Chapter I. Air-gas Lift

Principles of the Air-gas Lift as Applied to Oil Production

By H. R. PIERCE* AND JAMES O. LEWIS,t TULSA, OKLA.

(Fort Worth Meeting, October, 1927)

SINCE the sudden revival of the air or gas-lift and its extensive use in the oil fields, many questions have arisen as to principles and as to their application under the conditions actually encountered in the field. Much has been written regarding both theory and practice, especially on the air-lift as applied to lifting water. Many statements have been made as to the use and benefits of the gas-lift, some of which are reasonable but many of which are unreasonable, many specialty makers claiming to have devices which operate in almost miraculous ways and even to gener­ ate energy nature never possessed. These claims are often based upon Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 incomplete or inaccurate data. A review of the literature discloses many valuable articles, but it also discloses the need for setting out more clearly the different practical ends sought, and for working out specific engineering principles upon which to formulate the design and operation of gas-lifts to meet the desired ends within the limitations of field conditions. A check on some of the pub­ lished information has revealed basic errors in the sources of data upon which conclusions have been predicated and has led the writers to doubt the value of a large part of the compiled data. The writers have concluded, therefore, that it will be opportune to direct discussion to the sources of error in the data now being collected, to recommend for consideration by the engineers some methods of collecting and correlating data, and to point out some of the factors relating to the application of the air-lift principle to oil production as dictated by the different economic ends sought, and the limitations of working conditions that may be met. U ntH these several considerations are clearly understood by the engineers, and the unreliable data have been sifted out and dependable data substituted, there seems little chance of evolving satisfactory engineering control for the air-lift. In the following pages no distinction will be made between gas-lift and air-lift, as they are generically the same.

* Petroleum Engineer, Dunn & Lewis. t Petroleum Engineer, Dunn & Lewis. -19 20 AIR-GAS LIFT

THE ENDS SOUGHT The ends sought in applying the air-lift will always be practical and not theoretical, though in arriving at a satisfactory solution of a practical end it is absolutely necessary to have correct guiding principles. With­ out endeavoring to make a complete outline of the ends sought, we will give the few main considerations and some of the practical limitations. In many instances the primary consideration in applying the air-lift has been to increase the daily production to the greatest possible extent, regardless of all else. This is well illustrated at Seminole, where almost the only gage of efficiency, under the highly competitive conditions there, was the increase in daily production. A second reason has been a desire for a cheaper method of lifting the oil-both as to installation and operating costs. A third reason has been that in the very deep and crooked rotary holes which have been drilled in recent years, it was often impractical and Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 sometimes even impossible to use the old deep-well plunger pump. This problem was the chief reason for the revival of the air-lift in California. The fourth purpose has been the use of the air-lift as a means of increasing oil recovery as distinct from oil production. This results in a consideration of the effect of the back-pressure caused by the air-lift upon the expulsion of the oil from the sand as measured by the gas-oil ratio, though as pointed out in the following pages, the ratio of volumes of gas to oil alone is not a correct criterion of measuring recovery efficiency. It is obvious that these four main ends will not always be reconcilable, and that the design and operation of a gas-lift will differ with the different standards of efficiency. Of course, there will not always be a clear dis­ tinction between the ends sought; often there will be a combination of two or more factors, but always the final end will be what the operator thinks the most profitable manner of operation under the existing cir­ cumstances. It will be the problem of the engineer to work out the best method and then to convince the operator that it is the most profitable manner of operation; but to accomplish this the engineer must be well fortified with the data which will enable him to design and operate his apparatus so as to fit the specific need most satisfactorily.

PRACTICAL LIMITATIONS In working out these problems the engineer will be faced usually with very definite practical limitations. For example, tapered tubing will be limited to the size of the casing and it may not be possbile to use the most desirable graduations of flow pipe. Other limitations will be the back-pressure, the occurrence of water with the oil and the emulsibility of the oil with the water, the kind of well equipment already on the ground, the relation of the air-lift to gasoline H. R. PIERCE AND JAMES O. LEWIS 21 extraction, the rock pressure and natural gas volume, whether or not the lift can be made continuous or intermittent, and so forth. The main point is that theory must finally be adapted to the practical needs; the engineers need guiding principles and accurately gathered and analyzed data, which will enable the design of the proper installation with the least delay and the least necessity for experimentation.

THE THEORY OF THE AIR-LIFT

The elements of the air-lift are shown in Fig. 1. One arm of the U­ tube represents the submergence, the other the eduction, or flow tube. Water flows continuously into one arm where it reaches a level that counterbalances both the pressure at point of ejection in the flow tube and the pressure of the aerated column above the injection point, plus all other

pressure losses in the eduction tube. Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 It can be seen at once that the three pressures must be theoretically equal at the point of ejection, the weight of the water in the flow tube plus the friction being equal to the pressure and weight of the air less the fric­ tion in the air pipe and also equal to the weight of the water and air plus the friction in the air pipe and also equal to the weight of the water and air plus the friction in the flow pipe, but as the forces are more kinetic in the flow pipe the effect is a flow in that direction. In operation the water feeds continuously past the injection point and the air as continuously enters the upward-moving column and carries it up and out of the flow pipe. If the water is fed in more rapidly more air will be needed to remove it fast enough to maintain the same level in the submergence tube. Pressure failing, the submergence will be increased which will require a higher air pressure, and if the ingress of water is too rapid, it may overcome the air pressure and volume available and fill up both sides of the U-tube, thus stopping the flow. If the water flow is decreased, the submergence will be decreased, and the pressure at the injection point will be decreased likewise, but if the rate of water feed is dropped too low, the air will not lift the water and may pass alone up the flow pipe, and if the flow of the air is excessive the back-pressure caused by the friction and weight of the air in the pipe may exceed the pressure exerted by the water and cause some of the air to flow up the submergence end of the U-tube. It will thus be seen that the air-lift consists of a balancing and pro­ portioning of parts, pressures and volumes. For a continuous air-lift, the necessary elements are a pressure that will feed in the liquid to be lifted against the lift pressure and an equal pressure to feed in the air or other gas, the liquid pressure confining the gas pressure and directing its flow upward. The volume of the gas together with the pressure must contain the foot pounds of energy to lift the liquid against the frictional resistance, 22 AIR-GAS LIFT and after the wal;lte of energy caused by slippage or drop back of the water in the upflowing column. Thus the air pressures are controlled by the pressure resulting from weight and frictional resistance in the flow tube,

