US2751194A - Corrosion prevention - Google Patents

Description

June 19, 1956 G. H. ROHRBACK ET AL 2,751,194

CORROSION PREVENTION 4 SheeEs-Sheet 1 Filed Dec. 1, 1950 FIG. 4

PIC-1.1

INVENTORS GILSON H. ROHRBACK DWITE M. McCLOUD 6. M. ATTORNEYS FIG.2

R E T E M O T N E T O P 4 Sheets-Sheet 2 SELECTOR SWITCH m Q n G. H. ROHRBACK ETAL CORROSION PREVENTION June 19, 1956 Filed Dec.

FLOW STOPPED FLOW STARTED (HOURS) TIME OF TEST FIG. 6

6 136 .341. 5235 9:55p; m v

June 19, 1956 RQHRBACK ET AL 2,751,194

CORROSION PREVENTION Filed Dec. 1, 1950 4 Sheets-Sheet 3 FLOW STOPPED w FLOW STARTED 2 2 FLOW STOPPED 28/: i-LOW STARTED ANODIC POTENTIAL OF 251- RELATIVE To 231 F|G.8 x

\ INVENTORS GILSON H. ROHRBACK DWITE M. McCLOUD BY ZN fig 4M ATTORNEYS June 19, 1956 G. H. ROHRBACK ET AL 2,751,194

CORROSION PREVENTION 4 Sheets-Sheet 4 Filed Dec. 1 1950 3 R; C A :5

E m F K W 5%0 R H Q 0 T M m V NST mm ATTORNEYS United States Patent 2,751,194 CORROSION PREVENTION Gilson H. Rohrback, Whittier, and Dwite M. McCloud, Buena Park, Califl, assignors to California Research Corporation, .San Francisco, Calif., .a corporation of Delaware Application December 1, 1950, Serial No. 198,524 3 Claims. (Cl. 255-28) This invention relates to prevention of corrosion of structures exposed to flowing fluids which are electrolytes and particularly refers to improved configurations of such structures as sucker rods which are used in deep wells for remote actuation of pumps by the transmission of cyclic reciprocating motion from a power source at the surface of the earth to the pump plunger at the bottom of the well. These rods are exposed to the well liquids being pumped, including saline and other waters which tend to corrode the rods, particularly at the enlarged joints.

Heretofore, to facilitate interchangeability, the transmission of the large forces involved, as well as the screwing together and unscrewing the separate joints of a sucker rod string, the configurations and dimensions of the joints have been the subject of long-continued study, culminating in a substantial standardization of the structures by the American Petroleum Institute in its Standard No. 11-13, ninth edition, May 1942, entitled, API Sucker Rod Specification, issued by the Division of Production, Dallas, Texas. It has been found, however, that, in certain wells in which a quantity of saline water is produced with the oil, severe corrosion of these joint structures occurs within a matter of a few months or even weeks, requiring replacement of the entire rod string due to the weakened condition of the joint structure, including the enlarged or upset ends and the couplings connecting them. This involves a considerable economic loss in the rod replacement as well as the man-hours required and the loss of oil production from the wells.

This invention comprehends broadly the prevention of corrosion of structures exposed to flowing fluids which include electrolytes and is based upon the discovery that differences of velocity of the fluids w' h respect to adjacent portions of the structure, due to abrupt configuration ent. As will be last-named areas become electrically herein to prevent or at least greatly to minimize the cause of these potential differences and have been found by experience to be remarkably resistant to the hitherto experienced corrosion, particularly in Wells producing salt water in quantity along with the oil. The scope of the It is an object of this invention to provide not only the structures described but also an improved basis on which to design and to build other structures which are immersed in flowing liquids, and particularly those liquids which tend to corrode the structures, such as saline solufiorrsfound in deep oil wells.

Another object is to provide an improved joint construc- Z,75l,l94 Patented June 19, 1956 tion for sucker rod strings that are used in lengths up to several thousands of feet to pump oil and water from deep wells.

Another object is to provide an improved joint con struction that will not only be corrosion resistant but will also improve the hydraulic characteristics of the well pumping system, to minimize friction and turbulence losses and reduce the amount of water required to lift a given quantity of fluids to the surface for utilization.

Another object is to provide a joint construction for a well pumping system that will reduce the fluid turbulence which promotes the mixing of oil and water to form emulsions that must subsequently be broken by chemicals, heat, electric potentials, or other means.

These and other objects and advantages will be further apparent from the attached specification and drawings which illustrate the principle of the invention as well as preferred and alternative constructions of sucker rod joints embodying it.

In the drawings, Figure l is a of a conventional sucker rod to present standards.

Figure 2 is a transverse sectional view on line 2-2 of the rod arrangement of Figure 1.

