADWiddleton, ‘Gly Clark, 6d. Clark, 6d. Clark, iu Clark,” AD Middleton ADwiddletos, A OMiddleton: ADMiddleton, —— Brovsserd- Heb Broussard Hedért Broussard Heder? ADiidcteton,
No.7 No.5 Nolé No.10. No Not No.32'No.25  No.35 No.9 No.1 No.3 Nod No.5, No.6 No.1? No.55
il , i Mm , ima imal ’ M9 ao om ’ a oa mM O a y a ima a ml
£1, 29) E13 nN e134” 1. 31 £1. 33” et. 31 Ei. 31 1. 32 EN. 32 M4 E29 E129 is. E130" EN, 31 £1.30" q E130" u.30
- winre2 22] Shi | | Resistivity, 4
void — ~ meaty] | gone
5900 b- N* | te Toy i __Digeorbis —37—hie 15900
's 1-7Dis:
6000 }— i 6000
6100 F— 46100
e 4
ne a
41 6200 -—- —{ 6200
re
5 + 4
ws _ — 6300
6300 1
us b a
a
x 400 — —7 6400
wt 4
o
6500 [- 7716500
Fs 4
eo +f 4
S
a 4
a 6600 |- 6600
~ +f |
6700 }- TRS +6700
ws b
eo
6800 |-
6900 |-
7000 }-—
7100 F—
7200 f- re re re TRGLON. TR CR. CR TRL TR
eb MCR eae, CEE aon, ACRE son eee oe Gn ECE Gewberdny” con. Sabcfo, can tuacts. con Tee#S con Sees. Gk Sucka eka SAA. vy
Legend: ars se =e cin, Gack ;
on date slrfl production obi nner are erro
$5"? Setal Sepen atom aba Wantedraiocce fe porta” Horizontal scale: feet Sha eS tae
Depth baton ; . . 0 H n
sell mae plugged FIGURE 5.- EAST-WEST CROSS SECTION 8 - 8, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS. AAR RACER GBR SHEED aR. ABLE

DEPTH BELOW SEA LEVEL, FEET


/ South Gulf Oi Corp.
Meryetta Mayes j
—_ JMeverro | ay! Tidewater | Humble OiteRef | Humble Oi wer co | Humbie Oe Rat Co
- _ eu noite Humble Oi » Ref Co ; - nin | Membie os
Hirbie Gilat Co
1 (weodteg) ud M Lens
| (onto) 4G Blanke etal Janet Mpc Carthy et vir CA bans CM bony
H —— =
! it tt ne
\ Go 01 Cora Humble Ol Ref Co
JAMES Me GAHEY \ Magnolia Pet.co, ——— ED. RYEL
& _
BF. BARROW ‘Sun OU Co. Humble Oil a Ref Co.
8 . —_——_ Tumble Oo
” owt . a] rrsaraa
Cora Barrow ” Germ Oil Co.
T ry O.W.CARTER a
il = Great State
uit Ot orp. gut oncong@
ee es ‘HO Acom
sme ciaaK H «
wes EA SIS Browning
——_ =
ey Gulf, it Corp. « F Srowning
AC Toompsen
leara -
‘ ie meerorert 1 Prop ine
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Z “\ Navarro Oi
Anchor L Ranch Co no Nitcax tot fo an
Myante . moniter tat — ‘i Brop ine
“a peat pag > we es BE Quinn WL Poncirom Tr.
om Za =
- Gemance
Gut 2s C\
| e AR Gyn 7050, . _ Mumble Oba ReE Ca. |
| stodaera tarde Oa Ref 66.7080" ~~, TANORR |
ode " i Humble 0.0R.
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|
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RL Kimmins -~ . *
AB Shaver * “y wake © Nt Humble 1.
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LE Brvssorcrahd Medect Humble Oe Ref Go
Sun Oi Co, Humble Oi #ReF i Humble Oita Ref, co” | ws, |
i eReg co a
sy 8 =
2 7
Hurnble Ol we Ce, Humble Oi =a Go Sundilco
Ne Tyrrel Te a T. ¢| R. R.
Cy ” S|) &
AD Middleton Humble Oil eRef. Co. te
oe arene soma eso
{out on cap. ‘ Tones Nat Bank
i i te ‘00
H ' = = 7
: . ADMedleton
{shat Pet.core Humble O.0R, Humble it # Ref. Co. Gut on Corp. A.D Midleton JE Broussard | THLYoung Subd.
| sJoheber?
WHE Sudendort * n 72 | Humble oitaRer cdl®
: hte a
T 40 Miacieton Eo.Mavbovies Fred E Morrison Humble Oil set Co Humble Oi! Ref. Co AD heton
ANSON TAYLOR
SYMBOLS woo 9 me 70005000 _$00 Ile
5 Sst gta fem snc es
Quite, predcing from *hargiuine sand? Scole-foat
§ ceeeet peatiing from trie sendetone Dstum plane: Sea level. Contour interval: SOft.
© Ses ng far arginine send Contours drawn on top of Frio sendstone

$ bry ane ‘eoando’
FIGURE 2.~ STRUCTURE MAP, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS,



RI. 3579

The Merginulina formation contains one oil- and gas-bearing stratum,
the Marginulina sand, which is approximately 30 feet thick, .On December 1,
1940, only i2 gas and 38 oil wells had deen completed in the Marginulina
sand, as the chief interest before that date had been in obtaining oil produc-
tion from the more prolific Frio sendstone, Several wells completed as dry
holes in the Frio sandstone, however, were plugged back and completed as
producing oi] or gas wells in the Marginulina sand,

The Frio sandstone is the principal oil-producing formation in the
field and has been penetrated for a thickness of 1,479 feet by one well,
the Gulf 0il Corporation W, T, Knowles No, 1 in sec, kg, which was drilled
to a total depth of 8,749 feet, Gas was found in the Frio sandstone from
thetop of the Frio formation to the gas=oil contact, which was approximately
6,975 feet below sea level, The gas-oil contact, as established from drill-
ing and production records, was fairly level, although several wells had
gas-oil ratios higher than normal whendrilled to depths of more than 6,975
feet below sea level, Whenever this condition of high gas-oil ratios was
found an additional string of casing or a liner was set in the well to ex-
clude the free gas and thereby reduce the gas-oil ratio.

The main oil-producing zone in the Frio sandstone, which consisted of
sandstone with lenticular shale bodies, had a maximum thickness of 135 feet
between the gas-oil contact (6,975 feet below sea level) and the oil-water
contact (approximately 7,110 fect delow sea level).

An oil-water contact in a petroleum reservoir is established from core
analyses, drill-stem tests, and oil- and water-production records, Above
the level at which water is found in wells drilled into an oil- and gas<
bearing reservoir the reservoir usually contains little or no mobile water,
Below this level or so-called oil-water contact is a zone of indeterminate
thickness that contains oil and water; the content of mobile water increases
from virtually zerp at the oil-water contact to 100 percent at the lower
limit of the zone, The volume of.oil delow the oil-water contact cannot be
estimated accurately from core or drilling records because operators avoid
drilling wells below this level, and wells that are drilled imadvertently to
lower depths usually are plugged back, Thus any estimate of the reservoir
volume delimited by an oil-water contact should denote a minimum volume of
oil and gas because, unless trapped, the oil and gas contained in the inde-
terminate zone below the oil-water contact doubtless will migrate upward be-
cause of a natural water flood,

The oil-water contact in the Frio reservoir of the.Anahuac field was
established by drill-stem tests and according to the records of 31 wells
that either were drilled into the water-bearing part of the sand and were
plugged back or were abandoned as dry holes, The depth at which water was
found in these wells ranged from 7,102 to 7,722 feet below sea level, as show
on the isopachous map (fig, 7); 7,110 fect below sea level was selected as the

uy Leverett, M, C., Capillary Behavior in Porous Solids: Am, Inst, Min, and
Met, Eng, Tech, Pub, 1223, 1940, 17 pp. *

8231 -9-


R. I. 3579 Aucust 1941

UNITED STATES
DEPARTMENT OF THE INTERIOR
Harovo L. Ickes, SecreTaRY

BUREAU OF MINES
R. R. Sayers, Director

REPORT OF INVESTIGATIONS

PETROLEUM ENGINEERING STUDY OF THE ANAHUAC FIELD,

CHAMBERS COUNTY, TEXAS

BY

CHARLES B. CARPENTER AND H. J, SCHROEDER

AFTER THIS REPORT HAS SERVED YOUR PURPOSE AND IF YOU HAVE NO FURTHER NEED FOR IT, PLEASE RETURN IT TO
THE BUREAU OF MINES. USING THE OFFICIAL MAILING LABEL ON THE INSIDE OF THE BACK COVER.


