This paper describes new research into the role and importance of the carbonate/bicarbonate pH-buffer added to formate based well construction fluids along with a newly developed field method for determining buffer concentrations.
For many years it has been recommended practice to buffer formate brines with carbonate and bicarbonate for corrosion control. Bicarbonate is also the product of the dominant thermal decomposition reaction in formate brine. Since this is an equilibrium reaction, according to Le Chateliers Principle, this additive should cause the equilibrium to establish sooner. Past attempts to demonstrate this have been unsuccessful because it has been difficult to accurately reproduce downhole hydrothermal conditions in laboratory autoclaves. This challenge has now been overcome with specialist equipment that has made it possible to accurately simulate downhole conditions and prove that chemical equilibria do indeed establish downhole. Test results have shown that the equilibrium favors high formate concentrations, meaning that the presence of only relatively small amounts of carbonate/bicarbonate is required for the equilibrium to establish.
Along with the growing understanding of the importance of the carbonate/bicarbonate buffer, there has been a growing demand for a field method to accurately measure carbonate and bicarbonate concentrations as the standard API method for determination of alkalinity does not work in formate brines. The newly developed, simple field method consists of an accurate pH-determination combined with a standard phenolphthalein titration. The method has been extensively tested in the laboratory, and should be reviewed by API for possible inclusion in the API recommended practices.
This new understanding of the roles of the carbonate/bicarbonate buffer has provided us with a better basis for making recommendations as to how to buffer the brine and maintain the appropriate buffer levels for different applications. Combined with the new field method for determining buffer levels, these new insights provide the oil industry with a robust technology to meet the well challenges of the future.
Introduction
Formate fluids have unique physico-chemical properties that make them the ideal well-construction fluids for challenging well construction projects where extraordinary fluid performance is critical for economic success. They have been used in thousands of wells across the world since their commercial introduction in 1993. The natural attributes of the formate brines (high density, anti-oxidant, alkaline pH, lubricious, biostatic, low water activity, low TCT) make it possible to formulate them into high-performance well construction fluids with a minimum number of additives. Corrosion inhibitors, biocides, lubricants, antioxidants, oxygen scavengers, and solid weighting material are typically not required in formate fluid formulations. There is, however, one additive package that is essential in all formate-based well construction fluids, namely: a carbonate/bicarbonate pH buffer.
The need for a carbonate additive package for maintaining fluid pH control and reducing corrosion was understood from the earliest stages of formate brine development (Downs 1992, Howard 1995). Some of the other benefits of adding a carbonate/bicarbonate blend to formate brines have only become apparent after many years of use and testing. The purpose of this paper is to explain the full functionality and benefits provided by these two additives.
