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Collaborating Authors
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Common practice in the Mid-Continent until recently was to prolong thenatural flow of oil wells as long as possible by agitation and then swab for ashort period until the well was put on the pump. Comparatively recently thegas-lift has superseded the swabbing and agitating period and in some casesencroached on the natural flowing life, particularly where it was evident thatthe potential oil capacity of the well was greater than the gas supply, orlifting medium. The majority of wells are finished with either 61/4, 6 5/8 or 81/4-in. casingand connected to a separator through a flow line which is usually 6% in. Theflow line is generally designed to eliminate sharp turns and has reduced thegas-oil ratio and therefore increased the efficiency in a good many instances.Until recently all wells were allowed to flow without restriction against thepossible trap pressure until it was deemed advisable to put them on gas-lift.This resulted in a fairly efficient flow during the early life of the well ifthe well was rather large but was usually inefficient in small wells and in thelater flowing life of large wells, on account of the large area in the casingand therefore excessive slippage. Recently, considerable effort has been made to increase the efficiency of smallflowing wells by running tubing to eliminate as much slippage as possible. Withlarge wells there is a considerable trend toward back-pressuring, or beaning tofind out whether it is possible to reduce the gas-oil ratios and increase theflowing life. The greater number of fields that have been back-pressured have been producedin that manner through necessity, either to restrict production for prorationpurposes or, in fields producing by hydraulic pressure, to reduce waterencroaching. The recent Cromwell sand development in Little River is an examplewhere both back-pressuring by means of high-pressure traps and beans andflowing through tubing were used to conserve gas, reduce the high gas-oil ratioand prolong the flowing life.
Intermittent injection of gas in gas-lift pumping is a variation of thecommon practice in that the gas is delivered to the well for ashortperiod, called the "on time," followed by a period during which none isadmitted, called the "off time," instead of being admitted continuouslyat a constant rate or by intermittent flowing in which it is occasionallydelivered to the well in just sufficient quantity to flow off the accumulatedhead of fluid. The term "gas" is used throughout this discussion but itshould be remembered that in so far as the operation is concerned either air ornatural gas may be used. The equipment at the well is the same as commonly used in gas-liftinstallations and gas may be admitted to either the tubing or the casing. Itseldom occurs that all the wells on a lease can be pumped economically byintermittent injection from one header. Wells that have flow characteristicswhich permit their being handled together have their intake lines grouped at acentral compressor station. Well groups may consist of two or more wells. Thetotal volume of gas delivered by each group of compressors goes to one well ata time through a quick opening valve, each well connected to the headerreceiving its charge of gas in turn. Special cases may require a deviation fromthis arrangement. The distribution of the gas to the different wells iseffected by an automatic timing device which operates quick-opening valves onthe intake lines. The timing mechanism may be actuated by electricity, liquidor gas pressure. The essential factor in the operation of the timing mechanismis that as one intake valve is closed another must be opened. Water-Operated Timing Device Timing devices operate the intake valves either directly, or indirectly bypilot valves. Of the former type one that is operated by water and has givensatisfactory service is shown in Fig. 1 and can be made on the lease. Thehorizontal cylinder is a piece of 10-in. casing about 7 ft. long, closed ateach end and free to oscillate about a fulcrum at the center.