A critical step toward the optimized well completion efficiency is to minimize perforating debris. Conventionally zinc-case shaped charges have been used in perforating for their perceived benefits of low cost, low-debris, and dissolvability by acid. The low-debris characteristic has been proven to be a misconception. Charge detonation actually pulverizes a zinc case into fine powder with a minor amount of the zinc material melted and oxidized. The powder form zinc has higher tendency to exit the gun during perforating and retrieval of the gun string, which potentially causes formation damage and other operation problems. Shaped charges that truly result in a low amount of debris are therefore needed by the industry. In response to such a demand, low-debris steel-case shaped charges are developed.
Laboratory perforating tests under downhole conditions showed several mechanisms by which a zinc-case charge can cause operation failure and formation damage. (1) The fine zinc debris can be tightly compacted in the perforation tunnel and cause complete perforation plugging if the perforating job is not properly designed. The plugged perforation tunnel not only reduces the productivity and injectivity dramatically, but also causes reservoir fluid to bypasses the length of the perforation and converges into the entrance hole, increasing flow velocity and pore pressure gradient, and hence increasing sand production potential. (2) Zinc-case charges generate higher detonation pressure than steel-case charges due to higher reaction heat. Lower shot density has to be used to ensure underbalance for perforation cleanup. (3) When a kill well operation is required during completions, chemical reaction between zinc and CaCl2/CaBr2 based kill pill can cause failure in fluid loss control. Higher fluid loss will cause high completions cost due to loss of expensive completion fluid. In addition, high fluid loss volume leads to severe formation damage. (4) Expelled zinc debris is anodic to steel. Under common pH, temperature, and chemical environment inside a well, the redox reaction liberates atomic hydrogen, causing hydrogen related corrosion or enbrittlement to the downhole hardware. (5) Difference in performance of zinc-case and steel-case charges shows that the latter is a superior candidate particularly for deep penetrators. Given all other conditions are same, the steel-case charge is capable of deeper perforation.
To overcome the problems associated with zinc and to result in truly low amount of debris, steel-case shaped charges were developed. The unique shaped charge design combined with selection of special steel materials for charge cases allows all case fragments to be sufficiently large that they remain inside their hollow carriers, which are eventually retrieved back to surface after perforating. As a result, it not only achieves debris minimization during perforating, but also maintains consistency regardless of gun movement during retrieval operation.
This paper discusses the laboratory studies that compare the zinc-case and steel-case charge performances, the potential damage mechanism by zinc-case charges, and presents the extensive field history of the low-debris steel-case charges.