Description

Technical Field

The invention relates to a sucker rod for oil wells, more particularly, a steel element reinforced sucker rod which does not swivel under the reciprocal tension/compression load in the oil pumping operation.

Background Art

Sucker rods normally come in sections of 7.5 or 8.0 meters long. Most conventional prior art sucker rods are simply solid steel rods with two male fittings or with a male and a female fitting at their ends. At the oil well site, these are joined, section by section, via external couplings in the case of two male fittings or by putting the male fittings inside the female fittings, in order to form a rod of great length sufficient to reach the length of the well casing and operate a pump at the bottom of the well casing to force oil up the casing to the well head. A sucker rod assembly thus is an operating rod connected, at its lower extremity, to the pump element and, at its upper end, to the power motor and drive for the pump.

One problem well known to the art is that sucker rods become so long in length that they stretch longitudinally and get out of synchronization with the reciprocation of pump. Since some wells are 6,000 meters or more in depth, the cumulative weight of the aggregate connected sucker rods becomes so great to sustain and operate.

One solution to above problem is to replace the solid steel sucker rod with light weight sucker rod. For example, prior art US4205926 provides a sucker rod comprising an elongate central steel strand acting as load carrying element and an outer sheathing, a resin impregnated fiberglass wrap of helical character which is of light weight and great strength.

But there are drawbacks with this kind of sucker rods. Firstly, since the load carrying central steel strand is a twisted construction, the sucker rod may swivel under the reciprocal tension/compression load in the oil pumping operation. The swivel sucker rod may further loose the connections between rods to cut the strength of the connection or even worse to break the sucker rod assembly.

Disclosure of Invention

The primary object of the invention is to provide a sucker rod which does not swivel under the reciprocal tension/compression load in the oil pumping operation.

The second object of the invention is to provide a sucker rod of great length, sufficiently flexible to be wound round a reel.

According to one aspect of present invention, a sucker rod for oil pumping, comprises a polymer or resin as matrix material reinforced by at least two steel elements which are parallel to the axis of the sucker rod. The steel element can be a steel filament, a steel strand or a steel cord.

In case of a steel strand or a steel cord, the at least two steel elements are preferably twisted in different twisting direction. Most preferably the steel elements with different twisting direction are alternatively distributed on the cross section of the sucker rod.

According to another aspect of present invention, the sucker rod may have an approximately round cross section or may have a rectangular cross section. The thickness of the rectangular cross section of the sucker rod may range between 3mm to 20mm, while the width of the rectangular cross section of the sucker rod may range between 20mm and 35mm. The ratio between the width and the thickness of the rectangular cross section of the sucker rod may range between 1 to 10.

Brief Description of Figures in the Drawings

Figure 1 illustrates a cross sectional view of a sucker rod of approximately round cross section according to present invention.

Figure 2 illustrates a cross sectional view of a sucker rod of approximately rectangular cross section according to present invention.

Figure 3 illustrates the different twisting direction of twisted strand or cord.

Figure 4 schematically illustrates the manufacturing process of a sucker rod on an extrusion head.

Figure 5 schematically illustrates a cross sectional view of a sucker rod with rectangular cross section incorporating one of the best embodiments.

Figure 6 schematically illustrates a cross sectional view of a sucker rod with round cross section incorporating one of the best embodiments.

Figure 7 schematically illustrates a cross sectional view of a sucker rod with round cross section incorporating one of the best embodiments.

Mode(s) for Carrying Out the Invention

Figure 1 illustrates a cross sectional view of a sucker rod of approximately round cross section according to present invention. The sucker rod 2 comprises a polymer or resin as matrix material 4, and is reinforced by four steel elements 6s and 6z.

Figure 2 illustrates a cross sectional view of a sucker rod of approximately rectangular cross section according to present invention. The sucker rod 2 comprises a polymer or resin as matrix material 4, and is reinforced by six steel elements 6s and 6z.

