Closed-Loop Geothermal Heat Recovery Predictions: Impact of Time-Dependent Gradient Decay and Fracture Thermal Resistance

Abstract

This study examines the potential conversion of an abandoned well in the Krechba Field, Ahnet Basin, Algeria, into a closed-loop geothermal system using supercritical CO₂ as the working fluid. The research focuses on evaluating the effect of natural fracture properties on long-term heat extraction performance. A numerical model was developed to simulate conductive heat transfer along a 3000 m wellbore divided into 300 vertical segments, using geological, thermal, operational, and fracture-related parameters representative of the Krechba Field. Sensitivity analysis was conducted over a 30-year simulation period to assess the effects of fracture density, water saturation, fracture filling type, mass flow rate, and stratigraphic layer properties. The results show that higher fracture density and water saturation improve heat extraction, while mineralized or gouge-filled fractures reduce thermal performance. The study also shows that mass flow rate affects outlet temperature and heat delivery, and that improving fracture properties across all layers gives the highest thermal output. Overall, the results confirm that closed-loop geothermal systems using supercritical CO₂ can provide stable long- term performance in mature petroleum fields.