Cost-Effective Methods for Designing and Operating Fiberglass Sucker Rod Strings
Authors: Jacobs G.H. 
Published: December 2007

Jacobs, G.H., SPE Member formerly with Amoco Production Co.

Abstract
This paper describes procedures used by Amoco Production Company in a West Texas district to Production Company in a West Texas district to maximize the life of more than 200 fiberglass rod strings in service at depths between 5000 and 8000 ft. The paper describes rod string design methods, operating practices, and failure analyses for two major manufacturers' rods. Emphasis has been placed on showing procedures used in designing fiberglass rod strings for cost effective installation and for operating so as to minimize the number of rod string failures and, consequently, rod string operating costs. Actual case histories are used to illustrate the reduction in failure frequency which results from proper rod string design, operating practices, and proper rod string design, operating practices, and failure analysis.

Introduction
Amoco Production Company's Slaughter District is located about 35 miles southwest of Lubbock, Texas. The initial fiberglass sucker rod installation in the Slaughter District took place in 1976, making it one of the first Amoco districts to test fiberglass sucker rods. The district is currently the largest user of fiberglass sucker rods within Amoco Production Company. As of February, 1986, the district pumped 255 (15 percent) of its 1700 beam lift pumped 255 (15 percent) of its 1700 beam lift wells with fiberglass sucker rods.
The Slaughter District has historically used large beam pumping units rather than electric submersible pumps (ESPS) on moderately high volume wells (400-750 barrels of fluid per day - BFPD). This practice has minimized the need for frequent lift equipment resizing due to changing fluid production in secondary and tertiary recovery production in secondary and tertiary recovery projects. The district's philosophy of using projects. The district's philosophy of using beam lift rather than ESPS, wherever possible, is based on the higher overall efficiency of the beam lift system (which should yield lower power costs) and the lower maintenance costs for beam systems, based on the district's relative failure rates and cost per failure for beam and ESP systems. An engineering study is currently in progress to verify and quantify the cost effectiveness of each type of lift in one of the district's large, typical, producing units.
More than 90 percent of the fiberglass rod strings in this district are pumped with API 640 or 912 pumping units. These large pumping units will overload a conventional steel rod string in many applications, and the increased cross sectional area and lighter weight provided by 1.17 or 1.25 inch diameter fiberglass sucker rods reduces rod loading to within acceptable limits. The district justifies its use of fiberglass sucker rods by reduced rod failure costs.

DESIGNING A FIBERGLASS ROD STRING

Computer Aided Prediction
Designing a fiberglass rod string is not always a straightforward procedure. The additional elongation caused by the low modulus of elasticity of fiberglass sucker rods often introduces situations which would be almost impossible to predict without the aid of a computer. For example, it is possible to produce computer predicted cases on deeper wells in which the predicted cases on deeper wells in which the elongation of the fiberglass rods is sufficient to absorb almost the entire surface stroke of the pumping unit, producing almost no downhole stroke. Typically, this occurs with a large pump bore and a heavily loaded string. pump bore and a heavily loaded string.