Imaging high-enthalpy geothermal reservoirs using seismic noise interferometry

High-enthalpy geothermal reservoirs have been exploited for electrical power generation worldwide during the last century.  Despite the different definitions of high-enthalpy reservoirs in the literature, one can consider fluid enthalpies of around 800 kJ/kg as high. In terms of temperature, geothermal systems with more than 200°C at 1 km depth can yield high fluid enthalpies. Igneous-related geothermal reservoirs are an abundant though unexploited energy resource on Earth. The thermal energy stored in those reservoirs is much higher but the risk of drilling into a molten magma pocket is very high too.

While there are numerous geophysical exploration techniques developed for the oil and gas industry, few are directed to the geothermal sector, where profitability is much smaller. In this talk, we are going to show the potential of ambient seismic noise interferometry to image high-enthalpy geothermal reservoirs. This approach has been broadly used in many different scenarios but barely in geothermal settings. In particular, we study the Hengill area, which is located in Iceland and hosts three volcanic systems, several geothermal sub-fields and two large power plants, being one of them Hellisheiði (303 MWe, 200 MWt), one of the largest power plants in the world.

We compute a 3D shear-wave tomography from seismic noise records in the Hengill area and compare the results with several geophysical observables derived from borehole measurements in the region, such as steam ratio and formation temperature. Furthermore, we compare the results with a resistivity model obtaining an excellent correlation between both observables overall. We find some discrepancies in small areas that we interpret as a lack of thermal equilibrium. We also identify a promising site for future drilling projects.