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Description
Flow and chemical reactions on rough fracture surfaces can gradually change the aperture and permeability of a fracture, and hence in a long run, influence the productivity of a fractured geothermal reservoir. Most existing models, however, still assume smooth fractures and isothermal chemistry. In this work, a thermo-hydro-chemical (THC) model was developed for a single rough fracture, where temperature-dependent geochemical reactions are computed with PHREEQC and fully coupled to variable-density flow and heat transport processes.
In this study, the fracture geometry is constrained by laboratory data. A natural rock sample was first scanned to obtain its real fracture surface and extract key roughness statistics, such as aperture distribution and self-affine scaling parameters. These statistics are then used to generate artificial rough fractures that reproduce the measured characteristics but allow us to systematically vary fracture aperture and roughness. Within these geometries, the THC model passes the local, time-dependent temperature from the flow and heat transport solver OpenGeoSys to PHREEQC, so that speciation, reaction rates and fluid properties respond consistently to the evolving thermal field. The reaction network includes pressure-solution processes that slowly reduce aperture and modify permeability over time, without explicitly solving the mechanical equilibrium problem.
Numerical experiments show that combining realistic roughness with temperature-dependent chemistry leads to strongly localized patterns of dissolution and pressure solution, and to permeability evolutions that differ markedly from isothermal or smooth-fracture assumptions. All processes are implemented in the open-source OpenGeoSys--PHREEQC framework. This work forms part of the BMBF-funded RiskXclude project on quantitative risk assessment in fractured geothermal systems.
| Country | Germany |
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