17C-1 The Lease Pumper’s Handbook Chapter 17 Well Servicing and Workover Section C THE TUBING STRING C-1. Tubing and Casing Perforation. The relationship of casing perforations to tubing perforations is important to the performance of the well, whether it is a flowing well or produced by artificial lift. The lease operator must know if the tubing perforations are above or below the casing perforations or at the same level (Figure 1). This can be critical as to how much oil the well produces and the problems encountered. For wells that produce a lot of scale, production personnel operate under different philosophies. One school of thought is that the higher the tubing perforations can be raised above the casing perforations and still achieve a satisfactory daily production, the less the scale will break out of the water in the tubing and flow line. Some companies raise the tubing perforations a few feet above the casing perforations, while other companies move them much higher. Some operators maintain that if tubing perforations are placed even with or lower than the casing perforations, the reduction in reservoir pressure will increase daily production, resulting in increased income to solve the additional production problems. Other companies place the tubing perforations below the casing perforations even if it results in a tubing arrangement where the tubing has no mud anchor. A shop-made two-foot perforated joint is used. The joint is closed on bottom and has dozens of ½-inch holes drilled into it. A collar is used to screw it directly under the seating nipple. This reduces the length of the pump gas anchor to one foot or less. Companies favoring this method maintain that it gives the highest daily average production. The most typical tubing arrangement is to place the tubing perforations a few feet above the casing perforations. This maintains a small amount of liquid against the formation, instead of having a drained or dry matrix area at the bottom of the hole.
Figure 1. Relationship of well perforations. (courtesy of Harbison Fisher)
17C-2 The lease pumper should know the company preferences, and a supervisor is usually able to provide this information. C-2. Selection of Tubing Quality. Tubing is available in many different wall thicknesses and qualities of metal. Typical tubing ratings include: H-40. This is the most economical tubing and is used on wells that are not very deep. J-55. Most medium depth wells use this tubing. It will be found in wells up to approximately 7,000 feet deep. C-75. This tubing is not quite as common but gives dependable service where pipe better than J-55 is required. N-80. This pipe gives very good service in wells to approximately 12,000 feet or more in depth. P-105. This is an example of the heavier duty pipe needed for wells that are drilled deeper, where high gas pressures are encountered. Additional tubing ratings. Additional classifications go from x-heavy line pipe on the lower end to 110, 125, 140, 150, and 155 on the upper end. With wells exceeding 20,000 feet, special pipe has been developed. stamping may be a simple H, J, C, N, or P. On used tubing, this stamped number may be located after cleaning with a wire brush. Threads may be round or V-shaped, with 8 round serving as the standard thread today. Round threads are hot rolled into the metal and are much stronger than the older Vthread style. Changeover couplings are available when different threads are encountered.
Figure 2. The end of a joint of tubing showing the upset end and the tubing collar. C-4. Measuring Line Pipe and Tubing Diameter. New oilfield workers sometimes have trouble understanding the sizes of pipe and should memorize the following two rules: Line pipe is measured by inside diameter because it is associated with production volume. Tubing is measured by outside diameter because this is the inside diameter of lifting tools needed to run tubing into the hole.