US2875142A - Method and apparatus for cathodic protection of wells - Google Patents

Description

,Feb 24, 1959 J. E. LANDERS ErAL 2,875,142

METHOD AND` APPARATUS FOR CATHODIC PROTECTION OF WELLS ATTORNEY Feb. 24, 1259 J. E. LANDERS ET AL METHOD AND APPARATUS FOR CATHODIC POTECTION rOF' WELLS Filed June 2, 1955 2 Sheets-Sheet 2 I6 I2 V Y f I I '0 I i 92| 22 nl 24 2' I w 'vgl ylkf 2?,v 23 2| 24 24 8 IUI 8 INVENTORS. JAMES E. LANDERS gJOHN D. SUDBLJRY ATTORNEY ,cent to the lower METHOD AND APPARATUS FOR 'CATHODIC PROTECTION OF WELLS James E. Landers, El Para, rex., and Jahn D. Sudbury,

Ponca City, Okla., assignors to Continental Oil Company, Ponca City, Okla., a corporation of Delaware Application June 2, 1955, Serial No. 512,741 s claims. (ci. 2114-147) This invention relates to method and apparatus for use in connectionwithithe cathodic protection against electrochemical corrosionlofoil well structures and is more specifically directed to prevention of accelerated corrosion of an oil well casing in localized areas adjaextremity Ior base of the enveloping surfacevp'ipe. l l Throughoutthe following `specification and claims the ter'mftubiug is to'be understood to mean the pipe string of `2,21/A"in`ch diameter which traverses the length of the well and through which tiuidsA are produced. The term casing is the 5-7-inch `pipe set to the bottom of the wellmfor the purpose of protecting lthe hole from waters and loose earth." The term surface pipe is to be under stood to mean the 9-12-'i`nch diameter pipe which extends part way down into Vthewellfoutside of the tubing and is cementedto the earth formations `around the well.` f `The `term"cur`rent `is used to denote the ow of electrons froman area of negative c harge'toan area ofpositiveicliarge.` l v f c" When a well casingdraverses two formations, each congarganta Feb. 24,1959,

The difficulty encountered with this method of electrical protection of the casing is that, when the negative terminal of the current source is applied to the casing, the current divides, part of it flowing down the casing and part of it owing down the surface pipe.

As will be described in more detail later in this speci- 4 ication, electrons in most wells flow from the base of the taining waterof adiiferent salt concentration, an electrical potential dilference is developed between the two sections of casing in contact with the `two formations;` Electrons enter one ,of thesesections which, in combination with the surrounding formation and the interface, is referred to herein as thel anode area. The electrons then .traverse-the casing andilow' from` the second section which, in combination with the surrounding formation andlinterface, isn referred to herein as the cathode area. The electrons thencomplete the circuit by returning tol the `anode iarea through the formation. As the eleetrons flow through thecasing an electrochemical process causes hydrogen atoms to form in the cathode area and iron from the casing to dissolve in the anode area. The iron is dissolved by the formation of surface pipe tothe casing, causing as described abov corrosion of the casing at that point. V

It is, therefore, the principal object of our invention to provide a method and apparatus for preventing electrochemical corrosion of the casing adjacent to the base of the surface pipe.

`It is a more particular object yof our invention to provide a method and apparatus for maintaining an electric current between a well casing and an anode buried in the ground while maintaining the lower part of the sur face pipe substantially anodic with respect to the portion of the casing adjacent thereto.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, said invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiwell;

Figure 4 is a diagram of the electric circuit associated with the method illustratedin Figure 3;

Figure 5 illustrates an alternative method 4of our invention for cathodically protecting an encased well;

not removed, would eventually insulate the casing from the cathode area sufficiently `to prevent the flow of electrons. Hydrogen is usually removed, however, either by reaction with oxygen to form water, or by the action of hydrogen consuming bacteria.v

lf `the "electron `llow'is t permitted indefinitely, enough iron ,will beremoved from the casing in the anode area to corrode the `casing `and developleaks. This velectrochemical corrosion ofthe 4casing in the anode area may be prevented by connecting the negative terminal of a `directcurrent source to the casing and onnectingjthe positive terminal ofthe source to an anode buried in'the earth adjacent to the well. This .direct current source must he of sufficient magnitude that the casing at all points along its length is cathodic with Vrespect to-the anode buriedin the adjacent earth. 4If such is therese', electrons will flow from thecasing through-- the earth to' the `buried anode.` .0f lgreater l importance l'is 'the factthat no electrons will ow to any point of the casing, and, consequently, no electrochemical Figure 6 is a diagram of the electric circuit associated with the method., illustrated in .Figure 5; p

