Formate brines have been exposed to high-temperature conditions in over 150 HPHT wells since 1996. Over this time they have been exposed for lengthy periods (up to two years) to well temperatures up to 225°C / 437°F without any substantial change in composition or properties. In contrast, several laboratory tests have shown that formate in aqueous solutions can decompose appreciably at high temperatures. This section of the manual reviews the chemistry of formate decomposition and explains why a formate brine exposed to hydrothermal conditions in a deep high-pressure well is able to establish equilibrium with bicarbonate, whilst a formate brine exposed to the same temperature in a low-pressure laboratory autoclave with a gaseous headspace is not. These explanations are supported by testing conducted by scientists at the Woods Hole Oceanographic Institution in USA. Analyses of brines recovered from three different well construction operations are also presented and discussed.

 

Finally, the importance of the carbonate / bicarbonate pH buffer as a stabilising additive for formate brines under HPHT conditions is reviewed. Through the judicious addition of carbonate and bicarbonate, formate brines can be chemically stabilised under extreme HPHT conditions.

 

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