Coil tubing has been utilized in Canada for well servicing since 1965. Initial use was limited to routine service such as well cleanouts, gaslifts, and small acid treatments. This was due to limitations in equipment component strength, tubing size, and tubing tensile strength. Over the past several years we have pioneered the development of coil tubing services for more unconventional applications. In Prudhoe Bay, Alaska, coil tubing units are used as an alternative to service rigs. Coil tubing has increased in size from the original 19.05 mm (0.75"), to both 31.75 mm (1-1/4") and 38.1 mm (1-1/2").
Increased equipment strength has allowed the servicing of deeper, higher pressure wells, and the design of a variety of specialized down hole tools specifically for coil tubing. These improvements have allowed coil tubing to be used for such services as squeeze and plug back cementing, drilling, wireline work in deviated wells, fishing and selective acidizing.
The purpose of this paper is to discuss both the engineering and operational aspects of the more unconventional applications of coil tubing services.
Coil tubing has been used in a variety of applications for many years. Initially, it was limited to relatively simple applications such as well cleanouts, small acid treatments, and solvent squeezes. These limitations were due primarily to the minimal strength of the coil tubing and the load capacity of the units themselves. Another constraint was the restricted I.D. of 19.05 mm (0.75") coil tubing, resulting in low flow rates with corresponding high friction pressures.
In 1979, we entered the market with 25.4 mm (1") coil tubing. The tubing went through major changes in chemical composition, between 1982 and 1984, resulting in enhanced physical properties and H2S resistance.
Tubing sizes have increased to the present 38.1 mm (1.5"). The last two size increments of 31.75 mm (1.25") and 38.1 mm (1.5") have had a significant effect on the type of operations performed today. The increased size allowed higher pump rates, higher pressures, and larger fluid volumes. These in turn, have promoted squeeze and plug back cementing, drilling, drilling with underreamers, and wireline work in high angle or horizontal wells.
Historically, coil tubing applications have been limited by the internal burst pressure and tensile strength of the tubing itself. These limitations have restricted the operating depths and pressures for coil tubing. Today, with the larger diameter tubing, surface pumping pressures have decreased, and higher flowrates are attainable. The advantage of a larger diameter tubing is to reduce friction pressure and Increase flowrate as illustrated in figure 1. Table 1, presents a comparison of the improved coil tubing properties as larger coil tubing sizes became available.
There have been an number of improvements to coil tubing since it's introduction to increase the field rellabil1ty. These include:
Improved quality control procedures to detect flaws before the coil tubing is used in the field.
Heat treatment in the tube mill rather than batch heat treating of individual coils. This eliminates hat and cold spots in the coil.