In figure 2, the thickness T of the rectangular cross section of the sucker rod 2 is between 3mm and 20mm, while the width W of the rectangular cross section of the sucker rod 2 is between 20mm and 35mm. The ratio between the width W and the thickness T of the rectangular cross section of the sucker rod is between 1 to 10. The sucker rod with rectangular cross section has following benefits. Firstly, the sucker rod makes sure enough space between the rectangular sucker rod and the round oil well casing for oil pumping. Secondly, since the width W is bigger than the thickness T, the bending stiffness along the T direction is smaller to wound the sucker rod on a reel.

The steel elements 6s and 6z can be a steel filament, a steel strand, or a steel cord. A steel filament means a single steel wire. A steel strand means a group of single steel wires which is either twisted or untwisted. A steel cord is a group of single steel wires or steel strands twisted together.

Comparatively, a twisted structure is to be preferred to a non-twisted structure. A sucker rod reinforced with steel filaments or un-twisted steel strands is too loose to maintain a stable character under the reciprocal tension/compression load in the oil pumping operation. In contrast herewith, twisted steel strands and twisted steel cords can be a good choice to reinforce sucker rod to maintain a stable character under the reciprocal load in oil pumping operation. To counteract the swivel of the sucker rod, the twisted steel strands or cords may be twisted in different twisting direction, and more preferably steel strands or cords with different twisting direction may be alternatively distributed on the cross section of the sucker rod.

A typical steel composition for steel elements 6 may have a minimum carbon content of 0.65%, a manganese content ranging from 0.40% to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%), a maximum phosphorus content of 0.30%, all percentages being percentages by weight. There are only traces of copper, nickel and / or chromium. A typical steel composition for high-tensile steel elements 6 may have a minimum carbon content of around 0.80 weight %, e.g. 0.78 - 0.82 weight %).

Figure 3 illustrates the different twisting direction of twisted strand or cord. 3a shows a twisted strand or cord in Z direction, while 3b shows a twsted strand or cord in S direction. The strand or cord has an S or left-hand twisting direction if, when held vertically, the spirals around the central axis of the strand or cord conform in direction of sloop to the central portion of the letter S, while Z or right-hand lay if the spirals conform in direction of slope to the central protion of the letter Z. Going back to figure 1 and figure 2, 6s means the strand or cord is twisted in S direction, while 6z means the strand or cord is twisted in Z direction. Steel elements with different twisting direction 6s and 6z are alternatively distributed on the cross section of the sucker rod.

Figure 4 schematically illustrates the manufacturing process of a sucker rod on an extrusion head. Steel elements 6 are led into the extrusion head 8 from the left side, while polymer or resin matrix material 4 is fed into from the upper side. Inside the extrusion head 8, steel elements 6 are embedded into polymer or resin matrix material 4, and the finished sucker rod 2 is extruded to the right side. Since the extrusion process can be continuously carried out, a sucker rod with great length can be made and there is less need for couplings, external connections and fittings.

Figure 5 schematically illustrates a cross sectional view of a sucker rod with rectangular cross section incorporating one of the best embodiments. The sucker rod 2 is in a rectangular cross section. The matrix material 4 is Epoxy Vinyl Ester Resin. The steel element 6 is a steel wire with diameter 0.415mm and breaking load above 410 Newton. The sucker rod 2 is reinforce by 585 steel elements 6. The thickness T of the rectangular cross section of the sucker rod 2 is 5.1mm, while the width W of the rectangular cross section of the sucker rod 2 is 32mm. The ratio between the width W and the thickness T of the rectangular cross section of the sucker rod 2 is about 6.27. The 585 steel elements 6 are parallel to each other and evenly distributed in the matix material 4, in which 585 steel elements 6 are grouped into 9 layers in the width direction and each layer containing 65 steel elements 6. Each steel element 6 keeps approximately 0.04mm distance from the adjacent steel elements 6. The steel elements 6 which are next to the external surface of the matrix material 4 keep a distance about 0.5mm from the external surface of the matrix material 4. This sucker rod 2 has a breaking load above 200K newton.

