Effect of Cyclic Heat Treatment on Transport Properties of Hot Dry Rock

Abstract

Hot dry rock undergoes cyclic temperature variation during an enhanced geothermal system (EGS) operation, resulting in variations in reservoir rock’s transport properties and subsequently influencing the heat extraction efficiency of EGS. Therefore, the subject of this study was to systematically investigate the effect of cyclic heat treatment on the transport properties of granite, commonly employed in EGS, through the analysis of P-wave velocity, density, and scanning electron microscopy images. Besides, the effect of changes in the granite transport properties on EGS operation was also comprehensively discussed. The results indicated that the cyclic heat treatment led to an increase in granite permeability and a reduction in thermal conductivity. These changes primarily occurred due to the initiation and propagation of microcracks within the granite. Notably, higher-temperature heat treatments exhibited a more pronounced impact on granite properties. Additionally, a significant shift in the granite properties was observed within 450–550 °C, serving as a threshold temperature in this study. Due to the Kaiser memory effect and the blocking effect of the pre-microcrack on the subsequent microcrack, the effect of heat treatment on the properties of granite mainly came from the first heat treatment. Finally, the relationship models between heat treatment temperature and transport properties damage factors were obtained by fitting literature data.