US3645331A - Method for sealing nozzles in a drill bit - Google Patents

Abstract

A method for selectively sealing nozzles in an erosion bit without removing the bit from the well comprising the steps of (a) plugging the nozzles centrally located in the bit with sealants including at least one temporary plug so that the bulk fluid flow is through the peripherally located nozzles; (b) plugging at least one of the peripheral nozzles with pumpdown sealants; and (c) thereafter removing the temporary plug. Depending on its composition, the temporary plug can be removed by erosion, pressure, thermal decomposition, or combinations thereof.

Description

United States Patent Maurer et al.

[ Feb. 29, 1972 [54] METHOD FOR SEALING NOZZLES IN A 2,873,092 2/1959 Dwyer ..l75/340 X DRILL BIT 2,945,678 7/1960 Boudreaux et al. ...175/340 X 3,116,800 1 1964 K ...175 237 X [72] lnventors: William C. Maurer, Houston; Joe K. Heilammerer I heck" Bellaiw Robert L Graham 3,189,107 6/1965 Galle ....175/393 9 v 9 Houston, an ofTex 3,21 1,244 10/1965 Cordary ..l75/61 [73] Assignee: Esso Production Research Company Primary Examiner-Stephen .l. Novosad Attamey.lames A. Reilly, John B. Davidson, Lewis H. [22] 1970 Eatherton, James E. Gilchrist, Robert L. Graham and James [21] Appl. No.: 60,234 E. Reed [52] u.s.c| ..l75/65, 175/237, 175/393, [57] ABSTRACT 166/193 A method for selectively sealing nozzles in an erosion bit [51] Int. Cl ..E21b 7/18 i h t removing the bit from the well comprising the steps of [58] Field of Search ..175/61, 65, 67, 232, 237, 317, (a) plugging the nozzles centrally located in the bit with 175/318 422; 166/193 lants including at least one temporary plug so that the bulk fluid flow is through the peripherally located nozzles; (b) [56] References Cned plugging at least one of the peripheral nozzles with pumpdown UNITED STATES PATENTS sealants; and (c) thereafter removing the temporary plug. De-

I pending on its composition, the temporary plug can be 2,903,239 9/1959 Standndge ..175/393 X removcd b i re ure, thermal decomposition, or

Gage l X combinations thereof 2,987,130 6/1961 Mclntyre ...175/340 X 2,324,102 7/1943 Miller et a1 ..l75/237 X 12 Claims, 5 Drawing Figures Ill 1 l I 1 l 49 4| X I, I 1' I Q l w \l l i ""1 l u i l as 35 PATENTEUFEBZS I972 SHEET 1 OF 2 2:14 23 FIG. I-

. fljNVEN'TORS WILLIAM c; MAURER JOE K. HEILHECKER BY ROBERT L .,G RAHAM FIG. 2.

A TTORNE Y PATENTEDFEB29 I972 SHEET 2 OF 2 FIG. 4

FIG. 3

FIG. 5

INVENTORS WILLIAM C. MAURER JOE K. HE/LHECKER y RQBERT L. GRAHAM ATTORNEY METHOD FOR SEALING NOZZLES IN A DRILL BIT BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to erosion drilling. In one aspect it relates to an improved method for selectively sealing nozzles in an erosion drill bit.

2. Description of the Prior Art Erosion drilling is a technique for drilling boreholes using high-velocity hydraulic jets as the principal mechanism for inducing stresses in the formation rock. The basic components of the erosion drilling system include high-pressure pumps, a high-pressure drillstring, and an erosion bit provided with a plurality of flow nozzles. The erosion bit can also include auxiliary cutting devices such as cone cutters, drag bit blades, or a diamond crown. The principal source of energy, however, is derived from the high-velocity jets discharging from the nozzles.

The power output of an erosion drill can be expressed as follows:

P=0.0223 Ap where:

P is power output, horsepower;

p is differential pressure, p.s.i.; and

A is total nozzle flow area, square inches.

The differential pressure, p, is generally a fixed parameter controlled by the available pump pressure. For a given pump,

pressure then, and in accordance with the above equation, the power output P, is directly proportional to the nozzle flow area A, which is the summation of the cross sectional areas of the individual nozzle openings.

