Numerical Simulation of Noble Gases as Natural Tracers for Injection Returns and Reservoir Processes in Vapor-dominated Systems

Noble gases have attributes and properties that make them very attractive as tracers for fluid movement and reservoir processes. Noble gases are non-reactive and most of them have no sinks or sources in the subsurface, rendering them conservative. They occur naturally in waters that have been in contact with the atmosphere in concentrations that depend on their solubilities. As solubilities are temperaturedependent, noble gas concentrations in subsurface waters may be used to infer the temperature of recharge. Noble gases partition between aqueous and gas (vapor) phases, which alters concentrations in ways that may allow inferences on boiling and condensation processes in two-phase systems. We have developed a fluid property module for TOUGH2 that represents singleand two-phase mixtures of water, noble gases, and air. It includes dependence of solubilities and diffusivities on noble gas species, and on temperature and pressure conditions. This is applied to model transport of noble gases under conditions corresponding to The Geysers vapor-dominated system. Our calculations demonstrate that spatial and temporal variations in noble gas concentrations can be used to track the migration of injected fluids, and to evaluate the extent of vaporization of such fluids. Diffusive exchange between fractures and matrix blocks imparts a characteristic signature on breakthrough curves (BTCs) for noble gases, which may allow inferences on fracture spacing.