Figure 3 is a transverse sectional view on line 33 of the rod arrangement of Figure 1.

Figure 4 is a vertical elevational view of a standard sucker rod joint of Figure 1 after ,two months service in a well.

Figure 5 is a diagrammatic vertical sectional view of an electrical potential demonstration equipment to duplicate flow conditions along surfaces of abruptly changing section.

Figure 6 is a chart showing electrical potential difl'erences set up by fluid flow .in the apparatus of Figure 5.

Fgure 7 is a vertical sectional view of an electrical potential demonstration equipment to duplicate flow conitions along a conventional sucker rod joint.

8 is a chart showing typical electrical potential diflerences set up between adjacent parts of the equipment of Figure 7 by intermittent fluid flow such as is encountered in pumping wells.

Figure 9 is a vertical side elevation view of an improved sucker rod joint embodying this invention.

Figure 10is a transverse sectional view on line 10-10 of the sucker rod of Figure 9.

Figure 11 is a transverse sectional View on line 11-11 of the sucker rod of Figure 9.

Figure 12 is a vertical side elevation view of an alternative form of sucker rod joint embodying this invention.

Referring to the drawings and particularly to Figures 1, 2 and 3, reference numerals 20 designate the vertical elevational view joint constructed according are each upset or enlarged beginning at the upset start 21, enlarging through the upset curve 22 to the circular bead 23, thence reducing in section abruptly to the wrenchto which the rod end is Coupling 28, which connects the threaded pins of rods 20, is correspondingly internally threaded. Since these threads form no part of this invention they are notshown in this view (Figure 1)- Opposed wrench .flats 29 are usually provided on coupling 28 and terminate in shoulders 31, which form transition faces from the wrench flat 29 which is parallel to the rod axis, to the cylindrical surface of the coupling body. Coupling 28 is somewhat larger in diameter than the adjacent pin shoulder 26, leaving an annular face 30 at each end of'the coupling where it abuts the rod end. The primary reason for this enlarged diameter is to prevent wear of the rod ends due to rubbing against the well tubing, the coupling 23 generally being hardened to resist the abrasion that would otherwise occur.

Referring to Figure 4, which represents an actual sucker rod joint constructed as just described, after two months service in a pumping well at Inglewood, California; similar reference members indicate similar parts Where applicable. It will be noted, however, that the original upper bead reduction face 25 and even a part of upset curve 22 has become severely corroded at A and that the corrosion has proceeded longitudinally downwardly along the flat wrench face 24 and along the ridges or edges between adjacent wrench faces. Likewise, the original pin shoulder face 27 has corroded badly at B, and the coupling shoulders 30 have corroded at C. Even the end shoulders 31 of the relatively shallow wrench flats 29 on coupling 28 have pitted as at D and these pits have extended longitudinally along the cylindrical surface of the coupling. In short, it is believed that the typical joint corrosion pattern of sucker rods is accurately represented by Figure 4.

This invention presents a solution to this problem of corrosion, and is based upon a novel concept of the probable cause for the corrosion just described, viz, that it occurs most rapidly and severely at abrupt changes of diameter or even at relatively shallow shoulders where velocity of liquids, and particularly electrolytes such as saline water, decreases from rapid flow to slower flow or even to quiescent liquid conditions. To demonstrate this, the apparatus of Figure was constructed, in which a glass tube 32 was arranged with suitable valves and pump (not shown) to convey a saline electrolyte 33 in the direction indicated at varying rates of flow. Electrolyte 33 was chosen to have a corrosivity with respect to steel at room temperature that duplicated the corrosivity of the brines found in oil wells at the temperatures encountered in such wells. A bar of steel 34 of the composition commonly used for sucker rods was supported in tube 32 so that the liquid fllowed longitudinally with respect to its surface. The upper face 35 of the bar was provided with shallow depressions 36 and 37 in which quiescent volumes of the flowing liquid electrolyte 33 would be retained.

In order to determine whether or not electrical potential changes on the face of bar 34 and in depressions 36 and 37 could be caused by electrolyte flow through tube 32, electrical probes 38 and 39 were placed on the bar surface 35 as shown, and probes 40 and 41 were placed in the bottoms of depressions 36 and 37. Each probe consisted of a glass tube 42 with a constricted open lower end 43 and an enlarged upper end or cup 44. A saline electrically conductive gel 45 filled the lower end of the tube and was made of sufficient strength to prevent its running out of the open lower end 43. A standard saturated calomel half-cell reference electrode 46 was placed in a saline solution 47 above the gel 45 in the cup 44 of each probe, and connected by an insulated eonductor to selector switch 48, from which a single conductor 49 leads to one terminal of a standard potentiometer 50. The opposite terminal of the potentiometer was connected by an insulated conductor 51 to the body of bar 34. Thus, for any given electrolyte flow condition in conduit 32, the electrical potential of the exposed steel at the normal surface 35 and at the bottoms of depressions 36 and 37 could be easily measured with reference to the standard half-cells 46.

