CA1186948A - Sucker rod - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A rod assembly which includes a core component and a sheath component thereof which consists of an interlaced configuration of assemblies of non-metallic filamentary elements embedded in a polymeric matrix and of elongate cross section. A major axis of the cross section whereby the bending stiffness for bending deformations about an axis parallel to the major axis is less than the bending stiffness of the same core and sheath components in a radially symetrical configuration, and in which the load-elongation characteristics of the core and sheath assemblies are selected and the core and sheath assemblies are arranged so that the core and sheath components share substantially in the load bearing.

Description

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BACKGROUND OF THE INVENTION
~6-46 l ,:js I Conventional beam pumping installations for pumping fluid such as oil fLom underground locations utilize rods which are coupled in a continuous fashion to connect a surface pumping ~ unit to an underground or subsurface downhole well pump for the purpose of transmitting mechanical energy from the surface equip-¦
ment to the subsurface pump. The individual rods comprising the string are known as sucker rods and the plurality when coupled is referred to as a sucker rod string.

Subsurface oil well pumps are generally classified as either tubing or rod pumps. In the case of tubing pumps, the barrel is run on the tubing and the plunger is run on the rod strin~. Rod pumps have the advantage of being more easily removed for servicing and are less susceptible to damage in runnin but they offer less working area for the plunger since the max-imum bore of a rod pump is neccessarily less than the maximum bore of a tuhing pump fcr the same size tubing. In either case, howeve ~, pump travel length or plunger stroke is highly important in deter-mining output, since the plunger stroke for any given pump when multiplied by the product of stroke rate and plunger area gives the volumetric productivity.
In the prior art publication "Well Design: Drilling and Production, Craft, B.C., Holden, W. R., and Graves, E.P., Jr., ¦ Prentice-Hall Inc. 1962 "it is taught that the effective plunger l~stroke downhole differs from the polished rod stroke; it is de-llcreased by the effects of rod stretch resulting from fluid load ¦and rod mass; and is increased by the effect of plunger over-travel. Since the magnitudes of these increases and decreases in stroke length are affected by the mechanical properties of the rods it is evident that the effective stroke downhole can be modified by suitable manipulation of the rod materials and characteristics, and this possibility has lead to considerable development effort in this area. In particular, it is inter-esting that modern data-logging and computational techniques, s~ch as prescribed in SPE paper 588 by S.G. Gibbs presented at the Rocky Mountain Joint Regional Meeting, May 1963, of the Society of Petroleum Engineers of AIME permit the matching of sucker rod prop~rties and the make-up of the sucker rod string to the operational parameters of a given well to acheive highly favorable pumping conditions, and hence, enhanced opera-tional economics~
Sucker rods are currently manufactured in discrete lengths of either 25 ft. or 37.5 ft. and coupled together in the field. Couplings are expensive and introduce a mechanical weakness in the sucker rod string. References have been made to the use of wire cable without couplings. However, a cable, per se, lacks the appropriate bending rigidity to properly serve the end use~
Early sucker rods were of all-metal construction as exemplified by United States Patent No. 528,168 issued October 30, ~¦1894. Thereafter initial efforts to improve sucker rod perfor-mance were concerned with use of materia]s and design to resist corrosion and stress failure in view of the harsh environment of the well in which the rod is worked. These efforts are illustated in prlor art patents such as: United States Patent 3,486,557 issued in 1969 to Harrison showing a rod comprising an inner cable surrounded by an encasement of molded plastic or fiberglass in an unspecified configuration wherein the end of ¦the encasement has a conical recess to receive a splayed end of the cable which is held therin by metal introduced into the recess while molten and wherein the outer surface of the ~ .

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encasement is threaded to receive a connecting sleeve that serves to transfer load between adjacent sucker rods; United States Patent 4,063,838 issued in 1977 to Michael showing a sucker rod having a solid steel core wrapped with resin-impregnated glass filaments in which the filaments form a stratified structure and the load transfer is via the outer surface of the wrapping in a manner similar to that described by Harrison. In this latter concept, however, the sheath material contains only helically wrapped filaments and is specifically designed to sustain compressive load in an attempt to maintain the core in a state of tension after the curing step.

