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Publication numberUS3216497 A
Publication typeGrant
Publication dateNov 9, 1965
Filing dateDec 20, 1962
Priority dateDec 20, 1962
Publication numberUS 3216497 A, US 3216497A, US-A-3216497, US3216497 A, US3216497A
InventorsFast Clarence R, Howard George C
Original AssigneePan American Petroleum Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Gravel-packing method
US 3216497 A
Images(2)
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Description  (OCR text may contain errors)

' Nov. 9, 1965 G. c. HOWARD ETAL 3,216,497

GRAVELFPACKING METHOD Filed Deo. 20, 1962 2 Sheets-Sheet 1 CLARENCE R. FAST GEORGE C. HOWARD INVENTORS ATTORNEY N0V- 9, 1955 G. c. HOWARD ETAL 3,215,497

GRAVEL-PACKING METHOD 2. Sheets-Sheet 2 Filed Dec. 20, 1962 FIG. 5

FIG. 4

CLARENCE R. EAST GEORGE c. How/.mn mvENToRs Qwjfe/ ATTORNEY United States Patent O 3,216,497 GRAVEL-PACKING METHGD George C. Howard and Clarence R. Fast, Tulsa, kla., assignors to Pan American Petroleum Corporation, Tulsa, kla., a corporation of Delaware Filed Dec. 20, 1962, Ser. No. 246,627 4 Claims. (Cl. 166-19) This invention concerns a method of preventing the flow of sand particles into a well as fluids enter the well from an unconsolidated sand formation. More particularly, the method comprises placing a gravel pack in a washed-out section surrounding a well casing by first sealing existing perforations in the casing, then perforating the casing at the upper and lower extremities of the washed-out section, and thereafter circulating a slurry of gravel between the new perforations until the section has been completely packed with gravel.

A well producing from an unconsolidated formation is usually completed by first cementing a casing through the formation, and then perforating the casing and cement sheath at intervals throughout the producing zone. If the well is then placed on production without further treatment or safeguards, sand usually is carried into the well along with the formation fluids. Aside from the abrasive effects of the sand on the casing, tubing, and flowing or pumping equipment, a washed-out section or cavity develops outside the casing. This sometimes results in collapsed casing when caving occurs in the cavity.

Over the years, many techniques have been tried for placing a gravel pack in a well to separate and exclude sand fines from the produced fluids. In general, these packs have been put in place either by pumping a gravel slurry through all of the perforations simultaneously, with the carrier liquid filtering into the formation; or the gravel has been first deposited in the bottom of the well, and a screen is then lowered on a tubing and washed down into the gravel by circulating liquid to iluidize the gravel. In some instances, -the sand has been excluded satisfactorily. In others, success has been short-lived. Sand production resumed after only a brief period of exclusion.

An analysis of these successes and failures usually does not reveal that any particular type of gravel or screen for retaining the gravel is superior. Instead, the degree of success frequently appears to correlate with the extent to which the washed-out or open section of the unconsolidated formation has been filled with gravel. It is now believed that an incomplete fill-up of gravel between the unconsolidated formation and the screen enables the gravel particles to move about as fluids flow from the formation into the well. The result is that the small sand particles also can move through the loose gravel to the screen. The openings in the screen usually are large enough to pass these sand fines but not the gravel. As these sand particles enter the well, the section not filled with gravel enlarges. Continual movement of the gravel and sand abrades the screen until holes large enough to pass the gravel also may be worn in it, whereupon the well must be shut in and repaired.

We have now developed a method of gravel-packing a washed-out section outside of a perforated casing which produces a substantially complete fill-up of gravel, thereby preventing movement of the gravel particles as iluids flow through. Thus, instead of allowing passage of said grains, they are effectively filtered from the flowing iluids so that further washing-out cannot occur.

It is therefore an object of our invention to produce an effective and improved gravel pack in a washed-out section of unconsolidated formation outside of a perforated casing. Another object is to confine an unconsolidated 3,216,497 Patented Nov. 9, 1965 sand formation and substantially completely prevent movement of the sand particles as fluids flow from the formation into the well casing. A further object is to prevent the entrainment of solid particles in the fluids entering the well. A still further object is to prevent abrasion and erosion of the well equipment by sand particles entrained in the fluids entering a well from a producing formation.

