Claims
- 1. A combination comprising a string of sucker rods having threaded pins at their ends, couplings of like construction engaging said pins and joining the rods end-to-end, a subsurface pump, a string of inertia bars connected between the lowermost sucker rod and said pump for adding weight to the sucker rods and maintaining them in tension throughout a pumping cycle, and couplings of similar construction to said first-named couplings joining said inertia bars end-to-end;
- each of said inertia bars comprising:
- an elongated metal body of substantially the same diameter as the outside diameter of said couplings;
- integral threaded pins at the respective ends of said body; and
- shoulders at the base of said pins having locking engagement with said second-named couplings;
- said string of inertia bars having a neutral zone located about 80 percent of the distance up from the pump to the top of the string of inertia bars.
- 2. An inertia bar as defined in claim 1 in which said body has a length of about 15 to 50 feet.
Parent Case Info
This application is a continuation-in-part of our earlier application Ser. No. 665,845, filed Mar. 11, 1976, now abandoned which in turn is a continuation of our earlier application Ser. No. 216,401 filed Jan. 10, 1972, now abandoned.
This invention relates to an improved inertia bar for use in sucker rod string.
In the oilwell art, sucker rods are rods which extend down a well to a subsurface pump and are driven up and down to operate the pump. When a string of sucker rods is immersed in a well, there is a hydraulic up-force acting on the cross-sectional area of the rod, which force is equivalent to the hydraulic pressure at the bottom of the well. This force is a function of the depth of the well and can be substantial. For example, at a depth of 5000 feet in salt water with a specific gravity of 1.1, the up-force on a 3/4 inch diameter sucker rod is about 1060 pounds. This force tends to buckle the sucker rod string during their downstrokes and may induce fatigue failure.
As a means for overcoming the up-force on a sucker rod string, it is known to insert weights in the bottom of the string to serve as inertia bars just above the pump. The weights are intended to be of a magnitude to maintain the sucker rods under tension throughout the pumping cycle. Weights used heretofore have been old polished rods, a term which refers to the rod at the top of the sucker rods connected to the driving unit. Polished rods, not being designed for this purpose, are awkward to handle and are hit-or-miss as to where they locate the "neutral zone", that is, the level at which there is neither tension nor compression in the string.
The thread on the pin of a conventional polished rod is of the vanish type; that is, the thread fades out toward the body of the rod and there is no shoulder against which a coupling can be locked. The region at which the thread fades out becomes a stress notch where fatigue failure often occurs. As a consequence, when polished rods are used as weights at the bottom of a well, the pins are easily broken at the stress notches, or the couplings may become unscrewed accidentally since they are not positively locked against shoulders on the rods. As the number of polished rods is increased to add more weight, the tendency of the pins to break or of the couplings to become unscrewed becomes progressively worse. A conventional polished rod has no means for engagement by an elevator or a fishing tool. Hence when polished rods are used as weights, it is awkward to retrieve the pump from the well in normal operation, or to fish the loose parts from the well if a pin breaks or a coupling becomes unscrewed. Because of these problems, the number of polished rods used as weights has been limited to two or three. A rather long polished rod has a length of about 26 feet. The maximum length of weights attainable with polished rods is about 78 feet.
When a polished rod is used for its intended purpose at the top of a sucker rod string, the coupling at the top of the polished rod provides a shoulder which the hanger of a pumping unit may engage for supporting the rod string. Hence polished rods are of significantly smaller diameter than the outside diameter of the couplings and not of the full diameter which can be accommodated within the limits of the coupling diameter. Because of these limitations, it is exceedingly unlikely that the hit-or-miss location of the neutral zone would ever be within polished rods used as weights at the bottom of a well.
An object of our invention is to provide an improved inertia bar which is readily handled by conventional equipment used for handling sucker rods and which is accurately constructed for its intended purpose.
A further object is to provide an inertia bar which is dimensioned accurately to locate the neutral zone at a controlled level, typically about 80 percent of the distance up from the pump to the top of a string of such bars.
US Referenced Citations (8)
Continuations (1)
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Number |
Date |
Country |
| Parent |
216401 |
Jan 1972 |
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Continuation in Parts (1)
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Number |
Date |
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| Parent |
665845 |
Mar 1976 |
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Patent Grant
4221549
