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Kansas Geological Survey, Bulletin 38, pt. 12, originally published in 1941


The Otis Gas and Oil Pool, Rush and Barton Counties, Kansas

by Eugene A. Stephenson and John I. Moore

Department of Petroleum Engineering, University of Kansas

small image of the cover of the book; gray paper with black text.

Originally published in 1941 as part of Kansas Geological Survey Bulletin 38, pt. 12. This is, in general, the original text as published. The information has not been updated. An Acrobat PDF version (11 MB) is also available.

Abstract

The Otis gas and oil pool is located in central Kansas, in Rush and Barton counties, and embraces about twenty-three square miles. Production is obtained from the Lamotte sandstone of Late Cambrian age. The reservoir is estimated to have contained originally 207,900 million cubic feet of gas and 5,913,400 barrels of oil, both quantities expressed at standard surface conditions. As of May 1, 1941, a total of 93,100 million cubic feet of gas had been produced, of which 31,900 million cubic feet had been vented to the air in the production of oil, and the testing and blowing of wells; 1,411,386 barrels of oil had been marketed as of the same date.

Two elements of waste exist in the pool: (1) oil production with excessively high gas-oil ratios, and (2) premature decline in the reservoir pressures. Unless the oil is produced prior to the time when the static reservoir pressure becomes so low that operating pressure gradients sufficient to move the oil to the wells efficiently cannot be established, a large part of the otherwise recoverable oil will be left in the reservoir as underground waste.

In order to prevent such waste it is suggested (1) that the oil wells be allowed to produce at a rate sufficient to deplete the reservoir of its recoverable oil content before the withdrawal of too much gas for pipe-line sales, and (2) that the production of gas with the oil be prohibited unless such gas be recompressed and returned to the reservoir. The most feasible plan for such action lies in the unitization either of the entire pool or at least of that portion containing oil.

A tabulated summary of information pertaining to the Otis pool follows:

  1. Area of the Otis pool; gas (includes oil), 14,780 acres; oil, 455 acres; total 14,780 acres.
  2. Percent of area in Otis pool capable of producing gas, 100 per cent; capable of producing oil, 3.09 per cent.
  3. Original gas content of the Otis reservoir, estimated in cubic feet at 60° F. and 16.4 p.s.i. (pounds per square inch), 207,900 million cubic feet.
  4. Gas in reservoir on May 1, 1941, estimated under same conditions as in no. 3, 114,800 million cubic feet.
  5. Total pipe-line sales, as of May 1, 1941, corrected for deviation from ideal gas laws, and computed under same conditions as in no. 3, 61,196 million cubic feet.
  6. Original reservoir pressure at 109° F., 1,178 p.s.i.
  7. Volume of gas (measured at same conditions as in no. 3) required to form one cubic foot of gas at initial reservoir conditions in the Otis pool, 78.07 cubic feet.
  8. Original quantity of oil, estimated under reservoir conditions, 6,045,205 barrels.
  9. Original quantity of oil, estimated as stock tank oil, 5,913,419 barrels.
  10. Total oil produced from the Otis pool, prior to May 1, 1941, 1,411,386 barrels.
  11. Per cent of original quantity of oil in reservoir estimated to be commercially recoverable, 50 per cent.
  12. Amount of future recoverable oil, as of May 1, 1941 (estimated), 1,545,323 barrels.
  13. Reservoir pressure (equals average bottom-hole pressure), as of May 1, 1941, 702 p.s.i.
  14. Thickness of gas pay (estimated average), 30 feet.
  15. Thickness of oil pay (estimated average), 13.7 feet.
  16. Porosity of pay sand (estimated average), 12.5 per cent.
  17. Volume of gas vented with oil production, September 1, 1936, to May 1, 1941 (estimated), 26,100 million cubic feet.
  18. Reservoir volume of gas vented with oil, for same period, expressed as barrels, 59.5 million barrels.
  19. Volume of gas used in operating gas wells, and vented prior to oil production, cumulative to May 1, 1941 (estimated), 5,800 million cubic feet.
  20. Ratio of gas vented to oil recovered during the period of September 1, 1936, to May 1,1941, expressed at standard surface conditions (estimated), 18,750 cubic feet per barrel.

Introduction

Location

The Otis gas and oil pool is situated in west-central Kansas, partly in eastern Rush county and partly in western Barton county (fig. 1). The areal extent of the pool is approximately twenty-three square miles, and its wells are located in secs. 1, 2, 4, 5, 8-16, 21-27, 35, 36, T. 18 S., R. 16 W., and in secs. 15-21, 27-33, T. 18 S., R. 15 W.

Figure 1--Index map of Kansas, showing location of Otis pool.

Map of Kansas; Otis field is in west-central part of state.

Acknowledgments

Valuable assistance in the acquisition of pertinent information was rendered by T. A. Morgan, Director of the Conservation Division of the State Corporation Commission and by J. H. Page, chief gas engineer for the Conservation Division. Special appreciation is expressed to the U.S. Bureau of Mines and the Kansas State Corporation Commission for permission to use and quote from a confidential report entitled "Data Report on the Otis Field", by R. E. Heithecker, C. F. McCarroll, and W. F. Kreuger, to whom acknowledgment is also made. This report was based primarily upon field work done by these men in May and June, 1937, and it was published jointly in mimeographed form by the U.S. Bureau of Mines and the Kansas State Corporation Commission.

Generous cooperation was afforded to us by the Northern Natural Gas Company, by Morgan, Cobb, and Flynn, and by employees of these companies, particularly J. M. Fields, Jr., and John Hanley. Employees of other companies operating in the field gave freely of their time and of such information as they had acquired during their intimate contact with the unique problems of drilling, completion, reconditioning, and production.

The manuscript has been read critically by R. C. Moore, Director of the State Geological Survey, and by R. H. King and T. G. Payne, all of whom have made helpful and constructive suggestions.

Purpose of the report

The first study of the Otis pool by the authors was made in July, 1937, and a second more comprehensive examination was carried out in January, 1941, at the request of the Kansas State Corporation Commission. The pool has also been used as a field laboratory for four years, by students in Petroleum Engineering from the University of Kansas. The information obtained as of January, 1941, was used to forecast reservoir conditions as of May 1, 1941, but during the interim the engineers for the Commission completed the 1941 pressure survey of the pool. The calculations of the present paper have been thoroughly reviewed in the light of the last Commission survey (fig. 17), and some of the original computations have been slightly altered. No fundamental modifications have been necessary, however.

The Otis pool is predominantly a gas pool; however, it yields a small but significant quantity of oil on its southwest flank. Oil was discovered after the pool had produced gas for several years.

The Kansas proration regulations, which distribute the total state production among pools on the basis of the total potential of the wells in each pool, achieve their purpose admirably among most pools of the state, inasmuch as these pools have approximately similar characteristics. The Otis pool, however, differs radically from other Kansas pools. The energy which drives the oil to the well-bore is free gas rather than gas in solution in oil or the more customary water drive. Furthermore, the gas pressure has been steadily depleted by removal of gas for sale to the gas pipelines and by the production of gas along with the oil from oil wells. Considerable expense has been sustained by the oil-producing companies in efforts to lower the gas-oil ratios of their wells, but results have been only temporarily successful. Practically all the gas produced with oil has been reduced to approximately atmospheric pressure in the production process and has then been allowed to escape to the atmosphere.

