GB2137229A

GB2137229A - Internal sacrificial anode for subsurface conduit

Info

Publication number: GB2137229A
Application number: GB08404207A
Authority: GB
Inventors: Jerry Donald Hughes
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-02-23
Filing date: 1984-02-17
Publication date: 1984-10-03
Also published as: GB8404207D0

Abstract

A sacrificial anode (12, 58 a-d) is disposed within a length of metal conduit 14 (e.g. well casing or tubing) and in electrical connection therewith, such sacrificial anode constructed of a metal (e.g. Zn, Al, Mg) more active in the Galvanic series than the metal in the conduit, such that as an electrolytic fluid flows through the conduit, a galvanic cell is formed having the protected structure as the cathode. As potentially corrosive position ions reach the surface of the cathode or similar metals in contact therewith, electrons will flow from the internal sacrificial anode to the cathode, thereby neutralizing those positive ions and preventing or inhibiting such corrosion. <IMAGE>

Description

SPECIFICATION Internal sacrificial anode for subsurface conduit This invention relates generally to cathodic protection systems for preventing corrosion to a metal structure and more specifically relates to methods and apparatus utilizing an internal sacrificial anode to inhibit corrosion to a subterranean or submarine conduit.

Cathodic protection systems are utilized to prevent the corrosion of metal structures exposed to an electrolytic environment. One form of cathodic protection employs one or more sacrificial anodes electrically connected to the structure to be protected, each of such sacrificial anodes constructed of metal which is more active in the galvanic series than the metal in the structure to be protected. When the structure to be protected and the sacrificial anode are in the same electrolytic environment, a galvanic cell is established in which electrons flow from the sacrificial anode, thereby eroding it, through the electrical connection to the cathode, i.e., the protected structure.The protected structure thus does not corrode due to the availability of free electrons in that structure to, optimally, neutralize, or at least reduce, positive ions which reach the surface of the protected structure, thereby potentially corroding it.

Present systems using sacrificial anodes for corrosion protection of pipe lines and other similar structures rely upon the placement of sacrificial anodes exterior to the structure and awire or similar form of electrical connection connecting the sacrificial anodes to the structure. Such systems are not suitable for protection of other, deeper-lying structures such as well casing, tubing and/or associated instrumentalities because of the extreme difficulty of providing an electrical connection between the structure and the anode, and of placing the sacrificial anode within the same electrolytic environment as the casing, such electrolytic environment being subject to considerable alteration throughout the various formations through which the well and accompanying casing or similar structure extends.The absence of such corrosion protection can significantly increase the maintenance and operating costs of a well by necessitating the shutdown of the well and the removal and replacement of the casing, tubing or associated structures, such as hangers or liners, at intervals as frequent in some cases as every four years and in some extreme cases as frequent as every thirty days.

Accordingly, the present invention provides methods and apparatus whereby sacrificial anodes may be placed internally to a subterranean or submarine conduit to protect the conduit and/or structures located therein from corrosion.

A method and apparatus for protecting a metal conduit from corrosion wherein a sacrificial anode, composed of a metal more active in the galvanic series than said conduit, is disposed inside the conduit and in electrical connection therewith. In a preferred embodiment, a tubular sacrificial anode is pressed or threaded into place inside the conduit in axial alignment therewith.

The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section view of well casing incorporating an internal sacrificial anode in accordance with the present invention.

Figure 2 is a laterally cross-sectional view of the casing section of Fig. 1.

Figure 3 is a longitudinal cross-sectional view of a length of well tubing including an internal sacrificial anode in accordance with the present invention.

Figure 4 is a vertical cross-sectional view of a packer including a sacrificial anode internal to the casing in an alternative embodiment of the present invention.

Figure 5 is a top view of the packer of Fig.

4.

Referring now the drawings in more detail, particularly to Figs. 1 and 2, therein is shown in Fig. 1 a section of well casing 10 incorporating an internal sacrificial anode 12 in accordance with the present invention, illustrated in longitudinal cross-section and in Fig. 2 casing section 10 of Fig. 1 is illustrated in lateral cross-section. Casing section 10 includes a housing 14 constructed in accordance with well-known standards of casing design, for example, constructed of a suitable grade steel and with a suitable wall thickness to withstand the required rated pressure of the casing. Housing 14 has a first end 16 having a first given diameter and thread connection compatible with the nominal size of casing in which casing section 10 is to be placed.

Housing 14 then has a portion of enlarged inner and outer diameters forming an outer sleeve in which sacrificial anode 12, having a tubular form, may be located therein while maintaining a generally constant inner diameter through casing section 10 and the remaining conventional casing lengths to which casing 10 may be attached. Housing 14 is joined to an end cap 18 having a similar, but reverse facing thread than first end 16 of housing 14 such that upon assembly as shown in Fig. 1, casing section 10 displays one end of male threads and a second end of female threads exactly as a conventional length of well casing, but having enlarged central portion housing sacrificial anode 12. Housing 14 may be threadably coupled to end cap 18 or may be coupled thereto by other means, for example, welding.In the illustrated embodiment, sacrificial anode 12 is pressed or wedged into position in housing 14 thereby achieving the required electrical connection therewith, discussed further later herein. The coupling joints between houssing 14, and end cap 1 8 and the conventional casing lengths to which casing section 10 is attached will assure that the conventional casing lengths within the electrolytic environment will form the cathode in the cell. Sacrificial anode 14 may be of a variety of metals so long as such metal is more active in the Galvanic series than is the metal of which housing 14 is constructed. For example, for protecting a conventional steel housing (cathode), aluminum, zinc, and magnesium and magnesium-base alloys are highly satisfactory.

