Proper handling, both in transit and on site, are critical to avoiding damage to the tubing. This article provides an overview of inspection and handling considerations for tubing. API tubing is inspected at the mill in accordance with API Spec. Physical properties are checked and each length hydrostatically tested, normally to only 3,000 psi in the plain end (unthreaded) condition. Part of this inspection is to drift all lengths.
Designing the tubing for a well requires consideration of strength, load, performance, stretch, corrosion, coatings and many other factors. This page introduces each of these factors and includes some example tubing designs. A design factor is the specific load rating divided by the specific anticipated load. A design factor less than 1.0 does not necessarily mean the product will fail, and neither does a design factor in excess of 1.0 mean that the product will not fail.
In a PCP system, produced fluid flows from the pump to surface through the annular area between the rod string and tubing. High fluid viscosities, elevated flow rates, or restricted flow paths can result in large shear stresses developing in the fluid, which cause large frictional forces to act on the rod string. Fluid-flow effects can range from having a minor to a dominant influence on PCP system design. This is illustrated in Figure 1, which shows pressure losses for a range of flow rates and viscosities through a 100 m [328 ft] length of 76 mm [3.0 in.] Note that the pressure-drop values range from nearly zero to values that exceed the corresponding hydrostatic pressure.
The rod string and tubing are important components of the overall progressing cavity pump (PCP) system. In a PCP system, the rod string must be capable of carrying axial load and transmitting torque between the bottomhole pump and the surface drive. Therefore, rod-string design encompasses an evaluation of the axial tension and torque loading conditions for the full range of anticipated operating conditions. An appropriate size and grade of rod string can then be selected on the basis of appropriate design criteria, such as ensuring that the maximum calculated combined stress does not exceed the yield capacity or manufacturer's recommended values. Fatigue-loading considerations must also be addressed in certain applications.
The American Petroleum Institute (API) developed Specifications, Recommended Practices, and Bulletins for steel tubing that meet the major needs of the oil and gas industry.API This effort continues, and many of these documents (with modifications) have become International Organization for Standardization (ISO) documents. Currently, API and ISO are the international standards for products intended for worldwide use in the petroleum and natural gas industry. The information in API and ISO documents is covered here in some detail. API tubing sizes range from ODs of 1.050 to 4.500 in.
Casing and tubing strings are the main parts of the well construction. All wells drilled for the purpose of oil or gas production (or injecting materials into underground formations) must be cased with material with sufficient strength and functionality. Casing is the major structural component of a well. The cost of casing is a major part of the overall well cost, so selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration. Conductor casing is the first string set below the structural casing (i.e., drive pipe or marine conductor run to protect loose near-surface formations and to enable circulation of drilling fluid).
While downhole pumps and sucker rods are the chief components of a sucker-rod lift type artificial lift system, a number of other components are also used in the subsurface portion of the system. These include tubing, tubing anchor-catchers, tubing rotators, sinker bars, rod centralizers, and paraffin scrapers. Tubing provides detailed information on the design, selection, and use of tubing for production wells.
As with other artificial-lift systems, the basic objective in the design of a PCP system is to select system components and operating parameters (e.g., pump speed) that can achieve the desired fluid production rates while not exceeding the mechanical performance capabilities of the equipment components to facilitate optimal service life and system value. Figure 1 presents a "design process" flow chart that outlines the many factors and considerations that should be addressed in the selection of an effective overall system configuration and operating strategy. At each step, the designer selects certain operating parameters or specific equipment components and must then assess the impacts of these decisions on system performance. For example, selection of a particular tubing size is based on such design considerations as flow losses and casing size. Some considerations apply to more than one decision, as is the case with flow losses that affect pump, tubing, and rod-string selection.
The American Petroleum Institute (API) has numerous manufacturing requirements for tubing. Many API standards have also been adopted by the International Standards Organization (ISO). The tubing purchaser and designer should be aware of API requirements and testing procedures (see API Spec. All tubing should meet API minimum requirements. In critical wells, the purchaser may want to receive and review the manufacturer's test results.