Figure v7 is an elevational view partially in cross-sec,- tion of the' part of a well surrounding Vthe bottom of the casing wherein is shown theapparatus of our invention for practicing the preferred method of `uur invention; and,

Figure 8 is a horizontal' cross-sectional view of the apparatus of Figure 7 taken along the plane and in the direction substantially indicated by the line and the arrows at 8 8. l'

Broadly stated, our inventionappertains to oil well cathodic protection methods in which the well structure is maintained cathodic with n'respect to a protective anode buried in the ground and which are intended to eliminate all electrochemical corrosion in the well structure.` It comprises maintaining the potential of the: casing in the area of the base of the surface pipe at least as great as the potential at the` base .of the surface pipe., In brief, the casing is` maintained cathodic with respect to the base of the surface pipe, thereby eliminating the flow of electrons from the pipe to the casing andthe consequent accelerated `corrosion "in that area.

With reference now to Figure 1 wherein is illustrated the conventional method for cathodically protecting an encased well, 10 designates the surface pipe of the well and 11 designates the casing therein. lnsidegthe casing 11 is the conventional tubing` 12 through which production flows. The casing 11 and the. surface pipe 10 are made f 5? of metal and are electricallyconnected at the top of the well. An anode 14 is embedded in the earth adjacent to the well, and a source of direct current 15 is connected by a cable 16 between the anode 14 and a point 17 on the top o f the casing 1,1 as indicated.

With reference now to Figure 2 wherein is shown a diagram of the electric circuit of this combination, A represents the resistance of the surface pipe from the point where it joins casing 11 to vits bottom. B represents the resistance of the casing 11 from .the point where it joins the surface pipe 10 to a point 18 on it adjacent to the bottom of the surface pipe 10. C represents the resistance of the earth to the flow of current from the surface pipe 10 to the anode 14. D represents the resistance of the casing `11 and .the earth to current flow down the casing 11 and lthrough the earth to the anode 14 from the point 18, and E represents the resistance of the well fluids to current flow between the point 18 and the bottom of the surface pipe 1.0. V represents the electromotive vforce of the current source 15.

In this circuit, when V(+C) V(B+D) or in simpler form the potentials at the point 18 and the bottom of `the surface -pipe 10 are equal because the potential drop across `A equals the potential drop across B. When these potentials are equal, no current will ow through E, `andvhence neither the surface pipe 10 nor the casing 11 will be corroded. When is less than V(B |D) or in simpler form when s less than the potential drop across A -will be less than the potential drop across B and current will flow through E into the casing 11 causing corrosion of the casing 11 in the o neighborhood of the point 13. When the potential drop across A will be greater than the potential drop across B, and hence current will ow through E onto the surface pipe 10 causing corrosion of the bottom of the surface pipe 10.

vIn most wells, the values of the resistances A, B, C, and D are such that is less than ol nl a and the casing 11 is corroded in the neighborhood of the point 18. This situation could -be altered in any one of several ways. As A or D is increased or as B or C is reduced, the values of the ratios will approach each other until will become greater than i olle @Ik and .current will Ytlow from the point 18 vto the bottom of the surface pipe 11 corroding the latter.

