Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS3692110 A
Publication typeGrant
Publication dateSep 19, 1972
Filing dateDec 31, 1969
Priority dateDec 31, 1969
Publication numberUS 3692110 A, US 3692110A, US-A-3692110, US3692110 A, US3692110A
InventorsGrady Guido O
Original AssigneeCities Service Oil Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
In situ retorting and hydrogenation of oil shale
US 3692110 A
Abstract
Initiation and support of the in situ retorting and hydrogenation of oil shale is accomplished by injecting a retorting fluid at a multiplicity of points along a substantially horizontal plane in the chimney created by a nuclear explosion. The retorting fluid is injected through an injection well extending generally horizontally in the chimney, preferably in the uppermost portion thereof. A series of such injection wells may be employed in order to provide for the distribution of the retorting fluid substantially across the cross-sectional area of the chimney and subsequent hydrogenation and upgrading of the oil shale produced.
Images(1)
Previous page
Next page
Claims  available in
Description  (OCR text may contain errors)

United States Patent Slater ..166/263 1151 3,692,110 Grady [45] Se t. 19, 1972 [54] IN SITU RETORTING AND 3,102,588 9/1963 Fisher ..166/261 HYDROGENATION OF OIL SHALE 3,342,257 9/1967 Jacobs et a1. ..166/247 3,465,819 9/1969 Dixon ..166/247 [72] Invent Grady Tulsa Okla 3,490,529 1/1970 Parker ..166/247 [73]' Assignee: Cities Service Oil Company 3,513,913 5/1970 Bruist ..166/247 X [22] Filed: Dec. 31, 1969 3,533,469 10/1970 Parker ..166/247 X [21] App]. No.: 889,710 Primary Examiner-Stephen J. Novosad Attorney-J. Richard Geaman Related US. Application Data [63] Continuation-impart ofSer. No. 718,115, April [57] ABSTRACT 12, 1968,Pat. No. 3,601,193. Initiation and support of the in situ retorting and hydrogenation of oil shale is accomplished by injecting [52] US. Cl. ..166/245, 166/247, 166/248, 8 retorting fluid at a multiplicity of points along a sub- 166/259, 166/270 stantially horizontal plane in the chimney created by a 51 Int. Cl 3.13211) 43/24, E2lb 43/26 a nuclear e p The rewrting fluid is injected 58 Field of Search ..166/245, 247, 248, 256, 270, through an injection Well extending generally horizon- 166/259461, 263, 272, 299, 271 300 tally in the chimney, preferably in the uppermost portion thereof. A series of such injection wells may be [56] References Cited employed in order to provide for the distribution of the retorting fluid substantially across the cross-sec- UNITED STATES PATENTS tional area of the chimney and subsequent hydrogenad f th h 1 d d. 3,303,881 2/1967 Dixon ..166/247 and pgra mgo e 0115 a e pm 8 Claims, 2 Drawing, Figures PATENTEDSEP 19 m2 GUIDO O. GRADY, INVENTOR.

ATTORNEY.

IN SITU RETORTING AND HYDROGENATION OF OIL SHALE This application is a continuation in part of copending application, Ser. No. 718,115 filed Apr. 12, 1968, and now US. Pat. No. 3,601,193.

BACKGROUND OF THE INVENTION This invention relates to the recovery of hydrocarbons from oil shale. More particularly, it relates to the recovery of shale oil by in situ retorting of underground formations of oil shale.

The hydrocarbons contained in the vast deposits of oil shale, in the United States and elsewhere, cannot be recovered by conventional petroleum recovery techniques. The oil shalemust be subjected to destructive distillation or retorting to recover shale oil. Conventional mining followed by above ground retorting may be employed but is accompanied by various undesirable aspects well known in the art.

In situ recovery techniques are likewise confronted with significant problems. Such techniques have not been successful in recovering economical amounts of shale oil from the massive oil shale deposits that often are covered by an appreciable overburden. The very low natural permeability of oil shale has been a significant factor in the relative ineffectiveness of in situ retorting techniques. The oil shale must be extensively fractured or broken up into a rubble of relativelyhigh and uniform permeability in order to permit a nearly uniform heat permeation throughout a substantially impervious natural retorting chamber so as to release substantially all of the available shale oil from the area being treated.

