THMC-Coupled Simulation of Diagenetic Processes in Carbonate Geothermal Systems

The performance of geothermal reservoirs is fundamentally controlled by the evolution of porosity and permeability, which in turn is governed by diagenetic processes interacting with coupled thermal, hydraulic, mechanical, and chemical (THMC) processes. Diagenetic reactions may either enhance reservoir hydraulic performance - through mineral dissolution, secondary porosity generation, or dolomitization-related volume changes - or degrade it via mineral precipitation, compaction, and cementation, resulting in reduced hydraulic connectivity and geothermal productivity. A process-based understanding of these interactions and feedbacks is therefore essential for reliable geothermal resource assessment.

The Muschelkalk Formation in the Berlin–Brandenburg region of the North German Basin represents a promising geothermal target due to its favorable porosity, permeability due to brittle deformation, and temperature gradients at depth. However, its reservoir properties are strongly modified by diagenetic processes associated with halokinesis and fluid flow, including dolomitization, uplift-related deformation, and fluid-mixing corrosion. These processes generate pronounced spatial heterogeneity and uncertainty in reservoir performance, highlighting the need for a coupled, process-oriented modelling and analysis approach.

We developed a physics-based THMC-coupled modelling framework to investigate diagenetic controls on geothermal reservoir behavior from reservoir to basin scale using integrated geological and petrophysical data from the Muschelkalk Formation. The objectives of our study are (1) the analyses of THMC-coupled diagenetic processes in the Muschelkalk Formation and their effects on porosity–permeability evolution, (2) quantify the interaction between thermal, hydraulic, mechanical, and chemical processes and their influence on reservoir heterogeneity, and (3) assess the impact of these coupled processes on geothermal performance through reservoir- and basin-scale doublet simulations.

The modelling workflow is implemented using the GOLEM application (based on MOOSE framework) for coupled thermal–hydraulic–mechanical (THM) processes, which is coupled with PHREEQC to represent key geochemical reactions, enabling fully THMC-coupled model development and simulations. Despite the high computational demand of large-scale coupled modelling, this approach enables a comprehensive assessment of temperature, fluid flow, stress state, geochemistry, and petrophysical evolution. Overall, the study aims to provide a quantitative and process-based foundation for improving geothermal resource evaluation and long-term reservoir management in sedimentary basins.