Increasing A while reducing B As the point of contact 17, where the negative terminal of the current source 15 is connected to the casing, is moved down the casing, where the resistance of the casing between the point 17 and the point where it joins thesurface pipe is F, the potentialv drop be tween the point 17 and the bottom of the surfaeepipe is A-l-F l V(A+F+ C) and the potential drop between the points 17 and 18 is V(BF+D) Current will continue to flow from the bottom of the surface pipe 10 to the point 18 until the point V1'] moved far enough down the casing 11 so that or reducing this until The depth at which the bottom end of the surface pipe 10 is located is ascertainable from the records made when the Well was encased and is also ascertaina'ble by any one of a number of other ways. Y When the point 17 is moved down the casing 11 to a point 19 adjacent to the bottom of the surface pipe 10, as illustrated in Figure 3, the potential difference between the point where the negative terminal of the current source 15 is connected to the casing 11 and the point 18 becomes B B 0 V(B B C) and the potential' difference between the point where the negative terminal of the current source 15 contacts the casing 11 and the 'bottom of the surface pipe becomes A ri- B V(A+B+C) as illustrated in Figure 4. In this case, where 4the nega- Ative terminal of the current sourcefIS isiconnected t' the point 19 on the casing, current willowfroi the points 18 and `19, which coincide and describe 'a `circumferential line on the casing 11, tothe bottom of the surface pipe `10. This current `vvillcause` electrochemical corrosion of the bottom of the surface pipe 10, but such corrosion is permissible since it does` not hinder production of the well.

Increasing A `without reducing B With reference now` to Figure `5, the well equipment here illustrated is identical tdthat illustrated `in `Figure 1 except that here 'thetopfoftheeuiifljiipe 10 is separated from the casing 1 1 by an insulating collar 2 0. When the resistance Aof the Ycol lar 20 is designated as G, the potential at the point 18 on the casing y11is V B+f i and the potential f the lbottom 'ofl the surface pipe 10 is H ..-A-,FG

V`(A +Gl-C)` as illustrated in Figure 6. No current will flow between the point 18 and the bottom of the surface pipe when these potentials are equal, that is, when A-l-G B V(A+G+C) V(B+D) or reducing this, when In an ideal situation, the resistance of G would be carefully chosen so that this relationship would exist; but since the values of A, B, C, and D are difficult to determine, it is convenient to select a value of G for which A-lG C is certain to be greater than or equal to When this is the case, current will again ow from the casing 11 to the surface pipe 10 corroding the latter; but as mentioned above, this corrosion is permissible.

Reducing C is increased. When the value of the resistance C is reduced to a point where no current will flow between the point 18 and the bottom of the surface pipe. This procedure is not desirable because, since it eliminates current flow through the resistance E in Figure 1 by increasing the current through C relative to the current through D, this procedure necessitates the use of current source 15, the electromotive force of which is impractically large if a sufficient current to elect adequate cathodic protection is to be driven through the casing. Furthermore it is known that greater variations in potential, along the length of the surface pipe, occur as the anode is brought closer to 6 `the pipe. The objects of my invention cannot, therefore, be achieved by controlling' the distance between the anode andthe srface pipe.

AIncreasing D If a resistancey isumainta'ined between two adjacent sections of the casing 1.1 below thepoint 18, the value of D is increased, and hence `the value of no current wil1"ow between the point I8 and the bottom of the" surface-pipe, but `this procedure isless'desirable` for `the same reason s is the procedure of reducing thevalue of the. resistance C.

With reference now to Figure 7 wherein is shown Aa form'of apparatus `o'f our invention for the practice of the method of our invention illustrated in Figure 3, 10 designates the lower'part of the surface pipe;` 11 designates a section of` the casingadjaceht thereto; `and 12 `designates a section offthetubing referred to above.

Mounted on` the` tubing 5121 and secured; athereto` byiany suitable means are two insulating collars 21. Secured to the outside of the' collars 21 by any suitable means such as shrink fitting are two metal collars 22. Four convex springs 23 are secured between the collars 22 by bolts 24. The natural contour of the springs 23 is sufficiently convex that they iirmly contact the inner wall of the casing 11. The insulated cable 16 referred to above extends down into the well from the current source 15, and a terminal 25 on the end of the cable 16 is connected to the upper collar 22 by a bolt 26. When current is applied to the cable 16, it is conveyed to the casing 11 at a point adjacent to the bottom of the surface pipe 10 through the terminal 25, the collars 22 and the convex springs 23. The collars 22 are insulated from the tubing 12 because, if current were permitted to flow down the tubing 12, currents might llow through the well fluids, when presenti, between tubing 12 and the casing 11. Such currents, if they were permitted to exist, would cause corrosion of the tubing 12 or the inner surface of the casing 1.1 deeper in the well.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in v any of the following claims or the equivalent of such be employed.