The use of a nuclear explosive device placed near the bottom of the oil shale formation has been proposed.

Upon detonation of such a device, a large, substantially spherical hollow cavity would be created instantaneously. This cavity would be surrounded by a thick zone of fractured but nevertheless impervious shale or rock. The fractured rock located above the cavity would collapse so as to at least partially fill the cavity. Thus, a substantially cylindrical retort-like zone or chimney filled with crumbled and pervious oil shale would result. Such a technique is described, for example, in the Jacobs et al patent, US. Pat. No 3,342,257 issued Sept. 19, 1967.

The retorting of the shale in the chimney thus formed may be initiated and supported by drilling gas inlet and oil recovery wells to connect the chimney with on-surface gas pumping and shale oil recovery equipment. A hot or heated gas can thereafter be passed through the chimney of crumbled pervious shale, preferably in a downward direction, so as to decompose the hydrocarbons existing in the deposit in the form of kerogen and to distill shale oil from the shale in the chimney. The shale oil is collected and flows to the surface through the production well.

This technique offers distinct advantages over the methods heretofore considered. Nevertheless, the effectiveness of this technique is dependent on the success with which retorting of the oil shale in the relatively large chimney space can be accomplished. As a retorting gas is injected into the chimney through a gas inlet well connecting the chimney with the gas pumping equipment at the surface, a hot zone of sufficient temperature to support retorting is formed in the vicinity of the injection point. This hot zone can be advanced by the injection of additional retorting gas through the injection well. Initiating and supporting retorting of the shale in the chimney in this manner is not entirely satisfactory, however, because of the difficulty encountered in initiating and advancing the hot zone throughout the entire chimney volume. In the absence of satisfactory retorting throughout the chimney, the recovery of shale oil from the chimney will be unduly restricted, and the economic feasibility of utilizing this technique will be diminished.

After the eduction of the shale oil it must be subjected to treatment for removal of nitrogen and sulfur or for converting them to an innocuous form. This is generally accomplished by surface hydrogenation as described in Hamilton, et al., US. Pat. No. 3,233,668. Surface upgrading of this type is expensive and requires additional equipment and facilities for the in situ retorting operation. A means for providing in situ oil shale upgrading is a necessary criteria for in situ oil shale retorting success.

It is an object of the present invention, therefore, to provide an improved process for the in situ retorting of oil shale formations.

It is another object of the present invention to provide a process for increasing the recovery and quality of shale oil from underground oil shale deposits.

These and otherobjects are accomplished by the novel process of the present invention as hereinafter set forth.

SUMMARY OF THE INVENTION The present invention is directed to an improved method for initiating and supporting in situ retorting and hydrogenation of the oil shale in the chimney of fractured and crumbled shale of high permeability formed by the detonation of a nuclear device positioned ina subsurface oil shale formation. The present invention permits and facilitates the establishment of a hot zone substantially across the cross-sectional area of the chimney. This is accomplished by injecting the retorting fluid into the chimney space by means of at least one injection well positioned generally horizontally across the chimney. The retorting fluid is injected through the perforations in the injection well liner.

The injection of a retorting fluid into the chimney from a multiplicity of points permits the establishment of a hot zone at a temperature sufficient to support retorting across substantially the entire width of the chimney. By injecting retorting fluid through a series of injection wells thus positioned, the establishment of the desired hot zone substantially across and around the entire cross-sectional area of the chimney may be accomplished. The retorting fluid may in addition provide means for the in situ hydrogenation of the oil shale, thereby causing the upgrading thereof.

The support and advancement of the hot zone throughout the chimney face can-readily be accomplished by continued injection of the initial retorting fluid or a suitable gas into the injection wells and consequently into and around the cross-sectional area of the chimney. The injection wells called for herein may be positioned either in the upper or the lower portion of the chimney, with the distilled shale oil being recovered through production wells in accordance with normal techniques.

BRIEF DESCRIPTION OF THE DRAWING The present invention may be more fully understood by referral to the accompanying FIGS. in which:

FIG. 1 represents a side external view, partially in cross-section, showing the relative positions of an injection well and a production well in an illustrative embodiment of the present invention; and

FIG. 2 is a diagrammatical illustration of another embodiment of the present invention employing four injection wells positioned at 45 intervals around the oil shale chimney.