The purpose of this report is to present all of the available information concerning the Otis pool, so that it may provide a working basis for the development of improved means of operation and regulation.

History of the field

The discovery well in the Otis pool, the Milmac Oil Company No. 1 Eitel Estate well, located in the cen. NW sec. 11, T. 18 S., R. 16 W., was completed on March 26, 1930. The well had an initial open-flow capacity of 16,500,000 cubic feet of gas per day from a total depth of 3,507 feet. The initial rock pressure in this well was not ascertained, but other near-by gas wells completed soon afterward had maximum pressures of 1,064 pounds per square inch (absolute) at the well-head.

Oil was first produced from the Mid-Kansas Oil and Gas Company (Ohio Oil Company) No. 1 J. B. Mohr well, located in the NW sec. 10, T. 18 S., R. 16 W. This well was completed July 20, 1934, and had an initial production of 12,000,000 cubic feet of gas and 153 barrels of oil per day from a depth of 3,536 feet; the rock pressure at the well-head was reported to be 1,010 pounds per square inch. No other oil wells have been completed in the northern portion of the Otis pool. On February 16, 1936, oil was discovered in what is now the southern edge of the pool, by the completion of the Schermerhorn-Winton Company No. 1 Maneth-Brackley well, located in the NE sec. 22, T. 18 S., R. 16 W. This well had an initial production of 700 barrels of oil and 6,000,000 cubic feet of gas per day from a depth of 3,566 feet.

Table 1--Well Performance Data. Schermerhorn-Winton No.2 Maneth-Brackley, SE NE sec. 22, T. 18 S., R. 16 W. (U.S. Bureau of Mines)

Date Test
no.
Choke1
size
inches
Pressure, lbs. per sq. in. gauge Delivery rate per 24 hrs. Gas-oil ratio,
cu. ft. per bbl.
Gravity
Separator Casing Pressure
drop in
casing
Tubing Pressure
drop in
tubing
Oil, gross
bbls. at 60°F.
Gas, M cu. ft.
60°F.
Base 14.4 lbs.
Oil A.P.I.
at 60°F
Gas
Air = 1
May 25, 1987 1 (1) 187 711 118 149 690 36.6 1,225 33,470 34.5 .6652
Average for 8 hours after flowing 11 hours
June 2, 1937 7 7/64 104 727 112 665 174 31.4 1,315 41,900 34.6 .6792
Average for 10 hours after flowing 11 hours
June 3, 1937 8 15/64 106 671 168 593 246 50.7 1,680 33,100 34.9 .6772
Average for 11 hours after flowing 11 hours
June 4, 1937 9 25/64 111 562 277 446 898 94.7 2.130 22,490 35.1 .6882
Average for 11 hours after flowing 13 hours
June 5, 1937 10 35/64 106 470 369 317 522 131 2,400 18,320 35.3 .6672
Average for 10 hours after flowing 12 hours
June 6, 1987 11 2 1/23 57 342 497 145 694 182 2,760 15,160 34.4 .6702
Average for 12 hours after flowing 12 hours
May 26, 1937 2 (4) 134 587 252 658 181 37.4 1,285 34,360 34.7 .6814
Average for 8 hours after flowing 14 hours
May 27, 1937 3 - 150 512 327 628 211 84.3 1,695 20,100 36.2 .6744
Average for 8 hours after flowing 16 hours
May 29, 1937 4 - 135 346 493 360 479 151.7 2,415 15,920 35.2 .68014
Average for 8 hours after flowing 16 hours
May 30, 1937 5 - 132 194 645 266 573 223 2,710 12,150 35.2 .6784
Average for 9 hours after flowing 14 hours
May 31, 1937 6 - 58 91 748 137 702 230 3,090 13,440 34.4 .69054
Average for 8 hours after flowing 16 hours
June 8, 1937       8895   6985   Shut-in pressures (after flow tests)    
1 First tubing flow through 9/32-inch bottom-hole choke set at 3,450 feet.
2 Flowing through 2 1/2-inch tubing.
3 Full open choke.
4 Casing flow; controlled by regulating gate valve on flow line.
5 Highest pressure recorded in tubing was 698 pounds, but in calculating pressure drop in tubing during tests, a value of 839 pounds was used.

Table 2--Well Performance Data. Morgan, Flynn-Cobb No. B-1 Koriel, NW SW sec. 23, T. 18 S., R. 16 W. (U.S. Bureau of Mines). Well producing through casing.

Date Test
no.
Choke1
size
inches
Pressure, lbs. per sq. in. gauge Delivery rate per 24 hrs. Gas-oil ratio,
cu. ft. per bbl.
Gravity
Separator Casing Pressure
drop in
casing
Oil, gross
bbls. at 60°F.
Gas, M cu. ft.
60°F.
Base 14.4 lbs.
Oil A.P.I.
at 60°F
Gas
Air = 1
    0 0 8413 0 0 0 - - -
May 25, 1937 1 1/2 108 647 194 189.8 3,031 15,970 37.2 0.672
Average for 8 hours after flowing 14 hours
May 25, 1937 2 3/4 189 478 363 641.3 6,385 9,960 35.9 0.668
Average for 4 hours after flowing 5 hours
May 26, 1937 3 1-5/8 176 248 593 1,165.7 8,861 7,600 35.0 0.667
Average for 7 hours after flowing 10 hours
May 26, 1937 4 3 89 146 695 1,303.0 10,116 7,750 34.9 0.691
Average for 5 hours after flowing 2 hours
May 27, 1937 52 3./4 124 491 350 590.4 6,524 11,050 35.5 0.671
Average for 5 hours after flowing 10 hours
May 28, 1937 62 1/2 89 661 180 167.8 2,844 16,950 35.3 0.676
Average for 12 hours after flowing 12 hours
1 Approximate opening.
2 Pinching back after wide open flow.
3 Casing pressure recorded June 4, 1937. Pressure before tests 836.

Table 3--Well Performance Data. Morgan, Flynn-Cobb No. B-2 Koriel, SE NE SW sec. 23, T. 18 S., R.16 W. (U.S. Bureau of Mines). Well producing through casing.

Date Test
no.
Choke1
size
inches
Pressure, lbs. per sq. in. gauge Delivery rate per 24 hrs. Gas-oil ratio,
cu. ft. per bbl.
Gravity
Separator Casing Pressure
drop in
casing
Oil, gross
bbls. at 60°F.
Gas, M cu. ft.
60°F.
Base 14.4 lbs.
Oil A.P.I.
at 60°F
Gas
Air = 1
May 28, 1937 1 45/64 67 742 96 44.2 7,590 171,720 37.8 .673
Average for 12 hours after flowing 4 hours
May 29, 1937 2 66/64 96 708 130 58.3 10,290 176,500 37.6 .6695
Average for 11 hours after flowing 12 hours
May 30, 1937 3 85/64 90 539 299 136 19,380 142,500 35.6 .685
Average for 14 hours after flowing 9 hours
May 31, 1937 4 118/64 96 381 459 205.5 26,080 126,910 35.2 .685
Average for 12 hours after flowing 12 hours
June 2, 1937       8382   Shut-in pressures (after flow tests)    
1 Approximate opening.
2 Shut-in pressure, May 27, 1937, 827 lbs. (before flow tests).

