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. 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 is necessary to increase natural gas production. Furthermore, a casing pipe must be perforated for cementing when a well is abandoned to prevent natural gas from leaking along the pipe. In this study, we proposed a new system with abrasive waterjets for perforating steel tubing without using shaped explosive charges or coiled tubing. 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. To clarify the effect of ambient and driving pressures on both the mass of abrasives and the impinging time that are required for perforation, laboratory perforation tests were conducted under high ambient pressure. Main results obtained in this study are summarized as follows: (1) The developed system can perforate steel tubing with a thickness of 4.15 mm under a high ambient pressure of up to 7 MPa. (2) Both abrasive mass flow rate and the mass of abrasives required for perforation increase with ambient pressure. However, the increase rate with respect to the ambient pressure of the former decreases while that of the latter increases with ambient pressure. As a result, the impinging time required for perforation is minimum at a certain ambient pressure. (3) The mass of abrasives required for perforation and the impinging time required for perforation are governed mainly by two factors; cavitation number and the difference between driving and ambient pressures.