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FIG. I.-THE ELEMENTS OF THE AIR-LIFT. and the volume of the air is controlled by the energy required at that pressure to overcome the weight and friction in the flow pipe plus the quantity of air wasted by reason of slippage. H. R. PIERCE AND JAMES O. LEWIS 23

In a naturally flowing oil well, the oil is fed into the bottom of the hole by means of the gas pressure and there is sufficient volume and pressure of gas to make a natural air-lift from the bottom of the well to the surface, but when the volume or pressure of the gas becomes insufficient, it is necessary to add additional quantities of gas or air from the surface. Fig. 1 is diagrammatic and therefore the flow is shown as a series of separate slugs of water, whereas in fact a well-designed air-lift would show at the bottom of the well a mix­ ture of finely divided bubbles of gas in the water and near the top, droplets of water in the air. Fig. 1 also shows with approximate Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 accuracy the upward expansion of the air and the relative submergence which would be necessary to confine and balance the air inj ec­ tion and flow pressures. The summation of the thickness of the individual slugs will give the total length or weight of the column to which must be added the back-pressure resulting from friction. Fig. 2 shows the principles of the intermit­ tent gas-lift. The fluid accumulates in the bottom of the well in a chamber or in the hole of the well when the pressure is off the sand; air pressure is let into the well, which forces the fluid up into the eduction pipe, past an aperture in the pipe, through which a portion of the air enters and aerates the column above. The flow continues until all of the fluid is out of the well; then time must be given for a new charge of fluid to accumulate. It is apparent (Fig. 1) that if the pres­ sure is not maintained in the U-tube, or if the volume of air is too great, part of the air will flow up the short end of the U-tube, and will lift the water in that direction. This FIG. 2.-PRINCIPLES OF INTER- occurs in an oil well when the rock pressure MITTENT GAB-LIFT. becomes lower than the lifting pressure, the air or gas backing up into the sand and forcing the oil away from the well instead of lifting it out of the well. As a well becomes 24 AIR-GAS LIFT older, this condition is met at some point and it becomes necessary to design and operate the air-lift with lower and lower pressures until -"'----r'-,O I I I I I ---II' I I

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FIG. 3.-INCREASE IN CROSS-SECTION NECESSARY TO MAINTAIN CONSTANT VELOCITY IN UP FLOW OF WATER AND AIR. finally it may be necessary to flow the well intermittently; if the sand is open, a chamber. may be arranged in some practical manner while in H. R. PIERCE AND JAMES O. LEWIS 25

tight sands the well itself often will act satisfactorily as a chamber. Another, and more desirable plan, is to maintain rock pressure by injecting air or gas into the sand in near-by wells. Fig. 1 shows how the gas expands upstream in the pipe. The relative spaces. between the slugs of water show not only the expansion but the relative velocity of the flow of gas and water. To keep the velocity constant the cross-section of the pipe must be increased in proportion to the expansion of the gas. Fig. 3 shows with approximate accuracy the increase in the cross-section of pipe that would be necessary to maintain a constant velocity in the upflow of the water and the air.

ELEMENTS OF MECHANICAL EFFICIENCY The three main elements consuming energy in the eduction of the

liquid are (1) the weight of oil and the distance lifted, (2) the weight of the Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 gas itself and distance lifted, (3) the frictional resistance of the upward­ moving column of oil and gas, and (4) the slippage or dropback of the oil in the upward flowing gas. The first represents the useful work to be done, and, of course, is irreducible; therefore efforts to increase mechanical efficiency must primarily be directed to decreasing friction and slippage. Friction increases with velocity according to well-known laws, but in the air-lift it is difficult to determine a friction constant for a well, and as there is an acceleration of velocity from the bottom to the top of the flow pipe the computation of total friction is an involved problem. This acceleration is graphically shown in Fig. 1, in which the relative velocity is indicated by the distance between the water slugs. The problem of friction is thus to keep down velocity. Analyses of air-lift data by graphic and other means indicate that while friction increases with velocity, the slippage decreases with velocity; thus, where slippage predominates, friction tends to be the minor quan­ tity, and vice versa, so that when we reduce one the other increases and our final problem is in keeping these two factors balanced so that their combined loss is a minimum. The fundamental laws governing the volume of a gas at different pressures, and the work available in gas due to its state, are very simple, but the methods of directing these laws, or rather, the arrangement of our equipment to obtain the greatest benefit, is our complicated problem. The various factors of this problem are: 1. Physical laws governing flow. a. The complicated laws governing the variation of friction of flowing gases and fluids. Although similar, the laws governing the friction of flowing water or oil, and gases, are not identical. b. The fact that the law governing the flow of either changes at some critical velocity. c. The fact that the relative quantities of gas and oil or water, or all three, are variable in any flowing column; that is, the relation of 26 AIR-GA~ LIFT

quantity of gas and oil or gas, oil and watpr in the rising colulIln varies from top to bottom of the eductor tube. d. The change of the friction laws of the flowing constituents at some specific velocity. e. The variation of volume and pressure of the gas, or pistoning, or lightening medium of the column. 2. The fact that slippage varies with nature of gas and fluid to be lifted, with the changes in the quantity relation of gas and fluid, and with the state of the gas. 3. The fact that oil wells are not drilled and cased especially for gas­ lifts, and the rock pressure or submergences are not chosen for efficiency alone, but chiefly to allow the greatest quantity of oil to come into the well. Producing oil by gas-lift is not merely a gas-lift problem; it is insepar­ able from and really a minor part of the oil-production problem, and as a consequence is complicated and involved, because of the limits imposed Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 upon its operation. The gas-lift in its simplest form has never been thoroughly analyzed or reduced to mathematical calculation, therefore we do not hope for a complete or definite mathematical analysis of all its adaptations to oil production whereby all conditions can be reduced to a common denomina­ tor, but we do believe that the combination of experience in various fields on which an intelligent, consistent analysis has been made will do much to eliminate the mistakes so common in the application of air-lift to oil production in new or untried wells or pools. Although there is no doubt that oil can be produced in some instances more efficiently and more cheaply by the air-lift method than by other methods, in many cases a careful analysis at regular periods on the various wells on a lease would lead to more efficient work.