These objects are accomplished by the novel method which we have developed for placing a gravel pack in a well, which method comprises first sealing off all existing perforations, then carefully positioning new perforations with respect to the cavity to be filled, circulating a carrier liquid and gravel particles through these perforations to fill the cavity completely, and then resuming production through the gravel pack in such a way as not t0 disturb the pack particles. This method will now be described in more detail by referring to the accompanying drawings. In these drawings:

FIGURE 1 illustrates in cross-section a typical well having a washed-out section in the formation adjacent a perforated well casing;

FIGURE 2 shows the well with the original casing perforations sealed and new perforations made near the upper and lower extremities of the washed-out section;

FIGURE 3 shows a gravel-retaining screen and gravelplacement equipment in position to fill the washed-out section;

FIGURE 4 illustrates the removal of excess gravel from the well after the cavity behind the casing has been lled with gravel; and

FIGURE 5 shows the completed gravel pack with the production tubing re-installed to withdraw well fluids.

Referring to FIGURE l, an oil-producing formation 11 is penetrated by a well 9 having casing 10 therein. Fluids from the formation enter the casing through perforations 13 and are withdrawn from the well through tubing 14, suspended in well head 15. Gas may be drawn lfrom the well annulus via flow line 16 at the well head. A washed-out section or cavity 12 `surrounding the perforated section of the casing 10 is representative of the cavities which are produced by the flow of sand-containing fluids into the well, when formation 11 is a poorly consolidated sand.

In FIGURE 2 the well 9 is shown with the original perforations sealed and new perforations 17 and 1 respectively near the lower and upper extremities of cavity 12. An impermeable liner 25 covers the original perforations, which may also be filled with plastic plugs 30.

An apparatus which may be employed to circulate gravel into the cavity 12 is shown in FIGURE 3. The apparatus is attached to a tubing 24 at a threaded connection 23 in a valve body 22. A ball check 19 is held against the seat 21 of valve 22 by a spring 2t) to prevent back-flow of gravel into the tubing. A slotted or perforated screen 26 extends above the check valve assembly 22 to a point just above upper perforations 18 to prevent the ilow of gravel into the well. An annular seal 27 above the upper perforations 18 and between liner 25 and screen 26 prevents ilow of gravel and carrier liquid upwardly past the top end of screen 26. A lower annulus seal 28 on valve body 22 is provided to prevent flow of gravel slurry either upwardly from the check valve outlet, or downwardly past the valve outlet from upper perforations 18. The gravel slurry passing down through check valve 22 is thus forced to flow outwardly through perforations 17 during pack placement.

Preferably, but not necessarily, the outer surface of screen 26 is covered by a sleeve 29 over most of its length, from seal 27 downwardly to a point near but somewhat above the lower end of the screen. By thus leaving open 3, the slots in screen 26 immediately above valve body 22, a downward circulation of gravel slurry to completely pack the annular space between seals 27 and 28 and between liner 25 and screen 27 is assured. If sleeve 29 is omitted, however, gravel particles will usually settle out by gravity and lill this space, as the carrier liquid flows inwardly through the screen openings.

FIGURE 4 illustrates the removal of excess gravel from the well following placement of gravel 31 in the cavity 12 outside the casing 10. Tubing 24 is disconnected from valve body 22 at back-olf threads 23 and raised a short distance so that a flushing liquid can be circulated down the tubing and up the inside of screen 26 and casing 10 to discharge the excess gravel through flow line 16 at the well head. Also, a solvent for the material of sleeve 29 forming a temporary seal may be deposited inside the screen 26 after the excess gravel has been circulated out.

The completed gravel pack is shown in FIGURE 5 with production tubing 14 re-installed. Gravel 31 completely lls the cavity 12 which had been washed out in the unconsolidated formation 11. Spring 20 holds ball check 19 against valve seat 21 so that uids cannot enter the well at lower perforations 17 and carry gravel into the screen and tubing. Fluids entering the well through upper perforations 1S are iiltered through the gravel and screen between annular seals 27 and 28.

