CO₂ Corrosion of Steel in Formate Brines for Well Applications Available to Purchase

Formate brines are used as drilling, completion and packer fluids, among others due to their high specific gravity, their ability to stabilize polymers and their low corrosivity. The formate brines are basically non-corrosive to well materials due to their high pH (>9) and lack of oxidizing power. However, in case of CO₂ ingress, the carbonic acid will acidify the brine and react with the formate ion and form formic acid, and render the brine corrosive. In corrosion tests in K-formate with a CO₂ head space gas pressure up to 40 bar, initial corrosion rates of several millimetres per year were measured for carbon steel and 13%Cr steel by use of LPR (Linear Polarisation Resistance) and weight loss measurements. As a result of this initial corrosion, super saturation of iron carbonate will occur, resulting in formation of a stable and protective iron carbonate layer. Due to the high surface area-to- brine volumes in wells, the brine will within few days be saturated with iron carbonate, limiting the dissolution of this layer. This results in corrosion rates for carbon steel of 0.05-0.2 mm/year in the temperature range between 120 and 180°C. Addition of carbonate to the basic formate solution speeds up the formation and improves the quality of the carbonate layer. 13%Cr steel does not form the same protective layer, and has higher corrosion rates in the same temperature range. In contrast, Super 13% Cr steel (S13Cr) and higher grade stainless steels display very low rates.