RT. 3579

The subsurface ggyple described in table 8 was obtained by lowering a
sampling instrument, £2/ with valves open, inside the tubing in the well to
a depth of 6,910 feet (6,879 feet delow sea level), closing the valves and
then bringing the sampling instrument with entrapped sample to the surface
of the ground, ‘The subsurface sample under pressure was then transported
to the laboratory for analysis, :

In thé laboratory a portion of tae subsurface oil sample was transferred,
while in the liquid phase, to a special laboratory apparatus in which the
oil was confined over mercury at a pressure greater than its saturation
pressure, The saturation pressure of the sample at reservoir temperature
(177° F,) was determined by withdrawing small increments of mercury and
noting the percent-volume increase with corresponding reductions in pressure,
A plot of these data shows two intersecting straight lines, (See Fig, 11.)
The point at which these lines intersect or at which the slopes definitely
change marks the saturation pressure, which was 3,020 pounds per square
inch absolute,

After the saturation of the oil was determined the dissolved gas was
liberated from the sample at a constant temperature of 177° F. (reservoir
temperature) by the differential gas~liberation method, Pressure was reduced
in stages from saturation to atmospheric, and the volume of gas liberated
was measured, as well as the contraction of oil at the end of each pressure
stage, The total volume of gas liberated by the differential method (expressed
as gas in solution) when the pressure was reduced from 3,020 pounds per square
inch absolute to atmospheric was 545 cubic feet per barrel of residual oil,
Complete data for this analysis are showm in table 9 and figure 12, The
volume of gas that would have been in solution at reservoir pressure (3,235
pounds per square inch absolute) if the reservoir oil were considered to be
saturated with gas was estimated by extrapolation (fig, 12) to be 580 cubic
feet per barrel of residual oil,

Dissolved gas was again determined by the flash-liberation method, The
gas was liberated from solution in two major steps that closely simulate
producing conditions in the flow string, During the first step gas was
liberated by reducing both the pressure and the temperature from saturation
pressure (3,020 pounds per square inch absolute) and a temperature of 177° F,
to 91 pounds per square inch absolute and 60° F, (gas-oil separator pressure
and temperature), The volume of gas liberated in this step was 519 minus
20,5, or 498.5 cubic feet per barrel of residual oil (tadle 10), During the
second step gas was liberated by further reducing the pressure to atmospheric
and the temperature to 60° F, The volume of gas liberated in this step was
20,5 cubic feet per barrel, The total volume of gas liberated by the flash
method from saturation pressure (3,020 pounds per square inch) to atmospheric
pressure and temperature was 519 cubic feet per barrel of residual oil,
Complete data for this analysis are shown in table 10 and figure 13, The
volume of gas that would have been in solution at reservoir pressure (39255
pounds per square inch absolute) if the reservoir oil were considered to be
saturated with gas was estimated by extrapolation (fig, 13) to be 553 cubic
feet per barrel of residual oil,

237 Grandone, Peter, and Cook, A, B,, Collecting and Examining Subsurface

Samples of Petroleum: Bureau of Mines Tech, Papor 629, 1941, (In press, )

8231 ~ 26+



RI, 3579

Every precaution was taken to avoid blow-outs, and none occurred during
development of the field, While drilling was in progress a drilling fluid
weighing 9.5 pounds a gallon (specific gravity, 1.14) was circulated, Two
Dlow-out preventers ~ one held as a stand-by wit — were installed after the
water string (10-3/4- or 9-5/S=inch 0,D; casing) was set and cemented at an
approximate depth of 1,000 feet, The blowout preventers were not removed
until the last string of casing was set and the tubing was hanging in the
well,

Two casing programs were employed in the wells in the Anahuac field,
During the early development 10-3/%~inch water strings were set at an approxi-
mate depth of 1,000 feet with 216 sacks of cement, and 7~inch oi] strings
were set at an approximate depth of 7,050 feet with 330 sacks of cement,

The casing program adopted during later development of the ficld comprised
9-5/8-inch water strings set at an approximate depth of 1,000 feet with 185
sacks of cement and 5a1/2-inch oil strings set at an approximate depth of 7,050
feet with 300 sacks of cement,

All information gained from drilling and coring records in the Anahuac
field indicated that the Frio sandstone ~ the main oil-bearing formation -
was friable and poorly consolidated, and weasures were taken to prevent caving
or loose sand flowing into the wells, Liners were set in the open hole be-
tween the vottom of the casings and the total depth of the wells, ‘The liners
were set with a casing packer; slotted or galvanized-screen sections were
placed opposite the oil-producing beds to provide inlets, In a few wells the
casings were set and cemented at or near the total depth of the holes, and
inlets were provided by gun-perforating the casing, Experience proved, how-
ever, that the wells could be operated successfully without liners and screens,
and both were omitted during later development of the field,

Drilling Time

The subsurface formations are largely sand and shale and drilled easily.
The average time for drilling 194 wells to an approximate depth of 7,080 feet
was 21 days, Several wells, however, wore drilled in 11 days,

Drilling Costs

The estimated average cost of drilling and completing a well to an
average depth of 7,100 feet in the Ananuac field, as shown in table 2, was
$23, 000, This low cost may be attributed to the soft formations, the porta-
hility of the equipment, and the efficiency of the equipment and personnel,

8231 ~6-


RI, 3579
TABLE 1, - Summary of well data, Anahuac field, December 1, 1940

Wells drilled ....... cece cece eee eceeceeceeeecrececeseen 385
Oil wells producing from Frio sandstone ... wa 311
Oil wells producing from Marginulina sand .... a 38
Gas wells producing from Frio sandstone ... +4 1
Gas wells producing from Marginulina sand , seeeeed 12
Dry holes plugged and abandoned ......... sees 2
Flowing oil wells ... seeeeees wed 3

Pumping wells ........ deeee sees 2
Wells producing by gas Lift .......c cee ce cee ee rec ceecene 3

Drilling Methods

Virtually all wells in the Anahuac field were drilled by company-operated
tools of the medium-heavy, conventional rotary type,. The formations above
the oil-producing rock were soft and easily drilled,

Transportation of drilling equiptent was greatly impeded by the swampy
terrain until a well-designed system of roadways was built, Hard~surface
roads were constructed midway between lines,of wells with lateral roads of
plank to each well,

The drilling rigs used by one operator were convertible into portable
units, and frequently only 7 hours.was required to move an entire rig from
well location to another, The derricks were mounted on skids resembling sled
runners and pulled by large, track-laying tractors, The boi‘ers also were
“erected on skids and sometimes were moved without draining the water from
them, Circulating pumps, rotary tables, drilling engines, draw works, and
piping were assembled in wits for easy transportation by truck,

As a rule the following drilling equipment was used in the Anahvac field:

Derrick, 122 feet high with a 2l-foot base set on a steel svbstruc-
tvve 7 faet above the ground,
Draw works, 7-inch diameter shaft with roller bearings,
Rotary table, 27 inches (maximum opening, 27 inches),
Traveling block, 5-sheave with roller bearings,
Drill pipe, Yo /2 inches outside diameter,
(one 14— x 7-1/4 x 18-inch,

Slush pumps
(one 12= x 6= x.16-inch
Boilers, two 125-horsepower,
Blow-out preventers, two 9~5/8-inch and two 7- or 5-1/2-inch,

Subsurface pressures at any depth in the wells were equal approximately
to the normal pressure exerted by a colum of salt water (specific gravity,
1.07) of the same height, The estimated original hydrostatic pressure at a
datum of 7,015 feet below sea level (total depth, 7,045 feet) was 3,265 pounds
per square inch gage, A column of salt. water 7, O45 feet high would exert a
pressure of 7,045 x 0.463, or 3,262 pounds per square inch gage.

8231 ~5-


‘South Gut Oi Corp.
Moryatte Mayes : .
ad Tidewater =| Humble Oil # Ref. Humble Oil # Ref Co, Humble Oil #Ref. Co.
ume Humble Oil Ref. Co - -
wiles Jone ve CH tony citar:
LEWIS ovat es Carthy ot = ” a
“ne Gulf Oil Corp, ‘Humble Oi! #Ref. Co.
ts
JAMES Me GAHEY Magrotia PetCo ED RYEL
BF. BARROW Sum Oil Co
Humble O#R
Fm Sotila
ar Barrow SS Gem 01 Ga
YQ | O.W.CARTER a
J \ AN \ « Great State
" HOkewn . -
WM. CLARK WY eet WC Beidtan
os ele esata -
ronning
Nae Galt il Corp, ery Browning
+ 2 Thanosen
om \ SS OL eRe ca pw pemar
ONS Humble 0 &R,
care [+41 Womble Oia Ref Co
~
Os
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aN
Aocher Ranch Co orp AN a
NE ” PAN NS, Oe BE Quinn Ne onion Te
~ H =| ry
ore —_ Gem Cit Co.
hal Johnsen Gulr ol Humble Oil aRef.Co. |
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nm aa semester see mL ONS eet venison
“we als . ~ e Bn syphrett weno
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ow =| * . 8 . * * |
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.
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z wo. ” Munble Owe, vumbie Ot et co Var Oat co |
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pebraglean si nct ADMiadieton Humble Oi @Ret.ca
Sen i160, Humtte Set Co, ZA ” “
a aw
a ‘Humble Oil a Ref. Ce. Humble Olie Ref. Co. ‘Sun Oil Co,
MC Tyrrell R
o ela
AD Middleton UI Humble Oil» Ref. Co, «
i o@ raptor A Midleton JE Brouserd
{Gu com, ong Teves Natl Bank
i : 7 w vo
H i « . AD Midleton
| | shaupetcers. | Humble a¥n Humbe Oi abet. Co. Gulf Oi Corp. AD uiddleten Leprmusard | Tang Sit
HIE Sudender? + 7 Ta] Humbe Oana ca
i * 0"
AD Mideeton EOMeubovies Fred Merrion Humbe Oil eRe Co Humbe ta Ret Co Avneteten
ANSON TAYLOR
Sy MBO! 1090 2000 300040003000 I mite.
from Ore. Note: Estimated ori | static reservoir pressure March 1935 at
‘ieolaipg sone Seale-feet datum elevation 7015 7. below sea level, 3265 Ib. par sq. in. goge
“Merginuline na'sand” Iscbers represent subsurface pressures, Ib In. gage st detum elevation mi reservoir pre 3
TS AC below set ievel on Feb issoe Efalum slevetion TO Fe below seu vel 31 I pers, in. gage

FIGURE IS. - ISOBARIC MAP, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS, FEB.1,1940.



RoI. 3579

‘Proration

Proration in the Anahuac field since its discovery has been based upon
acreage (50 percent) and well potentials (50 percent). By this method
prorating oil production the total daily allowance for marginal wells2l/ is
decucted from the daily allowance for the field, as designated neriodically
by the Railroad Commission of Texas, and the remainder is divided among the
wells according to the acreage assiged to each well and the individual well
potentials. Well potentials in the Anahuac field are based upon flowing
tests through 1/4-inch-diameter positive chokes, and the initial production
tests are accepted as the potentials of most of the wells. The unit of
acreage of each well, or the, well spacing, is 20 acres. The daily allowance
per well for acreage is the ratio of the unit (20 acres) to the total pro-
ductive acreage in the field; and the daily allowance por well for potential
is the ratio of the individual well potential to the sum of the potentials
for all the wolls.

