Sylvain Brisson awarded the best Paper

Sylvain’s research, onducted as part of his master thesis at the EPFL Geo-Energy Laboratory – Gaznat Chair under the supervision of Brice Lecampion, investigates the nature of hydraulic fracture propagation during stimulation experiments at the Utah FORGE enhanced geothermal system test site. By combining analytical models and a fully coupled hydro-mechanical mixed-mode solver, the study differentiates between tensile hydraulic fractures and fluid-induced dilatant shear ruptures in two stimulation stages that mainly differed by the viscosity of the injected fluid.
In particular, the research shows that the hydraulic stimulation performed with a more viscous fluid (water with cross-linked CMHPG polymer) can only be explained by the propagation of a viscosity-dominated tensile hydraulic fracture, characterised by continued growth even after the end of the injection. In contrast, the injection stage using a less viscous slick-water fluid could correspond to either a toughness-dominated hydraulic fracture or a marginally-pressurized dilatant shear rupture, both exhibiting minimal post-injection propagation.
The growth of the fracture over time is inferred from the spatial distribution of associated microseismicity, which serves as a key observational constraint for the modeling.
This work demonstrates that relatively simple models—whether analytical or numerical axisymmetric—can yield valuable insights into the complex physical processes at play during EGS hydraulic stimulations, helping to interpret field data and guide the design of future operations.