Repurposing existing wells, including abandoned hydrocarbon wells, would significantly reduce the cost of deep geothermal developments. The Newcastle Science Central geothermal borehole, drilled in 2011 to a depth of 1820m, was intended to contribute to the energy mix of the Newcastle Helix redevelopment but the water yield proved insufficient. Our current project, Net Zero Geothermal Research for District Infrastructure Engineering (NetZero GeoRDIE), investigates the potential for repurposing the well as a closed loop coaxial deep borehole heat exchanger (DBHE) for heating and cooling. This study presents an uncertainty quantification of a conceptual DBHE at Newcastle Helix. The computational model is based on a recent fast semi-analytical solution, validated against a 3D finite element simulation undertaken in the open-source multi-physics software package OpenGeoSys. The model parameters are obtained from nearby geological data sources but a high degree of uncertainty surrounds several key physical and thermal parameters. We investigate the impact of geological uncertainties on the recoverable geothermal heat over the short and long-term (20 years) by using a probabilistic approach. We also conduct a global sensitivity analysis by computing the Sobol indices from the coefficients of a polynomial chaos surrogate model. The results indicate that, in the operational scenario considered, potential DBHE production is around 140 kW over a 20-year design life (P50). Over time, the sensitivity of the fluid temperature to the input parameters evolves, revealing that the thermal properties of the rock become more important over the long-term: the recoverable geothermal heat is mainly controlled by the thermal properties of the deeper geological formations. The results offer valuable insights into the feasibility of repurposing existing deep wells as closed loop geothermal systems.

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