For example, assume a field of 5,000 productive acres with 250 wells,
of which 20 are marginal, with a total potential field production of 147,700
barrels and a daily allowance for the field of 10,000 barrels to be prorated
upon the basis of acreage (50 percent) and potential (50 percent). The deily
well allowance would be computed as follows:

Total productive acres..... 5,000
Total wells.......-ee.ee 250
Total marginal wells.... aeee 20

seseee DDL, 147,700
do, 10,000

Daily potential oil production of field
Daily prorated oil allowance for field..
Deduction for 20 marginal wells (at 30 barrels

daily)... ee eee eee seee DOL, 600

Remainder to be divided according to acreage and

potential.......ceeeeee er ereee rc reccccrene soos DDL. 9;400
Allowance for acreage (50 nercent)...........6 » GO. L700
Allowance for potential (50 percent)......... wee dow 4.700
Acreage factor, 4,700 x 100 a, “percent 94.0

5,000
5 4,700 x 100 :
Potential factor 72100 x 100s, seeee 00s eee 2182
* “THT, 700 ° 3

21/ The term “marginal well" is defined by the Railroad Commission of Texas
as any pumping oil well capable, under normal unrestricted operating
conditions, of producing less than a predetermined mximum daily al-
lowance, but which would be damaged and prematurely abandoned if the
daily allowance were artificially curtailed below the predetermined
daily allowance. The maximum daily well allowance for marginal oil
wells 6,000 to 8,000 feet in depth, which would include all wells in
the Anahuac field, is 30 barrels. (Texas Oil and Gas Conservation Laws,
Railroad Corimission of Texas, December 1940, p. 42.)

8231 -22-


R.I. 3579

in connate water from a core occurs on account of the reduction of pressure,
except where the water saturation is very high." It is thought that a core
in which contamination by infiltration of drilling water is prevented will
contain approximately tho original quantity of fixed connate water when it
reaches the surface of the ground.

The Bureau of Mines ongincers examined four cores takon by the Humble Oil
& Refining Co. from its Marshall No. 4 well in the Anahuac field, These cores
wero part of the group used in the cxperimental work described by Schilthui s.l5/
Table 4 gives comparative results of the analyses made,in tho lnvoratory of the
Humble Oi1 & Refining Co. and of the Burcau of Mines. 26/

TABLE 4. - Comparative results of connate-water determinations,
Humble 0i1 & Refining Co., Marshall well Mo. 4,
Anahuac field, Chambers County, Tex.

Depth Fixed connate-water|Fixed connate=-water

at which ' Porosity, saturation, saturation,
Speci+| core was percent percent by volume |percent of pore space
men |obtained, Soy .

feet Be of wt! Eumble|3. of i! Humble [3. of ne! Bumble
788+4+/6,960-6,970 | 3149 28,9 | ito 13,0 Wed 45.0
789.00} 7 027 ,034 31.5 30.0 | 16,3 _ 13.2 BLT 4347
790.0017, 024-7 ,034 25.8 32.1] 17.7 9 68.6 46.5
791.-6]7,041.5-7,04F | 33.8 Rue. 12,2 W8 36.1 4345

Av=

erage 30.8 31.2 15.0 13.9 50.0 Wh7

I] Bureau of Mines.

The cores were cut while circulating an oil~oase, water-f
fluid consisting of 011 produced in the Anahuac field weighted with oil-wet
clay (fuller's earth) that had been used in refining mot oils, The drilling
fluid was treated with about 0.5 percent oleic acid and 1 percent sulfuric acid
(98 percent) by volume to aid dispersion of the clay and to increase the
tolerance to water, lLitharge was used whero necessary to increase the dm sity
of the drilling fluid. To prevent evaporation of connate water the corcs were
sealed immediately upon reaching the derrick floor with ordinary sealing wax.

ree drilling

The cores wore taken to tho Bureau of Mines laboratory, where they wore
analyzed by a modificationLl/ of the A.S.T.M. or Dean-Stark mothod for the
determination of water in petroleum products, In this method the sample is
boiled in a liquid (tolucne) that is immiscible with wator and has a boiling
temperature above that of water; the vapors are, condensed, and the water is

Eby Schilthuis, R. Js, Work cited in Footnote 12.

T6/ Taliaferro, D. B., and Spencer, G, B., A Method for Determining the Water
Content of Oil Sands: Bureau of Mines Rept. of Investigations 3535, 1940,
11 pp.

1/ Taliaferro, D. B., and Spencer, G. 3., York cited in footnote 16,

8231 -w.


Percent VoLume INCREASE OF SAMPLE.

4
3

2

2°)
I
\ (> 3020 Ib. per sq. in. absolute
we
S ee
fe) Lt
2700 2800 2900 3000 3100 3200 3300 3400 3500

Pressure, Le. PeR Sa. In. ABSOLUTE

FIGURE 1].- SATURATION - PRESSURE CURVE AT 177°F. FOR SUBSURFACE OIL SAMPLE,
ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.


RI. 3579 -

investigators! have reported evidence of the existence of fixed connate
water that fills the smell interstices of the rocks and, because of capillary
attraction and interfacial tension, is not removed under the differential
pressures set up during the development of a petroleum reservoir.

The importance of fixed connate water has been pointed out by Schiltimis,22/
who made an extensive study of tho subject and performed valuable experimental
work in the Anahuac, Hast Texas, and Tomball oil fields of Texas.

So far no method has been devised to measure quantitatively the connate
water in rocks as they exist underground. Therefore, all determinations mst
be made on cores after they have been cut and brought to the surface of the
ground. This procedure may cause two important changes in the core: (1) Con~
tamination resulting from infiltration of drilling water and (2) expulsion of
oil and water by the escaping gas as a result of reduction in pressure.

The best method of obtaining cores for examination at the surface would
be to cut the core while circulating a nonaqueous drilling fluid and to remove
the core from the well in a pressure-sealed vessel. jome progress has been
made in. the design of pressure=sealed core barrels,. but the method of taking
cores under pressure has not been perfected, .

It is possible, however, by circulating a nonaqueous drilling Fluig ‘such
as oil, to obtain a core- uncontaminated by drilling water. Soniltmislt)
states, "Because of the mechanism by which connate water exists within o1l-
and gasocaring strata, it is unlikely that: any appreciable amount of any loss

i Lindrop, N..T., and Wikolaeff, ¥. M., Oil and Water Content of Oi] Sands,
Grozny, Russia: Bull. Am. Assoc. Patrol. Geol., vol, 13, July 1929,
pp. 811-822, — : ; . : .
Brace, 0. L., Factors Governing Estimation of Recoverable Oil Reserves in
gona Fields: Bull. Am. Assoc. Petrol. Geol., vol. 18,.March 1934, pp. 3u3,
Horner, W. L,, Engineer Says Surface Methods. Can Produce as Much as Mining:
O41 and Gas Jour,, vol. 33, Feb. 21,1935, pp. 16 and 32,
Horner, W. L., Contamination of Cores: Oil Wockly, vol. 78, July 1, 1935,
pp. 29-30 and 71. ;
Gardner, J. H., Type of Well to Expect Determined vy Kind of Extra Gas or
Fluid in Core: Cil and Gas Jour., vol. 3l, Feb. 5, 1936, p. 15.
Lewis, J. A., and Horner, W. L., Interstitial Water Saturation: Oil Weekly,
vol. 83, October 19, 1935, pp. 36, 38, 40, Ye, and UL,
Pyle, H. C., and Jones, P. H,, Quantitative Determination of the Connate~
Water Content of O11 Sends: Am. Potrol. Inst., Drilling and Production
Practice, 1936, pp. 171-180. . . .
lef Schilthuis, R, J., Connate Water in Oil and Ges Sands: Trans. Am. Inst..
Min. and Met. Eng., Potrol. Devel. and Technol., vol. 127, 1938, pp. 199-214,
L/ Sewell, Ben W., The Carter Prossure Core Barrelg:Am. Petrol. Inst., Drilling
and Production Practice, 1939, pp. 69-78.
Taliaferro, D. B., and Eeithecker, R. E., Bureau,of Mines-A.P.1, Pressure
Core Barrel (Progress Report on Its Resign and Development): Bureau of
Mines Rept. of Investigations 3481, 1939, 20 pp.
LY schiltmis, R. J., Work cited in footnote 12.
8231 -B-


RI. 3579

12 gas from the Marginulina sand, and 23 were dry holes, ‘The oil~bearing
part of the Frio reservoir is delimited dy a gas-oil contact above and an
oil-water contact below, 9nd most of the wells were drilled to a depth
approximately midway beiieen the two contacts, The promilsive forces driving
oil through the reservoir and to the wells, therefore, are supplied by gas
expanding downward and by water moving upward, By conserving the free gas in
the gas cap end maintaining a low rate of oil withdrawal the reservoir should
be kept under hydraulic control,

The oil~ and gas-producing formations in most of the wells were cored
and surveyed with electric logging devices, and the records were helpful
in ifentifying oil-saturated strata and shale lenses, Analyses of 19 cores
of Frio sandstone indicated an average porosity of 30 percent, and permeability
determinations ranged from 9,6 to 590 millidarcys,

Laboratory analyses of four cores of Frio sandstone that were cut while
circulating an oil-base, water-free drilling fluid showed a fixed water
saturation of 50 percent, which is considered to be greater than the average
for most oil~dearing sandstones,

Examination of drill cuttings, coring records, and electric logs dis-
closed that the Frio sandstone is interspersed with numerous shale lenses
that cannot be correlated as uniform shale beds,. An estimate, made by
averaging the percentage of shale in all wells as determined from cores
and electric logs showed that shale represented 22,0 percent of the reservoir
volume,

The chief aim of this petroleum—engincering study was to point out means
of improving the accuracy of existing methods. of estimating reserves in
petroleum reservoirs by applying all the data that are collected during the
development of an oil field, ‘The fluid capacity of a petroleum reservoir
with beds of uniform thickness is the product of the area, thickness, and
oorosity of the beds, Calculations of the reservoir volume in fields similar
to Anahuac that have numerous favlts must be made in great detail to achieve
reasonable accuracy, The volume of the Anahuac reservoir was computed’ from
average end areas plotted on cross sections and from data shown on an iso-
pachous map; the results obtained by the two methods differed by only 1.39
percent,

The initial daily oil production of all Frio wells in the Anahuac field
measured through 1/4-inch-diame ter positive chokes ranged from 29 to 799
barrels, The initial gas-oil ratio ranged from 142 to 7,700 cubic feet a
barrel, but wells having an initial ratio above 1,000 cubic feet a barrel
were recompleted immediately in an effort to reduce the gas-oil ratio below
1,000 cubic feet a barrel, The gas-oil ratio of production, measured at gas-
oil separator pressure (91 pounds per square inch absolute), was close to
the measured volume (49%,5 cubic feet per barrel) of natural gas dissolved at
reservoir temperature and pressure per unit of residual oil as determined
on two samples of subsurface oil,

8231 ~36-






a, Pipe manifold for controlling flow of individual wells.

