Reactive Flow Experiments on Granite: Implications for Chemical Stimulation of Enhanced Geothermal Systems

Enhanced Geothermal Systems (EGS) rely on heat extraction from deep crystalline rocks, whose inherently low permeability requires reservoir stimulation to establish effective fluid circulation. Current stimulation strategies are largely limited to hydraulic methods, while chemical approaches remain underexplored in crystalline lithologies, even though natural hydrothermal analogues demonstrate that fluid–rock reactions can substantially modify pore structure and flow properties. 

Here, we investigate the reaction-driven evolution of porosity and permeability in low-porosity granite using controlled reactive flow-through experiments conducted under conditions relevant to chemical stimulation of EGS. Reactive fluids with modified regular mud acid (RMA), are continuously circulated through saw-cut granite cores, enabling direct monitoring of hydraulic property evolution during fluid flow. These measurements are complemented by post-experimental mineralogical and microstructural characterisation using electron microprobe analyses (EMPA), scanning electron microscopy (SEM), surface profilometry, and X-ray micro-computed tomography (µCT), conducted via the EXCITE network at the Ghent University Centre for X-ray Tomography. Previous batch experiments, presented separately at this conference (see Rüdiger et al., EGU2026), demonstrate that the used modified RMA fluid reacts preferentially with feldspar and mica, resulting in increased porosity. Building on these findings, the flow-through experiments examine how such mineral reactions progress under dynamic conditions and assess whether the newly formed porosity contributes to connected flow pathways and enhance permeability. In addition, the experiments further address the formation and stability of secondary phases and quantify the advance of reaction fronts into the granite matrix as function of time and flow. Together, these data allow to assess whether the substantial porosity increases observed in batch experiments are sustained under flow-through conditions, and how these changes affect both the magnitude and long-term stability of permeability enhancement.