Testing of MP35N springs was conducted in various downhole production and acid treatment environments containing H2S to determine the susceptibility to environmental cracking. The aim was to simulate the functioning of torsion springs used in the flapper seat of a Sub-Surface Safety Valve (SSSV) as part of a completion string in various well conditions. The MP35N springs were held using an Alloy 825 test fixture in a position equivalent to a flapper set in the open condition. The torsion spring is under maximum stress when the SSSV flapper is fully open. Distinct environments were designed for the corrosion tests to simulate different scenarios. These include normal production of hydrocarbons and acid flow back conditions during acid stimulation. Additionally, testing was conducted to see the effect of galvanic coupling and a possible way to induce hydrogen embrittlement. The results of testing in the various environments revealed that MP35N used as a torsion spring is very resistant to production environments containing H2S and exhibits good corrosion resistance in typical acidizing conditions during well stimulation but can be susceptible to cracking during acid treatment under very specific conditions.
MP35N (UNS R30035) is a Cobalt based alloy containing Ni-Cr-Mo and is considered a highly Corrosion Resistant Alloy (CRA). MP35N is used for various components where excellent corrosion resistance, high strength and good fatigue resistance is required. MP35N is generally considered to be one of the most suitable spring materials for downhole completion tools. The integrity of the torsion spring in the SSSV flapper is critical for successful, long-term operational functionality. Research has been conducted in the past to study the performance of MP35N as a spring material in the downhole environment. Kane et al studied the cracking mechanisms of Nickel and Cobalt based alloys in various corrosive environments.