The nozzles are arranged on the bit to provide a distributed pattern of water jets emerging therefrom. Centrally located nozzles direct water jets downwardly to erode the formation rock immediately below the bit, whereas peripherally located nozzles direct the water jets outwardly to cut a hole of sufficient diameter to permit advancement of the bit.

It is known that the differential pressure across the bit must be maintained at a threshold level in order to provide jet streams of sufficient energy to penetrate the rock. (See Paper Number SPE 2434 of the Society of Petroleum Engineers of AIME entitled Hydraulic Jet Drilling by William C. Maurer and Joe K. l-leilhecker.) The threshold pressure varies for different types of rock depending upon the cohesive strength of the rock; tests have shown, however, that the threshold pressure for all practical purposes must provide a jet velocity of at least 500 feet per second for rock penetration. At these high velocities, the jet streams tend to erode or washout the nozzles in a relatively short period of time. Nozzle erosion is particularly severe where the drilling fluid contains even small amounts of abrasives such as silica sand. Nozzle erosion not only reduces penetration rate but if the nozzle flow area is increased to a value which prevents the attainment of the threshold pressure for a given pump pressure, drilling by the erosion mechanism ceases. The bit then must be withdrawn from the borehole to replace the eroded nozzles even though other parts of the bit are in good repair. This involves withdrawing the entire drillstring from the borehole, replacing the bit or nozzles, and rerunning the drillstring into the borehole. This operation, known in the art as a round trip, is time consuming and expensive, particularly for deep wells.

In order to prolong bit life and thus reduce the frequency of round trip, attempts have been made to seal some of the nozzles by the use of sealing balls. Each of the sealing balls is introduced into the drilling fluid stream at the surface and is carried and lodged into a flow nozzle. The sealing balls are composed of a deformable material and are sized to effectively close the nozzle opening. By selecting the number of sealing balls in relation to the total number of nozzles, the nozzle flow area A can be effectively reduced to a fraction of its former value. Thus, for a constant power output P, the nozzle pressure p can be maintained above the threshold limit for a substantially longer period of time.

Field tests have shown, however, that the sealing balls tend to plug the inner nozzles, e.g., those located near the axis of rotation of the bit. This is probably due to the fact that the sealing balls are carried in the fluid stream at the point of maximum velocity, the center of the pipe. These tests have also shown that the drilling rate is substantially reduced when the inner nozzles are plugged. With the inner nozzlesplugged, the fluid flowing through only the outer nozzles cuts an annular groove in the formation rock leaving a central core which must be broken, drilled, or abraded by the auxiliary cutting devices such as the scraper blades or the diamond crown.

In theory, the plugging of some of the noules in order to reduce total nozzle flow area A appears to be sound but has not been successfully applied to date because of the resulting nonuniform distribution of nozzles left open.

SUMMARY OF THE INVENTION The present invention provides a method for selectively sealing nozzles so as to retain a uniform nozzle pattern. The method utilizes temporary plugs for sealing the central noale or nozzles thereby causing the fluid stream to carry permanent plugs to the outer nozzles. In practice, the temporary plug or plugs are first passed through the drillstring and lodged in the drill bit to close one or more of the inner nozzles. The permanent plugs are then passed through the drillstring and are carried in the fluid stream to the open nozzles. Finally, the temporary plugs are removed by a release mechanism which can be temperature sensitive, pressure sensitive or flow sensitive. For a given nozzle pattern, the number of temporary and permanent sealing plus can be selected so that the nozzles remaining open will be uniformly distributed; that is, inner nozzles as well as outer nozzles will be open.

The construction and composition of the temporary plugs will depend upon the type of mechanism employed. If a thermal release is desired, a plug composed of a thermoplastic material can be used. In this event, the thermoplastic selected has characteristic such'that it loses much of its strength as its temperature approaches the normal subsurface temperature of the strata being drilled. In operation, the thermoplastic plug is pumped down the drillstring and lodged in the appropriate nozzle which as indicated above will be one of the inner nozzles. The cooling effects of the circulating drilling fluid passing through the bit maintains the thermoplastic plug at a temperature below its softening point. The plug thus retains sufficient mechanical strength to provide a fluid seal in the nozzle in which it is lodged. With the plug sealingly lodged in the inner nozzle, the fluid flow is through the open nozzles permitting the permanent plug or plugs to be carried to and lodged therein. The number of permanent plugs is selected to seal some but not all of the outer nozzles. The permanent plugs can be composed of a tough resilient plastic material having a softening point substantially above the normal subsurface temperature of the strata. With the temporary and permanent plugs located, circulation is discontinued permitting equalization of the subsurface temperatures. As the temperature of the bit approaches the softening point of the thermoplastic material, the temporary plug loses its mechanical strength and becomes very pliable. Upon resumption of pumping operations, the pliable plastic material can be forced through the nozzle at relatively low pressures. The final pattern then comprises inner and outer nozzles open for conducting drilling fluid.