Figure 6 schematically illustrates a cross sectional view of a sucker rod with round cross section incorporating one of the best embodiments. The sucker rod 2 is in a round cross section. The matrix material 4 is Epoxy Vinyl Ester Resin. The steel element 6 is a twisted strand comprising seven steel wires. The diameter of the steel wires is 0.415mm. The breaking load of steel element 6 is above 2600 Newton. The sucker rod 2 is reinforce by 91 steel elements 6. The 91 steel elements 6 are parallel to each other and are distributed on five concentric circles. There are one steel element 6 located in the center as the center of the five concentric circles. The first circle comprises six steel elements 6, the second twelve steel elements 6, the third eighteen, the fourth twenty-four, and the fifth thirty. In short, the distribution of steel elements 6 can be expressed as 1+6+12+18+24+30. On each circle the steel elements 6 with different twisting direction are alternatively distributed. The gap between the steel elements 6 on the same circle is about 0.05mm. To counter-act the swivel, the steel element 6 in the center can be a group of 7 parallel steel wires with diameter of 0.415mm and breaking load above 410 Newton. The diameter of the sucker rod 2 is above 15mm and the breaking load of the sucker rod 2 is above 200K newton. Compared with the embodiment of figure 5, the embodiment of figure 6 has small number of steel elements 6 for the extrusion process, which may facilates quality control and machine efficiency.

Figure 7 schematically illustrates a cross sectional view of a sucker rod with round cross section incorporating one of the best embodiments. The sucker rod 2 is in a round cross section. The matrix material 4 is Epoxy Vinyl Ester Resin. The steel element 6 is a twisted strand comprising seven steel wires. The diameter of the steel wires is 0.415mm. The breaking load of steel element 6 is above 2600 Newton. The sucker rod 2 is reinforced by 127 steel elements 6. The 127 steel elements 6 are parallel to each other and are distributed on six concentric circles. There are one steel element 6 located in the center as the center of the six concentric circles. The first circle comprises six steel elements 6, the second twelve steel elements 6, the third eighteen, the fourth twenty-four, the fifth thirty, and the sixth thirty-six. In short, the distribution of steel elements 6 can be expressed as 1+6+12+18+24+30+36. On each circle the steel elements 6 with different twisting direction are alternatively distributed. The gap between the steel elements 6 on the same circle is about 0.05mm.To counter-act the swivel, the steel element 6 in the center can be a group of seven parallel steel wires with diameter of 0.415mm and breaking load above 410 Newton. The diameter of the sucker rod 2 is above 19mm and the breaking load of the sucker rod 2 is above 300K newton. Compared with the embodiment of figure 6, the embodiment of figure 7 has big improvement on the breaking load with small increase on the diameter.

Compared with prior art US4205926, present invention has following benefits. Firstly, the sucker rod does not swivel under the reciprocal tension/compression load in the oil pumping operation, because steel elements 6 are twisted in different directions and are alternatively distributed in the cross section of the sucker rod. Secondly, the breaking load of the sucker rod is higher, because all the steel elements 6 are parallel to each other, rather than twisted into a big rope, and the strength loss from the twist structure is limited. Thirdly, in case the sucker rod breaks, it is easier to remove the broken rod from the narrow oil well casing, becasue all the steel elements 6 are twisted in small strand with limited number of filaments, and the loosen ends of the broken steel elements 6 may not extend to much to block the narrow oil well casing. On the contrary, the sucker rod with big rope will really make a mass, because the loosen ends of the rope may extend under the huge torsion, which not only block the oil well casing but also ruin the oil well eventually. Fourthly, since the steel elements 6 are parallel to each other, the bending stiffness of present sucker rod is lower. The sucker rod according to present invention can be easily wound on a reel for transportation.