Abstract
Recently, projects have been proposed to engineer deep geothermal reservoirs in the ductile crust. To examine their feasibility, we performed high-temperature (up to 1000 °C), high-pressure (130 MPa) triaxial experiments on granite (initially-intact and shock-cooled samples) in which we measured the evolution of porosity during deformation. Mechanical data and post-mortem microstuctural characterisation (X-ray computed tomography and scanning electron microscopy) indicate that (1) the failure mode was brittle up to 900 °C (shear fracture formation) but ductile at 1000 °C (no strain localisation); (2) only deformation up to 800 °C was dilatant; (3) deformation at 900 °C was brittle but associated with net compaction due to an increase in the efficiency of crystal plastic processes; (4) ductile deformation at 1000 °C was compactant; (5) thermally-shocking the granite did not influence strength or failure mode. Our data show that, while brittle behaviour increases porosity, porosity loss is associated with both ductile behaviour and transitional behaviour as the failure mode evolves from brittle to ductile. Extrapolating our data to geological strain rates suggests that the brittle-ductile transition occurs at a temperature of 400 ± 100 °C, and is associated with the limit of fluid circulation in the deep continental crust.
Highlights
The energy potential of high-enthalpy geothermal resources serves as a catalyst to probe the viability of geothermal reservoirs located in, or adjacent to, the ductile crust[1]
The permeability of granite is well-studied and a number of studies exist that show that microcracks and macroscopic shear and tensile fractures serve to increase permeability when measured at room temperature[22,23,24,25,26,27,28]
We report on the mechanical behaviour and in-situ porosity evolution of initially-intact and thermally-shocked samples of Westerly granite at the high-temperature (600 to 1000 °C) and high-pressure conditions typical of deep geothermal reservoirs
Summary
The energy potential of high-enthalpy geothermal resources serves as a catalyst to probe the viability of geothermal reservoirs located in, or adjacent to, the ductile crust[1]. The permeability of granite is well-studied and a number of studies exist that show that microcracks and macroscopic shear and tensile fractures (i.e., brittle deformation) serve to increase permeability when measured at room temperature[22,23,24,25,26,27,28]. We report on the mechanical behaviour and in-situ porosity evolution of initially-intact and thermally-shocked samples of Westerly granite at the high-temperature (600 to 1000 °C) and high-pressure (effective confining pressure of 100 MPa) conditions typical of deep (depth of ~4 km) geothermal reservoirs. The results of our study will improve our ability to analyse heat transfer between magmatic intrusions and high-enthalpy hydro-geothermal systems, to model the migration of fluids through nominally ductile crust beneath volcanoes and, more generally, will aid in understanding the nature of permeability as a function of depth in the continental crust
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