Accessing Earth's vast geothermal energy requires understanding fracture networks under extreme mid-to-deep crust pressures and temperatures, where traditional brittle crust models become less applicable due to increasing ductile deformation.

This knowledge gap is critical for large-scale geothermal projects targeting supercritical reservoirs in the U.S., Iceland, New Zealand, and Japan. A key question remains: what are the physical properties of rocks at the brittle-to-ductile transition (B-D-T)? Limited data exist on porosity, permeability, seismic velocity, and electrical conductivity—both under hydrostatic conditions and deviatoric stress—at these depths, yet these properties directly impact fluid circulation, heat extraction, and geothermal viability.

To address this, we developed a gas-medium, internally heated apparatus simulating deep reservoir conditions, reaching 400 MPa and 1000°C, for 25 mm diameter, 50 mm length rock samples.

Our initial results show that at the B-D-T, pervasive grain-scale fracture networks form due to brittle-ductile interactions, causing dilatant behavior that transiently enhances permeability. This increased permeability, even under high pressure and stress, has major implications for fluid and heat recovery, expanding deep geothermal energy potential. Additionally, deformation in this regime is diffusive, meaning cracking is more distributed rather than localized, which may reduce induced seismicity risks during reservoir stimulation.

At higher temperatures, full ductile transition leads to rock compaction, reducing porosity and marking the cutoff for fluid circulation, setting a fundamental limit for geothermal energy production.

Next, we will assess seismic and electrical properties at the B-D-T to improve geothermal prospecting and geophysical data interpretation, enhancing deep geothermal energy extraction.