COLLECTING AND ANALYZING EVIDENCE

The data necessary to determine whether a gas-lift would be beneficial in a pool on which the gas-lift has not been tried or experience factors set up are as follows:

A. GENERAL 1. Depth of sand. 2. Nature of sand or producing formation. 3. Whether well is flowing or has ever flowed, and for how long (see B). 4. Size of hole and how cased. 5. Whether oil was produced by gas or water drive, or the relation of the two, as nearly as possible, if both water and gas were used. H. R. PIERCE AND JAMES O. LEWIS 27 6. The initial production and relative rate of decline of oil, gas, and water. 7. Present rate of production as compared to other wells in pool similarly located and drilled.

B. SPECIFIC INFORMATION 1. If the well is flowing. a. Rock pressure of shut-in well. b. Nature of flow; that is, steady or by heads, etc. c. Quantity of oil, gas, and water produced by well against different back-pressures held on the head of the well. Three tests of this nature are desirable, the data to be worked up in the form given in Table 1. 2. If the well is not flowing, the quantity of oil and gas that the sand will deliver should be plotted against the head in Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 feet of fluid above the top of the sand; that is, the oil level should be pumped, swabbed, or bailed down to three or more depths, and the quantity of oil and gas produced from these various places should be tabulated and plotted against pressure head of fluid above the sand. This pressure may be deduced from head and density of column, taking account of gas quantity in the column, gravities of gas, liquids, etc. Table 1 illustrates the authors' method of working up for comparison the data gathered on flowing wells. From such tabulation, compared with experience factors and certain fundamental curves prepared to fit in with the experience factors and data, deductions can be made, indicating the present efficiency relation to a deduced possible efficiency of flow under improved methods and conditions. The advantages then may bc balanced against the expense and time lost in making the needed changes.

TABLE I.-Method of Tabulating Data for Purposes of Comparison

Weill I Well 2

2,860 200 ~: W~r.,~r p~~Yd~~\,j,i: ::::... ::::::::::::::::::::::::::::::::::: 3. Total fluid per day, bbl ...... 2,860 200 4. Total fluid per sec., cu. ft ...... 0.1861 0.0130 5. Gas furnished per day, M ...... 480 650 ~. Natural gas per day, M ...... 1,305 185 1,785 835 8: i~~:l ::: ~:; ~ea:.: ~: it::::::::::::::::::::::::::::::::::::: 20.68 9.67 1~· Total gas-oil ratio, cu. ft. per bbl...... 624 4,175 11' Furnished gas-oil ratio, cu. ft. per bbl...... 168 3,250 . Natural gas-oil ratio, cu. ft. per bbl...... 456 925 g. We!ght total liquid per sec., lb ...... 9.55 0.68 14' Welght total gas per sec., lb ...... 1. 55 0.72 . Total. weight lifted per scc., lb ...... 11.10 1.40 28 AIR-GAS LIFT

TABLE I.-Method of Tabulating Data for Purposes of Comparison.­ (Continued)

Well 1 Well 2

15. Depth of tubing, feet...... 4,216 4,108 16. Submergence, feet...... 1,340 264 17. Lift, feet...... 2,876 3,844 18. Velocity at top, ft'/.ec. 1/2...... 114.0 41.4 19. Velocity at bottom, ft./sec. 1/1...... 8.44 7.42 20. Difference in velocities (V, - VI)...... 105.56 33.98 ------1------21. Pt (abs.) ...... 499.4 126.4 22. PI corrected (abe.) bottom well ...... 493.0 109.7 23. p. corrected (abe.) ...... 28.4 19.4 PI 24. Rlp...... 34.23 7.62 25. R2~: ...... 17.36 5.66

26. Total work in gas through RI, ft.-Ib./sec ...... 151,500 40,700 121,750 34,780 27. Total work in gas through R" ft.-lb./sec ...... Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 28. Total work lifting liquid + gas + acc., ft.-lb./sec ...... 33,834 5,707 2\). Total work lifting liquid, ft.-lb./sec ...... 27,450 2,t114 :~O. Total work lifting gas, ft.-lb./sec ...... 4,460 2,76S :n. Total work lifting acc. liquid + gas, ft.-lh./s,'" ...... 1,\)24 2,; :l2. Total work unaccounted for, ft.-lb./sec ...... 87,916 29,373 , , Work lifting liquid 33. EI = Work In gas through R;' per cent ...... 22.60 7.5;1 Work lifting gas 34. E, =Work in gasthroughR--;' per cent ...... 3.67 7.97 r: _ Work of acceleration 3". E, - 'W'ork-lil-£as-ihrough R, per cent ...... 1.58 .07 Work lifting liquid + gae + acc. 36. E, =--wo;'kiiigasihrouii11R~---' per cent •...... 27.85 15.57 Unaccounted for work 37. Wi>rkin gaS"tilfi>ugT,R, per cent ...... 15.72 84.43

PRESSURE AND VOLUME OF AIR Extensive experiments with the air-lift.in raising water have sh-own that for each depth, size of pipe, and quantity of fluid to be lifted there are a certain pressure and volume which will give the greatest efficiency of lift. If the pressure is exceeded, less air volume will be used, but the total horsepower will be increased, and similarly, if a lesser pressure is used, more air is needed, and the total horsepower consumed is greater. The results of these experiments disclose a curve of the nature shown in Fig. 4, which is taken from a paper by Davis and Weidner,! the sub­ mergence corresponding to relative pressure. An analogous curve can be made for the volume of air. There is a high point of efficiency with volumes and either more or less air will prove less efficient. If the volume becomes too great, not only will this relative efficiency be decreased but the actual quantity of the fluid lifted will be less because of the excessive energy consumed in lifting and overcoming the friction of the air itself.

