Numerical Modelling Of Thermal Energy Storage In Subsurface Geothermal Reservoirs For Efficient Energy Management

Abstract

Introduction:
The increasing deployment of renewable energy sources, such as wind and solar, has created a growing need for large-scale energy storage solutions. Subsurface geothermal reservoirs offer a promising alternative for thermal energy storage due to their high energy retention capacity and ability to support long-duration storage. However, their long-term performance under repeated heating and cooling cycles remains insufficiently understood, making numerical modeling essential for evaluating their potential.

Objectives:
This study aims to evaluate the thermal energy storage performance of deep geothermal formations, specifically focusing on how operational parameters, such as injection temperature, permeability, well configuration, and flow rate, affect thermal recovery efficiency and reservoir stability. The objective is to optimize operational strategies for enhanced energy retention and extraction over extended periods.

Methods:
A coupled thermo-hydraulic model was developed to simulate seasonal injection–storage–extraction cycles over 10 to 20 years in both porous and fractured reservoirs. The model examined a range of conditions, including injection temperatures (100-180°C), flow rates (20-60 kg/s), and permeabilities (50-300 mD). It tracked temperature evolution, pressure changes, heat losses, and mechanical responses, incorporating fracture networks to assess thermal breakthrough and energy storage capacity.

Results:
The results show that higher permeability significantly enhances recovery efficiency, from 0.68 at 50 mD to 0.87 at 300 mD. Single-well cyclic operations achieve recovery efficiencies of nearly 90%, while doublet systems yield 72-84%, depending on well spacing. Increased well spacing delays thermal breakthrough, with spacing from 100 m to 300 m extending breakthrough by approximately 35%. Injection of hotter water (up to 180°C) increases stored energy by 42%, though recovery decreases by 8% due to conductive losses. Mechanical deformation remains minimal, with vertical displacement under 5 mm.

Conclusion:
Geothermal reservoirs can serve as stable and efficient thermal energy storage systems when optimized operational parameters are used. Adjusting injection temperature, flow rate, and well configuration maximizes recovery efficiency and maintains long-term reservoir integrity. These findings support the potential of geothermal energy storage for integrating renewable energy into power grids and addressing seasonal energy storage needs.