GB2094414A - Swab cup - Google Patents

Abstract

A swab cup (5) for an oil well comprises a body (7) of elastomer bonded to a metal bushing (6). The surface of the body (7) is formed with a plurality of adjacent recesses (10) each having a concave arcuate bottom (11) and a recess depth of between 0.254 mm and 1.27 mm. A V-shaped groove (20) is provided at the upper end of the swab. <IMAGE>

Description

SPECIFICATION Swab cup This invention relates to a swab cup, and in the preferred embodiments provides a swab cup suitable for swabbing the tubing of an oil well.

In petroleum production, swabbing is a process whereby liquid in a well may be lifted for collection at the top of the well by means of a valved, pistonlike device. Typically, the device is carried on the end of a wireline and is raised and lowered within the well tubing to pick up a column of liquid and deliver it to the top of the well. In one form, the valved, piston-like device or swab cup is mounted slidably on a fluted mandrel so that, as the cup is lowered into the well the liquid therein is free to flow upwardly past the cup through the fluted portions of the mandrel but, upon reeling in the wireline, the cup shifts downwardly relative to the mandrel sealing against the mandrel to prevent the flow of liquid downwardly through the cup.

Accordingly, as the wireline is pulled upwardly within the well, the column of liquid above the cup is lifted towards the surface.

Representative different forms of all-rubber swab cups of the foregoing type are disclosed in United States Patents 3,166,334,3,318,607 and 3,467,753. Each of these cups comprises a generally cylindrical body formed of rubber or a like elastomeric material molded on a tubular support usually made of metal. The designation of the cups as being "all-rubber" means simply that the body lacks any internal support members such as metal wire supports found in other types of swab cups.

Theoretically an ideal swab cup would be one which drops freely when lowered into a well, which instantaneously seals against the inside of the well tubing when pulled'upwardly so as to pick up all of the liquid column in the tubing above the cup for delivery to the surface, and which does not wear regardless of the number of swabbing trips made in a well. In practice, however, an ideal all rubber swab cup is impossible to attain for many reasons. For example, the clearance between the cup and the inside of the tubing affects both the ability of the cup to freely fall and the seal between the cup and the tubing when the cup is lifted. Generally speaking, the greater the clearance between the cup and the inside of the tubing when being lowered, the easier the cup falls but the greater the volume liquid that may ieak past the cup when lifted.On the other hand, the closer that the size of the cup is to the internal diameter of the tubing the better it seals, but the more difficult it is for the cup to fall freely. In considering wearability, the degree to which the sealing surfaces of the cup wear against the inside of the tubing decreases in relation to the extent of such sealing area. All other factors being generally equal, a larger sealing area will wear less than a smaller area for the same cup load. But, small sealing areas, such as those in the form of a flexible lip at the top of the cup, tend to seal quickly so as to reduce leakage lost from the column of liquid above the cup when lifted. Larger sealing areas close to the internal wall of the tubing tend to keep the cup from being able to drop freely but are able to support larger loads without inverting.

One prior swab cup construction designed to obtain low pressure initial sealing while still providing sufficient wall clearance to allow for lowering the cup into the well is shown in the aforementioned United States Patent 3,166,334 wherein spaced deep annular grooves divide the cup into multiple sections whose sides are each tapered in identical fashion. United States Patent 3,487,753 also discloses a swab cup structure employing multiple spaced grooves in an elastomeric body whose side walls are tapered to define swabbing elements.

Experience has shown that the grooves defining multiple swab cup segments can serve as pockets where debris can collect and ultimately cause a cap to become stuck in the tubing. Moreover, the lips defined by the grooves tend to define areas of wear concentration and tearing.

The preferred embodiment of the present invention provides a swab cup which in-serviceuse seals more quickly, and is longer wearing than prior similar swab cups. Specifically, the foregoing is accomplished without the use of multiple grooves in an all-rubber swab cup and by forming the outer body surface configuration as a series of annular recesses having concave curved bottoms spaced from each other so as to define a series of annular ridges spaced from each other on the outer surface of the body longitudinally along the central axis of the cup. In use, this configuration has been found to fall easiiy within a well, to seal quickly under lower load, and to also resist wear much better than might be expected of prior allrubber cups.

