Indications of risks in geothermal systems caused by changes in pore structure and mechanical properties of granite: an experimental study

The effects of thermal treatment on the pore structure and mechanical property evolution of granite were experimentally investigated to evaluate the risks of traditional and critical geothermal systems. The microstructure of thermally treated granite was studied using an optical microscope and nuclear magnetic resonance, and the mechanical properties of thermally treated granite were studied under uniaxial compression. X-ray diffraction results showed that the mineralogical phase of granite hardly changed during thermal treatment from room temperature to 800 °C, but the granite experienced first a decrease and then an increase in porosity. Although overall porosity decreased at 200 °C, the proportion of pore throat radium beyond 6.3 μm grew increasingly with the treatment temperature, facilitating advective heat transport in the geothermal system. Intergranular microcracks caused by thermal anisotropic properties started at below 400 °C, and transgranular microcracks first appeared in feldspar at around 400 °C, finally extending to quartz at around 600 °C. The granite was strengthened due to decreased porosity at around 200 °C. Then, the granite was weakened and the failure transited from brittle to ductile due to increased porosity, where a sharp reduction occurred at 400–600 °C for α-β transition of quartz. The change in strength by cyclic heating under 200 °C was unnoticeable for temperature memory effect, but the cumulative strain at peak stress increased with the number of heating cycles. The changes in the pore and mechanical properties caused by thermal treatment favour geothermal systems which are not within the tectonic zone. However, increased porosity and strength weakening destabilize nearby faults due to normal stress release and strength weakening.