It is interesting to note that as early as 1959 United States Patent 2,874,937 to Higgins disclosed a sucker rod com-prised of glass fibers held together by plastic resin. Intensive work has been undertaken in the field of fiberglass sucker rod 15 `I
design. Fiberglass is not seriously affected by corrosion, poss-esses a low specific gravity and has a high tensile strength-to ¦ weight ratio compared to steel.
'I In Paper SPE6851 presented at a technical meeting of SPE

l~ of AIME, Denver in October of 1977 Watkins and Haarsma described ~l a continuous process for producing a high-volume-fraction glass l~ rod in which glass filaments are collimated, saturated with resin `I! ordered into a circular configuration and cured. The paper I presented data on the use of rods produced according to this '1I process~ The process has been referred to as the "pultrusion"
process and the resulting rods have been referred to as "pultrudel"
I iberglass/resin composite rods.
" Pultruded fiberglass sucker rods have a number of recognized positive attributes which include:
1 Higher Strength/Weight Ratio and Lower Rod Density than Steel Sucker Rods.

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l - 3 -, Lighter weight sucker rods allow the use of smaller ! pumpjacks and develop lower gear box loadings for a constant i rate of production compared with those required for steel rods. I
2. Good Corrosion Resistance/Low Electrical Conduct-11 ivity.
I Fiberglass/polyester composites have much greater resistance to corrosion than unprotected steel in the hostile environment found downhSole. The downhole environment includes crude oi], H2S, C02 and water at te~perature up to 2000F, and furthermore, enhanced oil recovery techniaues often result in ]0 increased concentration of corrosive elements. Rod strings con-sisting entirely of steel have been kno~m ~o have useful lives of less than three months when employed in corrosive environment wells.

3. Opportunity for Increased Oil Well Productivity.
Fiberglass possesses an extensional modulus that is approximately 1/3 that of steel. While fiberglass is con-sidered generally to be a stiff material, when fabricated into sucker rods and subsequently installed in a deep (approx. 3,000 to 8,0GO ft.) well, the resulting structure is sufficiently O ¦ compliant that the reciprocating motion of the rod string is affected to a considerable extent. That is, when the motion of the upper end of the rod string changes direction, the ratio of the inertial forces to the elastic forces is such that the lower end of the rod string tends to continue along the original direction. As a consequence the stroke of the lower end of the rod string can be considerably longer than the stroke at the upper end. This phenomenon, referred to as "overtravel,"
results in enhanced productivi-ty for a given pump stroke and rate.

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4. Relatively Simple to Fabricate.
¦ Fiberglass can be pultruded along with a variety of resin systems (for example, polyester, vinyl ester or epoxy) on a continuous basis through a constant cross-section die.

I The pultruded rods are then cut to length and adhesively bonded to metal couplings.

I While pultruded fiberglass sucker rods have the afore-mentioned a~tributes~ they also possess some significant short-comings. These include:
1. Coupling Bond.
Pultruded fiberglass sucker rods are bonded to the coupling at only one surface. This single interface between the composite rod body and the metal coupling is somewhat vulnerable and prone to premature failure.
2. Metal Couplings Exposed to Corrosive Environment.
Pultruded fiberglass rods are usually terminated with a steel coupling. This coupling is exposed to the sour environment of the oil well and is sub~ect to corrosion and to the possibility of stress-corrosion failure.
3. Reduced Torsional Properties The uniaxial character of the fiberglass in the pultruded rod does not provide strength in torsion. While sucker rods are not generally loaded in the torsional mode, torsional loads might be applied to unstick a downhole pump, and if the ¦l unsticking torque exceeds the torsional strength of the pultruded ¦ rod, it will fail ;n shear.
,1 4. Poor Compressive Properties Compression properties which are critical durin~
~i sucker rod use include: local axial compression which occurs whe I the rod ru~s against the tubing walI or if the downhole pump sticks; and compression impact if the rods part and the lower portion alls to the bottc.n of the well. Despite the inherent

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.nping of the motion of this free falling section by the oil in the tubiny, compression impact can cause temporary loading which ~is responsible for both fiber buckling and subsequent "brooming"
of the fiberglass. Usually, a pultruded rod is rendered useless when this occurs.
Local compression can also occur when the operator sets ,l the downhole pump to eliminate the condition known as gas pound.

I In this case, the pump is set to slightly tap the bottom and the local compression that results is small in magnitude, but is con ll tinual in nature, and it is reputed to cause premature failure O over the long term.