Figure 6 illustrates the electrical potentials set up at the points just mentioned by electrolyte 33 flowing at rates simulating those in a pumping well, which average about 2 feet per second, and when the electrolyte flow was stopped. It will be noted that, at the beginning and end of the time represented, when there was no electrolyte flow, the potentials of the steel at the ends of all the probes 38, 39, 40 and 41 were substantially the same at about 0.65 volt. However, when electrolyte 33 flowed along bar 34, the electrical potentials of the steel at the bottoms of depressions 36 and 37 became more anodic than the potentials at the face 35. From this demonstration it can be concluded that areas on the surface of a metal where the metal section changes to produce quiescent liquid volumes will be anodic with respect to nearby or adjacent areas where higher velocity of liquid flow occurs along the metal surface. It will be understood by one skilled in this art that the metal on electrically anodic areas will be electrochemically removed, resulting in the corrosion of the metal body.

To confirm this discovery, particularly as it applies to well sucker rods, the apparatus of Figure 7 was constructed, in which a hollow model of a sucker rod joint, generally designated 55, identical as to size and external configuration to the A. P. I. Standard joint of Figure l, was molded of an electrically insulating plastic material. Steel pin electrodes were molded into the model, machined flush with its outer surface, and were separately connected by appropriate conductors generally designated 56 to a potentiometer (not shown). For convenient reference to Figure l the locations of these electrodes are numbered in a manner generally corresponding to the several parts of the standard sucker rod joint of Figure 1, and will be referred to below by similar descriptive terms. Model 55 was suspended as shown in conduit 32, and electrolyte 33, consisting of a saline solution whose corrosivity duplicated that of salt water found in oil wells, was flowed intermittently in conduit 32 in the direction indicated. Figure 8 is a curve showing typical anodic potentials developed between electrodes located in sections of low velocity or quiescent liquid conditions and adjacent pins located in sections of more rapid flow of electrolyte 33. The following table gives the potential changes between different pairs of electrodes under flow conditions, and it will be noted that the anodic points are all in the quiet areas of fluid flow. The potential differences became zero when electrolyte flow was stopped.

Potential change with flow: 221 anodic to 231 251 anodic to 231 271 anodic to 261 301 anodic to 281 302 anodic to 281 272 anodic to 262 252 anodic to 232 Referring to Figures 1 and 4, the concept just discussed is quite well confirmed in practice, as it has been found that the areas of the rod where abrupt transitions of sections would produce relatively quiescent fluid flow adjacent areas of more rapid fluid flow, for example, at points 22 and 25 as compared to 23', 27 and 30 as compared to 28; and 31 as compared to 28; are those where the greatest corrosion has occurred. Furthermore, experience has shown that where a slight depression or: pit has formed at the edge of an area of uniform crosssection, for example, at the upper shoulder 31 of coupling 28, it will form a local quiescent zone for fluid flowing thereover, so that the pit will elongate or progress longitudinally along the surface, as shown at D. A similar efliect is noted by the corrosion extending along the upper wrench square 24 between A and B.

In order to embody this invention in a sucker rod or other structure subject to analogous corrosion conditions due to continuous or intermittent flow of electrolyte, the construction shown in Figure 9 has been devised. This is characterized by the absence of abrupt changes of section, particularly beads or shoulders, and sharp edges, adjacent to which there can exist quiwcent or low velocity fluid volumes that would render the surface areas of the structure electrically anodic with respect to the adjacent areas Which are In contact with faster-flowing liquids. In the case of well sucker rods, the upper and lower .end structures need not be exactly symmetrical. This is apparently due to the fact that the fastest relative fluid flow or motion with respect to the rod occurs only on the down stroke of the latter. When the rod is pulled upwardly, bringing with it the pump standing valve "in closed position, the liquid above the valve and around the rod rises at essentially the same speed as the rod. in the arrangement of Figure 9 this may be turned to advantage by retaining for the lower rod substantially the present standard configuration of part of the upset end, viz, the upset curve 60, thus permitting use of the presently constructed rod elevators which are curved to fit that shape.

Referring now to Figure 9, the upper end of lower rod 58 is upset or enlarged, the start of the enlargement being indicated at 59 and the upset curve being indicated at 66.

