US2398339A - Gas anchor - Google Patents

Description

OIL OUT FIG. 2

FIGB

E. V. WATTS GAS ANCHOR OWLY` F|G.l

,Filed April 3, 1945 ROTH LAYER MOVING RAPIDLY PRO H :NvENToR A'rToRNEY V. WATTS Patented Apr. 9, i946 NITED sTTs ENT OFFICE GAS NCHR Euclid V. Watts, South Pasadena, Calif., assignor to Socony-Vacuum i] Company, Incorporated, N ew York, N. Y., a corporation of New York Application April 3, 1945, Serial No. 586,288 1 claim. oL 10aa-203) solution in the oil by. decrease in pressure or, l0

occasionally, by turbulent ow of oil and gas through casing perforations or vother restrictions.

In the case of a newly tapped reservoir this froth formation may be beneficial in Vreducing the hydraulic head of the column of oil in the l5 tubing, thus causing a well to flow naturally which would have to be pumped if the froth were excluded and the tubing iilled with liquid` oil. But when pumping is required, the presence of the froth in the pump barrel materially reduces pumping eiiciency. Not only does the froth voc cupy uselessly a part of the displacement volume of the pump barrel, but if in suiiicient quantity, it may expand and contract with the plunger stroke to such extent as to causegas-lock and 275 total cessation of liquid delivery.

It is therefore customary, in cases in which froth tends to form at the pumping level, toprovide the pump with a so-called gas anchor.

This device, in its simplest and most common form, is little more than an elongated cup, open at its upper end except for an attachment tothe pump, and provided with an4 inner y tube (the skeeter bill) which extends from the pump intake to a point adjacent the lower end of the cup.

This device diverts away from the pump suction any large gas bubbles rising through the pool of oil in the bottom of the hole routside the A anchor. It also, in theory, permits gas bubbles inducted into or evolved within the cup toW rise Y and escape through the open upper end while a solid (in the sense of unbroken) annular column of oil passes down to the end ofthe skeeter bill and thus into the pump intake,

This theory is borne out in practice only in instances in which the upward velocity of gas bubbles relative to the oil exceeds the downward velocity of the annular column within the anchor and in which the iilm properties of the liq- 5o uid permit the rupture of bubbles as fast as they accumulate at the top of the anchor, thereby completing the separation of the gas phase from the liquid phase.

fa more or less xed quantity, being determined by the quantity of oil flowing into the pump intake and the cross-sectional area of the largest column which can be accommodated within the casing. The upward velocity is a function of theI viscosity of the oil and also of the size of the bubble, increasing (in an oil of given viscosity) as the square of the diameter of the bubble.

it follows, and is well recognized, that while these simple anchors function to advantage in wells giving small yields of light,lowviscosity crude, their eiliciency falls off very rapidly with 'increasing yield, enhanced velocity and reduced bubble dimensions. For these reasons they are of little if any utility in excluding gas from pumps handling the heavier class of crudes, or large volumes of any viscosity.

The structure described herein is an improvement over the conventional form of gas anchor in two major respects: iirst in materially increas-` ing the time required for the oil column iiowing downwardly within the anchor to travel any given vertical distance; second in reducing the vertical distance through which the gas bubble must rise through the oil body before escaping from it.

The simple structure of theinvention and the manner in which it functionsare illustrated in the attached drawing, in which Fig. 1 is a section through a casing and helical gas anchor, showing the pump intake tube in elevation;

Fig. 2 is a diagram used to illustrate the functions oi the structure of Fig. 1, and l y Fig. 3 is a section through a modied structure which utilizes the same' operating principle as that of the rst gure.

Referring first to Fig. 1, I0 is a well casing having perforations l l-I I through which oil enters, the oil carrying entrained gas. An anchor shell l2 surrounds the pump suction extension tube I3 and contains a sheet metal helix I4 closely ntting both thetube and the shell. The anchor is suspended from the suction tube by a disc I5 and the upper end of the tube screws into the suction end of a deep well pump not shown. A perforation l6 allows oil to pass from the exteriorfto the interior of the anchor shell and one or more perforations il permit the escape of gas and froth from the upper end of the anchor tube into the casing.d A

The opening I should be located more or less midway the length of the anchor tube l2 and should be in the i'orm of a deep slot situated slightly above the helix-that is to say,the lower The downward velocity is, in any given case, edge of the slot shouldv lie far enough above the illustrated diagrammatically in Fig. 2, which represents an extension ofthe entire lengthof the helical channel I9 included between the turns of the worm.

Oil with entrained gas enters the channel at I6 and flows downwardly over the upper face 20 of the worm, finally reaching .the lpool I8 from which oil is drawn by the pump. While so iiowing, gas in the forms of free bubbles and of froth, rises to the surface and the liquid ,volume of the stream diminishes. 'The free gas bubbles rise through the froth layer until they encounter the lower face 2l of the worm, which they follow upwardly to the point of escape l1, either as large individual bubbles or as a rapidly moving thin stream.

