The FuE project belongs to the sub-topic 4 "Geoenergy" of the topic "High-Temperature Thermal Technologies" of the Helmholtz programme MTET and is centrally located in Cluster 3: "Multi-physics modelling and materials", with the topic of the microstructure simulation of scaling, sedimentation and corrosion, as well as the coupled processes of flow, heat/substance transport and mechanics.

The research work includes the development of methods, for the description of anisotropic crystal symmetries, the simulation of large-scale 3D crystallisation processes of polycrystalline microstructures in porous rock structures, as a result of scaling and sedimentation, as well as the implementation of data analysis procedures, for the determination of effective relationships between the grain boundary dynamics, the changing porosity and the permeability through cementation processes. Another focus is on the implementation of coupled microstructure-mechanics models, for the calculation of thermomechanical and chemomechanical processes in polycrystalline rock fractures. In this topic area, modelling of corrosion processes is also to be established.

Description

Within the spectrum of renewable energy sources, geothermal energy systems are a significant element with increasing prospects. Process simulation does play an essential role in the efficient, secure and durable construction and use of the power plants and does represent an indispensable tool for a quantitative understanding of the complex hydraulic, thermal, mechanical and chemical processes in fractured crystalline reservoirs. The research work done within the cluster has the objective to investigate the influence of the three-dimensional fracture geometry and the surface structure on the flow properties, the loss of pressure and the heat transfer rate of a multiphase fluid in multiphysical simulations of fluid mechanics and the mass and heat transfer. Based on the hydrothermal calculations, the material models allow a description of the precipitation and crystallization processes at the fracture walls and a prediction of the change of the aperture and of the permeability resulting from this.

Circulating waters in deep geothermal power plants are not in equilibrium with the in situ conditions concerning P, T and chemical composition. Complex fluid-rock interactions include  mineral dissolution and/or precipitation reactions leading to:

• Changes in the long-term reservoir hydraulics
• The mobilization of reservoir-specific natural occurringchemotoxic and radiotoxic elements

Geothermal Energy Systems

As a central part of the Topic "Geothermal Energy Systems" within the Programme "Renewable Energies" of the Helmholtz Association, the Cluster focuses inter alia on the artefact-free geochemical monitoring and analysis of the circulating waters, the appropriate thermodynamic description of the brines, including the prediction of mineral precipitation, and their kinetics and the mechanistic understanding of technical solutions (e.g. inhibitor application) to prevent secondary phase formation (scales). Based on this approach, the sustainability of the geothermal power plants and the minimization of operational and waste disposal costs will be tackled and solutions will be provided.

Material corrosion

Material resistance at geothermal power plants is highly challenged due to chemical and mechanical attacks by geothermal brines. Material corrosion of metallic construction materials is therefore a major concern for operators in geothermal industry. A reasonable material selection and corrosion engineering can enhance power plant availability and decrease the operating costs. Thus, another objective of the cluster is to minimize corrosion processes of construction materials in geothermal power plants.

Multiphase flow Simulations

To estimate the dynamic chemical and physical conditions of the wellbore, computational multiphase flow simulations based on the equation of state are developed. This novel wellbore simulator enables the estimation of parameters such as temperature, pH, CO₂-, Cl^- - concentrations, and the process of degassing of the fluid. Knowledge of these parameters is a prerequisite for studying corrosion conditions and material behavior.

Material Simulations

Apart from experimental studies of the electrochemical processes, material simulations are developed to capture electrochemically induced phase transitions, which are employed for the simulation-aided prediction of corrosion and damaging processes for various materials and temperature conditions.

Hydrothermal Simulations and Hydrochemical Modeling

By means of hydrothermal simulations and hydrochemical modeling, the influence of the fluid properties on the precipitation and corrosion process can be investigated. An analysis of the corrosion tendency depending on the geometry factors of the construction elements forms the basis for an optimized construction of the power plant.

Another major challenge is to find suitable corrosion inhibitors for the often very complex and varying geochemistry at different geothermal sites. Besides the investigation and evaluation of new and existing inhibitors, corrosion phenomena of various materials are generally investigated in electrochemical investigations as well as in exposure tests.