Newsletter 2012.12 Index
Theme : "Fluid Machinery in Industry"
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Waterjetting technologies have been utilized in many industrial fields such as mining, civil engineering, material machining, surface cleaning, surgical tools, fire-fighting operation, and waterjet ships. In the mining field, the development of deep underground requires drilling earth crust composed of various rocks. Since waterjet drilling is a non-contact drilling method, the waterjet drilling may allow to use more lighter drilling system, and may promise longer life of drilling bit. Abrasive waterjet that is pure waterjet with abrasives is a powerful tool for cutting hard materials such as rocks, concretes and metals with a moderate pressure of less than 100 MPa. On the other hand, the performance of pure waterjets for hard rocks is insufficient when the pressure is not so high. However, the cost of abrasive waterjet is too high to excavate a large volume of rock mass. To put waterjet drilling system in practical use, we need to develop an inexpensive and powerful tool that utilizes only water. In this study, a new waterjets system that utilizes cuttings as abrasives for high ambient pressure condition was proposed (Fig. 1). The nozzle system consists of two injection type submerged abrasive waterjets systems and a guide pipe. Cuttings are sucked into the guide pipe at the open end by and aspirator action of waterjets and are transported to abrasive nozzles to serve as abrasives.
In the South Kanto natural gas fields, Japan, natural gas which is dissolved in water is produced mainly by gas lift through a perforated casing that is used as production tubing. Since the number of new production wells developed each year is restricted due to regulations relating to environmental preservation, a technique for improving poorly performing wells by perforating a casing for another natural gas formation is required to increase natural gas production. In this study, we proposed a new system with abrasive waterjets for perforating steel casing without using shaped explosive charges or coiled tubing (Fig. 2). Since slurry pumps are not economical for supplying abrasives from the surface to great depth, we adopted a batch type system with a tank for supplying abrasives.
In geothermal power plants that operate for a long time, geothermal scale that deposits on the inner wall of a casing can cause a decrease in well performance, and this problem is especially serious in injection wells, since the deposition of scale tends to increase with a decrease in temperature. Therefore, the periodic removal of scale has been needed to restore the performance of wells, and a method has been needed for removing scale as well as for preventing scale deposition. In this study, to develop a method for scale-removal with pure waterjets underwater, we conducted laboratory experiments on the rotational speed and scale-removal performance of two types of commercially available self-rotating nozzle systems with low and high rotational speeds. The average thickness of unremoved scale obtained by the low-rotational-speed nozzle system is less than that obtained by the high-rotational-speed system (Fig. 3). Thus, even with the same duration of cutting, a greater depth of cut is achieved with a decrease in the rotational speed of the nozzle system. This suggests that a sufficiently low rotational speed for the nozzle system is required to remove hard scale.
Waterjet, Drilling, Perforation, Scale Removal
Fig.1ﾂ Concept of the nozzle system utilizing cuttings as abrasives.
Fig.2ﾂ Abrasive waterjets nozzle system for perforation of oil well casing.
Fig. 3ﾂ Photos of the inner wall of specimens after the scale-removal experiments.