' DETAILED DESCRIPTION OF THE INVENTION In order to initiate and support retorting of the shale across and around the chimney formed by the detonation of a nuclear device in a subsurface in an oil shale formation, at least one injection well is positioned so that it extends into and preferably generally horizontally across the width of the chimney. The injection well liner contains perforations through which retorting fluid may be injected into the chimney. The retorting fluid is used to establish a hot zone in the chimney at a temperature capable of supporting retorting of the shale. The injection of the retorting fluid through a series or multiplicity of perforations in the generally horizontally extending injection well liner facilitates the establishment of a hot zone substantially across the width of the chimney. Injection of the retorting fluid through a generally vertically positioned injection well extending into the chimney would, on the other hand, result in the establishment of a more localized hot zone in the vicinity of the injection well outlet. Even with the use of a very high injection volume, the extending of the hot zone substantially across the width of the chimney would be difficult to accomplish. The present invention, therefore, permits the establishment and sup port of a hot zone that enhances the retorting of the shale and the recovery of the shale oil from the chimney.

In FIG. 1 of the drawing, the chimney of fractured and crumbled shale of high permeability is represented by the numeral 101. This chimney is formed by the detonation of a nuclear device generally positioned at the bottom of oil shale formation 102. Access well 103, which is drilled from surface 104 through overburden 105 and into oil shale formation 102 is employed initially in order to place the nuclear device in position for detonation. Slanted drilling rig 106 may be employed for the drilling of production well 107 positioned at the base of chimney 101. This drilling rig may also be employed in drilling the slant injection well 108 that is positioned in the upper portion and near the top of chimney 101. As can be readily seen from the drawing, injection well 108 is drilled so that the portion that extends across chimney 101 is positioned generally horizontally. Perforations generally represented by the numeral 109 are provided in the portion of injection well 108 extending across the chimney to provide suitable injection ports for the injection of retorting fluid substantially across the width of the chimney 101.

FIG. 2 illustrates an embodiment of the present invention in which four injection wells 208 are positioned in a radial pattern from the surface 204, around chimney 202 and access well 203. The four injection wells 208 are positioned at 45 intervals around the chimney 201. Each injection well 208 extends through oil shale formation 202 across the width of the chimney 201 and is provided with perforations on substantially the entire portion crossing said chimney 201. This arrangement provides for a multiplicity of injection ports substantially around and across the cross-sectional area of the chimney 201. As previously indicated, the injection wells 208 can be positioned in any convenient manner; however, as for example by passing the injection wells 208 across the chimney 201 in a substantially parallel configuration.

The injection well called for by the present invention can be provided by known drilling techniques. The injection well is drilled preferably as a slant well by the so-called slant-hole drilling technique. Details of this drilling technique, which involves tilting the drilling rig, are not included herein as this drilling technique is known and has been employed in conjunction with conventional petroleum producing methods.

It is also possible to provide one or more injection wellbore holes that extend downward from the earths surface in a generally vertical direction with radially diverging bore hole extensions extending into the chimney and across the chimney in a generally horizontal direction. The so-called directional drilling technique can be employed to provide the deviating bore hole extensions. Since directional drilling is also a well established technique that has been employed in conventional operations, further details of the manner in which the drilling is accomplished are not included herein.

In order to provide means for injecting the retorting fluid generally across the width of the chimney, perforated liner extensions may be employed in the portion of the injection well passing through the chimney. Alternately, plain liner extensions may be employed, and perforations may be made after the liner has been placed. Techniques for perforating the liner after it has been positioned in the formation are well known in the art. For example, pressure may be employed to blow out plugs in the liner or perforations may be made by chemical action.

Injection of the retorting fluid into the chimney space may be accomplished through one or more injection wells. Each well, of course, preferably will have a generally horizontally extending portion in the chimney. While one such injection well provides a series of injection ports across the width of the chimney, it may be desirable to employ two or more of such injection wells in order to more fully cover the cross-sectional area of the chimney. From about two to about six injection wells would normally be sufficient for this purpose although additional injection wells could also be employed. Coverage across and around the cross-sectional area of the chimney can conveniently be achieved by positioning four injection-wells at intervals at about 45 around the circumference of the chimney. By extending each of these wells substantially across the width of the chimney, thus crossing at the center portion of the chimney, a multiplicity of injection ports can be provided around the cross-sectional area of the chimney.