SUMMARY OF THE .INVENTION
The desirable attributes of pultruded fiberglass sucker rods can be realized and their shortcomings minimized by the l utilization of a unique combination of structural elements which ~ include various polymers, metals and ceramics. Towards this end, the present invention envisions a concentric structural combi-nation of elements, consisting of an elongate core component, and an elongate sheath component which consists of an interlaced ~ configuration of assemblies of non-metallic filamentary elements 0 embedded in a polymer matrix, the load-elongation characteristics ~of the core and sheath components being chosen so as to ensure that both components share substantially in the load-bearing under the working load conditions, with at least 50~ of the load being borne by the aggreate of the non-metallic elemen-ts, and the sheath 11 and matrix being disposed so as to substantially cover and protect ¦!the core and coupling components.
As an example of an embodiment of this invention, we eonsider a core component which consists of a steel wire rope eoverd with a sheath of load b~aring-fiberglass filaments oriented ~0 llpredominantly, but not exclusively, along the long~tudinal axis r _ 6 -I

of the wire rope, impregnated with a poly~eric resin and subse quently cured. Both structural elements of the rods, namely the core and the sheath, are involved ln the load bearing during uselr and the vulnerable metallic core and coupling components are protected from the potentially harmful environment of the well.
In order to acheive improved torsional and compressive properties we incorporated into the sheath component filamentary elements that are aligned at an oblique angle to the longitudina axial direction. These elements supply resistance to shear deformation of the assembly, and thus can increase the torsional strength by an appropriate design and also provide, under app-ropriate loadîng conditions, an inwardly-directed radial component of force that restricts the radial growth in the rod, and hence restricts or prevents "brooming." In order to produce a sheath structure that is as symmetrical as possible in its ~_ response to torsional strains it is helpful if the o~lique elements are aligned in both the plus and the minus angular directions as measured with respect to the longitudinal axis.
In a ~ilament wlnding process the o~lique elements form an interleaved assembly. It is also oE considerable value if the two sets oE oblique elements form an interlaced assembly, both with themselves and with such longitudinal elements as may be present. In this way not only is t~e structural integrity of each layer of the sheath material improved, but it is also possible to achieve the greatest measuxe of control over the circumferential location of the longitudinal elements, All the theoretical and practical considerations described above can be realized in the preferred embodiment of this invention, which utilizes a steel wire rope for the core and a triaxially braided fiberglass multilayered sheath, which provides the preferred interlaced configurat;on of assemblies of structural filaments which involve both longitudinal and oblique elements ordered in such a way as to provide adequate tensile, compressive and shear strength. In particular, .

~ 6~3 , ,he combination of wire rope and fiberglass tLiaxial braid allows the development of a struc-ture in which the load-elongation and ultimate elongation-to-~reak characteristics of both components are satisfactorily ,Imatched. Both core and sheath components are capable of independ-~ent adjustment of their tensile characteristics: the properties of the wire rope can be manipulated by choice of construction and by the use of transversely compliant core material; the properties of the braided sheath can be manipulated, 1nter alia, by the ~ choice of non-metallic filamentary material, by alteration of the O ratio of amount of longitudinal to oblique material in the system, by alteration of the angle of obliquity, and the overall density of the sheath assembly.
~he large number of design options permitted by this l paricular combination of core and sheath components provides considerable design flexibilityr and permits the realization of specific overall design parameters within the framework of a practically viable manufacturing technique. For example, while the embodiment described above uses a steel cable as the core and fiberglass as the outer sheath, in order to exploit to the ¦ fullest extent the material/process in-teraction,in this particular end-use application, it is possible that other end-use specifi-cations could be more readily met by the use of a~ternative llmaterials~ These might include ~or the core tow or rod made from ¦¦glass, carbon or other ceramic filaments, or from any o~ the !5 i¦available high stren~th organic filame,ntary materials, and for the ,¦sheath any of these or similar non-metallic materials. Geometric compatibility can be achieved by utilization of a wire rope with a transversely compliant core. Such a core enhances the elongatio l to break of the wire rope to the point where it is similar to :0 that of the fiber,lass overb=aid.
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ll In this invention the cross section of the hybrid tension member is modified in order to reduce the strains assoc-iated with bending deformations about a particular axis. This would allow for longer sections of rod which could also be coileo for shipment and storage. In this concept of the invention a circular wire or other core material is surrounded with a braid or wrapping in which the axially oriented fiberglass yarns are placed predominantly between the parallel planes tangent to opposite ends of a diameter of the core with a minimum of them O extending beyond the diameter of the core that is perpendicular to these planes~ The end result, after appropriate control of the axial yarns during formation, is an elongated cross section which may, for example, be rectangular or elliptical and which will have a lower bending stiffness for bending deformations about an axis parallel to the major axis o~ the cross section.
If the sheet material is braid the interlacings of the braid permit specific positioning of the load carrying ~arns relative to t~ core.

