This paper contains a thorough analysis of all available corrosion test results of formate brines combined with an extensive compilation of experience form 20 years of use of formate brines as completion and drilling fluids from hundreds of HPHT applications.

Formate brines were successfully introduced into the field as high pressure high temperature drilling and completion fluids in the early 90's. Today they are used in Extreme and Ultra HPHT applications worldwide. As they are the only group of non-hazardous brines that can reach the highest density requirements, they have been pushed to the extreme both when it comes to exposure temperature and exposure time. One of the biggest concerns when leaving completion brines downhole under extreme HPHT conditions is the compatibility with downhole materials.

The biggest corrosion concerns in completion brines are localized pitting corrosion and stress corrosion cracking (SCC) of the production tubing. C-steel casing material can suffer from pitting corrosion and sulfate stress cracking (SSC) and certain high nickel alloys can suffer from hydrogen embrittlement (HE) after extensive exposure to oilfield brines followed by sudden cooling of the material. Extensive laboratory testing of martensitic stainless steel has shown that this material can suffer from SCC in all oilfield brines. For formate brines, failures have typically been seen in long-term tests when very high yield stress has been applied and the pH buffer has been overwhelmed by CO2. 20 years of field experience at temperatures up to the upper recommended temperature limit of this material has shown an unbeatable track record of zero SCC failures of martensitic stainless steel. Duplex stainless steel tubing material has also never failed from SCC in formate brines.

Material compatibility is an increasing concern in future HPHT wells where downhole material is being exposed to even higher temperatures. All operators are cautious about challenging the temperature limits of completion brines and downhole materials. This review will be a useful tool for operators worldwide who are considering using formate brines for the first time in their future Extreme and Ultra HPHT applications.

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