3, Separators, gun barrel, and flow tanks.

Figure 14. - Centrally located tank battery on lease in
Anahuac field, Chambers County, Tex.


RI, 3579

87.78 percent methane, The deviation of 7 percent at an abandonment pres~

sure of 400 pounds per square inch wellhead pressure, was determined from

the compressibility curve, — trapolating on this curve to a pressure of
3,215.0 pounds per square inch and a temperature of 177° F. (637° F, absolute)
indicated a deviation of 9,4 percent. ‘The deviation at abandonment pressure of
400 pounds per square inch wellhead pressure of 7.0 percent was determined
directly from this curve, Deviation from Boyle's law at abandonment pressure
of 100 pounds per square inch wellhead pressure was disregarded, Table 11
summarizes the essential data for estimating the natural-gas reserve in the

gas cap of the Anahuac field,

TABLE 11. ~ Essential data for estimation of gas reserves
in gas cap of Frio sandstone

(Between top of Frio and _gas-oil contact)

Effective porosity (allowance made for connate water, 50 percent,
table 4) ..eccecceeeecerecececceesepercent = 15
Estimated original reservoir pressure, «+. pounds per square inch ‘ .
absolute 3,215,0
Estimated deviation from Boyle's law at reservoir pressure,.,,percent
Calculated temperature in reservoirscesssececceeccceeeeese oF absolute 637.
Assumed average temperature of static column of gas. do,
Assumed specific gravity of gas (air = 1,0) wccccecececcceccucecveces 70
Caleulated!/ absolute abandonment pressure in pounds per square
inch in reservoir with wellhead pressure of
400 pounds per square inch gage........ seesceceescees 490
Deviation from Boyle's law at abandonment pressure of 400 pounds
per square inch gage at wellhead ... eeoee. percent v4
Calculatedl/ absolute abandonment préssure in pounds per square
inch in reservoir with wellhead pressure of
100 pounds per square inch goge.. seseceeecasces 136

1/ Rawlins, 5, L,, and Schellhardt, M. A., Back-Pressure Data on Natural-Gas
Wells and Their Application to Production Practice: Bureav of Mines
Monograph 7, 1936, appendix 6, pp, 166-174,

PRODUCTION OF WATER

The slow and uniform rate of withdrawal of oil and gas from the Frio
oil-producing reservoir has resulted in an exceedingly well controlled up-
ward movement of modile connate water into the oil-bearing part of the
reservoir, On December 1, 1940, almost 6 years after the field was discovered,
13 oil~producing wells in the Frio sandstone and 1 oil-producing well in the
Marginulina sand were producing 2 to 90 percent of water (See fig, 7).. Ten
of the wells producing water from the Frio sandstone were on the edge of the
field near the productive limits of the reservoir, and their depth ranged
from 7 to 35 feet above the oil-water contact (7, i10 feet below sea level),
The oil-bearing part of the Frio sandstone thins toward the edge of the
reservoir (figs. 3, 4, 5, and 6), hence the edge wells were drilled deeper

8231 =~


‘29v0 m1 BS wRd-7 IOAUERE NI aUNSSIua ZOVETRY

-|3270
| 3260
{3280
{320
3230
{3220
3210
3200
3190
3180
3170
3160
3180
suo
3130
3120
3it0
3100

for

be waar nowsncoed i unr &
‘Soneeua wonwaeay m 850% TTT ane

e ¢ 2 2" © iad

1939

auction

Average daily prod
i

fon warrels cumulative pro
i
T
!

i
T

|
|
T
|
i

1938

‘GRAMBERS COUNTY, TEA:

FIGURE 10. ~ PRODUCTION, AND RESERVOIR PRESSURE CURVES, ANAHUAC FIELD,

=
2888 822 8 8 22 2 8°
228 gg 22°

“sTauwva 40 SNOMTTIN ‘NoLL2NGOwe TAILYTANND




. RI. 3579

TABLE 9, ~ Laboratory data from Anshuac’sudsurface oil sample
odtained August 20, 1937

Humble 0il & Refining Co, Tyrrell well No, 8
Anahuac field, Chambers County, Tex,

Data by differential gas liberation method at 177° F, (reservoir temperature)

Pressure, Oil sample Gas in solution
lb/sq.in, | Temp- | Volume, | Percent Cec, Cu, ft, per
absolute |erature| c.c, volume in cumu~ bol, of
OF, ‘lcrease of Density, |lative residval oil
‘|residual oillgm./c.c.
3,235 UY} ag - | 3/e8,9 3/o,7e8 |3/17; 324 3/580
3,020 2/ | 177 | 213.6) 27.5 +733 | 16,282 545
2,807 17 211.4] 26,1 - 4738 15, 27 512
2, 44g 177 207.2 | 23.6 - Th 13,37 Lug
2, 045 177 202,1 29.5 0756 11,112 372
1, 708 177 198.4 18.3 2704 9,376 314
1,412 177 195.3} 16,5 770 1, 158 260
1,132 177 192.2] 14,6 rand 6,388 214
837 177 183.9 | 12.6 787 4,876 163
588 177 186.3 | 11,2 +192 3,004 |. 1a
331 177 183.1 9.2 +799 2, 286 7
126 17 180,4 7.6 9803 1,077 36
4g 6 177. | 178.9 6.7 +807 506 17
14.5 ly 177.2 567 812 0 0
15 60 167.7 te} 2858 - -

1/ Static bottom—hole pressure at 7,011 feet below sea level (bottom of casing).

2/ Determined saturation pressure of oil semple taken at 6,879 feet below sea
level,

3/ Value obtained by.extrapolating data to 3,235 pounds absolute,

. After all the gas had been liberated from the oil by methods of
differential and flash liberation the quantity of residual’oil obtained
by each method was measured at 60° F,, and its volume under reservoir
pressure and temperature was computed,

The flowing pressure in the well, at 6,879 feet velow sea level, was
3,186 pounds per square inch absolute at the time of sampling (table 8).
As the saturation pressure of the oil sample at 6,879 feet below sea level
was 3,020 pounds per square inch absolute (fig. 11), or 166 pounds per square
inch lower than the flowing pressure, at the time of sampling, the reservoir
oil in the vicinity of this well may have been undersaturated with gas,

8231 -27-




RI. 3579

Intermittent flowing of wells in the Anahuac field, imposed by proration,
caused very rapid accumulation of paraffin, which when not removed restricted
its flow. At first the “eposited paraffin was removed from the tubing in the
wells by lowering and withdrawing a mechanically operated peraffin scraper,
but later automatic paraffin scrapers were installed in many wells, ‘These
rested on stcps near the bottom of the tuding while the wells were "shut in,"
When a well so equipped vegan flowing the scraper was propelled upward by the
fluid to the top of the tubing, and when the well Was closed in the scraper
dropped dow on the stop, Travel of the scraper up the tubing when the well
was flowing removed the thin coating of paraffin that had accumulated on the
wall of the flow string since the last flowing period, Automatic scrapers
remove the paraffin from wells flowed intermittently more cheaply then
mechanical operated scrapers, which require one or more men to operate; how-
ever, automatic scrapers must be run daily, as accumulations of thick coat-
ings of paraffin cannot be removed by then,

Each well in the Anshuac field is connected to a centrally located
separator and tank battery by individual flow lines, (See fig, 14.) The
well end of the flow line is connected to the Christmas tree of the well on
the downstream side of a positive choke, and the battery end of the flow
line is connected to a pipe manifold, Hach tank battery is equipped with a
large gas-oil separator through which the oil and gas from one or more wells
are flowed during routine operation, and a small gas~oil separator through
which oil and gas from an individuel well cen be flowed during testing periods,
An orifice meter is conrected to each separator to measure the volusze of gas
produced by the wells so that the gas-oil ratio may be computed,

An installation of this type at centrally located tenk batteries allows
flexibility cf well operation and permits the operetor to check regularly
the daily flowing rates and gas-oil ratios of each well, The manipulation
of several wells on a lease at a centrally located tank battery instead of
at individual wells has one disadvantage, however, as high wellhead pressures,
sometimes as much as 1,200 pounds a square inch, are maintained continuously
on all the flow lines between the wells and the tank battery, Although the
lines were buried a safe depth below the surface of the ground and a choke
was placed in the Christmas tree at eacn wellhead, a break in a flow line
could cause consideradle damage, ~

Depth-pressure Measurements

Reservoir pressures are based upon depth—pressure measurements made
periodically in wells by lowering a recording subsurface-pressure gage’
asf Sclater, K, C,, and Stephenson, B, R,, Measurements of Original Pressure,
Temperature, and Gas-0il Ratios in Oil Sands; Trans, Am, Inst, Min, and
Met, Eng,, Petrol, Devel, and Technol., 1923-29, pp. 119-136,

Reistle, C. E., Jr., and Hayes, E, P,, A Study of Subsurface Pressures
and Temperatures in Flowing Wells in the Hast Texas Field and the
Application of these Data to Reservoir and Vertical-Flow Problems:
Bureau of Mines Rept, of Investigations 3211, 1933, 17 pp. .

Hawthom,.D, G,,- Swosurface Pre es in Oil Wells and Their. Field of
Application: ‘trans, Am, Inst, Min, and. Met.’ Eng., Petrol, Devel, and
Technol,, vol, 103, 1933, pp. 13-169,

Berwald, W, B,, Buss, 2, and_Reigtle,_C, BE, Jr,, Bureau of Mines
eye Se biabhrecs Recordihe Subsurface’ Présstre Gage: Bureau of dines
Rept, of Investigations 3291, 1935, 10 pp.