Table 4--Well Performance Data. Carter Oil Company No. A-1 Schroeder, NW SW SE sec. 23, T. 18 S., R. 16 W. (U.S. Bureau of Mines). Well producing through casing.

Date Test
no.
Choke1
size
inches
Pressure, lbs. per sq. in. gauge Delivery rate per 24 hrs. Gas-oil ratio,
cu. ft. per bbl.
Gravity
Separator Casing Pressure
drop in
casing
Oil, gross
bbls. at 60°F.
Gas, M cu. ft.
60°F.
Base 14.4 lbs.
Oil A.P.I.
at 60°F
Gas
Air = 1
May 25, 1937 - - - 8442 0          
May 27, 1937 1 - 50 731 113 64.5 10,480 162,480 35.4 .6733
Average for 10 hours after flowing 11 hours
May 28, 1937 - 2 45 618 226 137.2 17,200 125,360 33.8 .672
Average for 10 hours after flowing 13 hours
May 29, 1937 3 - 52 451 393 231.0 24,160 104,590 33.4 .677
Average for 10 hours after flowing 15 hours
May 30, 1937 4 - 64 126 718 361.0 30,540 84,600 32.6 .675
Average for 11 hours after flowing 12 hours
May 30, 1937 5 -   5$ 794 347.0 32,9003 94,800 31.2 .684
Average for 8 hours after flowing 2 hours
1 Flow controlled by pinching casing gate, well not equipped with choke.
2 Shut-in pressure, June 1, 1937, 827 lbs, (after flow tests).
3 Gas measurement by closed pitot-tube method.

Table 5--Well Performance Data. Morgan, Flynn-Cobb No. B-1 Schroeder, SW NW SE, sec. 23, T. 18 S., R. 16W. (U.S. Bureau of Mines). Well producing through casing. Pressures and deliveries not stabilized during 24 hours.

Date Test
no.
Choke1
size
inches
Pressure, lbs. per sq. in. gauge Delivery rate per 24 hrs. Gas-oil ratio,
cu. ft. per bbl.
Gravity
Separator Casing Pressure
drop in
casing
Oil, gross
bbls. at 60°F.
Gas, M cu. ft.
60°F.
Base 14.4 lbs.
Oil A.P.I.
at 60°F
Gas
Air = 1
June 2, 1937 1 20/64 37 726 118 99 3,530 35,660 34.8 .665
Average for 9 hours after flowing 15 hours
June 3,1937 2 45/64 42 712 132 147 6,700 45,580 35.2 .677
Average for 9 hours after flowing 15 hours
June 4, 1937 3 78/64 71 570 274 411 14,060 34,210 34.8 .671
Average for 7 hours after flowing 18 hours
June 5, 1937 4 100/64 68 448 396 605 19,730 32,610 34.6 .669
Average for 10 hours after flowing 13 hours
June 6, 1937       8442   Shut-in pressures (after flow tests)    
1 Approximate opening.
2 Shut-in pressure, June 1, 1937, 827 lbs. (before flow tests).

On May 1, 1941, the pool included a total area of 14,760 acres, classified as proven gas territory; of this 3.09 per cent is productive of both oil and gas and the remainder of gas alone. On May 1, 1941, the field had 53 gas-producing wells and 17 oil wells, from which 58,853,599 cubic feet of gas (measured at 16.4 pounds per square inch and 60° F.) and 1,411,386 barrels of oil had been marketed.

Geologic Features

All the wells of the Otis pool produce from the Lamotte sand or from the "granite wash" where these rocks lie directly upon the top of the granite. The pool seems to represent a structural trap, bounded on the southwest by a topographically high portion of the pre-Cambrian granite surface. Definite proof of this is found in the line of dry holes that define the southwest edge of the pool. These dry holes encountered the granite at considerably less depth than adjacent producing wells. The producing zone rises northeastward to a point in the northeast part of the pool, where the zone is 70 to 80 feet higher than in the southwest portion.

Evidence pertaining to sand thickness is at best somewhat meager. Information from three sources is available-logs from the Kansas Well Log Bureau, records given in the U.S. Bureau of Mines (1938) Report to the Corporation Commission, and the electrical logs of four oil wells. These thicknesses are shown in table 6. In calculating average sand thickness the electrical logs were not utilized due to the fact that the electrical log may not show exact thickness of consolidated rocks; an average was taken of the thicknesses reported by the U.S. Bureau of Mines and the Kansas Well Log Bureau. Thus calculated, the average thickness of the oil pay zone is 13.7 feet and that of the gas pay is 30 feet. Very little information concerning the Lamotte sand is available from well cores. Only three cores in the sand have been taken and analyzed, and of the three wells from which these were obtained, one is unproductive and outside the oil-producing area, another produces only a very small amount of oil, and the third is a well of average production in the oil zone. The core from the non-productive well was not considered. The average porosity of the oil zone was calculated on the basis of the two remaining cores and determined to be 12.5 per cent (table 7).

Table 6--Thickness of the oil-bearing portion of the Lamotte sandstone in the Otis pool.

Well Location, sec., T. S., R. W. Depths of top and bottom of sand and thickness, in feet.
Kansas well
log bureau
U.S. Bur. Mines
(1937), drillers'
logs
Schlumberger logs
(Morgan, Flynn, Cobb)
Thickness
used by
authors
Crowell 2 NE 23-18-16 3541-3555 (14)     14
Koriel A-1 C NW 23-18-16 3525-3540 (15)     15
Koriel A-2 SW NW 23-18-16        
Koriel A-3 C S2 S2 NW 23-18-16        
Koriel A-4 SE SE NW 23-18-16 3527-3538 (11)   3527-3538 (11) 11
J. Schroeder A-1 NW SW SE 23-18-16 3522-3539 (17) 3522-3539 (16)
(less 1 ft. shale)
3541-3543 (2) 16
O. Schroeder B-1 SW NW SE 23-18-16 3523-3538 (15) 3523-3538 (15) 3534-3544 (10) 15
O. Schroeder B-2 NW NW SE 23-18-16 3510-3514 (4)     4
Woodward 1 SE SE SE 23-18-16 3536-3550 (14) 3529-3550 (21)   14
Maneth-Brackley 1 C NE 22-18-16 3548-3566 3548-3566 (18)   18
Maneth-Brackley 2 SE NE 22-18-16 3542-3550 (8) 3542-3550 (8)   8
Koriel B-1 NW SW 23-18-16 3535-3539 (4) 3536-3539 (3)   4
Koriel B-2 SE NE SW, 23-18-16 3533-3549 + 3533-3549 (16)   16
Koriel B-3 C N2 SW 23-18-16 3534-3550 (16) 3533-3550 (17) 3535-3546 (11) 16
Koriel B-4 NE NE SW 23-18-16 (3498-3509) (17)
(3544-3550)
    17
Moore 1 NW NW NW 25-18-16 3534-3556 (22)     22
Brack 2 Location?       11
Average of column     14.0 8.3 13.4
Average thickness = (14.0 + 13.4) / 2 = 13.7

Table 7--Porosity of the oil-bearing sand in the Otis pool.