The froth which rises to the surface of the oil stream consists of the smaller gas bubbles separated by av greater or lesser thickness of oil. So-

long as the proportion of oil in the mass'is sufficient to permit the bubbles to retain .their globular form, the froth has approximately the same fluidity as the oil, but as the oil drains out from between the globules they become closer together and the fluency diminishes. The nal stage in oil drainage is that at which the bubbles are separated only by extremely thin oil lms and the bubbles are distorted by mutual contact.

At Athis, stage the froth will not flow of its own volition, but is forced upwardly between the turns of the worm by the superior weight of the enteringy oil, occupying whatever space is left between the upflowing gas layer and the'downow, ing oil stream.

During this upward movement, which carries the froth past the oil inlet and toward or to the upper en'd of the helical channel, the froth is subjected to forces which tend to rupture the oil films, permitting the gas to escape and the oil to coalesce into globules. 'Ihese forces consist principally of the internal movements occasioned by relatively rapid movement of the interior of the helical column and retardation of the faces in contact with the walls, and of the agitation of the froth mass by largegas bubbles forcing their way over its upper surface. This froth resolving action continues throughout the length of the helical channel and under favorable circumstances may break down the froth as fast as it is separated from the entering oil. In that event only gas escapes from the upper end of the channel while the small amount of oil separated from the froth returns along the bottom of the channel to rejoin the entering stream. Ordinarily, resolution will be less than perfect and some very light froth, containing only a minute amount of oil, will be returned to the space between the casing and the anchor shell, but at a level materially above the oil inlet.

'The advantage of this structure over the con,

ventional form (which would be represented by removing the worm. from the form of Fig. 1 and admitting oil at the upper end of the anchor shell)l will be evident from the above description. In the prior art form the rising gas has to traverse the entire length of the downiiowing oil column before escaping. The critical velocity of oil flow is that which equals the rate of rise of the smallest gas bubbles which must be removed in order to bring the oil into condition to be pumped, and this critical velocity is not changed by lengthening the distance between the inlet point and the lower end of the shell. In the helical form the gas separates from the oil at an obtuse angle and must rise through an oil layer of less depth than the vertical distance between adjacent turns of the worm in order to escape from the oil flow into the froth mass, moving in the opposite direction and fromwhich it cannot return. Thus in the helical form the critical limitation is not the velocity of the oil stream, but the time of residence of the oil in the channel, and this time may be increased by lengthening the anchor up to such limits as are fixed by the desired location of the pump itself.

The most desirable leyel for oil inlet I6 will vary with the condition of the oil column in the casing and with the rapidity with which the gas 'and froth separate from the entering oil. The

portion of the anchor below the oil inlet performs the stripping of the oil, and the length of this portion must be suiiicient to effect a substantially clean separation before the oil enters the pump suction. On the other hand, the portion of the channel above the inlet performs the greater part of the froth resolution and thus the longer this portion of the channel the less will be the quantity of frth returned to the casing outside the anchor. Also, the greater the distance between the froth outlet and the oil inlet, the smaller will be the chance of froth circulating back to l the inlet with the entering oil.

`Generally speaking, oils of high viscosity or in y which the gas is very finely dispersed will require that the Oil inlet be at a relatively high level, while with less viscous oils or those in which the gas is coarselyr dispersed it may with advantage be placed lower. The optimum position for the inlet in any given well can be determined only by experiment with shells having inlets at different levels. y

A modified form of the device, using a different structure but the same functional principles, is shown in Fig.l 3. In this form the worin of Fig. l

, is replaced by a plurality of plates 23 dividing the annular space between shell I2 and pump tube i3 into chambers. These chambers communicate through downwardly directed tubes 2@ and upwardly, directed tubes 25, each of which should extend at least half way through the depth of the adjacent chamber. The inlet i6 for oil and gas may be placed immediately below the outlet l1 for gas and froth, as in Fig. 3, or at a lower level as in Fig. 1. The two communication tubes 2Q and 25' should be on the same side of the plate in which they are located, with the pairs of tubes alternating from side to side in descending from plate to plate.

Oil and entrained gas enter the inlet opening l and iiow across the plate to the downpipe 24,

passing thus to the lower portion of the chamber te lighter gas and froth ow over the liquid surface,

aaoasao in a contrary direction. and into the upper part of the chamber next above. i

'I'he gravity diilerence between the oil owing downwardly and 4the gas and froth being displaced upwardly is the soie' motivating force available to produce now and care must be taken not to make the communicating vpipes too narrow. This form of the device is highly eective in both stripping and breaking froth but has less capacity for any given form.

As the pressure drops across theI partitioning members in either or the forms described are. wholly negligible, either the worm oi' Fig. 1 or the plates and tubes of- Fis. 3 may be of light sheet metal so long as it is capable of resisting corrosion by saline well fluids.

I claim as my invention:

In a gasanchor: an inner tubular member: a

set of dimensions than the helical substantially closed shell and spaced from said tubular member to define a vertical ann nular space communicating with the interior of said tubular member at its lower end; partitions,

atleast approximately horizontal, dividing said space into a succession of chambers; means af-Y fording communication between the bottom of each chamberiand the lower part of the chamber -next below; means affording communication between the top of each chamber and Vthe upper part of the chamber next above; means for ad- Y mitting a stream of gas-containing oil intothe from said oil stream andflowing countercurrent thereto and thereove'r. A

EUCLID V. WATTS.

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