8231 - 30 ~



RI, 3579 *

and completed nearer the oil-water contact than most of the wells higher

on the structure where the oil-bearing part of the Frio sandstone is
thicker, ‘The three remaining Frio wells producing water were near the
ecnter of the structurc: two were completed within 26 feet and the third
within 46 feet of the oil-water contact, The only Marginulina-sand oil well
producing water on December 1, 1940, was near the edge of the indicated
Marginulina-sand reservoir,

The normal appearance cf water in wells completed near the oil-water
contact before it appeared in wells compicted a greater distance above
the oil-water contact is evidence of a wmiform upward movement of the oil-
water contact in the Anahuac field, Of the 13 Frio oil wells producing
water December 1, 1940, 11 were completed within 26 feet and 2 within 35
and 46 feet, respectively, of the oil-water contact, This indicated that
the maximum ascent of the oil-water contact in the 13 wells after oil and
gas had been withdrawn for nearly 6 years was 46 feet, and the average was
about 25 feet, A slight undulation of the oil-water contact, however, could
explain the early appearance of water in the two wells that were completed
more than.25 feet above the oil-water contact (7,110 feet below sea level).

SUMMARY AND CONCLUSIONS

The Anahvac structure was found by a reflection~seismograph survey made
by the Humble Oil & Refining Co, in May 1933. Subsequent development of
the field indicates that the subsurface structure is a nonpiercement-type
salt dome common to the Gulf Coast region of Texas,

The development and operation of the field are of great interest to
petroleum engineers, not only because it ranks fourth among the Gulf Coast
oil fields in estimated ultimate recovery but because it was developed upon
the basis of good engineering practices and in a systematic mamer, Ivery
modern technical and scientific aid was employed in the development and
operation of the field, including micropaleontology coring, electric logging,
determinations of porosity, permeability, and connate water, subsurface
sampling of crude oil, and periodic desthepressure measurements,

The reservoir rocks in the Anahuac field have been disturbed greatly
by intense faulting, which complicates the task of computing the volume of
the oil- and gas-bearing parts of the reservoir,

The wells were drilled with modern rotary equipment, and great care
was-exercised to prevent blowouts and "wild" wells, Drilling costs were
low in the Anahuac field as a result of good engineering design and super-
vision of drilling equipment,

Oil and.gas are produced from the Marginulina end Frio formation;
however, the oil-saturated Frio sandstone, which has a maximum thickness
of 135 feet, is the principal oil-producing reservoir, The productive area
of approximately 7,300 acres.and tne thickness and physical characteristics
of the Frio reservoir were determined carefully from the records of 385
wells, of which 311 produced oil and 1 gas from the Frio, 38 produced oil and

8231 ~35-


RI, 3579

ILLUSTRATIONS
Figure Follows page
1. Generalized geologic colwm, Anahuac field......... y
2 Structure MAP ....seceecceccecerccsceesseee a 4
é North-south cross section GG! ......... 8
. North-south cross section N-Nt ...........66 8
5. East-west cross section 83! ,, &
6, East-west cross section 14-11! , 3
T Tsopachous MAP ...ceececesreeee 8
8 Typical electric log of well .. . 10
2 Composite subsurface interpretation . . 16
10, Production and reservoir pressure curv: eoece 18
11, Saturation-pressure curve, subsurface oil sample...
le, Differential gas-liberation curves for suvsurface
O11 SAMPLE Loic e cece cece cree ee ee eens teecetenes 26
13. Flash gas-liberation curves for subsurface oil
SAMPLE .ecceeceevcsecevccevceeceneceeseteeeenes 26
14, Centrally located tank battery ... 30
15. Tsodaric Map ...csssceseseeereeree 30

InTRoDUCTION

The Bureau. of Mines selected Anamec for a petroleum=enginecering study
because it is one of the notable ficlds of the Gulf Coast region, It is out-
standing because the producing foraation is very ck; every modern technique
of drilling and development has been applied; complete records of all opera-
tions, including detailed subsurface information have dex 6; the field
has been developed systematically; and every effort has been sade to utilize
the reservoir energy efficiently,

Data and information pertaining to the development and cperation were
collected, compiled, and studied by the writers es if the field were under
single ownership, The chief aim of the study was to increase the accuracy of
the existing methods of estimating reserves by applying all the data collected
during the modern development of an oil field, These scientific aids include
coring; electric logging; micropaleontology; determinations of porosity,
permeadility of oil-bearing sands, and their connate water content; subsurface
sampling of the reservoir oil; and depth-pressure measurements, Because the
reservoir structure in the Auahuac field is extremely complex, special atten-
tion was given to methods of calculating the reservoir volume,

The Anahuac oil field, in the central part of Chambers County, Tex,,
about 6 miles east of the town of Anahuac, was discovered March 2, 1935, Al~
though it lies in the so-called salt-dome belt of the Gulf Coast region, salt
had not been reached in the deepest wells drilled before February 19U1, Most
geologists, however, consider the Anahuac structure to be a deep-seated or
nonpiercement-—type salt dome, The subsurface structure was found as a result
of reflection seismograph work performed in May 1933 on a vlock of land that
had been leased for exploration,

8231 -~2—


RI. 3579

Differential Flash
liberation, liberation,
at 177° FR, 17°F. to 80°R,

Indicated density of reservoir oil in flow
string at 6,879 fect below sea level,
gram per ce,. 0133 2736

Indicated density of reservoir oil at
7,011 feet below sea level,.gram per cc, 128 71

Gravity of residual oil at 60° F,.........
OAPI... 33-4 3304

Compressibility of subsurface oil sample 7
per 1,000 pounds per square inch,.percent 1,15

The following definitions of terms may prove helpful to an understanding
of the discussion of the laboratory data to follow:

Saturation pressure, ~ The minimum pressure capable of maintaining a
petroleum-hydrocardon fluid in the liquid phase at a given temperature,

Differential-gas-liberation method, - A gas-liberation process in
which the gases liberated during pressure reduction are removed from contact
with the oil as rapidly as they are formed,

Flashwliberation method, - A gas~liberation process in which the gases
liberated during pressure reduction remain in contact with the oil in the
system throughout the liberation process,

Barrel of oil, — The volume of 42 gallons of oil at 1 atmosphere pres~
sure and 60° F,

Residual oil, - 0il measured at atmospheric pressure and 60° F, remain-
ing at the end of a gas-liberation process,

Gas pressure and temperature base, - 14,4 pounds per square inch absolute
and 60° Fy

Dissolved_gas, ~ Gas liberated from solution in oil upon a reduction of
pressure or increase of temperature,

"Free" gas, ~ Gas produced with 1 barrel of residual oil in excess of
the gas dissolved in the oil at reservoir pressure and temperature,

Gas-oil ratio of production, — The volume of dissolved and free gas
measured at separator pressure and corrected to 14,4 pounds per square inch
absolute and 60° F, produced with 1 barrel of residual oil,

8231 ~ 2


RI. 3579

For one well per 20 acres and a daily potential production of 700
barrels of oil the well allowance would be computed as follows:

Allowance for acreage, 20 x 0.9........ seeeeebbl, 18.20
Allowance for potential, 700 x 0.03182.......+6 dO. 22.27
Daily well allowance...........

seevee Gee. 41.07
Gas-oil Ratios ‘

The initial gas-oil ratios of most wells producing in the Anahuac field
approximated the ratio of natural gag dissolved at reservoir pressure and
temperature per unit volume of residual oil as determined on samples of
reservoir oil. Initial gas-oil ratios of 315 wells at the time the wells
were completed averaged 728 cubic feet per barrel of oil produced.

In 1937 Bureau of Mines engineer s22/ studied two subsurface-oil samples
obtained from Humble Oil & Refining Co. Tyrrell well No. 8 The physical
characteristics of one of the samples and the data obtained by ‘physical tests
are given in table 8.

2/

TABLE 8 ~ Summary of field and laboratory data for subsurface, sample’

Humble Oil & Refining Co. Tyrrell well No, 8, sec. 52,
H&. C. 2. R, survey, Anahuac field, Chambers County,
Tex.; surface elevation of well, 31 feet; date completed,
June 11, 1936; sampling date, August 14-22, 1937

Well datas
Initial daily production.............. barrels of oil.... 681
Initial gas-oil ratio... .»--cubic feet per barrel L351
Total depth of well... ....s cece cece eevereree feet. 7,085
Depth of casing (oil string)....... 7,043
Top of producing sand (Frio)....... 6,820
Tuding (2-1/2-inch~diameter) depth.........2008 6,917

Sudsurface samples obtained at depth of 6,879 feet below sea
level. (Well shut in during sampling periods.)
Static pressure at a4denth of 6,879 feet below sea
LOVE]. ees sceeeeeeceseceeeee pounds per square inch gage 3,180
Static pressure at a depth of 7,011 fect below sea
Llevel........esseeeeeeeespounds per square inch absolute 3,235
Reservoir temperature at a depth of 7,044 feet below sea
Level (no flOW)..c. see cece cee c ccc esecatepercers ees Fave 177
Flowing pressure at depth of 6,879 feet below sea .
level.......2+....++006+epounds per square inch absolute 3,186
Gas-oil separator temperature......... sec e reece een Bees 80
Gas-oil separator pressure.pounds per square inch absolute 91
i Results of research work by M. A. Schellhardi, Peter Grandone,
W. H. Barlow, and A. B. Cook, Bureau of Mines, Bartlesville, Okla.,
and R. K. Guthrie, Bureau of Minés, Dallas, Tex. :

22] Physical tests conducted by i. A. Schellhardt and W. H. Barlow, Bureau of
Mines, Bartlesville, Okla., and R. K. Guthrie, Bureau of Mines, Dallas,

Tex. - :
Subsurface sample of reservoir oil_obtained_and analyzed by Peter Grandone
and A. B, Cook, Bureau of Mines, Bartlesville, Okla.

