Utilizing Rare Earth Elements as Tracers in High TDS Reservoir Brines in CCS Applications

In this paper we report the result of research associated with the development of a procedure necessary for utilizing natural occurring trace elements, specifically the Rare Earth Elements (REE) as geochemical tracers in CCS applications. Trace elements, particularly Rare Earth Elements (REE) may serve as in situ tracers for monitoring geochemical conditions and the migration of CO2-charged waters within CO2 storage system. The Center For Advanced Energy Studies (CAES) has been conducting research on the use of REE as a tracer and characterization tool in the laboratory at CAES at a CCS analogue site in Soda Springs, Idaho. Results from field and laboratory studies have been encouraging and show that REE and trace elements may be an effective tracer in CCS systems. In recent years, a series of studies using REE as natural groundwater tracers have been conducted successfully at various locations around the globe. Additionally, REE and trace elements have been successfully used as in situ tracers to describe the evolution of deep sedimentary Basins. Naturally occurring trace elements maybe useful MMV tools in CCS research because formation brine chemistry will be particularly sensitive to changes in local equilibrium caused by the addition of ScCO2. Because brine within CCS target formations will have been in chemical equilibrium with the host rocks for millions of years, the addition of large volumes of ScCO2 will cause the brine chemistry to evolve due to the pH change caused by the formation of carbonic acid. This CO2 driven change in formation fluid chemistry will have a major impact on water rock reaction equilibrium, which will impart a trace element fingerprint to the brine that can be used to monitor in situ reservoir conditions. Our research at the Soda Springs Site and in the laboratory has shown that the trace element signature imparted to the formation fluids by the introduction of ScCO2 is present, and can be measured and tracked as part of an MMV program. Additionally, this trace element fingerprint may serve as an ideal tracer for fluid migration, both within the target formation, and should formation fluids migrate into overlying formations.