It will be appreciated that the lateral wall thickness of sacrificial anode 14 is a matter of design choice and may be adapted in response to the metal of which sacrificial anode 14 is constructed and the corrosive potential of the intended environment casing section 10.

It will be appreciated that a unique feature of the present invention is that the transported fluid within the conduit serves as the electrolyte in the galvanic cell formed therein. In wells, particularly, areas of circulation or turbulence in the fluid flow exist which, in the case of water or other corrosive fluids, tend to promote corrosion of the casing or tubing adjacent such turbulence. With the present invention, these high-risk corrosion areas may be protected with optimum efficiency relative to that achieved through ue of externally located sacrificial anodes. In an intended usage, casing sections constructed in accordance with the present invention would be placed at spaced intervals within the casing string throughout known or suspected corrosion trouble areas, such as, for example, joints reducing or enlarging casing size or areas in which the casing is to be perforated.

Referring now to Fig. 3 of the drawings, therein is shown in longitudinal cross-section a section of well tubing 20 incorporating a sacrificial anode 22 in accordance with the present invention. Tubing section 20 includes sacrificial anode 22, housing 24 and an end cap 26 similar to those described for casing section 10, with the exception that sacrificial anode 22 is not pressed into housing 24 but, in an alternative method of mounting, is threaded thereto by means of a plurality of cooperatively engaging internal housing threads 28a and external sacrificial anode threads 28b proximate at least one end of tubing section 20. It will further be seen that housing 24 does not reduce in diameter at first end 30 but that the nominal diameter of housing 24 is approximately that associated with the flared coupling joint in a convetional tubing section.

In sn alternative, but less preferred embodiment, internal sacrificial anodes may be coupled to independent instrumentalities to be disposed within, and in electrical connection with, the structures to be protected. Referring now to Fig. 4 of the drawings, therein is shown an alternative embodiment of the sacrificial anode internal to a subsurface conduit in the form of a plurality of sacrificial anodes 58a, 58b, 58c, 58d coupled to a packer 40 disposed within a length of casing 42 and around a length of tubing 44 in a conventional operating configuration, illustrated partially in vertical cross-section. Because many methods and apparatus are well known for securing a packer in position within a well and because they are not of importance to the present invention, such securing apparatus, for clarity, are not illustrated in Fig. 5.Packer 40 includes a body member 46, a plurality of gripping teeth 48a, 48b, 48c, 48d, a sealing ring 50 and an activating system, represented generally at 52. Threadably disposed in upper and lower surfaces 54 and 56, respectively, are sacrificial anode plugs 58a, 58b, 58c, 58d, which serve to protect both packer 40 and casing 42 from corrosion in a similar manner to that described earlier herein, electrical connection between sacrificial anode plugs 58a, 58b, 58c, 58d and casing 42 being provided through gripping teeth 48a, 42b, 48c, 48d. It will further be appreciated that if activating system 52 establishes an electrically conductive path betweenn packer body 46 and tubing 44 then tubing 44 may also be cathodically protected by the illustrated embodiment.

Referring now to Fig. 6 therein is shown a top view of packer 40 of Fig. 5 showing a plurality of sacrificial anode plugs 58a, 58b, 58c, 58f, 589, 58h disposed in packer body 46 in an intended construction embodiment.

It will be appreciated that many alternative forms of attaching sacrificial anodes to packer body 46 could also be utilized and that the threaded plug construction illustrated is for purposes of example only.

Many modifications and variations besides those specifically mentioned may be made in the techniques and structures described herein and depicted in the accompanying drawings without departing substantially from the concept of the present invention. For example, as is evidenced from the above description, the sacrificial anode may be disposed within a variety of conduits, including pipe lines, and may be retained therein either directly to the conduit as described herein or in conjunction with a variety of forms of equipment to be located inside the conduit other than just the packer as illustrated.

Claims

1. Apparatus for protecting a metal conduit from corrosion, comprising a sacrifical anode disposed within said conduit, said sacri fical anode constructed of a metal bore active in the galvanic series than the metal of which said conduit is constructed, said sacrificial anode disposed within said conduit in electrical connection therewith.

2. Apparatus as claimed in claim 1, wherein said conduit is a length of well casing.

3. Apparatus as claimed in claim 1, wherein said conduit is a length of well tubing.

4. Apparatus as claimed in any one of the preceding claims, wherein said sacrificial anode is pressed into said conduit to achieve electrical connection therewith.

5. Apparatus as claimed in claim 1, wherein said sacrificial anode is threadably coupled to said conduit.

6. Apparatus as claimed in claim 1, wherein said sacrificial anode is of tubular form and is disposed in axial alignment with said conduit.

7. Apparatus as claimed in claim 1, wherein said sacrificial anode is disposed within said conduit by means of an independent instrumentality in electrical connection with said conduit.

8. Apparatus as claimed in claim 1, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.