Although it is not entirely clear at the present time exactly why the exemplary swab cup performs as it does, it is believed that the unique shape of the cup takes advantage of the drag generated during lifting the cup in the well to quickly effect sealing evenly against the interior of the well tubing along substantially the full length of the cup even under a relatively short column of liquid. This might be explained more specifically by considering what is believed to occur downhole during swabbing when using the present cup. After a swab cup has been lowered to a predetermined level in a tubing string, the swab cup is moved rapidly upward and this causes some of the liquid to flow around it and the pressure thus generated to deflect the outer surface of the cup radially outwardly into a sealing position relative to the tubing.The present cup shape is designed to utilize drag in sealing the cup in the tubing rather than relying upon dynamic lift to cause the swab cup rubber to deflect and seal.

Some of the drag is produced as a result of the form or shape of the top of the cup, form drag, while the remainder is drag resulting from the pressure losses occuring in the deceleration of high velocity flow, that is, drag related to boundary layer separation. The exterior shape of the exemplary cup is an attempt to create boundary layer separation beneath the top of the cup as quickly as possible while still providing adequate support for the rubber material adjacent the tubing in order to keep the material from being torn off or otherwise worn away quickly during in-service use.By forming the side profile of the cup as a series of arcuate recesses, the liquid flowing around the cup as it is moved upwardly may be accelerated and decelerated quickly while still maintaining a small amount of rubber at the desired maximum diameter for quick sealing without imparing the falling of the cup and yet also providing a substantial amount of rubber for wear, sealing and load handling. This repeated series of arcuate recesses allows for repetition of the process of converting substantially all of the flow energy into drag and thereby causing deflection of the rubber in the recesses for sealing between the cup and the inside of the tubing.

Advantageously, herein, the recesses are formed so that, in profile, there are arcs of circles having a chord length of approximately two inches (50 mm) and a maximum deflection (that maximum distance between the chord and the arc) in the approximate range of from 0.010 through 0.050 inches (0.254 to 1.27 mm), preferably about 0.030 inches (0.762 mm). The recesses are spaced from each with the centers of the circles defining the arcs at the same distance from the central axis of the body of the cup and so that adjacent arcs intersect approximately at the outer surface of the general cylinder defining the cup body.It is believed that with the recesses formed in this fashion, as load is applied to the cup body, the body deflects radially outward so as to substantially fill the recesses and seal relative to the interior of the tubing so that substantially all of the outer surface of the cup body forms a seal with the tubing.

Further advantage of the exemplary cup is that the unique shape enables the cup to be made without longitudinal seams, it being possible to pull the cup axially from its mold rather than having to split the mold in an axial direction in order to remove the cup.

These and other advantages of the present invention will be best understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings, wherein: Fig. 1 is a combined elevational and cross sectional view of a swab cup embodying the novel features of the present invention.

Fig. 2 is an enlarged, fragmentary, elevational view of a portion of the cup shown in Fig. 1.

Fig. 3 is a plan view of the cup shown in Fig. 1.

As shown in the drawings for purposes of illustration the present invention is embodied in a swab cup 5 such as is used in the petroleum industry for removing liquid from a well.

Specifically herein the cup is an all-rubber cup, that is, one which includes a tubular metal bushing 6 with an elastomeric, generally cylindrical body 7 molded thereon. In use, the cup fits in telescoped fashion over a fluted mandrel (not shown) or the like which is attached to a wireline for lowering within the tubing string of a well. As the cup is lowered into the well, the liquid therein is generally free to pass upwardly past the cup both through the tubular bushing within the fluted passages created between the mandrel and the bushing (see United States Patent 2,998,990) and between the outside of the cup and the inside wall of the tubing. When lifted, however, the lower end of the cup seals against the mandrel preventing the passage of substantial amounts of liquid downwardly from above the cup.Typically, sealing against the mandrel is effected by the lower end surface 9 of the body being compressed against a radial, annular surface projecting outwardly from the side of the mandrel. The magnitude of the liquid load on the body determines to some extent the effectiveness of the seal between the body and the mandrel. In addition to sealing between the body 7 and the mandrel, it is neccssary for a seal to be formed between the outside of the cup 5 and the inside of the tubing to keep the liquid above the cup from leaking back into the well. The efficiency with which the cup lifts the column of liquid above it from the well is determined primarily by how quickly the seals between the lower end 9 of the cup and mandrel and the outside of the cup and tubing are made after beginning to pull the cup upwardly.Generally speaking, the closer that the outside wall of an allrubber cup is to the inside of the tubing, the less liquid is lost from the column during lifting.

However, if such a cup is sized so as to actually engage the inside of the tubing before lifting, it is difficult to get that cup to fall properly. Wear also is a problem in that, as a cup becomes worn from use, greater load usually is required in order to effect sealing between the cup and the inside of the tubing and even then the sealing may not be complete so that a substantial amount of liquid may leak past the cup as it is lifted. Cups appear to be longer lasting as the area of wearing contact against the tubing increases. But, as this area for wearing contact with the tubing is increased in size and positioned closer to the inside of the tubing it makes it more difficult for such a cup to drop freely into the well.