Well and Location Cored selection,
depth in feet
Oil pay
depth an
thickness,
feet
Average
porosity
of pay,
in feet
Product,
porosity
times
thickness
J. Schroeder A-1
NW SW SE, 23-18-16
(casing set at 3521)
3519-3530 3522-3530 (8) 18.8 150.4
3531-3539 3531-3539 (8) 10.1 80.8
Woodward 1
SW SW SE, 23-18-16
3487-3509      
3523-3550 3536-3550 (14) 10.1 141.4
  Total (30)   372.6
Computation of weighted average porosity-Sum of porosity times: thickness (372.6) divided by total thickness (30) equals weighted average porosity (12.44). Figure for porosity used in computations is 12.5.

Petroleum Engineering

Requisites for engineering study.-The basic information needed for a precise engineering study of an oil field is not available in the case of the Otis pool. The operation of an oil pool on a sound engineering basis is dependent on such comprehensive materials and investigations as the following:

  1. Cores from all wells; the cored section should start a few feet above the top of the pay sand and continue through it.
  2. Measurements of porosity, permeability, and connate water of the core samples.
  3. A bottom-hole sample of the crude oil from the first oil well and from selected other wells.
  4. Laboratory examination of the oil, including determination of (a) solubility of the dissolved gas at various pressures at bottom-hole temperature, (b) composition of the dissolved gas, (c) changes in oil volume that accompany escape of the gas from solution, and (d) viscosity of the oil at reservoir conditions.
  5. Complete bottom-hole pressure and temperature surveys, at periodic intervals, at least twice a year.
  6. Segregation of cuttings and compilation of complete well logs; these should be made available to engineers of the State Corporation Commission and to the State Geological Survey.
  7. Complete and accurate records of all gas produced and utilized, and estimates of all gas lost.
  8. Complete and accurate records of all oil produced, and used, and estimates of all oil lost.
  9. Complete information concerning all casing, cementing, and perforating operations, squeeze jobs, quantities of dry cement and per cent of water used, together with depth at which pipe was set.
  10. Electrical well logs.

The acquisition of materials and information of the above types makes .it possible to calculate the total quantities of oil or gas, or both, in a reservoir even before the precise boundaries of the pool have been ascertained, and thus facilitates determination of the efficiency of the production practices employed.

Fundamental data of the character indicated above are most readily procured when a pool is operated as a unit, a plan which, as already demonstrated, is also more likely to lead to greater ultimate recovery of oil and gas, lower average development cost, and lower operating expense ( as well as increased revenue to the royalty owners) than can be achieved under such practices of development and operation as accompany competitive production.

Bottom-hole pressures

Casing head pressures have been measured at intervals throughout the life of the Otis pool, for the purpose of allocating the gas production. These pressures have been converted to bottom-hole pressure figures (referred to a datum plane of 1,600 feet below sea level) by means of the tables published by the U.S. Bureau of Mines (Rawlins and Schellhart, 1935). The resulting bottom-hole pressure data are plotted on figures 9 to 17 inclusive, and contoured at intervals of 10 pounds per square inch. From these maps the weighted average bottomhole pressure was obtained.

The method outlined by the U.S. Bureau of Mines is based on computation of the weight of the column of gas in the well, which, when added to the casing-head. pressure, gives the bottom-hole pressure. The tables published by the Bureau are based on the premise that ideal gas laws are applicable under the actual pressure conditions. The ideal gas laws are known to be inapplicable at high pressures, however, and therefore the average bottomhole pressure as determined from the contour maps has been corrected for deviation from the ideal gas laws (Brown, 1941).

The bottom-hole pressures of each well as of May 1, 1941, calculated from casing-head pressures and corrected for deviation is shown on figure 17. The weighted average pressure of the field was determined directly from this map.

Table 8--Weighted average casinghead and bottom-hole pressures in the Otis pool, based on field tests reported by the Kansas State Corporation Commission (text figs. 8 to 17). (Pressure expressed as pounds per square inch absolute)

Date Weighted average
casing head
pressure
Weighted average
bottom-hole
pressure,
uncorrected for
deviation
Weighted average
bottom-hole
pressure,
corrected for
deviation
1930 1064.0 1157.0 1178.0
Sept. 1, 1936 955.4 1039.0 1057.0
June 1, 1937 924.0 1000.5 1015.2
Jan. 1, 1938 865.6 941.5 955.2
June 1, 1938 855.6 930.6 943.3
Oct. 1, 1938 827.8 900.4 912.7
May 1, 1939 781.7 850.2 861.2
Oct. 1, 1939 759.9 826.5 837.1
May 1, 1940 682.1 741.9 749.7
May 1, 1941     702.2

Gas production

The pipe-line sales of gas are published monthly by the State Corporation Commission. These figures are based on the volume metered at pipe-line pressures, approximately 350 pounds per square inch, and calculated at a base pressure of 16.4 pounds per square inch and a temperature of 60° F. No correction is made for deviation from the ideal gas laws in the published figures.

The cumulative totals of pipe-line gas sales at dates corresponding to those of the pressure surveys of the field are tabulated in table 9 (column 1), and are presented graphically in figure 6. The appropriate factor required to correct for deviation from the ideal gas laws has been applied to obtain the true volume of the metered gas, which is shown in table 9 (column 2) .

For the purpose of calculating the over-all gas-oil rations, the reserves and other desirable quantities, it was necessary to estimate the amount of gas vented to the air prior to September 1, 1936, but of which no accurate record had been kept. After consultation with production men in the field, and comparison of various figures submitted by them, it seemed reasonable to assume that 3,000 million cubic feet had been vented in efforts to obtain commercial oil from wells that originally produced only small quantities. Estimates of the amount of gas vented during drillingin and well-testing operations (590 million cubic feet) were also available (table 9, column 3) and were utilized in the calculations.

Table 9--Gas production of the Otis pool, as reported periodically by the Kansas State Corporation Commission. (The dates used correspond to those of the various pressure surveys made by the Conservation Division of the Commission. Quantities expressed as million cubic feet).

Date (1) (2) (3) (4) (5) (6) (7)
Sept. 1, 1936 19,625 20,406 590 20,996 3,000 23,996 A-L
June 1, 1937 25,397 26,408 822       A-C
Jan. 1,1938 29,563 30,740 1,125       A-D
June 1, 1938 33,295 34,620 1,739       A-E
Oct. 1, 1938 34,094 35,451 2,037       A-F
May 1, 1939 39,791 41,375 2,188       A-G
Oct. 1,1939 41,568 43,222 2,314       A-H
May 1, 1940 50,342 52,356 2,440       A-J
May 1, 1941 58,854 61,196 2,800       A-W
(1) Cumulative pipe-line sales (measured at 350 p.s.i., absolute, and 60° F., corrected to 16.4 p.s.i., absolute, and 60° F., with deviation factor ignored).
(2) Cumulative pipe-line sales, cor-rected for deviation factor. (3) Estimated operating losses, cumulative.
(4) Pipe-line sales plus operating losses, cumulative. (5) Estimated vented gas (waste).
(6) Total gas removed from reservoir.
(7) Data in column (2) referred to points on fig. 2. The quantities indicated can be calculated by measuring along. the base line of the graph the horizontal distance between the points noted; each large-scale unit represents 10 billion cubic feet of gas.