Another technique utilizes erodible plugs. The temporary plugs composed of a highly erodible material such as copper, zinc, aluminum, or steel are passed at time-spaced intervals through the drillstring and separately lodge in an inner nozzle. The temporary plugs are configurated to prevent the formation of a fluidtight seal; that is, there is always a small amount of leakage past the temporary plug lodged in its associated nozzle. The bulk fluid flow thus is through the open nozzles permitting the permanent plugs to be carried and lodged therein. Upon resumption of erosion drilling, the drilling fluid ry plugs can be used and can be released by a variety of mechanisms. The erodible plug technique, however, is preferred because it can be used at any depth and is released by a fluid flow which can be controlled by surface operations.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of an erosion bit showing the lateral distribution of the nozzles and associated flow passages along a longitudinal cutting plane.

FIG. 2 is a bottom view of the bit shown in FIG. 1 showing a typical nozzle pattern for an erosion bit.

FIG. 3 is an enlarged sectional view of an outer nozzle shown in FIG. 1 illustrating the sealing relationship of the permanent plug lodged in the nozzle inlet.

FIGS. 4 and 5 show two types of erodible plugs adapted to lodge in the inlets of flow nozzles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1 and 2 an erosion bit provided with a diamond crown is seen to include a body 11 and a shank 12 threaded for connection to the bottom of a tubular drillstring (not shown). The lower end of the body 11 is threaded for connection to acrown 13. Thecrown 13 is normally manufactured separately and later connected to the body and welded in place. Alternatively, the body 11 andcrown 13 can be fabricated in a single unit. Although the present invention is described in connection with a diamond bit, it should be observed that the erosion bit can be used without auxiliary cutting devices, or scraper blades can be used in place of the diamond crown.

Thecrown 13 includes asteel blank 14 surfaced with ametallic matrix 16 in which the diamond-cutting elements or similar hard abrasive resistant particles 17 are embedded. Acentral chamber 18 having aninlet 19 is formed in thebit 10 and is provided with a plurality of downwardly directed ports. Each of the ports is sized to receive a flow nozzle. The nozzles can be welded to thecrown 13 or as shown in the drawings, can be threadedly connected thereto. With thebit 10 connected to the drillstring, drilling fluid passed down the interior of the drillstring into thechamber 18 emerges from the nozzles as high velocity jet streams.

Although the nozzle pattern can conform to any desired geometry, limited only by the space limitations on thebit 10, it has been found that the pattern should be such to provide generally equally distributed jet streams across the bottom surface of thebit 10. Thus, as shown in FIG. 2, thebit 10 is provided with a set of inner nozzles, denoted 21, 22, 23, and 24, and a set of outer nozzles, denoted 25, 26, 27 and 28, arranged about the longitudinal axis of thebit 10, with the inner nozzles 21-24 being closer to the longitudinal axis than the outer nozzles 25-28. The radial location of thenozzles 21 through 28 in relation to the axis of bit rotation can vary so that upon rotation of the bit during drilling operations, the jet streams emerging therefrom cut a series of concentric kerfs in the formation rock. Theinner nozzles 21 through 24 can be oriented so that the jet streams impinge against the formation rock directly below thebit 10 whereas theouter nozzles 25 through 28 can be tilted outwardly so that the jet streams cut a borehole of sufficient diameter to permit advancement of the bit. This arrangement is more apparent from FIG. 1showing nozzles 21 and 23 mounted inports 29 and 30, respectively;

andouter nozzles 25 and 27 mounted inports 31 and 32, respectively. The bottom surface of thebit crown 13 in the area of the port exit can be debossed to facilitate installation and removal of the nozzles. Radially extending water courses l5 and 20 interconnect the debossed areas.