1 G. J. Davis and C. R. Weidner: An Investigation of the Air-lift Pump. Bull. 667, Univ. of Wisconsin (1914), 86. (See Fig. 24.) H. R. PIERCE AND JAMES O. LEWIS 29 For the maximum efficiency at each depth of well, the percentage of submergence or the relative pressure to the height of the lift is greatest in the shallow wells and becomes less and less with increasing depth of the well. The maximum efficiency obtainable likewise becomes very much less with deep wells. Thus, in Fig. 4, for wells of increasing depths there would be a series of similar curves of less output or efficiency, and with the point of maximum efficiency shifting to the left, thus showing decreased percentage of submergence or relative pressure. In the oil fields the pressure and volume used will result not only from the considerations of the maximum efficiency obtainable but also

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FIG. 4.-lb;LATJON OF OUTPUT TO PERC~;NTAGJ<; OF ;;UIJM~;Hm;NCI". from consideration of the ends desired and the practical limitations; for example, if it is desired to get the maximum daily production, efficiency would be given little weight and all efforts would be directed toward increasing the rapidity of flow to the point where the increasing friction placed a limit on the capacities of the eduction pipe. In another case it might be desired to hold a considerable back-pressure on the well which might exceed the pressure of maximum efficiency, and in still other cases the declining rock pressure will necessitate the use of flowing pressures much below the air pressure of maximum efficiency. In fact, in each well the use of the air-lift goes through a series of changes, often starting above the pressure of maximum efficiency and finally ending at a pressure so far below the most efficient that an air-lift becomes unusable as a continuous process. 30 AIR-GAS LIFT Depth of the well obviously has a relation to both volume and pres­ sure, inasmueh as more foot-pounds of work will be needed to lift the fluid the greater height, and also as frietion and velocity will necessarily increase with the longer flow pipe. This results in the need for greater pressure and volume with depth and in a lower and lower percentage of efficiency as measured by the delivery of useful work. Size of pipe also has an influence on the pressure and volume of air needed. A pipe of large cross-section relative to the amount of fluid to be lifted will reduce friction, but increase slippage and necessitate excess volume of air. On the other hand, too small a pipe will cause excessive friction and result in high pressures with low efficiency. It may thus be seen that the pressures and volumes for maximum efficiency relate to the height of the lift and to the cross-section of the pipe relative to the volume being lifted. For each condition of depth and cross-section of pipe and relative quantity of fluid to be lifted, there Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 is a certain relation of volume and pressure that will give the greatest efficiency but the engineer applying the air-lift in the oil fields should know not only how to estimate the point of greatest efficiency, but also how to best meet other conditions such as low rock pressure, or how to get the greatest daily production from a large well, if that is desired.

USE OF TAPERED TUBING The use of tubing tapered downward is based upon the fact that as the gas approaches the top of the well the pressure becomes less; the gas expands and flows with increasing rapidity, thus greatly increasing the velocity. This is shown diagrammatically in Fig. 1, which represents a comparatively shallow well, the relative velocity being indicated by the space between the water slugs. In deep wells this velocity accelerates so mueh that the friction becomes enormous. At the bottom of the well the upward flow of the oil and gas is relatively slow, and probably much too slow for efficiency, considered from the standpoint of slippage. On the other hand, near the top of the well the flow is much too fast, considered from the standpoint of friction; therefore, the need is for a design which in effect will increase velocity at the bottom of a well and decrease velocity towards the top. These considerations, long reeognized, have led to the design of tapered tubing, which has been applied for many years in air-lifting water. The factors to be considered here are the practical design of tapered tubing under conditions met in the oil fields. To design a tube so that there would be a constant velocity from top to bottom would mean that the cross-section of the tube should be increased upwardly in proportion to the expansion of the gas, as shown diagrammatically in Fig. 3, but this is obviously impossible in deep oil wells, and in fact would theoretically not be efficient because it would H. R. PIERCE AND JAMES O. LEWIB 31 cause extreme slippage. Therefore the design must be a compromise between slippage and friction, and the downward tapering of the tubing must be based on the intermediate factor, making the most efficient design one with an increasing velocity upward but not so great as in a straight pipe. The use of tapered tubing, especially in a deep well, is necessarily restricted to the size of the casing. To design a tubing in the correct proportions from the limit of size at the top would make it far too small at the bottom of the well. As a matter of fact, in a well of considerable capacity, tapering can be used only by sacrifice of daily production, and sometimes of actual efficiency, as a smaller size of tapered tubing would so restrict the cross-section of the pipe as to unduly limit the flow and create high frictional resistance that would result in a high back-pressure. Therefore it may not be practical to use any tapered tubing early in the life of the field when the conditions are competitive.

As the volume to be handled grows smaller, it will become possible Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 to use tapered tubing effectively, which will result in a greater mechanical efficiency, if the tubing is properly designed, and also lower back-pres­ sures at the bottom of the well. In deciding on the taper, the engineer will have three problems: (1) the practical limitation of the largest size that can be used at the top of the well; (2) the smallest' size that can be used at the bottom of the well to handle the volume of fluid without undue frictional resistance, curtailment of production, or back-pressure; and (3) the proper proportion­ ing of the pipe between the maximum size at the top and the minimum size at the bottom. For efficient operation the graduations cannot be made at random, because there should be a mathematical relation between the lengths of the different sizes which will evolve the best compromise between the factors of slippage and velocity. There should be a gradual taper, not an abrupt change from one size to another, in order to minimize sudden changes in velocity. If the use of tapered tubing proves of sufficient importance, it would be possible in many instances to case the well with a string of pipe having larger sizes near the top, in order to take advantage of this principle in deep wells, where the need for it is greatest.

