Developing deep dry hot rock resources is widely regarded as crucial for addressing China’s energy security issues and achieving its “dual carbon” goals. However, extracting this type of energy is a significant challenge due to its location in complex geological environments characterized by high temperature and high pressure. The mining process is intricately linked to the multifaceted interactions of seepage, mechanics, and temperature. Consequently, establishing a thermo-hydro-mechanical coupling model that incorporates various physical factors is essential for studying the interactions and impacts of enhanced geothermal systems （dry hot rock）.
This study introduces a fully coupled thermo-hydraulic-mechanical model designed toinvestigate the effects of permeability and fault characteristics on the efficiency of geothermal extraction in hot dry rock reservoirs. The model thoroughly considers the interactions among multiple physical fields, including: ① reservoir deformation caused by changes in fluid pressure and temperature; ② variation of porosity, permeability, and fracture openings due to reservoir deformation; ③ temperature-dependent fluid properties （e.g., density, viscosity）; and ④ heat conduction resulting from fluid flow.
The Gonghe Basin in Qinghai Province is known for its abundant hot dry rock resources, notably the GR1 hot dry rock exploration well, which has the highest temperature recorded for hot dry rock drilled in China. This paper uses the GR1 well as a case study to develop a three-dimensional numerical model that examines how reservoir permeability and fault characteristics affect geothermal extraction efficiency in different production modes, specifically constant pressure and constant injection rate production. Additionally, we analyzed the combined effects of these factors on production efficiency and operational safety.
The research findings indicate:
1） In the constant pressure production mode, low-permeability reservoirs and barrier-type faults significantly reduce geothermal extraction efficiency. These features impede fluid flow, reducing the rate between injection and production wells and diminishing the overall energy output. While increasing reservoir permeability can boost geothermal extraction efficiency, however, simulations show that excessively high permeability can lead to rapid resource depletion within a short period, preventing full utilization and causing resource wastage, which is detrimental to sustainable exploitation. Therefore, it is vital to strike a balance between permeability enhancement for improved efficiency and sustainable resource management to avoid the swift exhaustion of geothermal reservoirs.
2） In the constant flow rate production mode, geological factors such as reservoir permeability and fault type have minimal impact on geothermal extraction efficiency. Since the flow rate is kept constant, the primary determinants of extraction efficiency are the water injection and production rates.
3） From a safety perspective, in the constant flow rate production mode, low reservoir permeability or barrier-type faults can substantially increase pore pressure response. This condition may lead to casing deformation and damage and could even trigger seismic events, posing construction safety risks.
4） For the enhanced geothermal systems in the Gonghe region of Qinghai, it is recommended to balance heat extraction efficiency with construction safety. In the constant pressure production mode, it is advisable to adjust the reservoir permeability to a range of 1×10−13 m2 to 1×10−12 m2 to strike a balance between heat extraction efficiency and resource sustainability. In the constant flow rate production mode, adjusting reservoir permeability to above 1×10−12 m2 is suggested to prevent excessive pore pressure from building up within the reservoir. In addition to modifying the reservoir permeability, it is also crucial to avoid barrier-type faults between the injection and production wells.
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