Numerous papers have dealt with the description and measurements of the erosion of perforation holes during a hydraulic fracturing treatment in single casing completions, but not much is known about the erosion of perforations in dual casing setups. This study addresses this topic and compares it to the erosion rate of single casing scenarios and how this is influenced by the backpressure, which is created by the fracture closure pressure.

The API 19B norm provides a guideline on how to test perforators under the most realistic downhole conditions. All casings used in our experiments were perforated in such a Section IV test set-up and subsequently installed in a specially designed high pressure flow apparatus. The casing holes were carefully measured, their hydraulic resistance was determined by a flow test and successively eroded by a slurry using high pressure pumping equipment. After each test, the holes were again geometrically measured, and their flow resistance was tested. In addition, the sand grain sizes were analyzed before and after the tests.

Our tests revealed a significant difference in the erosion characteristic of dual casing compared to single casing setups. Especially the diameter of the hole in the inner casing is critical for the progress of the erosion and the final hole diameters. Equal holes on both casings provide a better control of the treating pressures, especially after the first minutes of the treatment. The back pressure, which is created by the fluid in the fractures, influences mainly the flow rate through the perforation. For identical flow rates, the pressure differential becomes less with back pressure, however the erosion rate as a function of the cumulative energy pumped through the perforation, remains similar. Finally, the application and design of a bigger test cell was evaluated and will be discussed as well.

Although many perforating companies have started testing the charge performance for multiple casing completions, not much is known about the flow and erosion of two radially aligned holes in dual cemented casings during the fracture treatment and the influence of the back pressure created by the reservoir. The results will enhance the completion design and provide a better understanding of fracturing or refracturing through double-casings for hydraulic fracturing specialists and both operation and services companies.

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