Experimental studies on reactive transport processes in Enhanced Geothermal Systems (EGS)

Reactive transport processes are crucial in various geological settings, driving rock alteration, ore deposit formation, CO₂ sequestration and Enhanced Geothermal Systems (EGS). In EGS, these processes, triggered by chemical stimulation, result in dynamic changes in mineral composition and petrophysical properties. Porosity generation and maintenance of permeability are essential for EGS, as they enable efficient fluid flow and hence heat transport. However, the parameters that control the efficiency of chemical stimulation of low-permeable are incompletely understood and experimental studies are still scarce.

To simulate coupled reactive transport processes in low-permeable crystalline reservoirs and to investigate the change of the respective petrophysical properties, we conducted hydrothermal closed-system experiments on the lab-scale, stimulating granite with modified regular mud acid (RMA) under geothermal reservoir conditions.

We characterized and quantified chemical, mineralogical, and microstructural changes of granite samples exposed to reactive fluids, partly in three dimensions, using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), electron microprobe analyses (EMPA), Raman spectroscopy, X-ray micro-computed tomography (µCT) through the EXCITE network at the Centre for X-ray Tomography at Ghent University, and fluid chemical analyses. Furthermore, fluid pathways and distribution of secondary phases, after the fluid-rock interaction, in the granite samples are detected, offering insights into the reaction process and the influence of experimental parameters on the reactions.

Our results show that the experiments effectively stimulate granite and significantly increase interconnected porosity, driven by coupled mineral dissolution and the formation of denser phases replacing the original mineral assemblages. Depending on the fluid composition, secondary phases coat the initial phases or fill the newly-generated pore space. Key findings underscore the potential of reactive transport by laboratory chemical stimulation to affect substantially the petrophysical properties (porosity and permeability) of granites under geothermal reservoir conditions.