Oil production

All pipe-line runs of oil are reported monthly by the State Corporation Commission. These have been tabulated (table 10) semi-yearly, and for certain other specific dates utilized in the calculations, and are shown graphically (fig. 7). The figures are based on volumes in the stock tank, corrected to 60° F. Where reservoir displacement was computed, these volumes have also been corrected for expansion to the bottom-hole temperature. The solution of gas in the oil would also appreciably increase the volumes, but inasmuch as the effects of this solution factor on the oil in the Otis pool are unknown, the correction has been omitted from the calculations.

The total oil production from the Otis pool, as of May 1, 1941, is 1,411,386 barrels. The maximum rate of production was reached during the second half of 1938, although the rate for the ensuing six months was nearly the same. The average daily rate of production during the second half of 1938 was 88.6 barrels per well, for an average of 17 wells. At the present time the wells are producing at the statutory limiting rate of 15 barrels per well per day.

Table 10--Oil production of the Otis pool, compiled from reports issued periodically by the Kansas State Corporation Commission. (Quantities expressed as barrels of stock tank oil).

Date Average monthly
production for
semi-annual period
Semi-annual
production
Cumulative
production
Dec. 31, 1934 523 523 523
June 30, 1935 289 1,733 2,256
Dec. 31, 1935 127 760 3,016
June 30, 1936 1,618 9,707 12,723
Sept. 1, 1936     18,894
Dec. 31, 1936 4,857 29,141 41,864
June 30, 1937 12,560 75,361 117,225
Dec. 31, 1937 19,522 117,134 234,359
June 30, 1938 16,687 100,119 334,478
Dec. 31, 1938 45,697 274,184 608,662
May 1, 1939     801,325
June 30, 1939 45,297 271,780 880,442
Dec. 31, 1939 34,882 209,294 1,089,736
May 1, 1940     1,198,216
June 30, 1940 25,271 151,623 1,241,359
Dec. 31, 1940 23,118 138,706 1,380,065
May 1, 1941     1,411,386

Table 11--Monthly oil production of the Otis pool, and volume of gas produced with the oil in 1940, as reported by the Kansas State Corporation Commission. [The figures for gas produced with oil are approximately 45 per cent of the amount computed on the basis of the decline in reservoir pressure. The differences may be due to errors in the measurement of gas, gas retained in solution in the oil, or imperfect separation of gas and oil, or other unknown causes.]

1940 Monthly gas
production,
M cu. ft.
Monthly oil
production,
barrels
Calculated
gas-oil
ratio
Jan. 270,488 33,309 8,120
Feb. 202,651 27,681 7,320
Mar. 172,586 27,039 6,382
Apr. 141,670 20,451 6,927
May 164,446 21,225 7,747
June 201,735 21,918 9,204
July 240,336 26,396 9,105
Aug. 254,658 25,604 10,161
Sept. 244,322 25,627 9,534
Oct. 248,086 28,701 8,644
Nov. 187,072 23,672 7,903
Dec. 54,349 8,706 6,243
Totals 2,382,399 290,329 Average
8,206

Estimates of Reserves

Gas reserves

In the case of gas pools, estimates of recovery must take cognizance of the deviation of gases from ideal behavior, and must also recognize some average well-head pressure which marks the end of economically profitable operating conditions. In order that the effects of different final well-head pressures on the ultimate recovery may be shown, two such abandonment pressures have been assumed for use in the calculations,one of 350 pounds per square inch (gauge) and the other of 50 pounds per square inch (gauge). The volumes of pipe-line gas, gas not recovered from the reservoir and other quantities computed on the basis of these pressures, are compared in detail in tables 13 and 14, with appropriate references to points on a graph (fig. 2).

Table 12--Pipe-line gas sales, operating losses, gas vented and unaccounted for, current gas reserves (all as of May 1, 1941), and original gas content of Otis pool. (Bottom-hole pressure as of May 1, 1941, 702 p.s.i., at 109° F.)

  Million cubic feet of gas Read on fig. 2
(see table 9, note 7)
At 16.4 p.s.i.,
and 60° F
(from fig 2)
At 1178 p.s.i.,
and 109° F
(Column 1 / 78.07)
Pipe-line runs 61,200 784 U-W
Operating losses of gas wells,
plus gas vented prior to
oil production
5,800 74 X-Y
Gas vented with oil and
unaccounted for during period,
Sept. 1, 1936 to May 1, 1941
26,100 334 W-x
Gas present in reservoir,
as of May 1, 1941
114,800 1,471 Y-Z
Total 207,900 2,663 R-M

Table 13--Estimates of recoverable gas in the Otis pool for various operating conditions, based on an assumed abandonment pressure of 350 p.s.i. (gauge at casing-head) or 397 p.s.i. (absolute bottom-hole pressure).

  Million cubic feet of gas Read on fig. 2
(see table 9, note 7)
At 16.4 p.s.i.,
and 60° F
(from fig 2)
At 1178 p.s.i.,
and 109° F
(Column 1 / 78.07)
Original recoverable
gas volume
144,800 1,855 T-M"
Original unrecoverable
gas volume
63,100 808 M"-Z
Total operating losses
of gas wells
9,500 122 N"-M"
Ultimate pipe-line runs
at past rate of venting
86,000 1,102 T-P"
Ultimate pipe-line runs
if no more gas is vented
with oil production
110,700 1,418 T-O"
Remaining recoverable
gas at past rate
of venting
24,800 318 W-P"
Remaining recoverable
gas if no more gas is
vented with oil production
49,500 634 W-O"
Total volume of gas
that would be vented if
past rate of venting
were to be maintained
50,800 651 P"-N"
Gas which will have been
vented with oil production
if venting ceases May 1, 1941
26,100 334 O"-N"

Table 14--Estimates of recoverable gas in the Otis pool for various operating conditions, based on an assumed abandonment pressure of 50 p.s.i. (gauge at casing-head) or 70 p.s.i. (absolute bottom-hole pressure).

