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Abstract

Corrosivity data for halide and formate well brines that have been acidifies as a concequence of influx of large amounts of acid gas has been requested. 

The well materials C-steel, standard 13%Cr steel and Super 13%Cr steel were corrosion tested in potassium formate and CaBr[2] brines acidified with CO[2] gas, simulating a well situation with influx of acid gas. The acidification resulted in an initial accelerated corrosion of C-steel in both solutions, reduced to low levels by time due to formation of a protective iron-carbonate layer. Both C-steel and standard 13%Cr steel were attacked by severe localised corrosion in the bromide brine, whereas only limited general corrosion was developed in the formate brine. Super 13%Cr was uncorroded in both brines. 

Introduction

Both calcium halide (CaBr[2] and CaCl[2]) and formate brines (NaCOOH, KCOOH and CsCOOH) are basically non-corrosive over a wide temperature range in the oxygen-free environment of a well. This, however, requires the pH to be maintained at a high level (>7). 

Although very unlikely to happen in the heavily buffered formate brines, there has been raised some concern about what would happen to the less noble well completion materials if these brines would turn acidic as a consequence of a CO[2] influx.

An extensive laboratory test program has been carried out to address this concern. Currently, research focus is on the effect of CO[2] influx, while work into the additional effect of H[2]S is ongoing. Potassium formate (KCOOH) and calcium bromide (CaBr2) solutions, with corresponding specific gravities, were selected. These solutions represent the two main types of brines used as completion, packer and work-over fluids, namely formate brines and halide brines respectively. Results from the studies of potassium formate solution are reported in more detail elsewhere.[1] 

Stress corrosion cracking (SCC), which is known to be a problem in halide brines is not addressed with this work. The favourable behaviour of formate brines over halide brines with respect to SCC has been investigated and reported elsewhere.[2] 

Theory

Acidification of formate and halide brines

The classical theory for CO[2] corrosion in systems close to pure water, also applies to the (aqueous) high-concentration halide and formate solutions. The formate brines are heavily buffered with a carbonate/bicarbonate buffer (pH 9.5-11) when used in the field, and can therefore tolerate a substantial influx of acid gas before a pH drop can be experienced. Halide brines, independent of initial pH, are not compatible with pH buffer, and only a small influx of acid gas will cause an immediate drop in pH. In the unlikely event that the pH buffer in the formate brine should be overwhelmed, the pH will drop and the brine will be acidified the same way as the halide brines. Equations 1 to 3 below describe the mechanism for this acidification.