In carrying out our invention, production tubing 14 is rst removed from the well 9. Thereafter, perforations 13 in the well casing are plugged. We prefer to plug these perforations by one of the methods which have been developed recently for repairing leaks in casing. U.S. Patents 3,028,915 (Jennings) and 3,047,065 (Vincent) teach methods for placing a glass ber and resin liner inside a well casing so that the perforations 13 are lled by plugs 30 of synthetic resin or plastic material.

In the alternative, the perforations 13 may be sealed with a steel liner 25 as shown in FIGURE 2.

This method for sealing holes in casing utilizes a steel liner having longitudinal corrugations so that it is of reduced diameter and can be coated with an adhesive such as epoxy resin, then lowered into the casing and expanded by smoothing out the corrugations, producing a strong, cylindrical liner, tting snugly inside the casing. A portion of the resin is extruded into the perforations, forming the plugs 30 therein. A detailed description of this method for placing the steel liner inside a well casing is given in the paper by Vincent et al. in the Society of Petroleum Engineers of AIME, Production Research Symposium, Tulsa, Oklahoma, Apr. l2-l3, 1962, pages 21-28.

After liner 25 has been installed and perforations 13 plugged, perforations 17 and 18 are made near the lower and upper extremities of the washed-out section. lt is desirable that the perforations in each set be at substantially the same level. Shaped-charge casing perforators are presently available which can make as many as eight perforations in a single horizontal plane at one ring. One of these charges is fired at each level to produce the necessary perforations. As all of the formation fluids will enter the casing 10 through perforations 18 in subsequent producing operations, the perforations should have as large a total area as possible, so that the fluids pass through at low velocity. This helps avoid erosion of the screen 26.

With the liner 25 installed and perforations 17 and 18 made at opposite ends of the cavity 12, apparatus is assembled to place gravel in the cavity. The threaded connection 23 for attaching check valve body 22 to tubing 24 is preferably one that can be disconnected easily, such as is used in tubing safety joints. These joints typically have either rightor left-handed coarse threads which can be unscrewed at low torque when either tension or compression is applied to the tubing joint. Screen 26 is attached to the top of valve body 22 by welding or in any other suitable way. The screen may be woven wire,

wound wire, or a slotted tubular sleeve of any suitable type ordinarily used to support a gravel pack. Rubber swab cups or similar devices are attached to the outside of the screen and valve body to form the annular seals 27 and 28. Temporary sealing sleeve 29 is then applied to the screen from seal 27 downwardly to within a few inches of the bottom screen openings. This sleeve is desirably produced by coating the screen with a material which can be dissolved in oil, water, or other solvent. ln situations where there is no oil standing in the well and the gravel can be placed in an aqueous slurry, paraffin Wax or an oil-soluble hydrocarbon resin such as that taught in U.S. 2,067,073, Carmody, may be used to coat the screen. In situations where the screen will be contacted with oil, the coating should be a water-soluble material such as sugar, starch, polyvinyl alcohol, sodium chloride, or other similar compound which can be crystallized on the screen, then subsequently dissolved with water. Where both oil and water may be encountered during placement of the gravel pack, the screen can be covered with a thin sheet of metal such as magnesium, which is dissolved later with hydrochloric acid. Another suitable type of sealing sleeve 29 is that produced with a polymer such as polyvinyl chloride or polyvinyl acrylate which can be later dissolved with tetrachloroethylene.

This screen assembly is then lowered into the well on tubing 24 until upper annular seal 27 engages the upper end of liner 25. The Seal should enter the end of the liner, but care should be exercised to avoid lowering it as far as upper perforations 18. Although the length of screen 26 and the position of lower seal 2S and check valve body 22 relative to lower perforations 17 are not critical, it is generally preferred to place the check valve 22 only a short distance above the lower perforations. In this position the screen is opposite the upper perforations, with upper and lower seals 27 and 28 respectively directing flow from the upper perforations 18 through the screen openings.

