Formate brines have been successfully used in hundreds of HPHT drilling and completion operations over the past 12 years and have exhibited remarkable thermal stability. This has been confirmed by analyses of brines recovered from extreme HPHT wells after lengthy exposure (months/years). Laboratory testing cited in the literature has, however, indicated that significant formate brine decomposition to bicarbonate and hydrogen gas should occur at even relatively low temperatures if the brine is in contact with catalytic surfaces. The potential damage that such decomposition and the evolution of decomposition products could cause to metallic materials has caused concern in the industry.
The inconsistency between laboratory and field behavior has indicated that standard laboratory autoclaves are not suited for reproducing or simulating downhole conditions. Typical differences between these two environments are pressure, presence of gas cap, and amount and type of catalytic surfaces. In order to better simulate actual downhole conditions, experiments have been carried out in specialist equipment developed to examine hydrothermal chemistry. A non-catalytic gold test cell was used for the decomposition testing, immersed in a high pressure high temperature autoclave filled with water.
The flexibility of the gold cell has allowed experiments to be conducted and sampled at high pressure without the presence of an artificial gas cap. Initial tests have shown that any decomposition reactions taking place in formate brine under hydrothermal conditions will reach equilibrium with bicarbonate. This discovery has also given us useful information on how to formulate bicarbonate-rich formate brine compositions which remain stable even at very high temperatures. As bicarbonate is an important component in buffered formate brines, these formulations are not much different from the current brines already in field use.
Analyses of a range of formate brine samples that have been recovered from two HPHT wells have confirmed that formate reaches equilibrium with bicarbonate over time under hydrothermal conditions.
This paper reviews what has been learnt about formate decomposition under realistic hydrothermal conditions, and exposes new information on how the equilibrium reactions might impact on metal integrity and corrosion processes in HPHT wells.
The most important lesson learned from testing in the flexible gold cell is that traditional laboratory autoclaves operated with a gaseous head space are not suited for testing formate brines at high temperatures. In fact, test results generated in such autoclaves will be laboratory artifacts that do not simulate or reproduce what actually happens in HPHT wells.