Hydraulic properties of stressed granite fractures with heat-induced void alteration

Abstract

During the extraction of geothermal heat, the changes in temperature and pressure environments can alter the mechanical and hydraulic behavior of fractured rock masses, affecting the overall transmissivity and heat extraction efficiency accordingly. This study carried out a series of experiments, including X-ray diffraction (XRD), scanning electron microscope (SEM), uniaxial compression tests, and surface morphological scanning, to explore the influence of reservoir temperature on the mechanical behavior of granite and fracture surface topography. The results indicate that as the temperature increases, the uniaxial compressive strength, elastic modulus, and Poisson's ratio of granite show similar variation patterns that increase as the temperature approaches 200 °C and then decrease. In addition, with the increase in temperature, the overall surface elevation and roughness parameters generally decrease due to surface asperity degradation, with the maximum mean mechanical aperture found at 200 °C. Flow simulations were conducted to further explore the influence of temperature and pressure environments on fracture transmissivity and flow fields. It is found that the normalized equivalent hydraulic aperture BN of the tested fractures decreases progressively as the stress increases, and more scattered distributions of BN are observed under higher compressive stresses. Moreover, with the continuous increase in temperature, the average BN slowly increases to its peak of ~0.78 at 200 °C, then it decreases to ~0.76 at 300 °C and remains nearly constant. These findings can be important for assessing heat production performances during the geothermal extraction process with spatially and temporally varying reservoir temperatures and pressures.