Quantifying Reaction-Induced Porosity During KBr–KCl Replacement: 4D Synchrotron Tomography and Statistical Microstructure Descriptors

Fluid-induced mineral replacement reactions play a key role in controlling porosity generation and permeability evolution in geologic systems. However, the dynamic feedback between pore structure development and fluid transport remains poorly quantified. This study investigates the spatiotemporal evolution of reaction-induced pore space in the fluid-driven KBr–KCl system using time-resolved synchrotron X-ray tomography. Due to its high solubility and rapid reaction kinetics, the KBr–KCl system serves as an effective analogue for fluid–rock interactions in natural settings. We performed two operando experiments at the TOMCAT beamline (Swiss Light Source): one with direct KCl solution flow over a KBr crystal, and another using a pressurized X-ray-transparent cell. Machine-learning-based segmentation enabled quantitative analysis of porosity evolution through spatiotemporal correlation functions and transport property estimation. We identified a three-stage pore evolution process: (1) rapid pore channel formation along crystallographic axes with high reaction rates and a rough interface; (2) a transitional stage characterised by smoother interfaces and enhanced lateral connectivity; and (3) a steady-state regime where permeability continues to increase due to pore coarsening and reduced tortuosity. These results advance our quantitative understanding of how reaction-induced porosity governs dynamic fluid–rock interactions.