The various injection wells can be positioned across the chimney at various elevations if desired. Generally, however, the injection well or wells will be positioned near the top of the chimney. The injection wells can,

however, be positioned near the bottom of the chimney or at some intermediate position in either the upper or lower portion of the chimney. It is also within the scope of the present invention to inject the retorting fluid sequentially through injection wells positioned at more than one elevation in the chimney in order to facilitate the recovery of shale oil from the chimney space.

The heat generated by the explosion of the nuclear device is insufficient to retort any appreciable portion of the available and recoverable oil in the chimney. The retorting fluid, therefore, is injected into the chimney in order to heat the chimney space to a sufficient temperature to support combustion. The retorting fluid may be any suitable material or combination of materials that will provide the necessary heat to establish retorting conditions in the chimney. The retorting fluid may conveniently comprise a combustion gas formed by burning a combustible-containing gas, such as methane or natural gas, with air or some other oxygencontaining gas. It is within the scope of the present invention to burn a combustible gas or liquid above ground to form the combustion gas or to inject the combustible gas or liquid together with an oxygen-containing gas into the formation through the injection well so as to form the combustion gas underground. It is also within the scope of the invention to employ some other gas, such as a heatedoxygen-free gas, or steam as the retorting fluid.

The retorting fluid injected through the perforations in the generally horizontally extending injection well supports the hot zone formed across the width of the chimney, by conventional ignition or burning at the injection points, i.e. at the perforations, along the generally horizontally extending portion of the injection well in the chimney. The temperature required in the hot zone in order to support combustion varies with the pressure in the chimney while a temperature of at least about 600 F is ordinarily required at atmospheric pressure. Hot zone temperatures between about 700 F and about 2,5 F may conventionally be employed.

The amount of retorting fluid employed in the practice of the present invention is not critical and will vary depending on a number of factors, such as the retorting temperature employed, the extent of oil recovered, the initial shale temperature and the like. Retorting gas quantities of from about 20 to about 5,000 standard cubic feet into the relatively cool shale formation per cubic foot of crumbled shale in the chimney have been proposed although the size of the chimney developed will greatly influence the actual amount employed.

In another embodiment of the present invention, the hot zone may be established by the reaction of chemicals in the chimney, other than by combustion, so as to generate the desired quantity of heat. These methods are known to the art, for example, pellets of sodium hydroxide, possibly carried by non-aqueous solution, may be injected into the chimney through the perforations in the generally horizontally extending injection wells. Water or an aqueous acid solution may then be injected into the chimney through the same perforations so as to contact and react with the pellets. A tremendous heat of reaction would result with an evenly distributed heat force. Numerous other reactants will readily occur to those skilled in the art as suitable for injection into the chimney to provide, by means of their heat ofreaction, the heat necessary to support retorting ofthe oil shale. lrrespective of the particular reactants employed, the injection of the reactants through the perforations of the generally horizontally extending injection wells of the present invention permits the establishment of the hot zone substantially across-and around the cross-sectional area of the chimney.

After the hot zone has been established, it may be advanced in the desired direction by continuous or intermittent injection of a gas that advances the heat front through the chimney. Air or another oxygen-containing gas can conveniently be employed for this purpose. The oxygen content and rate of flow of the gas, together with the amount of residual combustibles in the formation, will determine the temperature and the rate at which the heat wave is propagated through the chimney.

It is a preferred embodiment of the present invention to provide means for the in situ hydrogenation of the oil shale contained within the chimney. This may be provided by the reaction of specific chemicals for heat generation as mentioned above. For example, magnesium pellets, may be injected into the chimney, together with water or other suitable carrying medium, through the perforations in the generally horizontally extending injection wells. Hydrochloric may then be injected into the chimney through the same perforations so as to contact and react with thepellets. Any of the many acid meta] reactions which release heat and evolve hydrogen may be employed. Such a technique would not only furnish the resultant heat of reaction, but would have an additional advantage as well. The reaction would result in the evolution of hydrogen that would hydrogenate the kerogen in the chimney, thus tending to upgrade the shale oil recovered from the chimney.