BRIEF DESCRIPTION OF THE DRAWINGS:
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Fig. 1 is an elevation of a typical conventional beam o pumping unit of the type used for pumping oil from a subsurface well and with which the present invention can be used;
Fig. 2 is a longitudinal sectional segmentary view of a sucker rod constructed in accordance with the teachings of this invention;
Fig. 3 is a cross sectional view taken along the line 3-3 in the direction of the arrows in Fig. 2 showing the various concentric layers which combine to form the sucker rod shown in Fig. 2;
Fig. 4 is an enlarged segmentary longitudinal view somewhat similar to the view of Fig. 2, but with portions of the layers remo~d to illustrate the internal construction of the ro~ ;
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' . ' ~ . ' : ; ~' ' ' ' '' ' ~186948 Fig. 5 is a diagrammatic view similar to Fig. 3 but somewhat mor~ detailed; and Fig. 6 is a view similar to Fig. 3 but of an alternate embodiment of the invention.

DESCRIPTION OF TEIE PREFERRED EMBODIMENTS
A conventional beam pumping system of the type used for pumping oil from a well and with which the present invention is used is shown in Fig. 1. The unit includes prime mover 10, surface pumping unit 12, sucker rod string 14 with sucker rods O constructed in accordance with the teachings of this invention, and subsurface or downhole pump 16.
The function of the prime mover 10 is to supply to the installation mechanical energy which is eventually trans-mitted to the pump 16 and used to lift fluid. The prime mover selected for a given installation r.lust have sufficient power output to lift fluid at the desired rate from the working fluid level in the well. Further, the load on the prime mover is a function of the weight of the sucker rod string 14. While pumping unit are counterbalanced, the weight of the sucker rod 0 affects not only the pr~ne mover but the size of the pumping unit and required mechanical energy transmission components.
And, of cour~e the load on the prime mover dekermines the energy requ;rement for pumping~
The subsurface pump 16 is provided to admit fluid from ,5 the formation in the well and to lift the fluid thus admitted to the surface.
The surface pumping unit indicated generally by the numeral 12 in the Figs. transfers energy for pumping the well 16 from the prime mover 10 to the sucker rod string 14. In doing ~0 this, it must change the rotary motion of the prime mover to reciprocating motion for the sucker rods, and it must reduce the . .. . . - ~-~r,,: .

