PHREEQC modeling approaches for characterizing lithium release from rock during geothermal plant operation

The poster presents the framework of the modeling part of the BMWi-funded project "Li-Fluids" (grant number: 03EE4034A). It introduces PHREEQC modeling approaches for simulating lithium-bearing minerals in geothermal reservoirs. By integrating lithium sources into PHREEQC, the release of lithium can be simulated. Additionally, a 1D dual-porosity approach is used to define reservoirs and calculate time-dependent depletion.

However, there are challenges. The accurate calculation of brines requires the Pitzer approach, which is limited in the number of minerals considered and does not include aluminum. Li-Mica, a modified form of K-Mica, appears to be an important primary source of lithium in the host rock. To address these limitations, other databases like the SIT database can be extended to include lithium-bearing aluminum silicates.

Saturation experiments were conducted up to 200°C using pure H₂O, approximating the observed Li⁺ concentrations from gold capsule experiments conducted by the project partner BGR. These concentrations ranged from a few mg/l Li⁺. Since Al³⁺ is necessary for calculating the saturation indices of Li⁺-bearing aluminum silicates, the use of pure H₂O was necessary to combine the BGR's laboratory experiments with PHREEQC's capabilities.

The next step involves implementing reservoir properties and developing an approach for modeling transport using concentrated saline waters frequently encountered in deep sedimentary basins in Germany. Van Genuchten's (1985) approach, which incorporates dual porosity similar to sedimentary rocks, will be utilized in PHREEQC. This hydraulic extension, combined with deep saline waters (e.g., the North German Basin), facilitates the extrapolation of findings from pure water experiments to a quasi-reservoir.