Evolution mechanism of permeability of hot dry rock under coupled effect of thermal fatigue and seawater interaction during coastal geothermal development

Abstract

Using seawater as a heat transfer fluid for developing hot dry rock (HDR) geothermal energy in coastal areas is a frontier of engineering worth exploring; however, only a few studies have considered this research topic. During the development of HDR based on enhanced geothermal system (EGS) technology, the thermal shock fatigue effect of low-temperature seawater on high-temperature reservoirs leads to changes in the permeability of the HDR, notably impacting the reservoir reconstruction design and heat transfer efficiency. Accordingly, the changes in pore structure, pore distribution, porosity, and permeability of HDR in the Guangdong-Hong Kong-Macao Greater Bay Area were investigated based on nuclear magnetic resonance (NMR) techniques and fractal theory after treatment to different temperatures and numbers of thermal shocks due to seawater interaction. The experimental results showed that with an increase in temperature and the number of seawater interactions, the pore structure distribution of the HDR underwent significant changes, with the proportion of mesopores and macropores increasing by 24.29 % and 10.52 %, respectively. Moreover, the porosity and permeability increased with increasing temperature and number of thermal shock cycles, especially above 300 °C. When the temperature increased from 100 °C to 500 °C, the total porosity increased from 0.98 % to 3.85 %, an increase of 292.9 %, while the permeability increased from 10−4 mD to 0.1 mD, an increase of nearly 1000 times. Changes in the pore structure, pore distribution, porosity, and permeability of HDR are caused by multiple damage mechanisms related to high-temperature nonlinear expansion, thermal shock effects, chemical erosion, and fatigue. Additionally, the permeability of the HDR after seawater treatment at the same heat treatment temperature and number of thermal shocks was higher than that after freshwater treatment. At a temperature of approximately 500 °C and 20 thermal shocks, the permeability of HDR under the action of seawater was approximately seven times that of fresh water. Therefore, these results suggest that using seawater as a heat transfer fluid has more advantages in enhancing the permeability of reservoirs.