Operational strategies and well design for reservoir thermal energy storage (RTES)

Abstract

The intermittency of renewable energy sources necessitates effective energy storage solutions. This study narrows in on reservoir thermal energy storage (RTES) as a system to bridge the supply–demand gap through the storage and recovery of heated water for periods ranging from daily to monthly timescales. By injecting hot water into subsurface formations and later retrieving it for use in electricity generation or direct heating, the viability of RTES is explored for load-levelling applications, typically on daily to monthly timescales rather than extended seasonal or multi-year storage. Many factors including formation parameters (e.g. permeability, porosity, thermal conductivity of rock, thickness and insulating layers), completion parameters such as injection well design, operational parameters such as injection and production rates, and schedule affect the facility's performance. The two key performance indicators (KPI) considered herein are the produced water temperature and the heat recovery factor. In this study, three operational strategies and four different well completions were investigated using a coupled fluid–thermal simulation model. Heat loss from the upper and lower boundaries of the reservoir was also studied. While the immediate impact of initial hydro-thermal charging on produced water temperature may not persist in the long-term operation, it does influence the overall system efficiency by reducing the cumulative heat recovery. Through detailed analysis, it is demonstrated that vertical injection across the entire well length offers a balanced approach between minimizing pumping costs and maximizing heat extraction efficiency. This study establishes basic calculations for developing innovative operational and completion strategies to maximize the long-term economic benefit of RTES.