  Million cubic feet of gas Read on fig. 2
(see table 9, note 7)
At 16.4 p.s.i.,
and 60° F
(from fig 2)
At 1178 p.s.i.,
and 109° F
(Column 1 / 78.07)
Original recoverable
gas volume
197,500 2,530 S-M'
Original unrecoverable
gas volume
10,400 133 M'-Z
Total operating losses
of gas wells
10,400 133 N'-M'
Ultimate pipe-line runs
at past rate of venting
114,300 1,464 S-P'
Ultimate pipe-line runs
if no more gas is vented
with oil production
161,000 2,062 S-O'
Remaining recoverable
gas at past rate
of venting
53,100 680 W-P'
Remaining recoverable
gas if no more gas is
vented with oil production
99,800 1,278 W-O'
Total volume of gas that would
be vented if past rate of venting
were to be maintained
72,800 932 P'-N'
Gas which will have been vented
with oil production if venting
ceases May 1, 1941
26,100 334 O'-N'
Pipe-line runs as
of Sept. 1, 1936
20,406 261 U-V
Pipe-line runs as
of May 1, 1941
61,196 784 U-W
Gas vented and unaccounted for
between May 1, 1939,
and May 1, 1940
7,700 98  

The estimates of future recoverable gas range from 24,800 million cubic feet (table 13, column 1, line 6) to 99,800 million cubic feet (table 14, column 1, line 7), dependent on operating conditions and on the abandonment pressures used as the economic limit. The first of the above estimates is based on an abandonment pressure of 350 pounds per square inch (gauge) at the well-head and the assumption that venting of gas with oil will continue at the same rate as in the past. The second estimate is based on an abandonment pressure of 50 pounds per square inch (gauge) at the wellhead and the assumption that no gas will be vented after May 1, 1941.

If no water drive exists, the total original quantity of gas in a reservoir may be estimated by plotting graphically the average static bottom-hole pressures of the reservoir, divided by the appropriate deviation factors, against cumulative gas removed at the date the pressure survey was made. This results in a straight line when the points are plotted on cartesian coordinates, the equation for which is as follows.

(P1 / Z1) - (P2 / Z2) = m (Q2 - Qt)

where P = bottom-hole pressure, lbs. per sq. inch
Z = deviation factor, applicable to the specific pressure
Q = quantity of gas reserve, billions of cu. ft. (at 16.4 lbs. per sq. inch and 60° F.)
m = the slope of the line.

or, as designated in figure 2:

(Pa / Za) - (P1 / Z1) = m (Qq + 3,590,000,000 - Qr)

The subscripts refer to the points on figure 2, and 3,590,000,000 represents gas lost, in cubic feet. The total original reserve could then be expressed by the following equation:

Qt = {(Pa / Za) / [(Pa / Za) = (Pa / Za)]} (Qq + 3,590,000,000)

This relation may also be stated as follows: The original or total gas content of the reservoir equals (1) total quantity of gas produced since discovery, multiplied by (2) the original bottomhole pressure (corrected for deviation), divided by (3) the decline in bottom-hole pressure (similarly corrected), during the production of the quantity of gas above.

The line A-L-M of figure 2 is based on the above equations. The quantity indicated along the horizontal scale of the graph, under A-M (or R-M) represents the original content of the pool. A-M' (or S-M') shows the quantity that could have been recovered from the reservoir had it been operated solely as a gas pool to an abandonment pressure of 50 pounds per square inch (gauge) at the well-head. Experience with many other fields has demonstrated that such an abandonment pressure may be reasonably anticipated, although it will be necessary to install additional compressor units in the field in order to boost the pressure at the wells to suitable line pressure. Such installations must precede the decline of working well-head pressures to the pipe-line pressure, which is approximately 350 pounds per square inch.

Figure 2--Graphs used in estimation of gas reserves, Otis pool, May 1, 1941.

Graphs used in estimation of gas reserves, Otis pool.

Several other significant features are portrayed by figure 2 (also table 12) , as follows.

  1. The actual pipe-line sales, corrected for deviation, are represented as of May 1, 1941, by the line U-W, which corresponds to 61,190 million cubic feet.
  2. The quantity of gas utilized in operating the gas wells, plus necessary losses prior to oil production, is represented by the horizontal distance between lines ALYM and ABXN. Since this quantity is cumulative, it gradually increases throughout the life of the pool. On May 1, 1941, it amounted to 6,100 million cubic feet (X-Y) .
  3. The gas vented with oil or otherwise unaccounted for since oil production began, is 26,100 million cubic feet, and is shown by the line W-X.
  4. The gas still present in the reservoir and recoverable, to an abandonment pressure of 50 pounds per square inch at the well-head, provided no gas is vented with oil after May 1, 1941, is 99,800 million cubic feet, as represented by the horizontal distance between points W and O'. If, however, gas is produced with the oil at the average rate that prevailed during the period from 1936 to December, 1940, then the amount of recoverable gas will be reduced to 53,100 million cubic feet, as shown by the horizontal distance from W to P'. The gas that would be lost in the future under such conditions would amount to 46,700 million cubic feet; the latter quantity would be in addition to the 26,100 million cubic feet believed to have been lost prior to May, 1941.
  5. Points B, C, D, E, F, G, H, J, and W, represent respectively the cumulative quantities of gas produced and marketed (corrected for deviation from the ideal gas laws) plotted against the corresponding bottom-hole pressures (divided by the deviation factors), as of Oct. 1, 1936, June 1, 1937, Jan. 1, 1938, and later dates as shown in table 9. Extrapolation of the line B-J to point P", or P', shows the relation of the estimated, future, marketed production to the bottom-hole pressure (corrected for deviation) under similar conditions of waste to those which prevailed up to May 1, 1940. The change in the slope of the line from B-J-P to J-W-O, indicates the wholesome influence of the restrictions which became effective about January 1, 1941, as a result of regulations by the State Corporation Commission. The general effect of these regulations was to increase the quantity of recoverable gas as of May 1, 1941, from 53,100 million to 99,800 million cubic feet. This increase in recoverable gas assumes 50 pounds per square inch as the abandonment pressure, which would probably mark the economic limit for the field. It does not take into consideration the effects of the return to the reservoir of the gas produced with the oil.

In general, the divergence of the two lines B-N' and B-P' represents the continual increase in the proportion of gas lost by venting with oil, as compared with the quantity that could have been produced in the past or might be produced in the future if the operations were based on engineering principles.

In many instances it is essential to know the volume of gas (measured at surface or standard conditions) required to form a cubic foot of gas at the pressure and temperature of the reservoir. This computation can be made readily if the reservoir pressure and temperature are known, together with the composition or the gravity of the gas, so that the deviation from ideal gas laws can be given proper consideration (Brown, 1940).

In the case of the Otis pool, the initial bottom-hole pressure was 1,178 p.s.i., absolute, and the temperature according to a Schlumberger electrical log was 109° F., and the gas had a specific gravity of 0.669, compared with air as unity. The deviation factor under these conditions is 0.841. By following the method of computation outlined by Brown, it is found that 78.07 cubic feet of gas measured at 60° F., and 16.4 p.s.i., would have a volume of one cubic foot under the initial conditions in the reservoir.

Oil reserves

Inasmuch as all the oil wells have been under restricted rates of output for the greater part of their lives, it is not possible to estimate their future production by the use of any of the various types of decline curves. Neither can the dependable methods based on the "material balance" procedures be used, owing to the lack of basic engineering information essential for such computations. Hence it has been necessary to rely on the method based primarily on the porosity and thickness of the sand, together with the areal extent of the oil-bearing portion of the reservoir. A summary of the results obtained by this method is shown in table 15, which also makes use of other data presented in tables 6 and 7.

Table 15--Original oil content of the Otis reservoir, oil produced, and oil recoverable.