As shown in FIG. 3, each nozzle includes ahollow insert member 33 mounted in ametal sleeve 34. Theinsert member 33 is composed of a wear resistant material such as one of the hard carbides, e.g., tungsten carbide, and can be secured to themetal sleeve 33 by known brazing techniques or by the method disclosed in US. Pat. No. 3,131,779 to D. S. Rowley et al. The nozzle can be constructed to be welded to the steel blank 17 or alternatively and as illustrated can be threadedly connected thereto. Theinsert member 33 has anopening 36 of predetermined size extending therethrough, the inlet of which can be tapered to provide for streamlined flow. Thesleeve 34 has ahex head 35 formed in its lower end to facilitate screwing the nozzle into the bit. With the nozzle secured to the bit, the lower exposed end is disposed slightly above the lower surface of the crown l3.

Erosion drilling will generally be commenced after the borehole has been drilled to a certain depth using conventional rotary drilling techniques. Theerosion bit 10 provided with the properly sized nozzles is run on the drillstring. Theopening 36 in each nozzle will depend upon several factors including available pump capacity and desired jet velocity. Ex perience has shown, however, that theopenings 36 of from 1/32 to 86 inch will serve satisfactorily for pump capacities up to 15,000 p.s.i. These sizes provide jet velocities of at least 500 feet per second which has been found to be the minimum level for jets to effectively cut formation rock. With thebit 10 located at the bottom of the borehole, the drilling fluid is passed through the nozzles at such a pressure to provide highvelocity jet streams. The drilling mechanism is a combination of jet streams cutting concentric kerfs in the formation rock and the bit crown abrading the islands between the kerfs. As the drilling proceeds the nozzle inserts 33 eventually become eroded which results in an increased total effective flow area through thebit 10. Efforts to plug some of the nozzles by pumping resilient plastic balls through the drillstring have not been entirely successful. Because of the velocity profile of the stream flowing through the drillstring, the balls tend to lodge only in the inner nozzles (nozzles 21 through 24 of the bit shown in FIG. 2). Although the effective nozzle flow area can be reduced by this approach, experience has shown that the drilling rate is also reduced.

The present invention solves this problem by using temporary sealing elements or plugs in combination with permanent sealing elements or plugs. Briefly the invention contemplates the placement of sealing plugs, at least one of which is a temporary sealing plug, in theinner nozzles 21 through 24, so that the drilling fluid passing through thebit 10 exits through theouter nozzles 25 through 28. The introduction of permanent sealing plugs into the drilling fluid then will be carried by the fluid stream to theouter nozzles 25 through 28. The temporary sealing plugs are constructed such that they can be removed by a release mechanism sensitive to a subsurface condition, permitting the resumption of erosion drilling operations. Depending upon the composition of the temporary plugs, the release can be induced by pressure, temperature, or flow.

The permanent sealing plugs can be composed of a polymeric material capable of plastic deformation at the subsurface temperatures encountered. Polyte trafluoroethylene (TFE) has been found to be ideal for this service since it has the characteristics of cold flow through a wide temperature range and has a useful temperature up to 500 F. Nylon can also be used. The plastic sealing plugs, usually in the form of a sphere or ball, are sized such to pass through the ports and to sealingly lodge in the tapered inlets of the nozzle. The low specific gravity of TFE (2.1) and nylon (l.l) permit the permanent balls to be carried in the fluid stream. As shown in FIG. 3, the plastic ball designated 38 seats in the inlet of the nozzle and, as pressure is applied thereabove, deforms to the contour of the inlet. Tests have shown that the spheres composed of TFE are capable of maintaining a pressure seal at pressure differentials up to 15,000 psi.

As indicated above the temporary sealing plugs, depending upon the type of release mechanism used, can take a variety of forms and can be composed of a variety of materials.

