Permeability of Lanhelin granite through the brittle-ductile transition.

With increasing depth, crustal rocks gradually transition in deformation type, from being brittle/cataclastic to being crystal plastic, as well as in deformation mode, from being localized (faults, shear zones) to being ductile (homogeneous flow). This transitional layer, commonly referred to as brittle-ductile transition (BDT) has recently become the focus of economic development with the advent of Superhot Rock geotherm Reservoirs (SHR). Superhot Rock geothermal projects (e.g., Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano) seek to extract heat from geothermal reservoirs where water reaches a supercritical state (≥ 400 °C). These could multiply the power output of geothermal plants by a factor ten, a progress that is critical in the context of the climate crisis.

However, SHR reservoirs are generally localized at the BDT in semibrittle rocks (rocks deforming through a mixture of brittle and crystal plastic processes) which hydraulic properties are poorly understood.

Here, we report experiments conducted in TARGET, a newly designed gas-confining triaxial apparatus located at EPFL, CH. We deformed cylindrical cores of Lanhelin granite of dimension 40 x 20 mm at a confining pressure of 100 MPa and temperatures ranging from 200 to 800°C and a strain rate of 10^-6 s^-1. While deforming, sample permeability was recorded using the pore pressure oscillation method with an oscillation amplitude of 5 MPa and a period of 2400 s.

Lanhelin granite transitions from being localized with the formation of a sample scale fracture to being ductile between 600 and 800°C. In the localized regime, samples have an ultimate strength of around 600 to 650 MPa. In this regime, permeability initially slightly decreases upon loading from its initial value of 10^-20 m² before increasing with continued deformation. Permeability eventually plateaus upon sample failure and remains constant with further deformation. In the localized regime, permeability increase never exceed 2x10^-19 m^2. In the ductile regime, sample strength is halved and, past the initial decrease upon loading, permeability increases monotically by more than an order of magnitude.

We interpret these data has being the result of sample bulk controlling the sample permeability. In our localized experiments, the fracture never connected the ends of the rock core but would concentrate all of the strain after nucleation, limiting permeability improvement by micro-cracking in the bulk. In the ductile regime, since no localization occurs, bulk permeability of the rock would continuously improve with strain. These results bear important implications for the engineering of permeability in semibrittle reservoirs as well as for the understanding of hydrothermal circulation in the continental crust.