Repurposing Onshore Wells For Geothermal Use: Uk Case Studies of Deep Borehole Heat Exchangers

Repurposing onshore wells for thermal energy extraction and/or storage presents an opportunity to progress the decarbonisation of the UK heat supply at a low-cost, with CAPEX minimised due to the mitigation of drilling expenditure by converting existing wells. In this study, we build on previous work, which has focused on repurposing hydrocarbon wells using GIS mapping and multi-parameter screening processes to identify abandoned or end-of-life oil and gas wells suitable for exploitation under conventional methods. Here, we adapt this further to focus on repurposing wells as deep borehole heat exchangers for closed-loop systems, rather than as traditional open-loop systems.

Through the application of screening survey criteria, 73 onshore hydrocarbon wells were identified as suitable to be repurposed as deep borehole heat exchangers. Of these, 25 wells are currently operational and a further 15 are approaching end of life. Higher heat loads are achievable in wells located in areas of increased geothermal gradient, or from wells drilled to greater depths, thus higher temperature systems remain the focus for repurposing, with high-ranking candidate wells located in fields such as Kirby Misperton, Malton, Crosby Warren, and Gainsborough.

Using high-ranking candidate wells as case studies, preliminary numerical modelling in MATLAB is used to estimate the potential of deep borehole heat exchangers as a means for repurposing hydrocarbon wells. Low-enthalpy geothermal resources are shown to provide a feasible alternative heat source to natural gas. For example, initial modelling of one of the case studies considers KM-8; an abandoned gas well located in the Kirby Misperton gas field of the Cleveland Basin in north-east England. A dual- continuum numerical model with finite-difference solver is used to simulate heat flow within the borehole heat exchanger; incorporating the heterogeneity of site-specific geology and borehole dimensioning. Thermal power requirements of a commercial-scale greenhouse are generated in Hortinergy – a greenhouse modelling software – for a typical growing season and used as demand signals in load-following proportional-integral-derivative control of closed-loop circulation flow rates. With the addition of a single heat-exchanger and heat pump system at surface, it is shown that the annual spatial heating demand of a commercial-scale greenhouse could be supplied using the thermal energy extracted from the repurposed well. The thermal power and performance of this case study, and others, are subsequently compared to other modelling and feasibility studies in literature.