Instead of the maximum diameter bead and the adjacent recessed wrench flats of the standard configuration (Figure 1), the upset curve 60 merges smoothly and substantially tangentially into what we prefer to designate as the upset cylinder 61 on which relatively shallow wrench flats 62 are formed. Desirably these flats do not meet in right angled sharp edges which are longitudinal to the rod axis, because it has been found, and is Well illustrated in the rod illustrated in Figure 4 on the opposite side from upper reference letter A, that longitudinally extending sharp or even slightly rounded edges formed by plane surfaces meeting at right angles, and particularly those adjacent to a circular section, such as the upper upset curve 21, cause cross-currents in liquid flowing along the rod, and where those currents leave the rod surface, areas of liquid quiescence or lower velocity are formed which are electrically anodic to the adjacent areas of the rod, promoting corrosion of the anodic areas as has been explained in detail above. More specifically it is preferred, in this example, to designate mathematically the relation between the opposed Wrench flat spacing and the upset cylinder diameter by stating that the former is greater than about 0.71 of the latter, this being the approximate relation between the side of a square and the diameter of a circumscribing circle.

The merging surface 64 of the ends of wrench flats 62 with the upset cylinder adjacent pin shoulder face 65 is preferably at a relatively low angle of not greater than about with the longitudinal axis of the rod, so that erably formed with an elongated taper of a length not difference in diameters of the adjacent cross-sectional areas. Additionally, the contour of the surface along the axis of 61, to avoid sudden promote corrosion. flats 62 can be used fluid velocity changes which would If desired, only two opposed wrench on either the -pin or box end of the rod construction shown, as illustrated .by the upper portion of the rod of Figure 9 and in Figure 1 1. Also, because a smooth .transition between sections ismore desirable at the upper end of the rod joint, due vto the fastest relative mo'tioncof the well liquid being upwardly .along the rod, the upset cylinder 61 adjacent the box end may taper uniformly as at 70 into the upper rod diameter 71, instead of :following the standard upset curve 60 which may be used at the lower end of the joint.

If it is desired to reduce to the minimum the opportunity for sucker rod joint corrosion due to relative liquid flow, the construction of Figure 12 may be adopted,

double pin rod with a smoothly contoured larger diameter coupling connected to the adjacent pin ends, and also uniformly tapered junctures 70 between both the upset cylinder diameters 61 and rod bodies 58 and 71.

In conclusion, it will be appreciated that comprehends broadly the prevention of corrosion of structures immersed in or exposed to relatively flowing liquids which include electrolytes, by suitable surface conthis invention the adjacent end of said enlarged portion, the external surface of said member gradually increasing in diameter from the external diameter of said abutting end to its maximum diameter.

2. A sucker rod in accordance with claim 1 in which said enlarged portion has wrench flats formed on its exterior surface, merge with said exterior surface.

3. A sucker rod having a cylindrical body, a diametrically enlarged portion of circular cross section on one end of said body, a pin protruding from the end of said enlarged portion and in axial alignment with said body, the said enlarged portion gradually decreasing in diameter until it merges with said body, and a member of circular cross section having a diameter greater than that of said Smith Apr. 24, 1894 Seevers Sept. 4, 1900 (Other references on following page) UNITED STATES PATENTS Smith Apr. 10, 1906 Parker Apr. 22, 1913 Gunn Dec. 20, 1919 Axtell Apr. 8, 1930 Bridges Mar. 15, 1932 Peterson Sept. 27, 1932 Davison June 23, 1936 Kitching Sept. 22, 1936 Watts Apr. 30, 1940 FOREIGN PATENTS Great Britain May 24, 1930 Great Britain Ian. 28, 1932 Great Britain May 3, 1943 OTHER REFERENCES Corrosion Handbook, by Uhlig, Wiley & Sons, 1948.,

page 15.

Claims (1)

1. A SUCKER ROD HAVING A CYLINDRICAL BODY, A DIAMETRICALLY ENLARGED PORTION OF CIRCULAR CROSS SECTION ON ONE END OF SAID BODY, A PIN PROTRUDING FROM THE END OF SAID ENLARGED PORTION AND IN AXIAL ALIGNMENT WITH SAID BODY, THE SAID ENLARGED PORTION GRADUALLY DECREASING IN DIAMETER UNTIL IT MERGES WHTI SAID BODY, AND A MEMBER OF CIRCULAR CROSS SECTION CONNECTED TO SAID PIN AND HAVING ONE END THEREOF ABUTTING SAID ENLARGED PORTION, THE ABUTTING END OF SAID MEMBER HAVING AN EXTERNAL DIAMETER COEXTENSIVE WITH THE ADJACENT END OF SAID ENLARGED PORTION, THE EXTERNAL SURFACE OF SAID MEMBER GRADUALLY INCREASING IN DIAMETER FROM THE EXTERNAL DIAMETER OF SAID ABUTTING END TO ITS MAXIMUM DIAMETER.

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