Repurposing a deep geothermal exploration well for borehole thermal energy storage: Implications from statistical modelling and sensitivity analysis

Borehole thermal energy storage (BTES) is an important technology to minimise greenhouse gas emissions by storing surplus heat from industrial processes, space cooling or even excess summertime renewable wind or solar energy. This paper investigates the efficiency of BTES via a single deep ex-geothermal exploration well in Newcastle, retrofitted using a coaxial deep borehole heat exchanger (DBHE) completion. Previously, few studies have investigated 1) the use of a single DBHE for thermal energy storage or 2) the retrofitting of an ex -geothermal exploration well; therefore, this study investigates deep BTES through numerical modelling on MATLAB by testing the impacts of 10 design parameters on operational performance of a DBHE using both global and local sensitivity analyses.Under base-case conditions, a DBHE operating at a depth of 920 m could achieve a heat extraction rate in excess of c.54 kW recorded at the end of a 6 month (winter) heat production phase. When applying a 6 month (summer) thermal charge phase prior to extraction (recorded as 250 kW at the end of the charge period), the thermal yield recorded at the end of extraction was increased to a minimum of c.69 kW. In total, over an annual cycle, 1.23 GWh of heat was injected into the formation, and 0.46 GWh was extracted. Across all local sensitivity simulations, the average heat extraction rate was increased by 9.5-55.6 kW following a 6 month period of charge. The global sensitivity analysis demonstrated that thermal recovery was most influenced by parameters such as the undisturbed geothermal gradient, flow rate, inlet temperature during charge and inlet temperature during extraction. Most of these are operational parameters, indicating deep BTES systems can be optimised through careful engineering. The study concludes that single DBHEs have some capacity to store surplus heat. However, the additional heat yield during extraction is only a modest proportion of the heat reinjected to the formation during the charging phase (calculated as <20 % using the new storage efficiency metric proposed in this study). This approach is only likely to be viable where there is a large source of surplus heat with little alternative value, and where there is an existing deep borehole suitable for retrofitting. If these conditions do not exists, more conventional, shallower, multi-borehole arrays are likely to be more suitable for BTES.