8231 - 23-




DEPTH BELOW SEA LEVEL, FEET

——-s 0 u TH—NO RT H——+

HumbleOeR. HumbleOaR, Humble OR, Humble O.eR, HumbleOeR, HumbleO#R. Humble aR, rumble wR, Hurcble Ow MumbleO aR, Mumple O.8R, Humb wun OitCe., und Humble 0.8R.,
AD Middleton, ADMidgleton, "GJ Clerk, GJ Clark,” "GJ Clark, BJU Ce G04 Clark. NC Tyrer We Tyrrell mersreel” “weryren” “wetyeren” Rinne” Rltnnie! Punnte,
No 20 No.2 Nol No.6 Noé No 4 No. 24 No.2? No 26 Ne No® Not No 13, No! No.3
Pyece pez" Pyeuse” ez” i wes” Pe aE! rye Pye’ Pye pena pene!
Porosity is i ¥ Porosity) | Resitivit
ilivolts > H 7 | Tote mtivartss | | Cohma. -
5900 [- | 4 xa ana 45900
L g SY hk = 4
6000 |- i = | || AS ~46000
| ois i 4
r : rani he ce
6100 - } > 48100
L + | i nS 4
6200 zit rh + {6200
pee 3 | \
L ier 4
|
6300 |- s r 3 be \ 16300
if hs 3| 1
6400 [- 4 | {6400
L +warg 9 |
3 j

T

6500 }- —1 6500

~ 6600

g
T

Aad antl

eee
—

Marg 5

i
|
|
|]
r 6700

}Huerg

i
Maton!

\
: |
6700 r } | = TT 3
ag
L / ~ 3,
+—tFrio wa ~
6800 |- Feige] 1 oH de Es > {6800
L So || < |
== fio
6900 | PZone “E = ss 6900
[ o i re |
jcor tact? | fa , pe ric —
7000 |- j = ; = a —{7000
a Sul gq ==
. UU a i 4 = LI LAE LI LJ 4
5 "Zone, z s
7100 | {Oil -weter contacts oi ee ee 4 Tl00
L Water 2 : : 5 ES: ae : J
7200 |— Camp. W136 Comp. 6-495 Comp. 35:98 Comp. 12-2738 Camp. 2.1296 Comp. 923:35 Comp. 35398 Camp G-M-40 | Camp. $2.49 Comp 4-28-39 Comp 2-9-38 Comp. 09°38 Camp S-3G Comp. 1-36 one. May 936 Cone ag 935 Comp. 1-238 17200
BP noe He ie” STP? FS" ose” FE? Sogo” SS Saas” FE joss” FO" Soa” «SE Goa” ST” SoaoTM SE Fobe” SS aes SE Sans ST? Goss Sor eee SP So ous?
1p @20bed. «IF Tsbps. IP GtSbad. IP I Soles IP dsehpd IP deioes Ih Torepa Ih davupa (2, iieopa It dettea. If detopa, IF bea. IRL dala IP, Gieea TP Toba Fea, 1k ithe
Legend Gon, Sento Gon. asecte Gon Saycfe Ga Gon, drecke che. toceFo. Gon tuscte. Gon asoco: con eaeeFo GOR c6acre Gon Satefo GOR. dsachs: Gon asicks. Goa, S3ecth Gon Gascte, Gen abeers Gon aseho. Gor, Tunete
m8 io 20
Come. + competion date LP initial ot production, bbls. per day
SOP: Si Sep below surrace  Gar-= Gesell ratio, cu fr per ba Morizontal scale: feet
PB Depth below surface to mich

melas paged beck FIGURE 3.-- NORTH - SOUTH CROSS SECTION G’-G’, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.

DEPTH BELOW SEA LEVEL, FEET


RI, 3579

of the tabulation revealed, however, that the gas~oil ratios of 227 of the
wells were below the solution ratio of 498,5 cubic feet a barrel, and the
ratios of 88 wells were above the solution ratio of 498.5 cubic feet a barrel
at 91 pounds per square inch absolute (table 10),

The operators made frequent gas~oil-ratio tests on all wells and con-
ducted semiannual gas-oil-ratio surveys of the field, As the results of
these surveys usually indicated an average gas-oil ratio lower than the gas-
oil ratio determined by laboratory tests on sudsurface samples, they were
considered to be only reasonably accurate, The surveys, however, offered
the operators a means of detecting increases in the gas-oil ratios of indi-
vidual wells and suggested the need of renedial work,

The increase in volume of residual oil from atmospheric to saturation
pressure (3,020 pounds per square inch absolute) was 27,4 percent by differ-
ential liberation (see table 9 and fig, 12) and 26,8 percent by flash libera~
tion (see table 10 and fig, 13), This means that 1 barrel of oil measured
at atmospheric pressure and 60° F, occupics a volume of 1.274 barrels in the
reservoir, based upon the differential gas-liberation method, and 1,268
barrels when the gas is liberated by the flash method, The shrinkage of the
reservoir-—oil owing to liberation of gas is:

1,274 - 1 x 100
1,27)

1,26
1.2

21,50 percent by differential liberation

and

1

8 - 1 x 100 = 21,13 percent by flash liberation,
8

Operation of Wells

Oil wells in the Anahuac field were equipped with 1/4-inch, maxinun—
diameter, positive chokes, Wells that tended to produce with high gas-oil
ratios were equipped with positive chokes smaller than 1/-inch diameter,

The flowing rates determined by initial production tests through 1/4
inch-diameter positive chokes ranged from 1,21 to 33 barrels of oil an hour,
Flowing rates under operating conditions, however, depended on the daily
prorated allowance per well (as determined by the Railroad Commission of
Texas), choice of operating personnel, and limitations of mechanical equip-
ment, .

The intermittent flowing of wells producing a peraffin oil often,
causes the inside of the flow string to become coated with por affin, 24/

au] Reistle, C. £., Jr,, Paraffin and Congealing-Oil Problems: Sureau of
Mines Bull, 34g, 1932, p, 20. :

8231 ~29-


RI, 3579

to a predetermined datum clevation in the reservoir or to a depth as near
that datum as possible and estimating the préssure at that point from a
plotted gradient, Depth-pressure neasuregeyts of several "key" wells in
a field are used to construct an isobar ic&®, map,

The reservoir pressure is computed either by averaging all the measured
pressures in the key wells or by calculating a weighted~average reservoir
pressure from the areas lying between the isobars on an isobaric map, A
weighted-average pressure based upon the arca and mean pressure between iso-
bars is more accurate than an average of all the measured pressures in the
key wells because it’ detter reflects the distribution of pressure throughout
the field,

Depth=pressure measurements were taken in 127 wells in the Anannec field,
Approximately 30 of the wells were considered to be key wells and depth~
pressure measurements were ‘taken periodically in them, The estimated original
static pressure in the reservoir vas 3,265 pounds per square inch gage
measured at a datum elevation 7,015 feet below sea level,

The estimated reservoir pressure in the Frio reservoir on February 1, 1940
at a datum of 7,015 feet below sea level, calculated from a weighted average
of the areas lying between the isobars on the isobaric map (fig, 15), was
3,178 pounds per square inch gage. The subsurface faults in the Frio sand-
stone are not shown on this map as there is little evidence that the fluids
in the reservoir are separated by sealed faults,

The average reservoir pressures for six surveys are plotted in figure 10,
as well as another curve showing Ioss in estimated reservoir presdure per
million barrels of cumulative oil production, ‘The estimated average reservoir
pressure on February 1, 1940, of 3,178 pounds per square inch indicated a
loss of 87 pounds per square inch from the estimated original reservoir pres-
sure of 3,265 pounds per square inch, As the cumulative oil production 2
February 1, 1940, was 12,960,857 barrels, the average pressure loss was 6.7
pounds per square inch per one million barrels of cumulative oil production,

METHOD oF ESTIMATING ULTIMATE RivoveaY OF CRUDE OIL

The ultimate recovery of crude oil from a field of the Anahuac type may
be estimated most accurately by the so-called porosity~saturation method,
In this method of calculating ultimate oil recovery the net volume of the
producing formation in the reservoir is computed from subsurface records of
the wells, and porosity, saturation, and shrinkage of oil are determined vy
analysis or experiment, Recovery is the only factor incapable of being de-=
termined by analysis or experiment and therefore must be based upon the records
of depleted or nearly depleted fields that have similar characteristics, or
upon theoretical considerations,

26/ An isobar is a line connecting points of equal pressure,
8231 ~3l~


SEA LEVEL, FEET

DEPTH BELOW

so UT H—N ORT H—

Humble O.eR, Humble O.#R, Humble O.eR, Humble 0.0R., Humble O.#R., Humble O.eR,, Humble QaR, Humble O.eR., Humble O.eR , Humble 0.08, Humble 0.8R., Humble OF,
Broussard -Hebert, Broussard: + Brousserd-Hebert, M.H Engels, MH Engles, HE Marshall, GCwmilcox, — C.G.John.on, CGJehnson, C6 Johnson, CG Johnson, EW Boyt,
No. 16 No.13 No.3 No.7 No.6 No.9 No.4 Nod No. 17X No 32 No.l4 No.l6 No.6 Nod No.5
aes” Poe.er’ (EI. 30" paee3o” —eL3i” q E130" [EL 30" r7ye1.30" (7) £1.30” ryenai” ea ena Pyese’ | yen33” (ese
ganranity. Resistivity, Porosity.) | Resistivity,
nite wilt) |eSiee J
| }Ioi, 1 +}Pis. 5 pot
5900 = ee ee el —{ 5900
I poi is. —— } |
=| |
6000 | oS js E —] 6000
6190 }— 1tbis. — 6100
= 1 tois,
F _ i ——Hois.__ 1
6200 |- j “> 76200
T tHet q
6300 |~ ) Hy Ni —{ 6300
43
6400 |- ae | 6400
|. _-—- Pa 4
6500 }- 4 | 6500
r }—Harg Marg. ieet. +
6600 }~ TN \ ~ 6600
[ \ 4
1 ar
6790 f- “——— —| 6700
| ——F —< :
6800 |- TM~ —{ 6800
5900 |- —7 6900
7000 }- sJuarg. 94. | 7000
7100 F- + 7100
7200 [- 2 Sere sin Same. 1637 i a a a $e? Tae 17200
od f a Ie sdipucr IP Minpa. — [R aabog, TR gg
Mignetend, Gon, Veh Gta Steefs! don, Socks! Gon. 3eeeFs con, Since asooucr 501. UCR Gao percey GAR Bosco Gon, eiacts. Gor, 205e%8
end: bon, Seeks, gee darcy
Comp.» Completion date LP__ = Initial il production, bbis.par day "C2, 2 =< =
BB. “Depth Selo surrace wien CON "ORO Rea vernantal scale fet geeseraey
walnae plisged beck FIGURE 4.- NORTH - SOUTH CROSS SECTION N-N, ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.

