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-tobrine 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.
One of the driving forces for the use of formate brines (K-formate, Cs-formate, Na-formate and their mixes) as drilling, completion, work-over and packer fluids is their compatibility with commonly used well completion materials. Their low corrosivity is related to their relatively high pH (9-10) and the lack of oxidizing power. This has been proven and accepted in all of the field applications where formates have been used.(1)
Some concern has, however, been raised when it comes to corrosion rates in formate brines in the presence of acidic gases such as CO2 and H2S. These gases will lower the pH and thereby increase the concentration of formic acid in the brine. The question is whether the presence of acid gases will increase the corrosion rates on C-steel and/or lower alloyed stainless steel to unacceptable levels. An extensive test program has been carried out to answer this question. In the present work focus is on the effect of CO2 ingress, whereas the additional effect of H2S (ongoing activity) is only briefly discussed.
Acidification of Formate Brines with CO2
The K-formate brine is the product of the reaction between potassium hydroxide (KOH) and formic acid (HCOOH). In water solutions the K-formate salt will be completely dissociated according to Equation 1.