Coupled flow, transport, and geochemical processes in fractures/fissures with a focus on geothermal and karstic systems

Coupling flow, transport, and geochemical processes in subsurface fractured media is challenging for several reasons and applies to different fields of applications. The focus in this project is on coupled processes related to two different fields, which we believe to have analogies in the governing processes: (i) geochemical reactions in fractured geothermal systems, as we find them e.g. in chemical stimulation and reinjection operations; (ii) CO2 density-driven dissolution. Advection and diffusion processes coupled to geochemical reactions lead to alterations of fracture morphology, fracture aperture, and, consequently, permeability. We want to know if dissolution of rocks facilitates a self-enhancing process that leads to the development of preferential flow paths. Pressure-driven or density-driven viscous flow competes with diffusive processes and interacts via the geochemical reactions with the hydraulic properties within the fractures. A conceptual, continuum-based model will be developed for the local scale of a fracture/fissure to study these effects. Perspectively, we need to identify effective processes to embed this into a multi-scale approach to address the field scales. For implementations, the numerical simulator DuMux is used. It provides modular coupling interfaces, which can be used to develop robust and efficient coupling strategies that take into account the different time scales of flow and reactions, which may also be specific for the reaction system and dimensionless numbers like Peclet, Damköhler, Reynolds, or Rayleigh. Both Navier-Stokes (for higher Reynolds numbers) and Darcy models are considered for fracture flow. Pore-scale models or pore-network models, developed and studied in collaborating projects of the applicants, can be used to generate pore-scale data which help to obtain better parameterizations of permeability. The vision is that with the help of machine learning algorithms, these findings can be incorporated into extended parameterizations or used to further develop models in the long term that allow a comprehensive analysis of geothermal systems.