Key Takeaways & Executive Findings
- •• Seawater thermal shock cycles significantly increase acoustic emission counts in hot dry rock, with 276% higher counts after 15 cycles compared to 0 cycles. • The b-value stabilizes after 5 seawater thermal shocks, indicating a transition to predominantly small fractures and reduced damage resistance. • Multifractal analysis of AE time series reveals that high-energy AE events dominate, pointing to shear damage as the primary failure mechanism. • The study supports using seawater as a heat transfer fluid in enhanced geothermal systems, conserving freshwater resources and promoting sustainable energy extraction.
Abstract
Analyzing the fatigue damage characteristics of hot dry rock (HDR) affected by seawater thermal shock cycles is required for the efficient exploitation of HDR and the conservation of freshwater resources. Mechanical and acoustic emission (AE) monitoring tests were conducted during the triaxial compression of HDR at different confining pressures, temperatures, and numbers of seawater thermal shocks to investigate the seawater damage of HDR. The test results indicated an increase in the cumulative AE counts with increasing temperature and number of seawater thermal shocks, and a decrease in AE counts with increasing confining pressure. The effect of the number of seawater thermal shocks was significant. The AE counts were 276% higher at 15 than at 0 seawater thermal shocks. The b-value increased with the number of thermal shocks and stabilized after 5 shocks. Most of the damage was small fractures, which reduced the rock’s damage resistance. The AE time series under HDR triaxial compression exhibited multifractal features. High-energy AE events dominated the damage mechanism of HDR, indicating shear damage to the HDR. Therefore, this study can provide a reference for seawater as a heat transfer fluid in the design of geothermal energy resource extraction.
1. Introduction
With the progress of human society, the growing energy demand continues to deplete the supply of non-renewable energy sources, such as oil and natural gas, and there is an urgent need to develop clean and efficient new green energy sources [1]. Geothermal energy is characterized by its wide distribution, large reserves, stable energy supply, low carbon emissions, and greenness. It is considered one of the most promising clean energy sources [2−6].
Hot dry rock (HDR) reservoirs have high densities and extremely low porosity and permeability (less than 0.001 mD) [7]. Despite the abundance of HDR geothermal energy reserves, large-scale development and utilization of deep geothermal energy is difficult [8]. After decades of exploration, the enhanced geothermal system (EGS) is considered the most effective and feasible HDR extraction method [9].
The EGS is an artificial geothermal system that utilizes reservoir fracturing to convert low-permeability, high-temperature rock into a geothermal reservoir by forming a complex network of cracks and injecting cold water for heat exchange, extraction, and utilization [10 −12]. However, the development of HDR requires a large amount of water for heat exchange, which is not suitable for areas with scarce freshwater resources, especially when developing HDR in high-energy-demand areas [13]. For example, the Guangdong-Hong Kong-Macao Greater Bay Area in South China, a pioneer demonstration area for China’s economy, has high pressure on energy requirements. Seawater has many properties similar to freshwater. Utilizing seawater instead of fresh water as a heat transfer medium can improve the sustainable development
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LI Cun-bao, LAN Ling, XIE He-ping, HU Jian-jun (2025). Acoustic emission response and rupture evolution analysis of triaxial compression damage of hot dry rock under seawater fatigue dissolution. Journal of Central South University. https://doi.org/10.1007/s11771-025-6029-9
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Frequently Asked Questions
What is the effect of seawater thermal shocks on the acoustic emission characteristics of hot dry rock?
Seawater thermal shocks significantly increase cumulative acoustic emission counts during triaxial compression, with 276% higher counts after 15 shocks compared to 0 shocks, indicating enhanced fatigue damage.
How does the b-value change with the number of seawater thermal shocks?
The b-value increases with the number of thermal shocks and stabilizes after 5 shocks, suggesting a transition to predominantly small-scale fractures and reduced damage resistance.
What is the significance of using seawater as a heat transfer fluid in enhanced geothermal systems?
Seawater can substitute freshwater for heat exchange, conserving freshwater resources and promoting sustainable geothermal energy extraction, especially in coastal high-energy-demand areas.
What are the main damage mechanisms of hot dry rock under triaxial compression after seawater fatigue dissolution?
Multifractal analysis of acoustic emission time series indicates that high-energy AE events dominate, pointing to shear damage as the primary failure mechanism in hot dry rock.
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