Effects of water on the brittle to ductile transition of sandstones

Geothermal energy is one of the renewable energy sources that can help mitigate climate change. Rocks are elasto-plastic materials at low pressure and temperature, and the deformation is accommodated along localized shear bands (brittle behavior); at high pressure and temperature rocks are elasto-visco-plastic and the deformation is homogeneous (ductile behavior); the transition between the two behaviors is called the brittle to ductile transition (BDT). Rocks capable of hosting geothermal fluids hot enough for electricity production (above 100°C) might be at or beyond the BDT.

The importance of the BDT of rocks stems from the fact that the largest earthquakes occur there, and it corresponds to a major decrease in the permeability of the crust. In the literature, the mechanical properties of rocks across the BDT are relatively well known, though only lately the effect of the presence of fluids and their chemical composition has been investigated by few research groups. However, certain fluid compositions might lead to mineral dissolution, precipitation, weakening and alteration, which in turn affect the mechanical properties of rocks.

To investigate the effect of water, fluid chemistry and alteration, triaxial experiments on a porous silicate sandstone (Adamswiller sandstone) with and without water at 100°C were done, to understand in detail the effect of water on deformation. To assess the evolution of the mechanical properties, complex electrical conductivity, permeability and ultrasonic seismic velocity were monitored in situ to fully describe the rock properties across the BDT.

The experiments resulted in a complete characterization of the failure and yield envelopes of Adamswiller sandstone across the BDT, together with the electrical conductivity, permeability and ultrasonic seismic velocities. The presence of water lowers both the peak stress and yield stress. Electrical conductivity decreases beyond the BDT due to porosity reduction, following the reduction of permeability.