Material Behavior and Computational Validation of Deep CO₂ Closed-Loop Geothermal Systems in Carbonate Reservoirs

Area of Science:

• Geothermal Energy Engineering
• Reservoir Engineering
• Carbonate Reservoir Characterization

Background:

• Closed-loop geothermal systems (CLGSs) are crucial for stable deep heat supply without groundwater extraction.
• System performance depends on wellbore, near-well interface, and formation coupling.
• Deep carbonate reservoirs present unique challenges and opportunities for geothermal energy.

Purpose of the Study:

• To develop and validate a thermo-hydraulic model for CLGSs in deep carbonate reservoirs.
• To compare the performance of different configurations (U-tube, coaxial) and working fluids (CO2, water).
• To optimize CLGS design and operation for enhanced thermal power and long-term stability.

Main Methods:

• Developed a three-domain thermo-hydraulic framework with updated CO2 properties (temperature, pressure).
• Explicitly accounted for wellbore-formation thermal resistance in the model.
• Compared U-tube and coaxial geometries with CO2 and water as working fluids under field constraints.
• Validated the model using field data from the D22 well in the Xiongan New Area.

Main Results:

• The coaxial configuration with CO2 achieved 186.3 kW average thermal power, 44.9% higher than water.
• CO2 systems showed a 3-5% reduction in wellbore heat loss.
• Outlet temperature predictions closely matched field measurements (0.52% difference).
• Long-term simulations (30 years) showed <8 °C temperature decline for CO2, outperforming water.

Conclusions:

• Field-calibrated modeling is essential for accurate CLGS performance prediction and parameter transferability.
• The single-well coaxial configuration with CO2 is optimal for deep carbonate reservoirs.
• CO2-CLGSs can enhance thermal output and ensure long-term supply stability, supporting engineering deployment.