Another means for providing in-situ hydrogenation may be supplied by the introduction of water into the chimney and subsequent electrolysis. thereof. Therefore, by theintroduction of water, say into the lower portion of the chimney, through a separate tubing string in the production well and subsequent hydrolysis thereof, the heated zone may be sustained by combustion of the oxygen diffusing thereto and the oil shale produced upgraded by the presence of the hydrogen atmosphere in the lower drainage position of the chimney. Numerous other suitable hydrogen generating means will occur to those skilled in the art for use with the present invention.

It has been suggested that a nuclear detonation may be used in the recovery of oil from Athabascatar sands and similar formations as well as from oil shale formations as disclosed herein. Recovery procedures would follow essentially the same pattern as in the recovery of shale oil. The present invention could also be employed in this procedure in a similar manner as set forth herein.

In yet another embodiment, the horizontally extend ing injection wells disclosed herein may be employed in the retorting of oil shale substantially across an area of fractured and crumbled shale formed by the detonation of conventional explosive devices.

While the. present invention has been described with reference to particular embodiments thereof, it will be appreciated that various changes and modifications can be made therein without departing from the scope of the invention as set forth herein.

Therefore, I claim:

1. In the recovery of shale oil from a subsurface oil shale formation by means of a nuclear explosion that creates a cavity in the formation that fills with collapsed oil shale to form a chimney of fractured and crumbled shale of high permeability from which shale oil is recovered through a production well while retort ing fluid is injected into the chimney through an injection well, an improved method of initiating and supporting the in situ retorting and hydrogenation of the oil shale comprising:

a. injecting the retorting fluid into the chimney through four injection wells positioned at approximately 45 intervals along the periphery of the chimney area;

b. hydrogenating the oil shale through in situ production of hydrogen; and

c. recovering the shale oil through at least one production well positioned in the chimney.

2. The method of claim 1 in which said retorting fluid is injected into the upper portion of the chimney and shale oil is recovered from the lower portion of the chimney.

3. The method of claim 1 in which the injection wells cross one another so as to provide a multiplicity of injection points throughout the cross-sectional area of the chimney.

4. In the recovery of shale oil from a subsurface oil shale formation by means of a nuclear explosion that creates a cavity in the formation that fills with collapsed oil shale to form a chimney of fractured and crumbled shale of high permeability from which shale oil is recovered through a production wellwhile retorting fluid is injected into the chimney through an injection well, an improved method of initiating and supporting the in situ retorting and hydrogenation of the oil shale which comprises:

a. injecting the retorting fluid into the chimney through perforations in the liner of at least one injection well in the chimney;

b. injecting water into the chimney; I

c. electrolyzing said water to produce free hydrogen to hydrogenate the oil and oxygen in situ to aid the supporting of the in situ retorting; and

d. recovering the shale oil through at least one production well positioned in the chimney.

5. In the recovery of shale oil from a subsurface oil shale formation by means of a nuclear explosion that creates a cavity in the formation that fills with collapsed oil shale to form a chimney of fractured and crumbled shale of high permeability from which shale oil is recovered through a production well while retorting fluid is injected into the chimney through an injection well, an improved method of initiating and supporting the in situ retorting and hydrogenation of the oil shale which comprises:

a. injecting the retorting fluid into the chimney through perforations in the liners of from about two to about six injection wells positioned in a parallel configuration across the width of the chimney;

b. hydrogenating the oil shale through in situ production of hydro en; and c. recovering e shale oil through at least one production well positioned in the chimney.

6. In the recovery of shale oil from a subsurface oil shale formation by means of a nuclear explosion that creates a cavity in the formation that fills with collapsed oil shale to form a chimney of fractured and crumbled shale of high permeability from which shale oil is recovered through a production well while retorting fluid is injected into the chimney through an injection well, an improved method of initiating and supporting the in situ retorting and hydrogenation of the oil shale which comprises:

a. injecting the retorting fluid into the chimney through perforations in the liner of at least one injection well in the chimney;

b. injecting chemical reactants into the chimney which react upon contact therein and generate heat and evolve hydrogen, retorting the fractured and crumbled shale in the chimney with said heat;

0. hydrogenating the oil shale in situ with the hydrogen produced by reaction of the chemical reactants, said hydrogen also tending to hydrogenate the kerogen in the chimney, thus upgrading the shale oil recovered from the chimney; and

d. recovering the shale oil through at least one production well positioned in the chimney.

