Saturated heating and quenching of three crustal rocks and implications for thermal stimulation of permeability in geothermal reservoirs

• Thermal stress was applied to three crustal rocks under water saturated conditions.
• Rapid quenching and slow cooling of rocks with water induced damage to samples.
• Physical properties were dramatically changed from thermal stressing testing.
• Permeability was increased in all samples with quenched samples showing most damage.
• Results are discussed with respect to other studies and geothermal applications.

Thermal stimulation is a reservoir enhancement technique that relies on the generation of permeable fractures by thermal gradients. Replication of thermal stimulation in a controlled laboratory environment provides insight to the physical changes that may be expected at the field scale and assist in stimulation optimization. We present a new methodology designed to replicate thermal stimulation in such an environment. We developed an apparatus to allow rocks to be heated to 375 °C at up to 35 MPa confining pressure and rapidly quenched with cold water to ambient temperature while maintaining system pressure. To test our methodology, we characterized three rocks that represent a range of geothermal developments and laboratory based investigations: Lanhélin Granodiorite (granite) from Brittany, France, Allandale Rhyolite from the Lyttelton Volcano, New Zealand and Halswell Basalt also from the Lyttelton Volcano. Our characterization consists of profiling microstructure, porosity, density, dynamic elastic moduli and permeability prior to thermal stressing. Results indicate that we have increased porosity, decreased density, attenuated elastic moduli and significantly enhanced the permeability of the rocks. Rapidly cooled and slowly cooled samples are presented with the rapidly cooled samples showing an increase in permeability. The results are compared to similar studies previously carried out under dry conditions and implications discussed for geothermal operations.