Fiberglass Sucker Rods in Beam-Pumped Oil Wells
Authors: Watkins D.L. , Haarsma John 
Published: December 2007

Fiberglass sucker rods can reduce polished rod loading and torque significantly and are virtually immune to corrosion. Sinker bars provide enough pump-plunger overtravel to compensate for increased rod stretch. These rods are strong enough for beam-pumping service; however, manufacturing defects must be eliminated before widespread application is feasible.

Introduction

Corrosion and stress combine in many producing wells to form a severe working environment for sucker rods. Although steel sucker rods have been the mainstay of beam-pumping systems for many years and have done an outstanding job in most beam-pumped wells, a certain percentage of producing wells develop severe failure percentage of producing wells develop severe failure frequencies. Because of this problem, many nonsteel materials have been examined recently to find a better structural material.

One such material is resin-bonded fiberglass. Not only is fiberglass unaffected by corrosion, but it is lightweight and has a high inherent tensile strength compared with steel. While developing a fiberglass sucker rod, 11 separate test strings were run in eight producing wells during the past 3 1/2 years. These wells producing wells during the past 3 1/2 years. These wells ranged from 4,180 to 6,360 ft deep and were selected because of their failure histories and corrosive tendencies. The wells represented a wide range of fluid production volumes. This study discusses the design of fiberglass sucker-rod strings, inherent properties of resin-bonded fiberglass material, and field performance in eight test wells through June 1977.

Description of Rod

The load-carrying ability of the fiberglass rod comes from the strength of individual glass fibers. Fig. 1 is a schematic of the pultrusion process used to manufacture fiberglass sucker rods. The process entails saturating bundles of continuous glass filaments with a thermosetting resin and pulling the wet mass through a heated forming die. In the heated die, a chemical reaction cures the liquid resin and changes it to a solid. The resulting product is a solid rod composed of resin-bonded glass fibers. Because of this construction, these rods have high tensile strength and are anisotropic.

These glass filaments generally are about 15 microns in diameter. Bundies of fibers (called rovings) contain thousands of fibers. As many as 150 rovings may be used in the sucker-rod construction. In fact, to process a sucker rod, more than 1.5 million individual fibers must be wetted effectively with resin and formed into a rod with good resin/fiber distribution over the whole cross-section.

Why use glass fibers? Consider the properties of individual glass fibers given in Table 1. Note the high tensile strength and low modulus of elasticity compared with steel. Because the fiberglass sucker rod is a composite, the maximum amount of glass attained is about 65 percent by volume. This volume gives a theoretical ultimate strength of about 300,000 psi and a tensile modulus of elasticity of about 7 million psi. In practice, we cannot attain the theoretical strength of 300,000 psi. However, the ultimate strength of the composite rod approaches 180,000 psi. Table 2 lists the dimensions and properties of fiberglass sucker rods and compares these with 7/8-in., Grade D steel sucker rods.

Fiberglass sucker rods are 37 1/2-ft long with a steel end connector at each end.

JPT

P. 731