New field method for monitoring buffer concentrations in formate brines
Formate brines are traditionally buffered with a mixture of water-soluble carbonate and bicarbonate salts. The ratio of carbonate to bicarbonate salt concentration determines the pH at which the brine is buffered, while the carbonate concentration determines the brine’s ‘buffer capacity’. This buffer system helps to maintain brine pH at the desired working level and controls corrosion in the event of an influx of acid gases into the well bore while the brine is not in circulation.
The buffer system also provides some important secondary benefits, including protection of polymers, reduction in formate decomposition rates and the absorption of influxes of toxic H2S gas. Given the importance of a competent carbonate/bicarbonate buffer to the proper functioning of formate brines it is essential that users have access to a simple field method for monitoring the brines’ buffering capability. Such a method has to provide measurements of the concentrations of soluble carbonate and bicarbonate in formate brines, regardless of the brine density and composition. And it has to be simple enough to use in the field laboratory where sophisticated analytical equipment is generally unavailable.
Unfortunately the standard API RP 13B-1 (1997) method for measuring alkalinity in drilling fluids does not work in formate brines as it requires titration to pH 3.1. Formate brines have inherent pH buffer capabilities that make it impossible to titrate the brine down to such a low pH value. The renowned Garret Gas Train (GGT) method is little better, being unable to differentiate between carbonate and bicarbonate.
Faced with this problem, researchers at Cabot Specialty Fluids have developed a novel method for measuring buffer levels formate brines. The methodology only requires an accurate determination of the formate brine pH with a pH probe, combined with standard phenolphthalein titration to pH 8.2. It exploits the relationship between pH and carbonate/bicarbonate ratio in the brine (see graph), which is independent of absolute concentrations of carbonate and bicarbonate and does not depend on type or concentration of formate brine. The method has already been thoroughly tested in the laboratory and is currently being validated in the field.