Patent Application
20080035329
A sucker rod string in a petroleum well extends from a power drive at the wellhead, down through the lengths of tubing and concentric casing used in a production system, and connects to the fluid pump at the lowest elevation in the system. The pump receives an accumulating petroleum inflow from the production zone somewhere above it and lifts a column of fluid up through the tubing. The power drive at the wellhead may be a reciprocating beam system, often called a horsehead, and the sucker rod string then reciprocates a downhole plunger pump which lifts fluid upward through the tubing. The power source may alternatively be a rotary drive coupled via the sucker rod string to a longitudinally extended progressing (or “progressive”) cavity pump.
At the wellhead, the fluid product raised through the tubing is diverted to a lateral outflow and the flow path is closed off by seals about a special interconnection rod which couples the upper end of the sucker rod string to the horsehead or rotary drive. This interconnection rod, called a polish (or polished) rod, must meet stringent requirements because it must provide a low wear durable seal as well as withstand the tensions and stresses engendered by the well operation. It must bear the weight of the sucker rod string as the string cycles, as well as the mass of the fluid column that is being lifted, and it must also resist transitory forces which occur during the pumping, whether reciprocating or rotary. These forces arise variably from impacts, inertial variations and stresses induced longitudinally or by torsion. Since the polish rod reciprocates through fluid seals at the wellhead, it must function uniformly without leaking or undue friction through the operating life of the polish rod. For these reasons, the polish rod diameter is typically larger in diameter than the sucker rods in the string, and is of a suitable high strength material. To provide affective sealing, the surface of the polish rod is spray metal coated with a hard alloy and ground and polished to provide a long life uniform surface which introduces little wear.
Over years of development and use in the oil industry, technical changes have been made to meet constantly increasing artificial lift requirements. Thus a relatively early (pre-1940) specification from the American Petroleum Institute (API) defined a polish rod in which one end had a forged upset area, including a shouldered element having a rod pin machined on it. The other end had no shoulder but terminated in straight threads of a specified API polish rod design. The thread design terminated the straight threads with 3 partial threads along a 9° (nine degree) taper. The shouldered polish rod specification became effectively obsolete while lift requirements were still relatively low because it was felt that there was no structural need for the shoulder. Consequently, both ends of the polish rod have connection threads of the straight API polish rod type without a shoulder.
As artificial lift requirements increased because of increases in the depth of wells along with greater demand for petroleum products, new sucker rod designs were developed for greater performance. It became evident that the polish rod thread configuration often became a weak link in the rod string under more demanding conditions. Shock and impact forces encountered with gas or fluid pounding appeared to be the most frequent failure consideration, because these forces excessively stressed the polish rod connection threads as they alone absorbed longitudinal stresses and impact forces on the rod. Such breakdowns are aggravated because, it is well known, movement between threaded members tends to destroy the connection.
Polish rod connection failures were evidenced, for both beam pumping and progressive cavity pumping, by “belling out” (radial expansion) of the coupling due to the prior sucker rod design. In beam pumping systems the 9° terminal taper and partial threads allowed the tapered threat sections to go deeper and deeper into the coupling, causing threads to shear or even split the coupling. In rotary systems gas absorbed in the stator elastomer, results in torque increasing, which may be accompanied by torque spikes caused by the ingestion, by the pump of formation solids, with failures like those mentioned above.
Applicant has shown that prestressing of connections in sucker rod systems can be used to advantage, as described in Carstensen U.S. Pat. No. 6,942,254 issued Sep. 13, 2005, and Carstensen patent application Ser. No. 09/961,391 filed Jul. 25, 2001. Improvement of the polish rod connection so as to take advantage of these beneficial expedients while providing the needed structural and sealing properties are desirable objectives.
An improved polish rod system in accordance with the invention is configured for precise dimensional positioning and structural interaction with a sucker rod system. It provides the structural properties needed and an extremely durable construction for load cycling operation. The improved polish rod integrates a shoulder surface at a position that absorbs stresses both static and dynamic, transmitted along the sucker rod string and in the rod couplings. This improvement enables the connection to take advantage of prestressing, with or without an intermediate torque button.