speed oE the ~rime mover to a rate suitable for pumping.
The surface pumping unit components shown in Fig. 1, in addition -to the prime mover 10, include V-belt drive 18, crank arm 20, pitaman arm 22, walking beam 24 pivotally connected to sampson post 26, horse's head 28 and hanger cable 30.
Polished rod 32 is connected to the hanger cable by clamp 34.
Rod 32 is projected within stuffing box 35 and the sucker rod string 14 is connected thereto.
Sucker rod string 14 is suspended within tubing 36 which itself is projected within the hole by casing 38. Flow-line 40 is indicated as being connected to tubing 36.
The sucker rod of this invention is shown in detail in the Figs.
The rod includes a concentric combination of steel wire helically stranded rope 42 containing a transversely compliant pol~mer core 44 and a triaxially braided fiberglass reinforced resin elongate sheath which is corrosion resistant and possesses a high strength-to-weight ratio. It is comprised of a plurality of concentric layers such as for example those designated in the Figs. by the numerals 48, S0, 52, 54,56, 58 and 60.
Wire rope 42 (3/8" fiber core? is a stranded structure of low tensile modulus which is comparable to that of fiberglass and is of high tensile strength. The transversely compliant polymer core increases the strain-to-break property of the wire rope so that it is in the immediate range of the strain-to-break property of the longitudinally ordered fiberglass structural elements.
The resulting combination of structural elements pro-vides a tensile structure wherein each component bears axial loading at similar ratios of the ultimate load and strain to brea ;
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n a structurally efficient manner.
The utilization of braiding allows oppor~unity for pump overtravel for many configurations with high strength-to weight ratios. The braided sheath increases the torsional streng h and provides "off-axis" reinforcement and improves the compress-ive properties of the combinat;on.
In the example given braid layer 48 is a triaxial braid with cross yarns at 45 to the rod axis. There are thirty-two yarns with six~een having a right hand obliquity and sixteen 0 having left hand obliquity (16 X 16). Each yarn possesses a linear density specified by a yield of 2500yds/lb. There are eight longitudinal yarns interlaced with the oblique yarns having a linear density specified by a yield of 112 yds/lb.
Layers 50, 52, 54, 56 and 58 are all the same with a construction somewhat like that of layer 48~ that is, 16 X 16 cross yarns at 45 to the rod axis and having a linear density 5pecified by a yield of 2500 yds/lb. There are 8 longitudinal yarns interlaced with the cross yarns. The linear density of these yarns ls specified by a yiel~ of 56yds/lb. It has been 0 found undesirable to use lengthwise yarns with this high linear density in layer 48 since t~ere is not sufficient room to accom-omodate the cross sectional area of these yarns in a single, compact layer. Fig. 5 Illustrates the lenghtwise yarns held in position in the triaxial ~raid portion of the sheath which insure the integrity of the structure.
The final layer 6Q is a 48 X 48 braid of conventional construction utilizing yarns specified by a yield of 2500 yds/lb.
The layer contributes to the torsional strength and provides a smooth outer surface to the rod assemblies.
During braiding, a resin system is applied to the rod 0 structure to impregnate the fiberglass. The number of layers ,~. . . . . .

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o~ fiberglass yarns which are braiaed the ratio of linear densities of axial yarn to cross yarns, and the braid angle can be ad~usted over a wide range to affect total system modulus I and hence plunger overtravel. Further, this is accomplished ¦I while maintaining the sucker rod strength within a range suitable for oil we l pumping. The steel wire rope and the obli~ue sets ¦ of ply fiberglass yarns contribute to the torsional strength _f the rod. Also it may be desîreable in certain applications to include a filamentary component in the external layer of the sheath which by its nature and disposition will mechanically protect the interior load bearing elements.

The unique feature of this invention is that the composite rod :L4 is so formed as to have in cross section a major axis such as X-X as shown in Fig. 3 to provide a bending stif~ness about an axis parallel to clXiS X-X which is lower than for a structure made from the same components arranged in a radially symetrical configuration. This will allow for coiling of the rod and the elimination in many instances of the use of couplings. By way of example the cross section in the preferred O embodiment is elliptical as seen in Fig. 3.
A further embodiment of the invention is shown in Fig. 5 wherein components similar to components shown in Fig. 3 are indicated by th~ same numeral but provided in each instance with I a 'Iprime'l. The embodiment shown in Fig. 5 is identical in all ¦ respects to that shown in Fig. 3 except that the cross section configuration is rectangular, with rounded corners.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rod assembly including a core component and a sheath component thereof consisting of an interlaced configuration of assemblies of non-metallic filamentary elements embedded in a polymeric matrix and of elongate cross section, a major axis of said cross section whereby the bending stiffness for bending deformations about an axis parallel to said major axis is less than the bending stiffness of the same core and sheath components in a radially symmetrical configuration, and in which the load-elongation characteristics of the core and sheath assemblies are selected and the core and sheath assemblies are arranged so that the core and sheath components share substantially in the load bearing.

2. A rod assembly in accordance with claim 1 in which said interlaced configuration of non-metallic elements form a braided structure.

3. A rod assembly in accordance with claim 1 or 2 in which said sheath is a triaxially braided structure.

4. A rod assembly in accordance with claim 1 in which the filamentary elements of said sheath are fiberglass and said core component is a stranded cable of low tensile modulus and high tensile strength.

5. A rod assembly in accordance with claim 4 in which said core component includes a transversely compliant center core whereby the extension-to-break characteristic of the core component is increased.

6. A rod assembly in accordance with claim 5 in which said sheath is resin impregnated fiberglass and said core component is a stranded cable wherein the strains to break of the sheath and core component are substantially equal.

7. An assembly as defined in claim 1 wherein said sheath component has an external layer which includes a protective filamentary component.