Experimental investigation on triaxial mechanical and permeability behavior of sandstone after exposure to different high temperature treatments

Geothermal energy offers great advantages in cost, reliability and environmental friendliness compared with conventional fossil fuels; geothermal energy is recommended and has been identified as a renewable and alternative energy source. Two of the pre-requisites for the exploitation of geothermal energy are reservoir and cap rock, and the mechanical properties and permeability behavior of reservoir and cap rock have a great influence on the exploitation of geothermal energy. This study presents a series of experimental results that analyze the effects of temperature (25, 100, 200, 300, 400, 500, 600, 700 and 800 degrees C) on the physical properties and mechanical and permeability behavior of sandstone. According to the physical test findings, the critical temperature (T-c) that induces changes in the mechanical and permeability behavior of sandstone was identified as 400-500 degrees C. A more obvious decreasing trend in those physical properties, i.e., weight, density, P-wave velocity, S-wave velocity, dynamic elastic modulus and Poisson's ratio, can be observed with the increase in temperature after T-c compared to that observed before T-c The triaxial compression coupled with the transient pulse permeability test results showed that increasing temperature leads to an increase of cohesion and decrease of internal friction angle before T-c and the opposite trends were observed after T-c A decrease of nearly 20% in elastic modulus was observed after 800 degrees C compared with room temperature. The initial permeability of sandstone under certain pressure conditions was found to increase nonlinearly with the increase in temperature. Those findings are further discussed in the SEM and XRD analysis, according to which the material composition and state of sandstone as well as the micro-structure changed dramatically with the increase in temperature. Furthermore, a series of empirical relations between the temperatures and physical and mechanical properties of sandstone were derived, and are expected to aid in geothermal energy extraction from super-critical temperature resources.