With the screen assembly positioned as described above, a slurry of gravel in a carrier liquid is pumped down the tubing, through the check valve 22 and into the casing adjacent the lower perforations 17. The slurry passes through these perforations into cavity 12 surrounding the casing. After the slurry fills cavity 12, it then enters perforations 1S, and gravel filters out against the screen 26 between annular seals 27 and 28 as the carrier uid flows through the unsealed section of the screen above the lower seal. After the annulus between the screen and liner segregated by these seals has been filled with gravel, further circulation of the slurry packs the gravel in the cavity with the carrier-liquid ltrate either passing out into formation 11 or being removed through perforations 18 and the screen 26, and then up the casing annulus. With circulation in this manner, the gravel has a tendency to pack the cavity from the top to the bottom, thereby producing a uniform pack. The upward flow of the slurry agitates or fluidizes gravel particles which otherwise might settle toward the bottom of the cavity and produce a bridge before the cavity is completely lled with gravel. When the cavity 12 becomes packed with gravel, continued circulation of the slurry iirst packs the casing 10 below the check valve 22 and then gravel begins to back up in the tubing 24. When a pronounced and continuing increase in injection pressure signals this condition, slurry circulation is discontinued.

After the pack has thus been placed, it is necessary to withdraw slurry circulation tubing 24 without permitting movement of the gravel pack. This is accomplished simply by disconnecting the tubing at back-olf connection 23 so that the check valve 22 retains the gravel. After tubing 24 has been disconnected, it is lifted about one foot above the valve body and gravel-free liquid is circulated down through the tubing and up through the screen and casing annulus to remove the excess gravel, as shown in FIGURE 4. As an alternative to direct circulation down the tubing, up the annulus and out flow line 16, reverse circulation down the annulus and up the tubing may be used to remove the eXcess gravel.

Following the gravel clean-out step, a solvent for temporary seal sleeve 29 is circulated down the tubing and discharged inside the screen above the check valve. The solution of the sealing material in the solvent may be removed either by circulating it from the well or by displacing it into the formation. It is to be understood that the thickness of sleeve 29 is small enough that dissolving it does not cause or allow any appreciable movement of the annulus pack particles.

The gravel-placement tubing 24 is then withdrawn and replaced by production tubing 14 as shown in FIGURE 5. Although fluids from the formation can enter the well only through the upper perforations 18, the entire area of the screen 26 is open to let the iluids flow into tubing 14. The gravel used in forming the gravel pack according to one invention may be any of a number of different hard, inert materials including nutshells, sand, gravel, limestone, glass, and the like. For the most effective sand restraining ability, the particle size of the gravel should be selected on the basis of the particle size of the sand to be restrained. Gravel, 85 weight percent of which passes a 40-mesh and is retained on a 60-mesh screen, U.S. Standard sieve, is suitable for many of the unconsolidated sand formations in the United States Gulf Coast area. Also, it has been found that the particles in the pack are less likely to move and allow sand passage if they are angular in shape rather than rounded. Thus, we prefer to use gravel produced by crushing larger particles, and then screening to recover the desired particle sizes. The carrier uid may be water, crude oil, or a petroleum fraction such as fuel oil.

Although we have described our invention as a procedure wherein the gravel slurry is injected at the lower end of the washed-out zone, it should be understood that the zone could be packed by injecting the slurry at the upper perforations and withdrawing the ltrate at the lower perforations. To place the pack in this manner, it is necessary to place a sand-retaining screen in the well bore below the upper perforations, so as to intercept gravel entering the lower perforations from the cavity 12, and then connect the upper perforations to the tubing outlet. That is, apparatus may be used which will enable the gravel pack to be placed as the slurry is circulated vertically through the cavity 12, with `at least some of the ltrate returning to the well rather than passing into the formation.

Our invention therefore should not be considered as limited to the embodiments described in detail, but its scope is properly to be ascertained from the appended claims.

We claim: 1. A method of gravel-packing a washed-out section in an unconsolidated formation penetrated by a well casing having perforations opening into said washed-out section comprising:

placing a liner in said casing to seal said perforations, making multiple upper or lower perforations through said casing and liner near the upper and lower extremities respectively of said washed-out section,

lowering a uid conduit in said well, said conduit having a gravel-retaining screen attached to the lower end thereof with a check valve in said screen in cornmunication with said uid conduit, said screen having near each of its top and bottom ends an annular uid seal between it and said liner and a soluble material sealing its openings except near said bottom end,

positioning said screen opposite said upper perforations and above said lower perforations, with said topend Seal @Qing 'lbQVtall@ said bottom-end seal being below said upper perforations,

circulating a slurry of gravel in a carrier liquid through said conduit and check valve and thence through said lower perforations into said washed-out section, while at least a part of said carrier liquid returns to said well through said upper perforations and screen, thereby depositing said gravel in said washed-out section and against said screen,

disconnecting said flow conduit from said screen and depositing a solvent for said soluble sealing material in said screen, and

thereafter removing said solvent and dissolved sealing material from said screen.