The invention herein resides in the unique shape of the outside of the swab cup 5 so as to provide a cup which falls freely when lowered into a well but seals more quickly and is much longer lasting in service use than might be expected of prior cups similarly used. Although the reasons for the improved performance of the exemplary swab cup may not be fully understood, the approach taken in the design of the cup was to construct the outer surface thereof so that more utilization was made of drag resulting from movement of the cup upwardly in the well to effect initial sealing and to thereafter rely on such drag to maintain sealing rather then to rely simply upon the dynamic load imposed on the cup from the column of liquid above the cup.Toward this end, the body 7 is formed with a series of external, annular recesses 10 axially spaced from each other and having concave, arcuate bottoms 11 whose closest adjacent tangent lines 13 intersect at an obtuse included angle a. This angle is believed important as defining structure which (1) stimulates early boundary layer separation (and therefore high pressure drag) for liquid flowing between the cup and the inside of the tubing, and which (2) provides sufficient material support in the elastomeric material of the body so as to keep radially extended portions of the cup from being torn off or quickly worn away in service use.It is suspected that the high pressure drag resulting from boundary layer separation causes radial deflection of the elastomeric material comprising the body of the cup, such material being compressed to expand outwardly filling the recesses 10 and thereby sealing with the inside of the tubing against the leakage of any substantial volume of liquid past the cup when pulled upwardly. As the working shape of the cup is compressed in this manner, the force with which the cup engages the inside of the tubing is distributed relatively equally over substantially the entire outer surface of the cup, even over the surface areas otherwise defining the recesses, so that areas of concentrated wear are avoided, thereby substantially lengthening the useful service life of the cup.

In the present instance, the outer shape of the swab cup 5 may be more specifically described with the bottom 11 of each concave recess 10 having upper and lower sides 14 and 1 5 of slopes which are mirror images of each other.

Specifically, herein, the sides 14 and 1 5 are curved, being defined by the arc of a circle having a radius R and an origin at O as is shown in Fig. 1 and rotated about the axis 1 6 of the body so that configuration of each recess is as a surface section of a toroid. In the exemplary swab cup 5 shown in Fig. 1 , the recesses resulting from this construction are exaggerated in depth. For a regular size cup suitable for use in 2 3/8" (60 mm) tubing and having a nominal diameter of 1.980 inches (50 mm) a recess depth of 0.030 inches (0.764 mm) produced by a circle radius R of 16.65 inches (0.423 mm) is satisfactory. Recesses of other depths generally within the range of 0.010 inches (0.254 mm) through 0.050 inches (1.27 mm) also might be used to produce favorable results.Prior experience indicates that when the body of the cup is undersized relative to the inside diameter of the well tubing by an amount of generally less than 0.010 inches (0.254), then difficulty may be experienced in getting such a cup to fall properly. Similarly, where the body of the cup is undersized from the inside diameter of well tubing by an amount generally greater than 0.050 inches (1.27 mm), difficulty may be experienced in obtaining a seal between the cup and the tubing.

As shown, there are three of the arcuate recesses 10 spaced axially from each other with their curvature origins Oi, 02 and 03 equidistant from the next adjacent origin. The origin 03 of the lower recess is radially spaced from the bottom end surface 9 of the cup body 7 and the upper origin 0, is spaced vertically below an upper end surface 1 7 of the cup. Specifically, with the cup dimensioned as above, the origins 01,02 and O3of recesses are spaced approximately two inches apart. Moreover, bottoms 11 of adjacent recesses are curved so as to intersect each other at the nominal diameter of the cup body 7 thereby forming annular ridges 19.As may be seen in axial profile of the cup body, the closest possible tangent lines 1 3 of the curves of adjacent recesses intersect each other at an obtuse included angle a (see Fig. 2) so as to provide a substantial mass of elastomeric material at the ridges 1 9. At the uppermost recess 10, the intersection of the curvature of the recess with the plane of the upper end surface 1 7 of the cup is such that the tangent line 13 to that curve at such intersection forms a relief angle B which is an acute angle as measured relative to a direction parallel to the axis 1 6 of the cup body 7. Herein, the angle B is the complement of one-half the included obtuse angle a, the included obtuse angle between two intersecting tangent lines 13.