DEPTH BELOW SEA LEVEL, FEET


RI, 3579,
August iota, :
REPORT OF INVESTIGATIONS

UNITED STATES DEPARTMENT OF THE INTERIOR ~ BUREAU OF MINES

PETROLEUM ENGINEERING STUDY OF THE ANAHUAC FIELD,
caagars county, mxast/
By Charles BE, Carpenter=/ ana ER o. Schroeder!

CONTENTS

Introduction ..esee se see eee er cece tes erecereserreees sone
Acknowledgments .......%. .
Geology of Anahuac field .
Development .......-+.3--
Drilling methods...
Drilling time ...
Drilling costs ..
Subsurface interpretations .
Coring operations .....
Electric logging ....
Porosity and permeability tests ..
Connate water ...... eee eee .
Determinations of shale ,
Calculation of reservoir volume .,
Average end-area method ...
Isopachous map ........
Production of oil and gas ....
Initial production ..
Production of oil ..
Proration .......
Gas-oil ratios .
Operation of wells ......
Depth-pressure measurements ¢.......+.66--
Method of estimating ultimate recovery of crude oi] ...,.,
Method of estimating natural gas reserves in Frio sandstone ..
Production of water .....
Summary and conclusions .

v/ The Bureau of Mines will welcome reprinting of this paper provided the
following footnote acknowledgment is used: "Reprinted from Bureau of

Mines Report of Investigations 3579."
2/ Petroleum engineer, Bureau of Mines, Dallas, ‘Tex.
3/ Assistant petroleum engineer, Buredu of Mines, Dallas, Tex,

8231,


Rel. 3579

Initial 2lhour oi1 production of all wells in the Anahuac field ranged from
29 to 799 barrels and the zgas~oil ratio from 142 to 7,700 cubic feet a
barrel, Whenever the initial gs-oil ratio for a well vas.more than 1,000
cubic feet a barrel the well was recompleted, usually by drilling deeper and
setting a liner, in an effort to reduce the gas-oil ratios.

Production of

The cumulative 011 production for the Anahuac field to October 1, 1940,
was 14,633,482 barrels. Monthly production figures for the field are show
in table 5 and a production-time curve in figure 10.

Tables 6 and 7 give distillation analyses of samples of crade 011 pro-
duced from the Frio sandstone and the Marginulina sand made by the Bureau
of Mines Hempel method. The results of these enalys2s show that the gravity
of the oil produced from the Frio sandstone was 35.40 A.P.1. and the gravity
of the oil produced from the Marginulina sand 37.6° A.?.I.

TABLE 5, ~ Oil-production statistics, 2/ Anahuac
field, Chambers County, Tex.

Year . Monthly | Average deily Cumulative
and Wells, production, production per produc tion,
month No. bbl. well, bol. bl.
19358
January. .ee - - -
February... - - =
March...... 2 4259 59
3 7547 ah
5 18,517 119
TUN. eee eee 7 2,986 119
July...e.ee 9 29,983 107
Aueust.....] 13 29,827 4
Septomber.. 17 Yl. 907 38
October.... 23 53, 314 15
November... 29 75,550 87
December... 38 103,793 8B 393,153
1936:

January...» us 126, 60 i)
February... | 52 141,90 ou

64 182,452 92

1) 204,973 91

3 eur 392 93

91 ako , ules 8B
Tuly...e.6. | 100 223,270 72
Avgust..... | 112 250,351 Te
Soptembor.. | 123 225,301 61
Octobor.... | 131 251, 744 62
Novenbor... | 138 22) 021 oy
Decombor... | 1h | 270 ,263 $1 2,976, uu5

L/ Includes oil production from the Marginulina sand and the Frio sandstones
8231 - -1g-


RI, 3579

approximate elevation of theoil-water contact throughout the field, Itsy
should be recognized, however, that this oil-water contact is undulated
and is not a definite horizontal interface between oil and water,

Because of the higher recovery ovtainable by displacing oil with water
instead of gas it is desirable whenever possible to produce oil under a water
drive,©/ In the Anahuac field a special effort was made to attain this
objective by drilling the wells, wherever possible, to a depth approximately
midway between the gas-oil contact and the oil-water contact. By conserving
the free gas in the gas cap and maintaining a low rate of withdrawal the
operators hoped that the reservoir would remain under hydraulic control,
with the underlying water supplyitig the propulsive force as well as a
natural water flood behind the oil,

The Anahuac field may not be operated always at a rate equivalent to
the inflow rate of encroaching water, and the gas cap may expand as a result
of reduced reservoir pressure, This contingency, however, was foreseen by
the operators, and the casings in all wells were set far enough below the
original gas-oil contact to allow the gas cap to expand downward (at least
35 feet) before "free" gas can enter the wells, ‘The original gas~oil contact,
as determined from subsurface records, vas 6,975 feet below sea level, and
the casings in all wells drilled to the Frio formation were set at or lower
than 7,010 feet below sea level, which wes 35 feet below the original gas-oil
contact,

The aim of the operators is to obtain themaximum benefit in oil recovery
from the water underlying the oil in the reservoir by maintaining a level oil-
water contact as the water rises and displaces the recoverable oil, Theoreti-
cally, the greatest recovery of oi] would be had if a level oil~water contact
rose and ultimately met a level gas-oil contact at its original elevation
and flushed the recoverable oil and its dissolved gas from the oil-bearing
sandstone, This is difficult to accomplish because any lag in the upward
movement of the oil-water contact as the oil and dissolved gas are withdrawn
would reduce the reservoir pressure, thereby causing the overlying gas cap
to expand, which, in turn, would depress the gas-oil contact, Under such
conditions some recoverable oi] would remain in the upper part of the oil-
bearing sand invaded by the expanding’ gas cap, This recoveradle oil could
not be flushed out by the water after the oil-water contact and gas-oil
contact had met, unless the gas cap were contracted,

5/ Beal, Carl H., Geologic Structure in the Cushing Oil and Gas Field,
Oklahoma, and, Its Relation to the Oil, Gas, and Water: Geol, Survey
Bull, 658 (in cooveration with the Bureau of Mines), 1917, pp. 48-61.

6/ Moore, T, V., A Review of the Principles of 041-Reservoir Performance:
Presented at 10th midyear meeting, Au, Petrol, Inst., May 29, 190,
Fort Worth, Tex,; Drilling and Production Practice, 190, Division
of Production, 4m, Petrol, Inst,, 1941 (in press), pp. 97-10l; pre-
print, p. 3.

8231 -10-






RI. 3579

An average figure was used to make a de@uction for shale from the corputed
volume of the reservoir by the end-area method. Shale represented 22.0 percent
of the volume of the Anahuac reservoir.

A composite interpretation of the oil-producing zone cf the Frio sandstone
in the Anahuac field is shown in figure 9. ‘This east-west interpretation was
prepared by averaging the thickness of the gas-oil sand, 011 sand, shale, and
unpenetrated zone (vetween the bottor. of the wells end the oil-water contact),
as reported in 190 wells that were plotted on the north-south cross sections.
In the graphic interpretation the average shale content of the oil-producing
zone penetrated by these wells has been superimposed on the area representing
the penetrated oil zone. The shale occurs in lenses throughout the zone, and
the relative position at which it is show graphically has no significance.

Calculation of Reservoir Yolume

Knowledge of the volumetric content of petroleum reservoirs is essential
in estimating reserves of oil and gis and in evaluating the forces supplying
the energy required to move theoil through the reservoir rocks to the wells.
Therefore, calculation of such volumes of a reservoir is oneof the most important
tasks confronting the engineer who is making a vetroleum-engineering study.

The fluid capacity of a petroleum reservoir is the product cf the area,
the thickness, and the porosity of the producing rocks. The variation in
thickness of the producing stratum, especially in fields dmilar to Anahuac
that have numerous fmlis, renders,difficult computation of the reservoir
volume. The volume of tho Anakuac reservoir was computed from average end-
areas plotted on cross sections and from data suppliel by an isopachous mp;
in each method the results are summations of smaller volumes

Average end-area method

The end-area method of computing volumes is used by civil ongineers in
highway and railroad construction to calculate the volume of earthwork in
cuts or fills. The end-arca forma follows:

v= Gi +4) x a,

2
where
Vv = volume of a solid between planes 1 and 2,
Ay = area of cross section of solid at plane 1,
Ap = area of cross section of solid at plane 2, and
ad” = distance between planes 1 and 2.

Although the forma is only approximately correct, it is recommended for
use in computing the volume of petroleum reservoirs because of its simplicity
and substantial accuracy in nost calculations.

8231 ~16—


C. A. Lord well No. 1
7,047 267 feet
Humble O11 & Refining Co.

TABLE 7
Sample No. 37210

Analmac field
Marginulina sand

RI. 3579

Texas
Chambers County
Section 50

GENERAL CHARACTERISTICS

Specific gravity, 0-837
Sulfur, 0.13 percent

Saybolt Universal -riscosity at: 77°F., 45 sec.
Saybolt Universal viscosity at 100°F., 41 sec.

