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1154 EFFECT OF BACK PRESSURE ON WELLSsmall pumping wells in Osage County, Okla., and the results show aconsiderable reduction in the ratio of gas per barrel of oil by producingthe wells against a back pressure. Although Mr. Swigart's work withback pressures is the only information on this subject that has been published, other tests made with back pressures against flowing wells have notonly failed to show a reduction in gas ratio but show an actual increasein this ratio. The Mid-West Refining Co. conducted a series of tests onflowing wells in the Salt Creek field, under the direction of E. L. Estabrook, which showed that straight back pressures did not lower the gasvolume per barrel of oil.T. F. Morgan, in the September, 1923, number of Mining and OilBulletin of California says:

Restriction of oil production has undoubtedly assisted greatly n the conservationof gill , whil its general effect hili been to increlll e the ratio of gaB to oil.This applies to the large flowing wells in the fields of the Los Angeles. basin.

The experiments to be described in this paper confirm the conclusionsof Mr. Morgan and Mr. Estabrook, as applied to small wells. Possiblythe reason for the difference between these results and those of Mr.Swigart is that he was working with pumping wells only, while pumpingand flowing wells are combined in these tests.

BACK PRESSURES

The experiments were carried out in the Brock Field in township 5south, range 1 east, Carter County, Okla., about 9 miles southwest ofArdmore. The field was discovered in January, 1920, and has beendeveloped slowly, now having thirty producing wells. All the productiveleasehold is owned by the Amerada Petroleum Corpn. As the companyoperates a casinghead-gasoline plant in the field, means were availablefor metering the gas from separate wells, separate leases, and the entirefield.The production comes from a series of sands in a shale-and-sand zonefound from 1300 to 2000 ft. The average thickness of the sands is about20 ft. and the average initial production of the wells about 45 bbl. Thesand is fine-grained and compact but not very hard. Only two wellsin the field have been shot; shooting caused the sand to continue cavinginto the hole to such an extent that it required weeks to clean out andno benefits in production were obtained.Back pressures were first applied, to some of the wells, by maintaining15 lb. back pressure on a Ballard trap, the gas from the trap being usedfor drilling purposes. s the back pressure did not affect the productionof the .wells in the trap it was decided to try higher pressures and

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J M LOVEJOY 1155measure both oil production and gas volume. At this time most of thewells were Howing through tubing set with a packer; a few were Howingthrough casing and some were pumping. Each well was then equippedwith aspring release valve that could be set for any pressure desired anda pressure gage was attached to the lead line from each well.The results of the various back pressures used have been recordedfrom Jan. 1 to Sept. 15. The curves Fig. 1, show that during Januaryby building up pressures of 15 lb. and later releasing the back pressureentirely there was little effect on the wells. In the period beginning

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FIG. 1.Feb. 10, with no back pressu:ce, there is a gas ratio of 1160 cu. ft. per bb1.On April 1, with a back pressure of 32 lb. the ratio was 1320 cu. ft. perbbl. On May 6, with no back pressure the lowest ratio of the series oftests was recorded being 1070 cu. ft. per bbl.; the highest ratio is in theperiod beginning July 10, when with a back pressure of 76 lb. 1620 cu. ft.of gas was required to produce a barrel of oil.The higher back pressures shown after June 20, were not appliedpurely in the interests of science but because pro-rating became effectiveat this time and it was necessary to curtail production. .

t is interesting to note the generally higher gasoline content perthousand cubic feet of gas during the last two months of the tests withthe wells producing against about 50 lb. back pressure. Trus may bepartly accounted for by the fact that during the very hot weather gas

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DISCUSSION 1157proper times, so that a continuous operation is possible. The results ofthe tests already made are encouraging and we trust that further workwith this method will show as good results.From Sept. 1 to 20 inclusive,well flowing through tubing and withno back pressure,

Gas passed , 554,500 cu. ft.Oil produced 551 bbl.Average daily oil production. . . . . . . . . . . .. 27.5 bbl.Gas per barrel of oil. 1006 cu. ft.