  Cu. ft.,1
at 109° F.
Barrels
at 109° F.
Cu. ft.1
at 60° F.
Barrels
at 60° F.
Space originally occupied
by oil in the reservoir
(43,560 x 455 x 13.7 x .125)2
33,941,408 6,045,205 33,201,485 5,913,419
Estimated total amount
recoverable (50 per cent)
16,970,704 3,022,602 16,600,742 2,956,709
Oil production as of May 1, 1941 8,100,963 1,442,839 7,924,362 1,411,386
Estimated remaining amount
recoverable as of May 1, 1941
8,869,741 1,579,763 8,676,380 1,545,323
Oil produced during period
Sept. 1, 1936, to May 1, 1941
7,992,315 1,423,488 7,818,083 1,392,456
Oil produced during period
May 1, 1939, to May 1, 1940
2,278,045 405,736 2,228,384 396,891
1One cubic foot of oil at 109° F., = 0.9782 cu. ft. of oil with an A.P.I. gravity of 35 degrees at 60° F.
(National Standard Petroleum Oil Tables, U.S. Bureau of Standards, Circular C410, p. 43, 1936.)
2 Based on assumptions that (1) no gas is dissolved in the oil, (2) no connate water is present,
and (3) no shrinkage of oil takes place due to escape of gas from solution.

The total quantity of oil originally present in the Otis pool is estimated to have been 5,913,409 barrels, expressed as stock tank oil, of which 50 per cent is considered to be recoverable. Of the latter amount, 1,411,386 barrels had been recovered by May 1, 1941; this leaves 1,545,323 barrels remaining to be recovered. All the above calculations and estimates ignore the presence of connate water in pore spaces of the rock and of dissolved gas in the oil. If either or both of these factors should be taken into consideration the volume of oil expressed as stock tank barrels would be reduced. The estimates of recoverable oil purposely ignore the great difficulty that will accompany efforts to produce it without an inevitable increase in the over-all gas-oil ratios.

Figure 3--Conversion chart showing relation of bottom-hole pressure to bottom-hole pressure divided by the deviation factor, for calculating ordinates of fig. 2.

Conversion chart showing relation of bottom-hole pressure to bottom-hole pressure divided by the deviation factor.

Figure 4--Graph showing relation of bottom-hole pressure to deviation factor.

Graph showing relation of bottom-hole pressure to deviation factor.

Figure 5--Graph showing bottom-hole pressure decline by years (1931-1941).

Graph showing bottom-hole pressure decline by years (1931-1941).

The original volume of gas in the Otis pool occupied 2,663 million cubic feet, at reservoir conditions, and comprised 98.74 per cent of the total reservoir space, while the oil occupied 33,941,000 cubic feet or only 1.26 per cent; therefore the pool must be regarded as a gas pool rather than an oil pool.

It has been established both theoretically and experimentally that where oil and gas are present in the same sand, the difficulties of completing and operating the wells are such that they usually produce excess quantities of gas, even under most careful control. Hence it is necessary to maintain relatively low pressures at the separators; otherwise the wells produce virtually all gas and no oil. Gradually the permeability of the pay diminishes with respect to oil, and at the same time it increases with respect to gas; thus in the fluid produced the proportion of gas tends to increase and the proportion of oil to decrease. Ultimately a point is reached where gas alone is produced, even though 30 to 60 per cent of the total oil originally present still remains in the sand. Long before this point is reached, however, the operation of the wells probably will have become unprofitable, owing to the compression cost of the increased volume of gas that must be handled if the gas is to be returned to the reservoir or sold directly to pipe-lines.

Gas-Oil Ratios

High gas-oil ratios have characterized the oil production of the Otis pool since the date of its discoverey. As a result of most careful testing by engineers of the Bureau of Mines in 1937 (Heithecker, McCarroll and Kreuger, 1937), these ratios were found to have ranged from 7,600 cubic feet per barrel to 176,500 cubic feet per barrel. The results obtained during the tests by the U.S. Bureau of Mines engineers are shown in tables 1 to 5 inclusive, which are reproduced from the report referred to on page 348.

In order to reduce the high gas-oil ratios that had prevailed in the Otis pool, several operators undertook expensive reconditioning operations at the wells. Some degree of success attended these efforts, but it has been necessary to repeat the reconditioning process several times. After these treatments the gas-oil ratios have continued to be extremely high, as determined from well tests, from the pressure decline of the field, and from consideration of the actual quantity of gas marketed and accounted for from the reservoir.

From May 1, 1939 to May 1, 1940, the oil production amounted to 396,891 barrels, and the estimated quantity of gas produced, but not accounted for, was 7,700 million cubic feet, equivalent to a gas-oil ratio of 19,400 cubic feet per barrel, measured at standard conditions (tables 14, 15, and 16). During the first four months of 1940 (table 11) the Corporation Commission figures show that 787,395,000 cubic feet of gas were vented while 108,480 barrels of oil were produced; these volumes represent a gas-oil ratio of 7,256 cubic feet per barrel. The latter in itself is a high gas-oil ratio, but is less than the average quantity computed by us and regarded as unaccounted for during the same four-month period. According to Corporation Commission figures, the average quantity of gas vented during 1940 was 8,206 cubic feet per barrel, whereas the decline in pressure of the reservoir indicates that the gas unaccounted for (including vented gas), or the over-all gas-oil ratio for 1940, was about twice that amount. Within the first four months of 1941, during which time the pool was shut-in to a maximum of 15 barrels per day per well, the gas oil ratio dropped to 3,546. This improvement is clearly shown on figure 2 by the divergence of line J-W towards the right and away from J-P'. Point W represents a higher pressure and a smaller cumulative gas volume than would have resulted if the oil wells had been permitted to operate with such high gas-oil ratios as had prevailed before May 1, 1940.

Table 16--Comparison of gas-oil ratios in the Otis pool at reservoir and at surface conditions, based on a total reservoir volume of 2,696,941,000 cubic feet.

  Original reservoir gas-oil ratio Surface gas-oil ratio
Cu. ft. gas at
1178 p.s.i. and
109° F. to cu. ft.
oil at 109° F.
Cu. ft. gas at
1178 p.s.i. and
109° F. to bbls.
oil at 109° F.
Cu. ft. gas at
16.4 p.s.i. and
60° F. to cu. ft.
oil at 60° F.
Cu. ft. gas at
16.4 p.s.i. and
60° F. to bbls.
oil at 60° F.
Ratio of original gas to
original oil in reservoir
78.46:1 440:1 6,262:1 35,157:1
Ratio of original recoverable
gas to original recoverable oil,
at abandonment pressure of 350 p.s.i.
109.31: 1 614:1 8,723:1 48,973:1
at abandonment pressure of 50 p.s.i. 149.08:1 837:1 11,897:1 66,797:1
Ratio of remaining gas to
remaining oil on May 1, 1941
56.92:1 320:1 4,542:1 25,500:1
Ratio of remaining recoverable gas
to remaining recoverable oil on May 1, 1941,
at abandonment pressure of 350 p.s.i.
71.48:1 401:1 5,705:1 32,032:1
at abandonment pressure of 50 p.s.i. 144.08:1 809:1 11,502:1 64,582:1
Ratio of total gas removed to
total oil recovered during
period from Sept. 1, 1936, to May 1, 1941
110.23:1 619:1 8,812:1 49,478:1
Ratio of gas vented and unaccounted
for to oil recovered from Sept. 1, 1936,
to May 1, 1941
41.79:1 235:1 3,338:1 18,744:1
Ratio of gas vented and unaccounted
for to oil recovered from May 1, 1939,
to April 1, 1940
43.02:1 242:1 3,455:1 19,400:1

A thorough study of the relationship between the volume of gas reported as marketed (fig. 2, B to W) and the quantity that the pressure-decline curve indicates was produced (B to X) shows that since Sept. 1, 1936, soon after the discovery of oil, the average gas-oil ratio has been approximately 18,750 cubic feet per barrel.

