Application of Fiberglass Sucker Rods
- S.G. Gibbs (Nabla Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- May 1991
- Document Type
- Journal Paper
- 147 - 154
- 1991. Society of Petroleum Engineers
- 7.4.3 Market analysis /supply and demand forecasting/pricing, 4.2.3 Materials and Corrosion, 3 Production and Well Operations, 4.1.5 Processing Equipment, 3.1.1 Beam and related pumping techniques, 4.1.2 Separation and Treating, 7.2.2 Risk Management Systems, 3.1 Artificial Lift Systems, 1.10 Drilling Equipment
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Fiberglass sucker rods are assuming a place in artificial-lift technology.This paper briefly describes the manufacturing process and gives some designand operational hints for practical applications. It also describes somemathematical modeling modifications needed for fiberglass wave-equation designprograms.
Fiberglass sucker rods were introduced during the 1970's. Initially, theprincipal application was thought to be in corrosive wells (fiberglass isprincipal application was thought to be in corrosive wells (fiberglass is notsubject to corrosion). As time passed, however, fiberglass rods gained broaderapplicability. A helpful characteristic was that fiberglass rods, when matedwith steel rods in proper proportions, could in some cases produce longer pumpstrokes than stiffer, all-steel strings could. Also, produce longer pumpstrokes than stiffer, all-steel strings could. Also, it was often possible toincrease production without changing the surface pumping unit becausefiberglass rods are light and often produced less load pumping unit becausefiberglass rods are light and often produced less load on the unit structureand reducer than steel rods. Several disadvantages of using fiberglass rodssoon became apparent, though, such as premium cost, greater difficulty ininstallation design, and somewhat greater care requirements in fieldoperations.
This paper briefly describes the process for manufacturing fiberglass rods.In addition, it gives hints about rod design, pumping unit affinity, pumpselection, and spacing. Operational pumping unit affinity, pump selection, andspacing. Operational considerations discussed include the need to avoid fluidpound, comparative power consumption with steel rods, and use of the fiberglassstress range diagram. Finally, conclusions are drawn concerning applicabilityand operation of fiberglass rod strings. The Appendix outlines helpfuldesign/diagnostic methods and the modification of wave-equation solutions forsteel strings for fiberglass applications.
The limiting element in the rod pumping system is the rod itself. Thus, anynew technology that improves the rod string will increase the depth andproduction capability of the rod-pumping method. This improvement has profoundimplications in view of rod pumping's role as the most widely usedartificial-lift method. pumping's role as the most widely used artificial-liftmethod. Manufacturing Process
The rod body is constructed by pultrusion. This is a simple, yetinteresting, procedure. The rod is initially continuous and is cut intosegments later for ease of handling in conventional pulling unit derricks. Fig.1 schematically shows the pultrusion process. Many boxes of fiberglass strandscalled rovings are arrayed in a rack, and the groups of rovings are drawn intoa bath of thermosetting resin. The rod-body modulus of elasticity can becontrolled (within limits) by adjusting the ratio of resin to fiberglass.Afterward, the strands pass through a bushing, which groups them into a bundleand strips some of the excess resin. Curing begins in a microwave preheater andends when the rod is pulled through a heated die. preheater and ends when therod is pulled through a heated die. This die completely forms the finished rodbody. Throughout the process, the rod is drawn by a puller mechanism. Eventhough the process, the rod is drawn by a puller mechanism. Even though thebasic materials are individual strands of fiberglass, the finished product hasthe appearance of a perfectly round, homogeneous rod, product has theappearance of a perfectly round, homogeneous rod, The descriptive termadvocated by industry is the reinforced-plastic sucker rod, although theproduct is usually called a fiberglass sucker rod.
A power saw cuts the rod body into segments. Finished length is (usually)37.5 ft when couplings are attached. Thus, two fiberglass rods are as long asthree 25-ft steel rods. This is a convenient length for many oilfield serviceunits. Fiberglass rods are also available in 30-ft finished lengths.
The coupling that joins the segmented rods is premachined and attached tothe rod body by an adhesive. Fig. 2 schematically shows a typical coupling.Actual coupling configuration differs among manufacturers. A system of wedgeswithin the coupling grips the rod body when axial load is applied. Adhesiondoes not play a major role in attaching the coupling. Instead, the adhesivematerial merely fills the space between the wedges and the rod body andtransfers the gripping force by compression and shear. The active wedge anglesare smaller when tension is applied. These small angles distribute the grippingforces over a longer interval, decreasing the crushing effect on the rod body(and hence reducing the chance of a pull-out failure). Note that steeper wedgeangles apply if a compressive load is imposed on the rod. These steeper anglescause a greater crushing tendency and a greater likelihood of failure. Animportant consideration in installation design is to incorporate steel rodsbelow the fiberglass to keep the fiberglass rods in tension. The individualrods are configured with pins on both ends. The API Spec. 11C specifiescoupling dimensions, materials, rod-body diameters, makeup, and handlingprocedures.
Installation Design Considerations
Installation design is more critical for fiberglass rods than for steel rodsbecause the greater elasticity of fiberglass rods makes them more sensitive todownhole friction and pump loads. Large downhole loads cause rod stretch anddecrease pump displacement. Wave-equation-based computer programs are best fordesign and diagnosis of fiberglass installations.
In this paper, pumping system performance is depicted with dynamometercards. These are plots of rod load vs. rod position. Surface cards show surfaceloads and positions and reveal polished-rod power and structure load. Whenconsidered with counterbalance information and unit geometry, surface cards arealso used to compute torque on the unit reducer. Similarly, pump cards showdownhole pump load vs. position. The horizontal width of the pump cardindicates total stroke of the pump and hence displacement. The shape of thepump card gives pump mechanical condition, degree of pump fillage, and otherdiagnostic indicators.
Rod-String Design. Relatively heavy, stiff steel rods must be run on bottomof the fiberglass rod interval. The role of the steel rods is two-fold.
1. Fiberglass rods are weak when loaded in compression. Thus, steel must beused on bottom to absorb the bucking forces should the pump hit down or stickon the downstroke. As mentioned, the coupling wedge system is configured fortension. If the fiberglass rods are subjected to compression, the rod bodycould be crushed beneath the wedges and a pull-out failure could occur. 2. Theheavy steel rods on bottom help to increase the pump stroke. Inertia of ftsteel makes the pump rise higher before reversal at the top of the stroke takesplace. Similarly, inertia of the steel rods makes the pump travel lower beforebottom reversal occurs. The combination of greater upward and lower downwardtravel extends the stroke, thus increasing displacement of the downholepump.
Ordinary steel sucker rods or larger-diameter steel sinker bars are suitablefor use below the fiberglass interval. An equivalent weight of eitherconventional steel rods or steel sinker bars usually produces about the sameperformance. Fig. 3a shows dynamometer cards predicted with the digitalcomputer in a 7,000-ft well with 4,900 ft of 1-in. fiberglass rods combinedwith 2,100 ft of ordinary 1-in. steel rods on bottom (6,090 lbm of steel onbottom). By contrast, Fig. 3b shows the same well equipped with 5,985 ft of1-in. fiberglass rods and 1,015 ft of 1.5-in. steel sinker bars (6,090 Ibm ofsteel on bottom). As shown, mechanical performance and pump capacity areroughly the same.
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