SaltPower is the World’s first commercial energy production technology based on osmosis. This novel technique has been practically proven in Sønderborg, Denmark, where saline geothermal water from Gassum sandstone aquifer at 1.23 km depth has been exploited to provide renewable energy to the district heating network. In this approach, once the heat is extracted from the brine, the brine is planned to be reinjected in the same sandstone aquifer thereby potentially eliminating challenges associated with disposal. Recently, one of the few publications in this area, showed that the potential of hypersaline brines for energy production can be by pressure retarded osmosis (PRO). In the PRO process, the osmotic pressure difference between the draw (high salinity fluid) and the feed (low salinity fluid) drives water molecules from the diluted fluid to the concentrated brine. By applying hydraulic pressure to the draw solution, the water flux across the membrane is pressurized and it can be converted into electricity by the help of a hydroturbine. Another set of researchers reported that hypersaline PRO process can be operated at pressures up to 70 bar with power densities above 5 W/m2 making SaltPower electricity generation economically viable.
The idea of getting more energy from heat depleted geothermal brines by a PRO process is appealing. Nonetheless, the larger amounts of diluted brine must be handled properly to avoid environmental issues. Reinjection of the diluted brine from SaltPower energy generation can be considered as a disposal option; however, questions regarding the overall reservoir assurance remain still unclear due to the complex fluid–fluid and rock–fluid interactions that are taking place upon reinjection. To address this, Aalborg University researchers: Postdoctoral fellow Dr. Jacquelin E. Cobos and Professor Erik G. Søgaard studied the fluid–fluid and rock–fluid interactions upon geothermal brine reinjection through isothermal titration calorimetry (ITC) and core flooding experiments. Their work is currently published in the research journal, Geothermics.
In their approach, the research team assessed the dynamic rock–fluid and fluid–fluid interactions that take place in a geothermal reservoir upon brine reinjection. First, the rock sample considered as a reference formation for core flooding experiments was collected. Next, highly saline brine from Thisted geothermal energy plant in Denmark was obtained. The researchers then carried out isothermal titration calorimetry experiments, followed by core flooding procedure.
The ITC experiments were reported to show the dynamic rock–fluid and fluid–fluid interactions that take place in the reservoir upon brine reinjection. The direct implication of those dynamic interactions was seen to be the stabilization of clay particles loosely attached to the pore surfaces. Moreover, formation damage was observed when Thisted and diluted Thisted brines were reinjected into Berea sandstone core plugs. More so, the authors established that the Fe (III) oxides precipitation inside the porous media was the main reason for that permeability reduction.
In summary, the Cobos-Søgaard study explored the synergy between isothermal titration calorimetry (ITC) and core flooding experiments with the goal being to provide mechanistic information about the rock–brine and fluid–fluid interactions when a geothermal brine is reinjected into a sandstone reservoir. Remarkable results that were in good agreement with the literature were reported. In a statement to Advances in Engineering, Professor Erik G. Søgaard explained their results could serve as the foundation of a reinjection scheme for diluted geothermal brine coming from SaltPower electricity generation.
Jacquelin E. Cobos, Erik G. Søgaard. Study of geothermal brine reinjection by microcalorimetry and core flooding experiments. Geothermics: volume 87 (2020) 101863.