SURFACE FLOW LINES As the velocity towards the top of the well increases, the factor of frictional resistance is of even greater moment at the surface of the ground than in the well itself. Shaw has called attention to surface equipment deSigned to reduce frictional resistance to a minimum, and incidentally to reduce the back-pressure at the top of the flow column in the well. This is applying correct and acceptable practices which have been used in the lifting of water, where the equipment for separation of air and water has received close attention. The value of reducing frictional resistance 32 AIR-GAS LIFT at the top of the well by having large flow lines and separating the gas from the oil as close to the well as possible is exceedingly important, as can be shown by theoretical considerations backed by experience; for example, if the flowing pressure is 20 atm., and the pressure at top of the flow column in one case is 4 atm. and in the other case 2 atm., the expansion in the first instance is 5 times and in the second instance 10 times, with a resultant of 1.43 times the energy delivered. Of course, the useful work delivered is not increased 1.43 times, because of the increased friction caused by the increased velocity, but the over-all efficiency in almost every case will be increased considerably by keeping down the top pressures by proper design of surface flow equipment. The reduction in flow pressure at the bottom of the well is likewise reduced to an important extent.

OTHER FACTORS RELATING TO EFFICIENCY Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 There are many other factors of the efficiency of the air-lift of which space will not permit more than mention. These include size of air and flow pipe, designs of footpieees, air injectors, the relative merits of annular and tubular flowing, the solubilities of air and gas and their relative advantages, the use of booster holes, intermittent air-lifts, and many others. The air pipe can always be smaller than the flow pipe, yet there can be a surprising loss in pressure in small air pipes in deep wells which should be guarded against. To handle much fluid the flow pipe should be large, but if there is little fluid the slippage losses will be excessive, and in fact the air-lift may be inoperative. Where the fluid becomes small, tapered tubing should be used. Whether to use the central pipe as the air or flow pipe is mostly a practical matter as to the quantity of fluid to be flowed, the corrosion problems, and so on. Footpieces have been elaborately investigated and found to have but small influence on effi­ ciency. The values of different designs of air injectors have also been exaggerated. Too few reliable and comparative data have come to the writers' attention as to the relative merits of gas and air to warrant the expression of any opinion. The intermittent air-lifts involve practical differences rather than theoretical differences.

GAS-OIL RATIO There has been much interest displayed as to the effect of the air-lift upon oil recovery. The evidence considered has consisted of curves showing the decline before and after the application of the lift and data showing the difference between the gas-oil ratios before and after use of the lift. Evidence and opinions have differed, and a general confusion is apparent as to both principles and results. The misunderstanding as to the use and significance of the gas-oil ratio is especially notable, therefore H. R. PIERCE AND JAMES O. LEWIS 33 it has been deemed advisable to set out what the writers believe to be the correct principles and use of this factor. Some years ago the junior author3 suggested the use of the volume of gas compared to the volume of oil as the measure of recovery efficiency, which since has been commonly termed the" gas-oil ratio" or" gas factor," but in applying this principle some of the fundamental elements have apparently been entirely overlooked, one of which is that the gas-oil ratio is a measure of relative efficiency only where other conditions are the same, and, secondly, that efficiency finally comes down to a matter of both pressure and volume of gas; that is, to the energy contained in the com­ pressed gas which expels the oil. In the voluminous discussions of gas­ oil ratios, we have seen no consideration of the fundamental principle, which is that the oil represents the useful work done by the energy con­ tained by the gas, measured by volume and pressure in terms of horse­ power or foot-pounds of work. Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 As a practical thing, the gas-oil ratio, as measured in volumes, can be used when limited to conditions otherwise comparable, which is in measuring efficiency of methods of operation on the same well at about the same time. Where, however, the gas-oil ratio is used to compare the efficiency of a process as applied to two different fields, 'two different properties, or even two different wells, it does not take into account the differences in the energy represented by a given volume of gas. Under such circumstances, therefore, it is essential that the pressure factor be taken into account and that the relative efficiencies be represented on a true comparative basis-that is, in foot-pounds of work-but in compar­ ing the efficiencies in recovery between different wells, pools, or properties, it must be borne in mind that the efficiency of recovery is mostly deter­ mined by underground conditions, and that in measuring the efficiency ()f recovery at a well, one measures primarily the frictional resistance and slippage in the forcing of the oil through the sand to the well, and second­ arily, the effect of the manner of operation of the well upon these recovery factors. Distinction should be kept between the energy used in over­ coming natural conditions and the relative efficiency induced by manner of operation, the former being mostly uncontrollable by the operator. There is also much confusion among engineers and operators as to the function of the gas-oil ratio. The opinion seems to be prevalent that the gas-oil ratio is a cause of efficiency, whereas it is only a measure of effi­ ciency, just as the thermometer if! a measure of a change in temperature but does not itself cause a change in temperature. The value of the gas-oil ratio is that it provides a simple and practicable means of measur­ ing the sum of the results from the complex relation of many factors underground and in the well, which include the pressure, nature, and

3 James O. Lewis: Bull. 148, U. S. Bur. of Mines (1916) 118. 34 AIR-GAS LIFT volume of the gas, the distance the gas and oil hllve to travel to the hole, the resistance of movement of the oil through the sand by reason of f;:iction, viscosity, adhesiveness to the sand grains, and other factors which consume or waste energy. Inasmuch as the gas is a means for forcing the oil out of the sand, we are interested in so applying the means as to get a maximum quantity of useful work from it, this useful work being represented by the oil delivered to the tanks. In the sense as outlined here, the use of the gas­ oil ratio, which more correctly could be termed the gas-pressure-oil ratio, is as fundamentally correct as a similar use in measuring the effi­ ciency of the gas-lift alone or in a steam engine, gas engine, or any other mechanical contrivance deriving its energy from an expanding gas. We believe the question as to its utility as a measure of oil efficiency has come about through a misunderstanding of its fundamental principles and the resulting misapplication and confusion as to its utility. Until Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 engineers clearly understand and correctly apply these principles the confusion will persist.