A polish rod in accordance with the invention is externally configured to have the needed principal length and body diameter for the stress on the sucker rod string to which it is to be attached. The polish rod body is machined, ground and polished and spray coated with hardened material in accordance with prior practice, but includes a small diametral reduction for a predetermined length adjacent its lower end that has precisely defined end transitions. A shoulder fitting or sleeve of slightly smaller internal dimension than the reduced rod dimension but of the predetermined length is attached on the polish rod body, after being heated sufficiently to shrunk into place with secure engagement. A corner radius of curvature on the upper end of the shoulder sleeve engages a matched curvature at the corresponding end of the predetermined length of the polish rod, to aid in seating and retention. The sleeve includes a tapered section that diverges from its upper end to a mid-region transition and thereafter is of uniform cross section to its lower end. This end defines a transverse end shoulder in a transverse plane that is precisely aligned longitudinally with the end of a pin neck section in the polish rod. The pin neck provides stress relief and adjoins the terminating straight thread section which is compatible with the female thread in a sucker rod coupling. When the pin end is fully in threaded position in the coupling, the coupling end engages the transverse shoulder surface of the sleeve.
With this arrangement, a sucker rod coupling attached to the pin end and abutted against the shoulder surface can be precisely prestressed, in the fashion described in the aforementioned Carstensen patents, to resist the initiation and growth of microcracks and displacements between the threads during repetitive cycling under stress. Alternatively, the combination may include a torque button positioned between the end faces of a the polish rod and an opposing sucker rod end mounted in the coupling. Similar interconnections can be used at both ends of the polish rod, if a direct connection is not used, except that at the upper end the sleeve is inverted (relative to the orientation at the lower end) and the pin end is coupled directly or via an intermediate device to the primary cable.
A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:
Alternatively, as is well known, the sucker rod system can be driven circumferentially by a rotary drive (not shown) in which event the pumping structure at the downhole location is of the progressive cavity type. Load variations with the progressive cavity drive reflect not only the nominal metal and petroleum masses and inertial loads but also transient variations introduced by solids and gases in the product.
The primary cable 22 in this example couples directly to the upper end of a polish rod 36 having a conventional length which extends through a circumferential seal 38 into the vertical tubing 40 supported in part by an encompassing tubing head 42 which contains the polish rod seals. The polish rod 36 is not only about 1.5× the diameter of the sucker rods in the string but has a suitable length, such as 10 ft. to 36 ft. above the tubing head 42. Below the tubing head 42, the installation extends downhole in conventional fashion, within the tubing 40 sections that are concentric within a casing 46. The joined lengths of sucker rod sections 48 are centrally positioned within the tubing 40.
The general downhole construction (as seen in
In the polish rod 36 assembly, shown in greater detail in
Along its upper section the fitted sleeve 62 includes a downwardly diverging tapered section 70 which merges smoothly in a mid-region of the sleeve 62 into a constant outer diameter section terminating at the transverse and shoulder 67 surface. On the end portion of the polish rod 36, the neck or relief area 66 merges to a straight threaded section 74 mating with the internal female threads on the sucker rod coupling 50.
When assembled, as also seen in
As seen in
The polish rod assembly, as previously expressed, has a diameter that is greater than the diameter of the sucker rod with which it is being used. Other sizes of sucker rods and polish rods may be used, in accordance with the following chart given as an example only.
The incorporation of a physically united sleeve 62 on the reduced diameter length of the polish rod involves using a sleeve 62 which is 0.003″ to 0.004″ less in diameter than the outer diameter of the reduced diameter length. The sleeve 62 is heated to between 600° and 750° F. to expand it sufficiently to slip over the dedicated length of the polish rod. When in proper position, which is precisely defined by the curved ridge 60, the sleeve 62 is in effect physically united with the polish rod 36 on cooling, and withstands the loads that encountered on cyclic operation.
With polish rods provided in accordance with the invention, cyclically varying loads, such as are encountered with a modern pumping system, are buffered by coupling to shoulder contact as well as contact at the threaded flanks. Furthermore existing parts inventories of polish rods can be employed after modification at service centers or even on site.
Although modifications and variations of devices in accordance with the invention have been described, the invention is not intended to be limited to such but is to apply to all expedients within the scope of the appended claims.
This invention relies for priority on previously filed provisional application No. 60/837,563, filed Aug. 14, 2006 by Kenneth J. Carstensen and entitled “Interconnect Rod for Sucker Rod String”.
| Number | Date | Country | |
|---|---|---|---|
| 60837563 | Aug 2006 | US |
