Optimizing and Monitoring Cyclic Stimulation through Lithology-Controlled Thermal Cracking

Enhanced geothermal systems rely on increasing permeability and pore surface area in hot rock. Cyclic thermal shocking enhances both by inducing thermal cracks through repeated rapid cooling. Because the process requires resources like water, optimizing stimulation efficiency involves identifying lithologies most responsive to cracking and quantitative descriptors of stimulation results. We tested micrite, granodiorite, and alkali trachybasalt, applying up to ten cycles, monitoring crack evolution via time-lapse electron microscopy, pressure-dependent permeability, and compressional/shear wave velocities. Rocks with minerals of contrasting thermal properties, large crystal grains, and irregularly shaped vugs show the most persistent permeability increases. Velocity reductions are greatest below 10 MPa and diminish with additional cycles. Results demonstrate that Vp/Vs effectively quantify cyclic thermal damage, linking rock microstructure, pressure, and crack evolution to guide optimization of geothermal reservoir stimulation.