DETERMINING FACTOR OF EFFICIENCY OF PRODUCTION

In order to point out the fallacy of basing oil-recovery efficiency on gas-oil ratios only, we will work out two specific problems, representin/!: approximately the conditions existing in two pools, taking one well from each pool. Well A is 4000 ft. deep and has 1200 lb. absolute rock pressure; it is flowing with 200 lb. absolute pressure at the sand face and 15 lb. absolute pressure at the well head. It is producing 3000 bbl. of oil per day with 1,500,000 cu. ft of gas of a gravity of 1. Well B is 2000 ft. deep and has 325 lb. absolute rock pressure in the sand; it is flowing with 122 lb. absolute pressure at the sand face and 33 lb. absolute back-pressure at the well head. It is producing 600 bbl. of oil with 600,000 cu. ft. of gas per day of a gravity of 1. In case A there is required 500 ft. of gas to produce 1 bbl. of oil from the sand, and in case B, there is required 1000 ft. of /!:as to produce 1 bbl. of oil from the sand. Assuming isothermal work by the gas

CASE A CASE B Number of foot-pounds required to expel oil from sand...... 5,560,000,000 1,110,000,000 Foot-pounds per barrel...... 1,85.1,000 1,855,000 (Values are approximate-worked out by lO-in. slide rule.) That is, the actual energy used to expel 1 bbl. of oil from the sand is practically the same in A as it is in B, even though twice the number of cubic feet of gas per barrel of oil was used in B. H. ~. PIERCE AND JAMES O. LEWIS 35 Still assuming isothemal work by the gas CASE B Foot-pounds (approximate) of work in gas in expanding from sand to surface...... 2,680,000 2,680,000 but this quantity of gas or energy was not sufficient to keep the wells flowing steadily under the above conditions, and it was in each case necessary to supply additional air, as follows: CASE A CASE B Additional gas supplied per day, cu. ft ...... 1,500,000 600,000 Total gas used per day, cu. ft ...... 3,000,000 1,200,000 Therefore: CASE A CASE B Foot-pounds (approximate) of work used in lifting oil...... 16,000,000,000 3,200,000,000 Foot-pounds (approximate) per bbl...... 5,360,000 5,360,000

Hence: Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 CASE A CASE B Efficiency of lifting oil only, gas assuming isother- mal expansion, with oil of 86 sp. gr...... 22.3 per cent. 11 .15 per cent.

ESTIMATING COMPRESSOR CAPACITY AND HORSEPOWER OF DRIVING UNIT

The following problem is given to show the size of compressor and engine required to furnish the additional gas necessary to flow the two representative wells in the previous example. Assuming adiabatic compression, and allowing for loss in friction and slippage in the compressor transmission and mechanical efficiency of the machine CASE A CASEB Horsepower of gas engine to pull compressor to deliver necessary air...... 335 112 Assuming 14 per cent. thermal efficiency and a gas of 1200 B.t.u. per cu. ft. CASE A CASE B Cu. ft. per day required as fuel for gas engine.. . .. 129,000 41,000

ENERGY IN TERMS OF HORSEPOWER

If it were necessary to furnish all the energy used by these two wells in the expulsion of the oil from tha sand and raising it to the surface, we would have to install units as follows: CASE A CASE B Unit required, hp ...... 1214 287 Per barrel of oil, hp ...... 0.405 0.478 Table 2 gives a complete analysis of the energy distribution of these two wells. 36 AIR-GAS LIFT

TABLE 2.-Complete Analysis of Energy Distribution from Wells A and B

A B

Well depth, ft. from surface. .... 4,000 2,000 Back-pressure or pressure in the sand, lb. per sq. in. abs...... 1,200 325 Back-pressure against face of sand or pressure at bottom of well, lb. per sq. in. abs...... 200 122 Discharge pressure or pressure at well mouth, lb. per sq. in. abs...... 15 33 PRODUCTION Oil daily, sp. gr.. 86, bbl...... 3,000 600 Per Bbi. of II Total Horse- i Horsepower Gas with oil from the sand, Oil : power 1 per Bbl. of Oil M cu. ft ...... 1,500 600 Additional gas added at bot­ I I tom to cause well to flow, ----~---I------!------c- M cu. ft ...... 1,500 600 Total gas necessary to flow oil from bottom of well to sur- B face, M cu. ft...... ~()()O___ _ 1,200 _~_Bl~I_BJ--,-J ENE}RGY ISOTHERMAL EXPAN- i I I I 1 I Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021

5,573 1,160 [I Ti:~~fl._l~l.~~~~.~~~~:.~~l~-.I 857I 117 3 I 0.039J 0.0406 To raise oil from bottom of 1. 11. 934 1 . 24.4 [' well to surface, million ft.- lb ...... !_1_6_,1_0_0_ 1_3_,_25_7__ 5_._37_0 _5_._42_5_ 338. 5 !~1 0.1130 0.1140 1 Total energy required to expel I 1 I I oil from sand and lift it to I surface, million ft.-lb ...... 21,673 4,417 7.2271 7.359 455.8 I 92.8 . 0.1520 0.1546 WOR:"RACTUALLY ACCOUNTED II II I Moving oil through sand to well bore, million ft.-Ib. . . . .? ? ? ? I Lifting oil to surface from bot- tom of well million ft.-lb. . . 3,610 361 1. 203 602 76. 01~.Jl.:.Q~53_ 0.1275 EFFICIENCY OF I I Expulsion of oil from sand, I II,I E&~ie~~~\)f·liiti~g·~; fi;';';;;'i? ? I ? [! Ii oil from bottom of well to 11.08 !.22.41 Ii .-~s~u~rf~a~c~e~,~p~er~ce~n~t~.~.~..~.~.~.~ ..~.~.~~2=2~.~4~1~~ 11.08 ii 22.41 11.08,22.41 11.08 ADI~~~~~!~~ LB·C~M!~~!I~~ I SHOULD ALL THE ENERGY I BE FURNISHED BY A COM­ PRESSOR TO I :[ Expel oil from sand...... 6,360 1,314 I 2.120 2.190'!133.71 27.6. 0.0444: 0.041; Lift oil to Burface, assuming I ! I ' 14.4 lb. intake ...... 19,900 .1,190 i 6.640 8.650,419.01109.0 10.1395.0.182 Furnish total energy for ex- pulsion and lifting ...... " 26,260 ::-1~--::;!i~1136.61 0.183r:228 ADDITIONAL WORK ADDED BY ADIABATIC COMPRESSION IN Ii ---- o-o]~--- FURNISHING Additional gas necessary to cause well to flow...... 9,950 2,595 3.320 4.325Ii209.5 54.6 I 0.0!11