7. The process of claim 6 in which said reactants are magnesium and hydrochloric acid.

8. In the recovery of shale oil from a subsurface oil shale formation by means of a nuclear explosion that creates a cavity in the formation that fills with collapsed oil shale to form a chimney of fractured and crumbled shale of high permeability from which shale oil is recovered through a production well while retorting fluid is injected into the chimney through an injection well, and improved method of initiating and supporting the in situ retorting and hydrogenation of the oil shale which comprises:

a. injecting the retorting fluid into the chimney through perforations in the liners of injection wells positioned at more than one elevation in the chimney space, said injection into the wells being sequential;

b. hydrogenating the oil shale by in situproduction of hydrogen; and

c. recovering the shale oil through at least one production well positioned in the chimney.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3084919 *Aug 3, 1960Apr 9, 1963Texaco IncRecovery of oil from oil shale by underground hydrogenation
US3102588 *Jul 24, 1959Sep 3, 1963Phillips Petroleum CoProcess for recovering hydrocarbon from subterranean strata
US3303881 *Nov 22, 1963Feb 14, 1967Nuclear Proc CorpUnderground nuclear detonations for treatment and production of hydrocarbons in situ
US3342257 *Dec 30, 1963Sep 19, 1967Standard Oil CoIn situ retorting of oil shale using nuclear energy
US3465819 *Feb 13, 1967Sep 9, 1969American Oil Shale CorpUse of nuclear detonations in producing hydrocarbons from an underground formation
US3490529 *May 18, 1967Jan 20, 1970Phillips Petroleum CoProduction of oil from a nuclear chimney in an oil shale by in situ combustion
US3513913 *Apr 19, 1966May 26, 1970Shell Oil CoOil recovery from oil shales by transverse combustion
US3533469 *Dec 12, 1968Oct 13, 1970Phillips Petroleum CoMethod of insulating the roof of a subterranean cavity during retorting
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3766982 *Dec 27, 1971Oct 23, 1973Justheim Petrol CoMethod for the in-situ treatment of hydrocarbonaceous materials
US4027917 *May 16, 1975Jun 7, 1977Occidental Petroleum CorporationMethod for igniting the top surface of oil shale in an in situ retort
US4072350 *Jan 27, 1977Feb 7, 1978Occidental Oil Shale, Inc.Multi-stage method of operating an in situ oil shale retort
US4076312 *Jul 30, 1976Feb 28, 1978Occidental Oil Shale, Inc.Method and apparatus for retorting oil shale at subatmospheric pressure
US4105251 *Jul 29, 1977Aug 8, 1978Standard Oil Company (Indiana)Dispersion of catalysts with explosives for in situ mining of carbonaceous minerals
US4119345 *Apr 4, 1977Oct 10, 1978Occidental Oil Shale, Inc.In situ oil shale retorting process using introduction of gas at an intermediate location
US4243100 *May 4, 1979Jan 6, 1981Occidental Oil Shale, Inc.Operation of in situ oil shale retort with void at the top
US4977959 *Jun 5, 1986Dec 18, 1990Institut Francais Du PetroleMethod of positioning of plugs or screens by horizontal drilling
US6814141 *May 9, 2002Nov 9, 2004Exxonmobil Upstream Research CompanyMethod for improving oil recovery by delivering vibrational energy in a well fracture
US8056624 *Jul 19, 2007Nov 15, 2011Uti Limited PartnershipIn Situ heavy oil and bitumen recovery process
US20030042018 *May 9, 2002Mar 6, 2003Chun HuhMethod for improving oil recovery by delivering vibrational energy in a well fracture
US20100065268 *Jul 19, 2007Mar 18, 2010Uti Limited PartnershipIn situ heavy oil and bitumen recovery process
US20160215604 *Jan 28, 2015Jul 28, 2016Schlumberger Technology CorporationWell treatment
Classifications
U.S. Classification166/245, 166/247, 166/272.7, 166/259, 166/248
International ClassificationE21B43/263, E21B43/16, E21B43/25, E21B43/247
Cooperative ClassificationE21B43/2635, E21B43/247
European ClassificationE21B43/263F, E21B43/247