DISTILIATION, BURFAU OF MINES HEMPEL METHOD

Dry distillation

Temperature, Cut, ‘Sum,. Sp. er.
°c, per-  per- of cut
cent cent
“Up to 50
50 - 75 a
75 - 100. 3,2 3542 0,694,
100: = 125 3.5 §.7 o740
125 ~ 150 ee 10.3 0759
150 = 175 6 1.9 1175
175 ~ 200 5.3 20.2 +738
200 = 225 Tel 218 2802
225 = 250 9.1 36. 2815
250 » 275 11.6 4g.o0 2831
Vacuum distillation at 40 mn,
Up to 200 8.3 8.8 0.843
200 225 9.2 18.0 9852
225 = 250 5-5 23.5 2855
250 ~ 275 5 28.5 237
275 = 300 5.9 38 + 837

Carbon residue of residuum, 5." percent.

Barometer, 745. mm. -

“4. P

72,4
5967
ae
51.1
higy2

45,2.

Neh
38.8

3 ot
36
32,3
3004
22.0

A. P. 1. gravity, 37.6°

Color, brownish-green

First drop, 29°C. (s4°Fr.)

. I. Viscosity Cloud Temperature,
at 100°F, test, oF,
oF,
Up to 122
122 ~ 167
167 = 212
212 - 257
257 ~ 302
302 — 347
347 = 392
392 = 437
437 - luge
4g2 - 527
ho. 15 Up to 392
hg 35 392 ~ 437
62 5 437 ~ 4eo
93 65 482 - 527
170 85 527 - 572

Carbon residue of crude, 0.8 percent.
APPROXIMATES SUMMARY

Percent Sp. gr. Viscosity
Light gasoline 3.2 0,694
Total gasoline and naphtha 20.2 0.757
Kerosene distillate 16.2 2309
Gas ofl 26.1 - she
Nonviscous lubricating distillate 12.2 2 854~, 875 50-100
Medium lubricating distallate Ty 875.892 100-200
Viscous lubricating distillate 0.7 2 892=.89 Above 200
Residuum 15-5 2938
Distillation loss 1.7 -
8231 ~21e


RI, 3579

Optimum control of the reservoir in the Anahuac field can be accom-
plished best by wmiform withdrawal of reservoir fluid, The rate of upward
movement of the oil-water contact should keep pace with the rate of oil with-
drawal, so that each volume of oil will be displaced by an equivalent volume
of water, This procedure should cause forces within the reservoir to remain
in efficient balance, Too rapid withdrawal may cause premature incursion of
water and result in coning and consequent entrapment of oil, or it may cause
the gas cap to expand with an attendant waste of reservoir energy through
dissipation of gas owing to greater gas~oil ratios,

Coring Operations

The oil~bearing formation was cored in virtually all the wells drilled,
The exceptions were a few wells drilled during later development, Both con-
ventional and wireline core barrels were used with approximately the same
degree of success,

Because of poor consolidation of the Frio sandstone, the cores usually
were friable and could not be distinguished from tightly packed sand, It
was possible, however, to identify the shale streaks plainly and to determine
quite accurately whether or not the sandstone sections of the cores contained
oil and gas, .

Electric Logging:

Virtually every well in the field was surveyed by an electric logging
device from a depth of 1,000 feet below the surface of the ground to the
bottom of the well, Graphic electric logs furnished valuable aid for sub-
surface correlation and for indicating the character of the fluids in the
formation, Interpretations of the electric logs agreed closely with the data
obtained by coring, especially in regard to identification of shale strata,
as shown by a typical electric log (fig. 8).

Porosity and Permeability Tests

Eighteen cores of Frio sandstone taken from drilling wells were analyzed
in the Bureau of Mines Laboratory at Bartlesville, Okla., for their porosity8/
and permeability4/, and table 3 presents the results, It will be observed
7/ Schlumberger, C, and M,, and Leonardon, B, G,, Electrical Coring; A

Method of Determining Bottom~Hole Data by Electrical Measurements: Am,

Inst, Min, and Met, Eng, Tech, Pub, 426, 1932, 38 pp.

Deussen, Alexander, and Leonardon, E, G,, Electrical Exploration of Drill

Holes: 4m, Petrol, Inst,, Drilling and Production Practice, 1935,

pp. 47-59. :

8/ Taliaferro, D, B,, Johnson, T, W., and Dewees, B, J., A Method of
Determining Porosity: A List of Porosities of Oil Sands: Bureau of
Mines Rept, of Investigations 3352, 1937, 24 pp.

9/ Johnson, T, W., and Taliaferro, D., B,, Flow of Air and Natural Cas
Through Porous Media: Bureau of Mines Tech, Paper 592, 1938, 55 pp.

8231 -~l-


TABLE 2, — Estimated cost of. drilling and completing 7,100-foot

RoI. 3579

well in Anahyac field

Labor:
Company accomt:
3 drilling crews, 15 days at $133.92 ........606

Building derrick and pipe racks ..
Dismantling derrick and pipe racks ........e-e08

Contract:

Digging cellar, placing foudations, and cement-
ing two strings of casing ..... seen eee neces
Coring equipment ...........e eee eee
Digging slush, reserve, andwater pits .
Electric logging service .............
Backfilling pits ..:..
Total Labor COSt 1... ceesreccereerereeccecere

Material:
New purchases?

One 12-1/4-inch Dit .....ceece cence reeecneceecese
Seven 8-5/-inch bits at $17.65
One 6-3/4-inch bit ............
_ UW coreheads ...........e sc eee
475 sacks cement at 80 cents .....,...6--
* Rig lubrication at $7.57 per day (15 days),
Pump parts at $5 per day (13 days) .......
Total cost of new purchases ............0006

Pipe and appurtenances (through warehouse stock)?

Pipe:
1,080 feet 9-7/S-inch 0, D, casing at $1.73 ....
qT: 28D fees 5-1/2-inch 0. D. casing at 81 cents,.
6, feet 2-inch tubing at 29 cents ..........

420 feet 3-1/2-inch liner at 72 cents.........
30 feet 3-1/2-inch galvanized screen at $3.50

Handling charge on pipe, 5 percent .........+-4-
Total cost of pipe ... ee eee e eee renee neces

8231 ~7T-

$2, 008, 80
350,00
175.00

385,00
143, 83
182,00
503,50

| 125200
3,873.13

19, 60
123,55
18,56
12,00
380,00
113.55
65,00
732.26

1, 816,50
5, 720,50
2,015.50

66,40
105,00
9, 733-90

“486.70
$10,200.66

$3, 873.13


RI. 3579
METHOD OF ESTIMATING NATURAL~GAS RESERVES IN FRIO SANDSTONE

The natural-gas reserve in the Frio sandstone above the gas~oil con~
tact can_be estimated by the porosity~area method, using the following
formula: :

_ 460 + 60)| (Pe) (1 +N.) _ (Pa) ( 14a).
@ = 154560 volte} | Gor ) © “Goo * GH)» "F00),

where Q = cubic feet of gas at a temperature of 60° F, and under a pressure
of 14,4 pounds per square inch absolute;

43,560 = conversion of acres to square feet;

V = net volume of reservoir rock, cubic feet;

s = effective porosity, percent. (expressed as a decimal);
Tf = temperature in producing formation, oF;
Pf = absolute pressure.in formation ata depth of 7,000 feet,

pounds per square inch;

Nf = deviation of gas from Boyle's law at pressure Pf, percent;

it

Pa = absolute abandonment pressure in formation, pounds per square inch;
and

Na

deviation of gas from Boyle's law at pressure Pa, percent,

As previously explained, the outline of the Frio sandstone was extremely
irregular as a result of faulting, hence the volume of the reservoir could
not be calculated accurately by multiplying area ty thickness, Accordingly,
the volume of the gas cap in the Frio sandstone was determined from cross
sections by the end-area method, which also has been described,

The estimated original reservoir pressure in the gas cap in the Anahuac
field was 3,200 pounds per square inch gage, ‘The porosity of the sandstone
. and the reservoir temperature were determined experimentally,

Whe gas in the gas cap was not analyzed, By assuming that it is a dry
gas composed largely of methane the geviations from Boyle's law were esti-
mated from a compressibility curve: for a dry natural gas that contained
27/ This formula was used by Bureau of Mines engineers in making preliminary

estimates of the natural gas in the Pennsylvanian horizon of the

Oklahoma City oil field (Report of Investigations 3217), in estimating

the natural-gas reserves in the Michigan stray sandstone horizon in

central Michigan (Report of Investigations 3313), and in estimating
the natural~gas reserves from the Layton, Oolitic, and Oswego~Prue

horizons in the Oklahoma City oil field (Report of Investigations 3338).
28/ Sage, Bruce, H., and lacey, William .N,, Volumetric and Phase Behavior

of Hydrocarbons: Stanford Univ, Press, 1939, p. 187.

8231 © ~33-



“4,09 4¥ 119 Wharsay “eg wad “14 -ND ‘NoLLMTIOS NI SYD

2000 2400 2800 3200

1600
Pressure, Le. PER Sa. IN. ABSOLUTE,

FIELD, CHAMBERS COUNTY, TEXAS.

1200

800
FIGURE I2.- DIFFERENTIAL GAS - LIBERATION CURVES FOR SUBSURFACE OIL SAMPLE, ANAHUAC

400

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2 8 2 8 8 8 8
DD HBd SWVED ‘11Q GaLVuNIvG-SvO 40 ALISNIG
g 3 g g g g g g
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8 5 he 8
ae a
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IQ Awnaisay 40 ASVAYON| BMATOA INIOUS


“4909 4v 719 Twnaisay “Teg wad 14'ND ‘NOILMIOS NI S¥Q

° 2 2 °
8 8 8 8 8 8 8
R 3 R g 8 g 2
"29 Bad SNVED ‘ID GaLvuNIVG-svO 40 ALISNaG
3 ° 2 3 ° 3 2 2
3 g 8 g 8 3 g g
8 3 8 3 2 Rg = £
- ~ Ee mv eos non 99 sok 98 sunssoad upwares oneas- {Lyf og
y %
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8
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s
8 2 8 &* &$ & 8 S$ ££ BS GF 8 © Fw

IQ Twnaisay 40 aSvaHON| BWANA LNZDUad

Pressure, La. per Sq. IN. AssoLuTe

FIGURE 13. - FLASH GAS-LIBERATION CURVES FOR SUBSURFACE OIL SAMPLE,
ANAHUAC FIELD, CHAMBERS COUNTY, TEXAS.