We, therefore, particularly point out and distinctly claim as our invention:

1. In the process of cathodically protecting the casing in an encased well in which the surface pipe extends downwardly for only a portion of the depth of the well and in which the casing is maintained cathodic with respect to a protective anode buried in the ground and spaced from the well structure, the step comprising connecting a direct current source between said anode and a point on the casing adjacent to the bottom of the surface pipe.

2. In the process of cathodically protecting the casing in an encased well in which the surface pipe extends downwardly for only a portion of the depth of the well and in which the casing is maintained cathodic with re spect to a protective anode buried in the ground and spaced from the well structure, the step comprising maintaining an electrical insulator between the top of said surface pipe and the casing.

3. In the process of cathodically protecting the casing in an encased well in which the surface pipe extends downwardly for only a portion of the depth of the well and vin which the casing is maintained cathodic with respectv toa protective anode lburiedin the ground and spaced from the well structure by a direct current source the negative terminal of which is connected to the casing and the positive terminal of which is connected to an anode buried in the earth adjacent to the well, the step ofrmaintaining an insulator between two adjacent sections of the casing below the level of the bottom of the surface pipe, the magnitude of the resistance of the surface pipe between the point where it joins the casing and its bottom divided by the resistance of the earth structures surrounding thesurface pipe between the surface pipe and the anode, being at least as great as the ratio, of the resistance of the casing between the point where the surface pipe is connected thereto and the part of the casing adjacent to the bottom of the surface pipe divided by the resistance of the casing, said insulator, and the earth structures surrounding the well between said part of the casing and said anode.

4. The apparatus for preventing electrochemical corrosion of the casing of encased wells which comprises in combination an anode buriedin the earthadjacent to said encased well, means electrically contactingthe casing of said well adjacent to the lower end of the surface pipe of said well, and a source of direct electric current connected between said means and said anode. 5. In a process for cathodically protecting oil well .casing in which a current is applied to the casing from an external power source to maintain a flow of electrons from the casing to an anode buried in the earth out lof contact with said casing and in which said oil well casing is enveloped for a portion of its depth by .a surface pipe, the improvement which comprisesmaintaining the potential of the casing in the areaadjacent to the Abase of the surface pipe at least as great as the potential of the base of the surface pipe so that no electrons ow from the base of the surface pipe to the adjacent area of the casing.

References Cited in the tile of this patent l v STATES PATENTS I Menaul Mar. 7, 1939 2,244,322 Zoller et al. June 3, 1941 2,310,757 Waener Feb. 9, 1943 2,359,239 Newton Sept. 26, 1944

Claims (2)

1. IN THE PROCESS OF CATHODICALLY PROTECTING THE CASING IN AN ENCASED WELL IN WHICH THE SURFACE PIPE EXTENDS DOWNWARDLY FOR ONLY A PORTION OF THE DEPTH OF THE WELL AND IN WHICH THE CASING IS MAINTAINED CATHODIC WITH RERESPECT TO A PROTECTIVE ANODE BURIED IN THE GROUND AND SPACED FROM THE WELL STRUCTURE, THE STEP COMPRISING CONNECTING A DIRECT CURRENT SOURCE BETWEEN SAID ANODE AND A POINT ON THE CASING ADJACENT TO THE BOTTOM OF THE SURFACE PIPE.

2. IN THE PROCESS OF CATHODICALLY PROTECTING THE CASING IN AN ENCASED WELL IN WHICH THE SURFACE PIPE EXTENDS DOWNWARDLY FOR ONLY A PORTION OF THE DEPTH OF THE WELL AND IN WHICH THE CASING IS MAINTAINED CATHODIC WITH RESPECT TO A PROTECTIVE ANODE BURIED IN THE GROUND AND SPACED FROM THE WELL STRUCTURE, THE STEP COMPRISING MAINTAINING AN ELECTRICAL INSULATOR BETWEEN THE TOP OF SAID SURFACE PIPE AND THE CASING.

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