Monitoring two-phase fluids in geothermal fields using seismic noise interferometry

Harvesting geothermal energy often leads to a pressure drop in reservoirs that promotes the formation of steam. In some reservoirs, steam coexists with liquid water forming two-phase fluids, as happens in the Hengill geothermal field. This field is located in a triple junction of three large tectonic features in Iceland, 30 km east of the capital Reykjavik. The accumulation of steam in the top part of the reservoir forms a so-called steam cap. While steam caps are valuable energy resources, they also alter the reservoir thermodynamics and entail diverse risks such as land subsidence. Therefore, monitoring the steam content in reservoirs is essential for both operational and economic perspectives. However, this is an inherently challenging task and quantifying the steam content from indirect and surface-based measurements is still an unsolved matter. Here, we present a new method for indirectly sampling the steam content in the subsurface using the ever-present seismic background noise. We analyse the seismic velocity changes in the area, estimate the land subsidence via Interferometric Synthetic Aperture Radar (InSAR) and work with in situ borehole data. We observe a consistent annual velocity drop in the Hengill geothermal field and establish a correlation between the velocity drop and steam buildup. This study introduces seismic noise interferometry as a powerful tool for monitoring two-phase fluids in the crust with minimal infrastructure, only one seismic station. Beyond geothermal sites, the methodology could extend to diverse geological settings, such as volcanoes, CO2 storage sites, hydrocarbon reservoirs, among others.