2. A method of gravel-packing a washed-out section in an unconsolidated formation penetrated by a well casing having perforations opening into said washed-out section comprising:

placing a liner in said casing to seal said perforations,

perforating said casing and liner with upper and lower perforations respectively; near the upper and lower extremities of said washed-out section,

placing a screen in said liner opposite said upper perforations, the upper portion of said screen being sealed with a temporary plugging material, said screen having means respectively above and below said upper perforations for effecting a fluid seal between said liner and each of the ends of said screen,

injecting a slurry of gravel in a carrier liquid into said well below said screen, whereby said slurry enters said lower perforations and at least part of said carrier liquid returns to said well through said upper perforations, while depositing gravel in said washedout section,

continuing to inject said slurry and remove carrier liquid until said gravel lls said washed-out section, and

thereafter removing said temporary plugging material from the upper portion of said screen. 3. A method of gravel-packing a washed-out section in an unconsolidated formation penetrated by a well casing having perforations opening into said washed-out section comprising:

placing a liner in said casing to seal said perforations, making multiple upper and lower perforations through said casing and liner near the upper and lower eX- tremities, respectively, of said washed-out section,

placing in said liner at least opposite said upper perforations a gravel-retaining screen having a check valve in its lower end, a Huid seal between the body of said check valve and said liner above said lower perforations, and a uid seal between said screen and said liner above said upper perforations,

circulating a slurry of gravel in a carrier liquid through said check valve and said lower perforation into saidV washed-out section, with at least part of said carrier liquid returning to said casing through said upper perforations and screen after depositing said gravel in said washed-out section, until said section is substantially completely gravel-filled, and

removing the excess of said gravel from said well casing.

4. The method of claim 3 wherein said gravel comprises angular particles at least weight percent of which pass a lO-mesh and are retained on a 60-mesh U.S. Standard sieve screen.

