Top-Down Cooling and Its Role in Sustaining the Laugarnes Geothermal Field, Iceland

Abstract

The Laugarnes geothermal field is a low-temperature system (<150 °C) that supplies an average of 80 MW_th for district heating in Reykjavík. Over nearly a century of operation, the field has maintained stable pressures and discharge temperatures. Despite its longevity, the processes sustaining the reservoir's response to production are still not fully understood. This study analyzes long-term production and field data, revealing a "top-down" cooling pattern attributed to the recharge of cooler surface waters into shallow reservoir rocks. This phenomenon is then integrated into a new conceptual model of the field. Energy balance calculations – accounting for basal conductive heat flux, drawdown-induced surface recharge, and the associated cooling of the rock – demonstrate that formation cooling constitutes a significant heat source during production. A 3D numerical model, calibrated against natural-state temperature and pressure history, reproduces the observed temperature changes, further supporting the importance of heat extraction from the rock formation. The findings suggest an alternative paradigm for the understanding heat budget and transport in Laugarnes and highlight the broader relevance of recharge-driven heat extraction in low-temperature systems. Therefore, considering this process is crucial for accurately characterizing low-temperature systems and ensuring their sustainable use.