In one embodiment the temporary plugs can be composed of an easily eroded material such as copper, zinc, aluminum or steel. in this embodiment the plug in the form of a sphere or ball is constructed to provide a small leakage past the ball when lodged in the nozzle. The small amount of leakage past the ball can be achieved by providing the ball with an irregular surface or by providing the ball with a plurality of diametric holes. Referring first to FIG. 4, aball 39 composed of copper, zinc, or other erodible material has a roughened or irregular outer surface. Such a surface can be formed by molding or by working the outer surface of the ball as by filing or cutting to form a roughened surface. The irregular surface prevents the ball from providing a fluidtight seal when lodged in the nozzle. Under a high differential pressure then there will be a small amount of fluid flow around the ball seated on the inlet ofopening 36. As the upstream pressure increases as when erosion drilling is resumed, the flow around the irregular surface quickly erodes theball 39 away. The same result can be achieved by drilling a plurality of smalldiametric holes 40 through a ball 41 (see FIG. 5). The spacing and number ofholes 40 are such that one will register with opening 36 with theball 41 seated in a nozzle. Here, the flow is through the ball If a mechanism of thermal decomposition is used, the temporary sealing plugs can be balls composed of a material such as a thermoplastic having a softening point at or below the subsurface temperature of the formation being drilled. Thermoplastics that can be used as temporary plugs include lowdensity polyethylene, olefin copolymers such as ethylene ethyl acrylate and ethylene vinyl acetate, polyvinyl butural, polystyrene, to name but a few thermoplastics that have relatively low softening points (between 115 F and 200 F). Of course for deeper wells, thermoplastics having higher softening points can be used. These thermoplastic polymers, are easily molded into the desired shape and generally soften or lose their mechanical strength at temperatures at or below their softening points, permitting them to be dislodged from the nozzles. During the placement of the thermoplastic sealing balls, the drilling fluid maintains the plastic material in the solid condition. Thus the thermoplastic balls lodged in the nozzles provide a fluidtight seal permitting the subsequent placement of the permanent sealing balls. With all of the balls placed, the circulation of the drilling fluid can be discontinued causing the thermoplastic materials to lose their mechanical strength as the temperature of the bit approaches the formation temperature. The pliable thermoplastic balls then can be forced through the nozzles by increasing the fluid pressure in thebit 10. The specific gravity of the temporary plug should be large enough to cause some settling in the drilling fluid. Although the thermoplastics listed above are relatively light (specific gravity range: 0.9 through 1.1), most are available in filled form having specific gravities from 1.3 to 1.7.

In practice, the erosion bit as installed will be used until the drilling rate has reduced sufficiently to indicate severe nozzle erosion. The sealing plugs, at least one of which is a temporary plug, are pumped through the drillstring to plug the inner nozzles. 1f the nozzle pattern of the bit is that shown in FIG. I, the plugs for sealing theinner nozzles 21 through 24 can include two temporary sealing plugs and two permanent sealing plugs. These plugs are pumped down at time-spaced intervals each lodging in one of the inner nozzles. Two permanent plugs introduced into the drilling fluid stream at timespaced intervals then will be carried to the outer nozzles. Thus when erosion drilling is resumed, two of the inner nozzles are temporarily plugged, two are permanently plugged, two of the outer nozzles are permanently plugged, and two are open. Upon resumption of erosion drilling, the temporary plugs are quickly removed leaving a pattern wherein two inner nozzles and two outer nozzles are open thus providing distributed jets for cutting concentric kerfs in the formation rock. The total nozzle flow area has been reduced by 50 percent enabling.

the bit from the well which comprises: closing the inner noz-' zles with plugging means so that the bulk flow of fluid is through the outer nozzles, said plugging means including a temporary plug for at least one inner nozzle; flowing a plugging element thro ugh the drillstring and into an outer nozzle to pressure seal said outer nozzle; and thereafter removing said temporary plug from said at least one inner nozzle.

2. in a well-drilling operation wherein a drilling fluidis flowed through a tubular drillstring and a bit provided with inner and outer flow nozzles, said. nozzles being arranged about the longitudinal axis of said bit with said inner nozzles being closer to said axis than said outer nozzles a method for reducing the total flow area of said nozzles without removing the bit from the well which comprises: passing a plugging element into each of said inner nozzles to substantially close the inner noules so that the bulk fluid flow is through the outer nozzles, at least one of said plugging elements in said inner nozzles being a temporary plugging element; passing a'per-fl manent plugging element into an outer nozzle to pressure seal V said outer nozzle; and thereafter removing said temporary plugging element.

3. The invention as recited in claim 2 wherein said temporary plugging element is composed of an erodible material and is configurated to prevent the formation of a fluid tight seal when lodged in its associated nozzle; and wherein the step of removing said temporary plugging element is performed by passing fluid past said temporary plugging element lodged in its associated nozzle at such a rate to erode said erodible material.

