Experimental Investigation of Thermal Shock Effects on Fracture Nucleation and Propagation of Igneous Rocks

Abstract

The thermo-mechanical response of igneous rocks to rapid temperature fluctuations governs both the heat-extraction efficiency in Enhanced Geothermal Systems (EGS) and the permeability development in engineered geological hydrogen systems. This study experimentally investigated the effects of thermal shock on the strength, stiffness, and fracture evolution of granite and ultramafic rocks to establish cross-domain insights into subsurface energy systems. Cylindrical granite and ultramafic specimens were characterized using micro-computed tomography (micro-CT) measurements, then subjected to controlled thermal shock treatments consisting of heating to 200 °C followed by rapid quenching in water at 0 °C or 20 °C to simulate cold-fluid injection into hot geothermal reservoirs. Micro-CT analysis reveals systematic porosity increases following thermal treatment, with heating alone producing minor increases of ~0.07–0.43 percentage points, while rapid quenching results in substantially larger porosity gains of up to ~1.0 percentage point in granite and ~0.8 percentage points in ultramafic samples. These microstructural changes are accompanied by reductions in elastic stiffness, with P- and S-wave velocities decreasing by approximately 5–15% and Young’s modulus decreasing by ~15–40% depending on lithology and quenching severity. Granite exhibits larger relative porosity increases and greater stiffness degradation under thermal shock, whereas ultramafic samples retain higher absolute velocities and moduli, indicating greater resistance to thermally induced damage. Together, the porosity–velocity–modulus response provides experimentally constrained insight into thermally driven fracture evolution and its role in permeability enhancement in enhanced geothermal systems.