References Cited by the Examiner UNITED STATES PATENTS 2,207,334 7/40 Reynolds et al. 166--19 2,224,630 12/40 Dean et al. 166-51 X 2,844,205 7/58 Carothers 166--35 X CHARLES E. OCONNELL, Primary Examiner.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2207334 *Mar 20, 1939Jul 9, 1940Union Oil CoMethod and apparatus for placing a filter body in a well
US2224630 *Sep 11, 1939Dec 10, 1940Socony Vacuum Oil Co IncScreen pipe with fragile lining
US2844205 *Dec 20, 1955Jul 22, 1958Exxon Research Engineering CoMethod for completing and servicing a well
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3289761 *Apr 15, 1964Dec 6, 1966Smith Robbie JMethod and means for sealing wells
US3295603 *Apr 30, 1964Jan 3, 1967Continental Oil CoMethod and apparatus for production well completion
US3353599 *Aug 4, 1964Nov 21, 1967Gulf Oil CorpMethod and apparatus for stabilizing formations
US3999608 *Sep 22, 1975Dec 28, 1976Smith Donald MOil well gravel packing method and apparatus
US4018282 *Feb 26, 1976Apr 19, 1977Exxon Production Research CompanyMethod and apparatus for gravel packing wells
US4018283 *Mar 25, 1976Apr 19, 1977Exxon Production Research CompanyMethod and apparatus for gravel packing wells
US4202411 *May 24, 1978May 13, 1980Baker International CorporationMagnesium oxide react with magnesium chloride to provide impervious matrix
US4239084 *Jul 11, 1979Dec 16, 1980Baker International CorporationAcid soluble coating for well screens
US4498543 *Apr 25, 1983Feb 12, 1985Union Oil Company Of CaliforniaMethod for placing a liner in a pressurized well
US4673039 *Jan 24, 1986Jun 16, 1987Mohaupt Henry HOil and gas; produce formation fluids into conduit
US4932474 *Jul 14, 1988Jun 12, 1990Marathon Oil CompanyStaged screen assembly for gravel packing
US4995456 *May 4, 1990Feb 26, 1991Atlantic Richfield CompanyGravel pack well completions
US5062484 *Aug 24, 1990Nov 5, 1991Marathon Oil CompanyMethod of gravel packing a subterranean well
US5269375 *Jul 28, 1992Dec 14, 1993Schroeder Jr Donald EMethod of gravel packing a well
US5287923 *Jul 28, 1992Feb 22, 1994Atlantic Richfield CompanySand control installation for deep open hole wells
US5309994 *Jun 17, 1993May 10, 1994U.S. Army Corps Of Engineers As Represented By The Secretary Of The ArmyMethod and apparatus for installing a well
US5330003 *Dec 22, 1992Jul 19, 1994Bullick Robert LGravel packing system with diversion of fluid
US5355956 *Sep 28, 1992Oct 18, 1994Halliburton CompanyPlugged base pipe for sand control
US5409061 *May 4, 1994Apr 25, 1995Bullick; Robert L.Gravel packing system with fracturing and diversion of fluid
US5526881 *Jun 30, 1994Jun 18, 1996Quality Tubing, Inc.Preperforated coiled tubing
US5622211 *Jun 7, 1995Apr 22, 1997Quality Tubing, Inc.Preperforated coiled tubing
US6367547Apr 16, 1999Apr 9, 2002Halliburton Energy Services, Inc.Downhole separator for use in a subterranean well and method
US6793017 *Jul 24, 2002Sep 21, 2004Halliburton Energy Services, Inc.Method and apparatus for transferring material in a wellbore
US7204316Jan 20, 2004Apr 17, 2007Halliburton Energy Services, Inc.Expandable well screen having temporary sealing substance
US7451815Aug 22, 2005Nov 18, 2008Halliburton Energy Services, Inc.Sand control screen assembly enhanced with disappearing sleeve and burst disc
US7673673 *Aug 3, 2007Mar 9, 2010Halliburton Energy Services, Inc.Apparatus for isolating a jet forming aperture in a well bore servicing tool
US7775285Nov 19, 2008Aug 17, 2010Halliburton Energy Services, Inc.Apparatus and method for servicing a wellbore
US7963331Jan 21, 2010Jun 21, 2011Halliburton Energy Services Inc.Method and apparatus for isolating a jet forming aperture in a well bore servicing tool
US8215385 *Jul 9, 2007Jul 10, 2012Cooke Jr Claude EApplication of degradable polymers in sand control
US8272443Nov 12, 2009Sep 25, 2012Halliburton Energy Services Inc.Downhole progressive pressurization actuated tool and method of using the same
US8276675Aug 11, 2009Oct 2, 2012Halliburton Energy Services Inc.System and method for servicing a wellbore
US8439108 *Jul 6, 2012May 14, 2013Claude E. Cooke, Jr.Application of degradable polymers in sand control
US8662178Sep 29, 2011Mar 4, 2014Halliburton Energy Services, Inc.Responsively activated wellbore stimulation assemblies and methods of using the same
US8668012Feb 10, 2011Mar 11, 2014Halliburton Energy Services, Inc.System and method for servicing a wellbore
US8668016Jun 2, 2011Mar 11, 2014Halliburton Energy Services, Inc.System and method for servicing a wellbore
US8695710Feb 10, 2011Apr 15, 2014Halliburton Energy Services, Inc.Method for individually servicing a plurality of zones of a subterranean formation
US20120267101 *Jul 6, 2012Oct 25, 2012Cooke Jr Claude EApplication of Degradable Polymers in Sand Control
US20130206393 *Dec 19, 2012Aug 15, 2013Halliburton Energy Services, Inc.Economical construction of well screens
WO1994002707A1 *May 3, 1993Feb 3, 1994Marathon Oil CoMethod of gravel packing a well
WO1994015064A1 *Dec 21, 1993Jul 7, 1994Robert L BullickGravel packing system with diversion of fluid
WO2005073506A1 *Jan 11, 2005Aug 11, 2005Halliburton Energy Serv IncExpandable well screen having temporary sealing substance
Classifications
U.S. Classification166/278, 166/295, 166/296, 166/376, 166/51
International ClassificationE21B43/04, E21B43/02
Cooperative ClassificationE21B43/04
European ClassificationE21B43/04