4. The invention as recited in claim 3 wherein said temporary plugging element is spherical in shape and configurated to have an irregular outer surface.

5. The invention as recited in claim 3 wherein said temporary plugging element is spherical in shape and has a plurality of holes extending therethrough, the arrangement of said holes being such that at least one hole is in registry with the nozzle for any position of said plugging element in said nozzle.

6. The invention as recited in claim 3 wherein said erodible material is aluminum.

7. The invention as recited in claim 3 wherein said erodible material is copper. I

8. The invention as recited in claim 2 wherein said temporary plugging element is composed of a deformable material such that in the unstressed condition said temporary plugging element provides a fluid seal preventing the flow of fluid through said nozzle, and wherein the step of removing said temporary plugging element is performed by increasing the pressure in said bit to deform and force said-plugging element composed of deformable material through said nozzle.

9. The invention as recited in claim 8 wherein said deforma ble material is a thermoplastic having a softening point near the normal subsurface temperature of the strata being drilled.

10. The invention as recited in claim 2 wherein said permanent plugging element is composed of a solid polymeric material. Y

11. The invention as recited inclaim 10 wherein said polymeric material is polytetrafluoroethylene. I

12. The invention as recited inclaim 10 wherein said polymeric material is nylon. Iv

Claims (12)

1. In a well-drilling operation wherein a drilling fluid is passed down a tubular drillstring and through a bit provided with inner and outer flow nozzles, said nozzles being arranged about the longitudinal axis of said bit with said inner nozzles being closer to said axis than said outer nozzles, a method for reducing the total flow area of said nozzles without removing the bit from the well which comprises: closing the inner nozzles with plugging means so that the bulk flow of fluid is through the outer nozzles, said plugging means including a temporary plug for at least one inner nozzle; flowing a plugging element through the drillstring and into an outer nozzle to pressure seal said outer nozzle; and thereafter removing said temporary plug from said at least one inner nozzle.

2. In a well-drilling operation wherein a drilling fluid is flowed through a tubular drillstring and a bit provided with inner and outer flow nozzles, said nozzles being arranged about the longitudinal axis of said bit with said inner nozzles being closer to said axis than said outer nozzles, a method for reducing the total flow area of said nozzles without removing the bit from the well which comprises: passing a plugging element into each of said inner nozzles to substantially close the inner nozzles so that the bulk fluid flow is through the outer nozzles, at least one of said plugging elements in said inner nozzles being a temporary plugging element; passing a permanent plugging element into an outer nozzle to pressure seal said outer nozzle; and thereafter removing said temporary plugging element.

3. The invention as recited in claim 2 wherein said temporary plugging element is composed of an erodible material and is configurated to prevent the formation of a fluid tight seal when lodged in its associated nozzle; and wherein the step of removing said temporary plugging element is performed by passing fluid past said temporary plugging element lodged in its associated nozzle at such a rate to erode said erodible material.

4. The invention as recited in claim 3 wherein said temporary plugging element is spherical in shape and configurated to have an irregular outer surface.

5. The invention as recited in claim 3 wherein said temporary plugging element is spherical in shape and has a plurality of holes extending therethrough, the arrangement of said holes being such that at least one hole is in registry with the nozzle for any position of said plugging element in said nozzle.

6. The invention as recited in claim 3 wherein said erodible material is aluminum.

7. The invention as recited in claim 3 wherein said erodible material is copper.

8. The invention as recited in claim 2 wherein said temporary plugging element is composed of a deformable material such that in the unstressed condition said temporary plugging element provides a fluid seal preventing the flow of fluid through said nozzle, and wherein the step of removing said temporary plugging element is performed by increasing the pressure in said bit to deform and force said plugging element composed of deformable material through said nozzle.

9. The invention as recited in claim 8 wherein said deformable material is a thermoplastic having a softening point near the normal subsurface temperature of the strata being drilled.

10. The invention as recited in claim 2 wherein said permanent plugging element is composed of a solid polymeric material.

11. The invention as recited in claim 10 wherein said polymeric material is polytetrafluoroethylene.

12. The invention as recited in claim 10 wherein said polymeric material is nylon.

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