From Sept. 21 to 25 inclusive, with apparatus attached allowing wellto flow entirely open each time a pressure of 50 lb. was reached,Gas passed , 129,405 cu. ft.Oil produced 155 bbl.Average daily oil production . . ' 31 bbl.Gas per barrel of oil.. . . . . . . . . . . . .. 834 cu. ft.

Not only was the gas volume of oil decreased by 172 cu. ft. per bbl.,but the actual production of the well was increased from 27.5 to 31 bbl.The well would make two flows about 20 min. apart and then wouldnot flow again for nearly 2 hr., although in some cases during the 2-hr.period the well would build up enough pressure to cause the valve torelease but would not be followed by a flow of oil. This well was equippedwith 3-in. tubing. It is thought that even better results will be obtainedby this method from wells equipped with 2-in. tubing.

CONCLUSIONSThe writer hesitates to present concIusions. t is difficult to believethat back pressure on flowing wells is a waste rather than a conservationof natural forces. Yet this is the result of our tests on straight backpressures, although the tests on the stop-cocking method give encouragingresults. t is hoped that more work will he oone along these lines andthat true conservation and consequent methods of increasing the ultimaterecovery will soon be discovered.

DISCUSSIONE. L. ESTABROOK Casper, Wyo. (written discussion).-This subject

is part of the general problem of the efficient utilization of the liftingpower of gas in oil wells, the study of which is likely to lead to increasedrecoveries of oil from sands with greatly decreased costs of production.That the author's results do not coincide with those of other investigatorsillustrates a fundamental difficulty in generalizing on this problem.Every oil well is individual; it varies from every other well in the amountand relative proportions of oil and gas being produced, as well as in rock

Ptltroleum Production Engineer, Midwest Refining Co.

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1158 EFFECT OF BACK PRESSURE ON WELLSpressure, sand conditions, offset wells, etc. The size of the casing ortUbing through which the well is produced, the size of the flow nipple,the location of the packer, the location of the perforations through whichthe oil and gas enter the tubing also affect the results in such tests.The studies that have been made indicate that we need to know muchmore about the various conditions that affect the efficiency of the use ofthe lifting power of the gas in a producing oil well. We need descriptionsof the visible evidences of efficient and inefficient production so thatfield methods may be improved without the necessity of metering the gasor measuring the pressure of every oil well. Probably all will agree that asteady stream of oil completely filling the opening through which it flowsand permitting the escape of no undissolved gas, represents the mostefficient utilization of the gas of a flowing well. This standard may beeasily attained in certain wells but is difficult, or perhaps impossible,witii others in which the natural proportion of gas to oil is higher. Inthe Salt Creek field, some wells produce as little as 400 cu. ft. of gas perbarrel of oil, while in others it has not been possible to reduce the gasvolume below 2000 cu. ft. per barrel.

H. C. GEORGE Ardmore, Okla. (written discussion).-From e x p e r i ~ments with back pressure and stop-cocking wells in the Brock field, Iconclude that the more favorable results secured by stop-cocking, ascompared with constant back pressure, were due to the fact that withstop-cocking the wells were entirely closed n more than half of thetime, so that no gas escaped j when the well was opened, the release ofpressure usually resulted in a flow of oil with the gas. On the otherhand, when the back-pressure valve was set for a given pressure, much ofthe time gas was escaping through the valve without being accompaniedby a flow of oil. By both of these methods, as used at the Brock field,the oil flowed by heads.The use of flow nipples, or the use of tubing of only sufficient internalsectional area to accommodate the oil production by continuous flowingwould have shown a still lower production of gas per barrel of oil thaneither of the methods used. This belief is not based upon any experimentsthat I have made but on the results observed at flowing wells in Pennsylvania where Hi-in. tubing was used, results observed at severallarge wells in the Mid-Continent flowing continuously for a period ofseveral months, and the use of flow nipples in some of the large wellsof California.

In oil-field practice, that size of tubing used in a flowing well invariablyhas been of the size that the operator expected to use later for pumpingthe well. This has been the main consideration in making the i n s t l ~lation of tubing at the flowing well and has in most cases resulted in theuse of tubing too large for the flowing well.

_ Petroleum Engineer, U. S. Bureau of Mines.