As an aid to falling of the cup 5 within the tubing, the lowermost recess 10 intersects with an annular taper on bevel 1 8 which slants radially inward therefrom to the lower end surface 9 of the cup body 7. At the other end of the cup, the upper end surface 17 of the cup body 7 includes an annular V-shaped groove 20 formed therein as an aid to initial sealing of the cup against the inside wall of the tubing when first lifting the cup in the well tubing. Wherein, the groove 20 includes an annular wall 21 and a concentric outer annular wall 23.In profile as shown in Fig. 2, the inner wall forms an acute angle C slanting inwardly toward the axis 16 of the body relative to the axial direction from the bottom of the groove 20 upon progressing upwardly and the outer wall 23 forms an acute angle D greater in magnitude than the angle C but extending in an outwardly direction relative to the central axis of the cup. Herein, the angle C is approximately 100 and the angle D is about 380. Moreover, the upper end of the outer wall 23 is spaced radially inward from the upper ridge 1 9 so that the body material located radiall outward of the groove just beneath the upper ridge 1 9 so that the body material located radially truncated upper end portion forming a ring surface 25 in the upper end surface 1 7 of the cup body.

With this configuration, initial sealing between the cup body 7 and the inside of the well tubing is aided as the cup is lifted but the lip is particularly well supported against excessive flexure, wear and tearing in service use.

Advantageously, the foregoing described structural configuration for the cup 5 enables the cup to be dropped easily in a well while providing quicker sealing and less wear when pulled up.

Initial sealing is somewhat enhanced through the use of the upper end groove 20 but the use of the concave arcuate recesses 10 is believed to provide a configuration which advantageously makes use of pressure drag to increase performance characteristics of the cup. With the lowermost recess 10 comprising generally onehalf the curvature that appears in the two uppermost recesses, the lower end of the cup is particularly adapted for falling without impairing the desired drag characteristics obtained from the inwardly directed curvature of the upper side 14 of the recess in enhancing boundary layer separation while still providing sufficient elastomeric mass for supporting the lowermost ridge 19.

Claims (12)

1. A swab cup having a tubular bushing with an elastomeric body bonded thereon, said body having an external axially extending profile comprising a series of concave recesses each having an arcuate bottom of substantially similar curvature and a maximum deflection radially inward approximately within the range of from 0.010 to 0.050 inches (0.254 to 1.27 mm).

2. A swab cup according to claim 1 wherein the closest tangent lines for immediately adjacent bottoms of said recesses intersect each other at an obtuse included angle.

3. A swab cup according to claim 2 wherein said intersection of said tangent lines substantially coincides with the external surface of said body.

4. A swab cup according to claim 2 or claim 3 wherein said obtuse included angle is in the neighborhood of one hundred seventy three degrees.

5. A swab cup according to any of claims 2 to 4 wherein the upper end of the swab cup includes an upwardly-opening annular groove defining a peripheral lip, and wherein the uppermost one of said recesses forms an angle of relief defined by the acute included angle between a line tangent to the bottom of the recess and an axially extending line at any point on the periphery of said lip, said angle of relief being approximately equal to the complement of one-half of said obtuse included angle.

6. A swab cup according to claim 5 wherein said upwardly opening groove has a generally Vshaped profile with an outwardly slanted wall and a radially inward wall more steeply slanted with respect to the axis of said cup body than said outwardly slanted wall.

7. A swab cup according to any preceding claim wherein said arcuate bottom is circular and has a radius in the neighborhood of sixteen inches (0.4 m).

8. A swab cup according to any preceding claim wherein said maximum deflection is not less than 0.010 inches (0.254 mm).

9. A swab cup according to any preceding claim wherein said maximum deflection is no greater than 0.050 inches (1.27 mm).

10. A swab cup according to any preceding claim wherein said maximum deflection is 0.030 inches (0.764 mm).

11. A swab cup having a tubular bushing, an elastomeric body bonded externally on said bushing, said body having a generally cylindrical outer surface and generally radial upper and lower ends, an upwardly-opening annular groove formed in said upper end and defining a peripheral lip, a first annular recess formed in said outer surface, said recess having a bottom having an upper side sloped generally inwardly upon progressing downwardly along said outer surface and a lower side sloped substantially as a mirror image of said upper side so as to extend generally outwardly upon progressing downwardly along said surface said bottom having a maximum deflection radially inward generally within the range of 0.010 through 0.050 inches and a profile length in the neighborhood of two inches, a second annular recess substantially like said first annular recess formed in said body immediately below said first recess with said second recess having an upper side forming an obtuse included angle with said lower side of said bottom of said first recess, and a partial third recess substantially like the upper half of said first recess.

12. A swab cup, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.