Horsepower = 33,000 ft.-lb. per mm. All values were worked out on a slide rule and are approximate, but are accurate enough for all practical purposes. Gas quantities are all reduced to quantity at 14.4 lb. and 60° F. All energy units arc based on these conditions also.

RELATION OF AIR-LIFT TO OIL-RECOVERY EFFICIENCY

It has been.necessary to divert from the subject in hand in order to prepare the ground for a discussion of the relation of the air-lift to effi­ ciency in oil recovery. There have been some statements of mysterious effects of the air-lift on recovery as measured by gas-oil ratios and there H. R. PIERCE AND JAMES O. LEWIS 37 have also been statements that the air-lift has been detrimental as measured by the same criteria. So far as the writers can see, the air-lift has two effects or. the effi­ ciency of oil recovery: one, by reason of changes of back-pressure against the sand, and the other by reason of its effect upon the condition at the bottom of the hole. The last factor is comparatively of less importance, and therefore will be covered by stating that the gas-lift tends to remove the sand, mud, and other matter from the hole, thus keeping the hole clean. However, it can also be so applied as to paraffin up a sand and thus harm the well. It seems doubtful whether the gas-lift has any important effect on recovery other than the back-pressure applied against the face of the sand; therefore, the discussion of the effeet of the air-lift on efficiency of oil recovery comes back to the same controversial question as to the man­ ner of operation and utility of the back-pressures. Without presenting Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 evidence or analytical reasons for their opinion, the writers wish to say that they believe that the back-pressure will be beneficial or not wholly in accordance with the degree of pressure applied, and the manner of applying it in relation to the local conditions. When the junior author discussed this subject in Bulletin No. 148, of the Bureau of Mines, he called attention to the fact that too high a back-pressure would be detri­ mental, but, on the other hand, there was much evidence that at some point there would be a back-pressure for each condition which would be beneficial just as there has been shown to be an optimum submergence or confining pressure for the air-lift with relation to the other con­ current conditions. A high back-pressure prohibits the full expansion of gas and thus lessens the amount of energy delivered by it. On the other hand, if the gas is allowed to expand to its maximum extent, the additional work delivered will be largely consumed in greater velocity and, therefore, greater friction, so that the additional useful work theoretically available will be small. Furthermore, there is good reason to believe that the higher velocities created will result in greater slippage of the gas through the sand. Also, Henry L. Doherty has demonstrated that taking the gas out of solu'tion from the oil will have an important and seemingly detri­ mental effect on the physical properties of the oil. These lines of evidence are reasoned out by a .careful consideration of the results from applying back-pressure both under artificial pressure and under natural pressure. The seemingly irreconcilable results reported by various observers of back-pressure both when used with air-lifts and else­ where can be explained by the fact that account must be taken of the limited range in which back-pressure can be advantageously used, which, if either less or more, will be harmful. 38 AIR-GAS LIFT With respect to the term "back-pressure" there has been a similar confusion of thought as with the term" gas-oil ratio." The virtue of the pressure held against a well is in the change in differential pressure created between the pressures in the sand and in the well. It is this differential pressure that causes the movement of the oil and gas, and that is important. We would have clearer thoughts on the subject if we thought in terms of differential pressure rather than in terms of back-pressure. That the high differential pressures between the oil sand and the pres­ sure at the bottom of the hole are largely used up otherwise than in recovery efficiency may be inferred from observational data on the rela­ tion of the gas-oil ratio to declining production. At first thought it would seem that, because there was less energy in each thousand feet of gas when the pressure declined, the volume of gas necessary to move each barrel of oil would be increased greatly. A further reason for thinking that this Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 supposition would be true is that in a more depleted sand a greater pro­ portion of the energy is wasted by slippage, but as a matter of fact we find that taken as a whole the gas-oil ratio does not increase greatly as the pressure goes off, and in some instances the volume of gas with each barrel of oil decreases over a considerable period of time. It would appear from this evidence that the increased energy released by holding a low back-pressure and thus a high differential pressure is used up mostly in forms of work that do not increase recovery. Experience in repres­ suring sands leads to similar conclusions. There is probably a curve of efficiency in recovery for each well with respect to the differential pressure that is analogous to the relation l::ietween pressure and efficiency in the air-lift as shown by Fig. 4. If the writers are correct in their conclusions, the effect of the air-lift upon oil recovery is to be measured by the back-pressure created by it and the air-lift should be designed and operated at a pressure to conform with the back-pressure giving the best results in oil recovery. These considerations, however, are limited by the practical ones of the competitive conditions at the well and therefore the necessity for getting the highest daily production. Not until a great deal more observational data have been collected and analyzed can it be known how to work out the use of back-pressures and how to design the air-lift so as to get the hack-pressures desired, and at the same time have an efficient air-lift within the limits imposed by the size of the casing and other economic and practical conditions. It may at first thought seem that the air-lift, which, in a deep well, may be imposing several hundred pounds of pressure at the bottom of the well, would create a back-pressure instead of lessening the back­ pressure, but, actually, a naturally flowing well may have a greater back-pressure on the sand than is necessary to lift the oil by the air-lift, DISCUSSION 39

and this is true even with a pumping well. If the oil flowing into the well is not removed as fast as it enters, it heads up and exerts a strong back-pressure, as has been shown in discussing Fig. 1. This will take place if the pump has not sufficient capaeity, or in a flowing well if the pressure and volume of the gas does not supply enough energy to lift the oil rapidly. Under these eonditions the air-lift, by removing the accumulated column of oil, will actually reduce the pressure on the sand.

