String Design Research Articles

Torque and Drag in Directional Wells-Prediction and Measurement

Summary A computer model has been developed to predict drillstring torque and drag, and a versatile rotary torque meter has been built to use in calibrating the model. The principle of the predictive model is that torque and drag forces in a directional wellbore are primarily caused by sliding friction. Sliding friction force is calculated by multiplying the sidewall contact force by a friction coefficient. Realistic sliding friction coefficients were determined from field data by using the same predictive computer model. These field data were gathered using novel torque and hookload indicators that are accurate, portable, and easily installed. Good agreement between friction coefficients calculated from different loads in the same well, as well as agreement between those for different wells, indicates the validity of the predictive drillstring model. Sliding friction is concluded to be the major source of torque and drag in directional wells. For water base mud systems, typical friction coefficients range from 0.25 to 0.40. Introduction Drill string drag is the incremental force required to move the pipe up or down in the hole; torque is the moment required to rotate the pipe. Drag forces usually are given relative to the string weight measured with the string rotating but not reciprocating. Measured from the rotating string weight, the pickup drag usually is slightly greater than the slack-off drag. The magnitudes of torque and drag are related in any particular well; high drag forces and excessive torque loads normally occur together. There are a number of causes for excessive torque and drag, including tight hole conditions, sloughing hole, key seats, differential sticking, cuttings buildup caused by poor hole cleaning, and sliding wellbore friction. With the exception of sliding friction, these causes are associated with problem conditions in the wellbore. Conversely, in wells with good hole conditions, the primary source of torque and drag is sliding friction. Torque and drag from any source tend to be more troublesome in directional holes. In very deep, highly deviated wells overcoming torque and drag can be critical to the successful well completion. The capability to predict frictional loads on drillpipe has two main benefits. First, deep, highly deviated wells can be planned to minimize torque and drag. Use of torque and drag as criteria to select the most appropriate well path will help ensure successful drilling operations to total depth. Second, more complete knowledge of drill string loading allows use of improved drill string design techniques. Drill string components can be chosen by using a systematic approach that considers the extra forces involved. Torque and Drag Prediction Technique Mathematical Model. A lumped-parameter model provides the basis for the prediction of torque and drag. Both torque and drag are assumed to be caused entirely by sliding friction forces that result from contact of the drill string with the wellbore. Other less important sources of torque and drag are not considered in this model. Two factors affect sliding wellbore friction-the normal contact force and the coefficient of friction between the contact surfaces. The product of these two factors represents the magnitude of the sliding friction force. The normal contact force between the pipe and hole wall depends on several factors. This paper considers only two contributions to nominal force-the effects of gravity on the pipe and the effects of tension acting through curvatures in the wellbore. These forces, and their contributions to normal force, are shown schematically in Fig. 1. Other factors such as pipe bending may contribute small normal forces but are not considered here. The sliding friction coefficient is the ratio of the friction force to the normal contact force. In reality, this value depends on specific contacting materials and on the degree of lubrication at various places in the wellbore. However, in this paper all these effects are expressed as a single characteristic friction coefficient representing average conditions in a particular wellbore. Determination of this lumped-parameter coefficient is fundamental to practical application of this model. Computer Calculations. The following paragraphs describe the calculation of torque and/or drag forces when the sliding friction coefficient is given. This calculation is made directly. The reverse calculation, where a friction coefficient is determined from given torque or drag data, is done by assuming a friction coefficient and iterating to match the data. In either case, drill string description and wellbore survey data are required.

Fiberglass Sucker Rods in Beam-Pumped Oil Wells

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

Determination of permafrost thickness in wells in northern Canada

Accurate interpretation of geophysical data, design of safe casing strings for drilling and production operations, and determination of the potential for existence of gas hydrates in and immediately below the permafrost zone are only possible if the thickness of the permafrost zone can be accurately determined.Freezing of interstitial water in soils has little effect on many of the physical properties of the soil, however, large changes in elastic properties and electrical resistivity do result. This makes it feasible to determine the thickness of the permafrost zone using techniques which measure changes in acoustic and electrical properties of the soils adjacent to the wellbore. This paper discusses the characteristic response which occurs as the borehole logging tool passes upwards from unfrozen sediments into the permafrost zone. It is this response which indicates the base of the permafrost zone and allows determination of permafrost thickness.Conclusive evidence of permafrost thickness and confirmation of permafrost thickness inferred from acoustic and electrical data is obtained from temperature data obtained subsequent to well suspension. Techniques utilized to obtain these temperature data are outlined in this paper.

Fret positions and string parameters for fretted stringed instruments

A serious problem with exact intonation on most fretted string instruments is caused by the fact that each string is used to produce many notes, undergoing changes in effective length and in tension, and that one and the same fret is used to stop more than one string. To compensate for the increased tension when a string is stopped, the effective string length must always be a little larger than what one would expect from an inversely proportional relation to pitch. The optimum amount of compensation, however, is usually not the same for each string but depends on action height, string tension, and string elasticity. Optimum compensations were computed for nylon and steel guitar strings and results were compared with both a simple logarithmic fret layout and the more traditional “rule of 18.” Fretboard and string designs, securing exact intonation over the entire range of the instrument, are discussed. [Work supported by a grant from Norlin Music, Inc.]

Formulas for Canister and Pipe Design in Underground Nuclear Emplacement

In this paper, pressure vessel theory and practice are applied to the analysis and design of long emplacement strings and canister hardware used in a complex environment of underground nuclear tests. Design formulas are given for the collapse pressure of canisters and piping under external soil pressure and for the axial response of gussetted flanges supported by the theoretical and experimental results. The paper is intended primarily as a practical treatment of a complex problem of structural response with the aid of user-oriented, closed-form solutions.