Experimental investigation of triaxial compression and permeability of gritstone in geothermal environment

In this paper, the characteristics of microscopic failure and macroscopic mechanics of gritstone are studied experimentally under geothermal conditions. Different hydrothermal and cooling treatments are conducted to simulate geothermal environments. Permeability and triaxial compression tests of treated gritstone specimens are employed to obtain complete stress–strain curves and permeability and to discuss the mechanical properties of gritstone under various high-temperature, high-pressure, and cooling conditions. The results show that, in the hydrothermal study, the triaxial compressive strength of gritstone increases as the pre-treating pressure. In the cooling study, the specimen cooled in an autoclave had the lowest strength and the largest peak strain, while the air-cooled specimen had the greatest strength. Furthermore, pre-treatment with water at high pressure and temperature had a significant influence on the permeability of specimens. The permeability increased with increasing pre-treatment water pressure. In the cooling study, specimens cooled in an autoclave had the highest permeability. The microscopic characteristics of gritstone were analyzed by scanning electron microscopy (SEM) images. The SEM images showed that internal defects, such as microcracks induced by hydrothermal and cooling treatments, led to differences in macroscopic behavior. X-ray electron diffraction (XRD) data reflected the changes in the composition and content of specific minerals before and after treatment, and these changes were related to hydrothermal coupling. Analysis of the micro and macro characteristics showed that the temperature and pressure in the geothermal environment affected cracking and dissolution in gritstone, stimulating the initiation and propagation of cracks.