NEED FOR BETTER INFORMATION Tests made on the deliveries of air compressors have disclosed that assumptions as to their delivery, horsepower, and effieiency founded upon manufacturer's claims and ratings are very unreliable, and conclusions based upon such data should not be accepted. As operated in the field,

a compressor has not the same efficiency as when tested on the floor of the Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 factory. Analogous errors have been noted in other observational data. Progress in air-lift engineering will not be satisfactory until the informa­ tion gathered in the field is more complete and dependable.

DISCUSSION R. R. BRANDENTHALER,' Bartlesville, Okla.-This paper is one of the best that have been written on the principles of the air-gas lift. Certainly it emphasizes the need for fundamental experimental work and accurate, pertinent data as a basis for future development. I was much interested in the comments regarding the necessity for setting out more clcarly the different practical ends sought and for working out specific engineering principles upon which to formulate the design and operation of gas-lifts to meet the desired end. Unless this is accomplished, there is not likely to be any material advancement in the development of the gas-lift. The differences in opinion relating to the gas-lift are sufficient justification of the statement that there are basic errors in the source of data upon which conclusions have been predicated. Unfortunately, economic necessity has centered interest primarily on obtaining the maximum daily oil production, little recognition being given the possible effect on ultimate production. Some records have been more complete than others, but there is still much to be desired as regards complete and accurate records. Lewis and Pierce bring out an important consideration; that is, the question of velocity in the oil line. This was demonstrated by the small gas-lift model exhibited by the U. S. Bureau of Mines at the Tulsa Exposition. Varying the velocity had a decided effect on the type of flow obtainable. The Bartlesville Station of the U. S. Bureau of Mines is planning a series of experi­ ments with natural flow and the gas-lift. A derrick has been erected, of which the crow's-nest will represent the surface of the ground; a large tank placed under the derrick floor will represent an oil structure. While we all recognize the possible limitations in the height of lift the total lift being 85 ft., we believe that pertinent fun­ damental data may be obtained in the contemplated experiments by working with smaller sizes of tubing and casing. For each experiment it will be possible to recharge

* Petroleum Engineer, U. S. Bureau of Mines. 40 AIR-GAS LIFT the reservoir tank with the same volume of oil and gas, hence quantitative comparisons can be made between experiments. The various companies in the Mid-Continent will be notified prior to each experi­ ment, so that they may send representatives to witness the experiments. Suggestions concerning the contemplated experimental work will be much appreciated. W. E. WRATHER, * Dallas, Tex.-Has tapered tubing been used in the Seminole field? J. M. LOVEJOY, t Tulsa, Okla.-I think it has been used in the experiments.

MEMBER.-Have lift operations been tried through 4-in. tubing tapered down to a string of 2 inch?

E. V. FORAN,t Breckenridge, Tex.-The Marland Oil Co. has made some experi­ ments under the conditions just mentioned, in a well that has made from 300 to 1000 bbl. a day from 2500 ft. in depth. The results of that will be given a little later in the session. Downloaded from http://onepetro.org/TRANS/article-pdf/77/01/19/2178228/spe-927019-g.pdf by guest on 24 September 2021 C. V. MILLIKAN,§ Tulsa, Okla.-In further answer to Mr. Wrather's question: The Amerada Petroleum Corpn. has conducted some experiments along that line. On wells 1300 ft. deep making from 50 to 100 bbl. flowing through 2-in. regular tubing, the working pressure was reduced from 100 lb. to 50 lb. by using a tubing graduated 2, 2% and 3 in. At one well in Seminole making 150 bbl. of fluid through 2%-in. tubing, the working pressure was reduced about 20 lb. by using a graduated string of 2, 2% and 3 in. There was no change in volume of fluid handled at either place.

MEMBER.-The man in attendance at the exhibit at Tulsa, in talking of tapered tubing, said that in a well 4200 ft. deep ideal conditions would call for 2%-in. tubing at the bottom with a gradual increase to 42 ft. in diameter at the top. I told him I did not think there would be any production with such conditions.

E. H. GRISWOLD,II Ponca City, Okla.-We have found that with both natural and gas-lift flowing wells, there exists a critical back-pressure for each well. Pressure above and below this critical pressure tends to increase the gas factors. The critical pressure varies with the life of the well and frequent adjustments are necessary. It is thought that the use of back-pressure has often been condemned because pressures in excess of the critical point were applied. Some wells are pinched beyond their critical pressure by the accidental or unavoidable use of too small a .flow pipe and of course are not subject to the use of increased pressures. It is rarely economical to change the size of flow pipe more than once or twice in the gas-lift life of a well. Controlled back-pressures have been used to maintain efficient flow during the periods between changes of tubing sizes. Velocities as low as 15 ft. per second and as high as 100 ft. per second have been found at the well head on different wells at their most efficient points. Just what the most efficient velocities are for various combinations of gas and oil production, size of flow pipe, etc., have not been determined; but it is believed that with the data now being assembled some general correlation at least can be made.

* Consulting Geologist. t Vice-president, Amerada Petroleum Corpn. t Production Engineer, Marland Oil Co. of Texas. § Petroleum Engineer, Amerada Petroleum Corpn. II Petroleum Engineering Dept., Marland Oil Co. of Oklahoma.