Both the decline curve and the application of the average gasoil ratio (18,750 cubic feet per barrel) to the total oil production of the pool (1,411,386 barrels), show that 60,354,000 barrels of reservoir space have been voided by the combined volume of oil produced and gas vented therewith. This means that 12.6 per cent of the original reservoir space has been voided in the production of oil that originally occupied only three-tenths of one per cent of the total reservoir space (table 17).

Table 17--Summary of on and gas data for the Otis pool, expressed as percentage of reservoir volume.

Gas per cent
Reservoir volume occupied originally by gas 98.74
Reservoir volume occupied originally by recoverable gas with 350 p.s.i. as the abandonment pressure (1855/2696.9 x 100) 68.78
Reservoir volume occupied originally by recoverable gas with 50 p.s.i. as the abandonment pressure (2530/2696.9 x 100) 93.81
Reservoir volume occupied originally by gas sold, as of May 1, 1941 (784/2696.9 x 100) 29.44
Reservoir volume occupied originally by gas lost in well operations, as of May 1, 1941, plus gas vented prior to oil production (74/2696.9 x 100) 2.75
Reservoir volume occupied by gas vented and. unaccounted for, during period from Sept. 1, 1936, to May 1, 1941 (334/2696.9 x 100) 12.38
Reservoir volume occupied by all gas produced, as of May 1, 1941 (1192/2696.9 x 100) 44.19
Reservoir volume occupied by gas remaining, as of May 1, 1941 (1471/2696.9 x 100) 54.55
Oil per cent
Reservoir volume occupied originally by total oil 1.26
Reservoir volume occupied originally by recoverable oil (50 per cent of total oil) 0.63
Reservoir volume occupied originally by total oil sold, as of May 1, 1941 0.30
Reservoir volume occupied originally by oil remaining in reservoir and recoverable as of May 1, 1941 0.33
Gas and Oil per cent
Reservoir volume occupied originally by all oil and gas produced as of May 1, 1941 44.50

From an economic standpoint, the excess gas production of 26,100 million cubic feet, measured at standard conditions of 16.4 pounds per square inch and 60° F., is equivalent, at 4 cents per thousand cubic feet, to $1,044,000. First-hand information is lacking as to the selling price of the oil per barrel from the Otis pool; but if the price has been of the order of magnitude of 70 cents per barrel, which has been the case for at least a part of the crude oil, it would seem that more money has been lost in the production of gas with oil than has been received from the sale of the gross barrels produced.

Figure 6--Cumulative pipe-line gas sales to 1941, Otis pool.

Cumulative pipe-line gas sales to 1941, Otis pool.

Figure 7--Cumulative oil production to 1941, Otis pool.

Cumulative oil production to 1941, Otis pool.

Conclusions

It is hereby proposed that an allocation plan applicable in the future to the Otis oil production should include the following objectives:

  1. Elimination from consideration of wells, now classified as oil wells, that are not in condition to produce oil or are producing gas only. This proposal need not be applied if the oil area should be unitized.
  2. The return to the reservoir of all gas produced with the oil, except that needed as fuel for the compressors. The location of the injection wells should be at least one mile from the edge of the oil pool or from the nearest oil well.
  3. Foundation of the allowables, in so far as possible, upon the net volumetric reservoir displacement for which each oil operator is responsible.
  4. Development of a method of allocation that will (a) permit the oil operators to produce as much oil as possible under a method of proration that conforms with the laws of the State of Kansas and with the rules of the State Corporation Commission, and (b) conserve the energy of the reservoir by returning to it all gas produced with the oil.

It should be noted that the volume of gas produced even at the gas-oil ratios which have been used by the Conservation Division of the Corporation Commission is many times the volume of oil produced, where both volumes are expressed at the initial pressure and temperature of the reservoir (table 16). No possible method of allocation can ever fully compensate for the over-production of gas.

The difficulties that attend efforts to produce oil from a reservoir whose contents are virtually 100 per cent gas (expressed at reservoir pressure and temperature) are almost insurmountable under competitive operating conditions.

The value of gas is two-fold: (1) as an essential commodity in modern civilization, and (2) as an exhaustible source of energy for the expulsion of oil. The aims of a true conservation program should be directed toward the realization of both values. It is believed that these objectives can only be achieved in the case of the Otis pool through separate unitization of both the gas and the oil areas. Such unit plans or agreements should have incorporated therein provision for the return to the reservoir of all gas produced with the oil which is not utilized as fuel for the compressor plants.

References

Brown, George Granger, 1941, Deviations of natural gas from ideal gas laws: The Oil Weekly, Dec. 30, vol. 100, no. 4, pp. 30-40, and Jan. 6, vol. 100, no. 5, pp. 26-34.

Heithecker, R. E., McCarroll, C. F., and Krueger, W. F., 1937, Data report on the Otis field: U. S. Bureau of Mines in cooperation with the State Corporation Commission of Kansas.

Rawlins, E. L., and Schellhardt, M. A., 1935, Back pressure data on natural gas wells and their application to production practices: U.S. Bureau of Mines, Monograph 7.

U.S. Bureau of Standards, 1936, National Standard Petroleum Tables: U.S. Bureau of Standards, Circular C. 410, p. 43.

Figure 8--Areal outline map of oil- and gas-producing areas, Otis pool.

Figure 9--Bottom-hole pressure contour map, September 1, 1936.

Bottom-hole pressure contour map, September 1, 1936.

Figure 10--Bottom-hole pressure contour map, June 1, 1937.

Bottom-hole pressure contour map, June 1, 1937.

Figure 11--Bottom-hole pressure contour map, January 1, 1938.

Bottom-hole pressure contour map, January 1, 1938.

Figure 12--Bottom-hole pressure contour map, June 1, 1938.

Bottom-hole pressure contour map, June 1, 1938.

Figure 13--Bottom-hole pressure contour map, October 1, 1938.

Bottom-hole pressure contour map, October 1, 1938.

Figure 14--Bottom-hole pressure contour map, May 1, 1939.

Bottom-hole pressure contour map, May 1, 1939.

Figure 15--Bottom-hole pressure contour map, October 1, 1939.

Bottom-hole pressure contour map, October 1, 1939.

Figure 16--Bottom-hole pressure contour map, May 1, 1940.

Bottom-hole pressure contour map, May 1, 1940.

Figure 17--Bottom-hole pressure contour map, May 1, 1941.

Bottom-hole pressure contour map, May 1, 1941.


Kansas Geological Survey
Placed on web March 9, 2015; originally published November 28, 1941.
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