Repurposing a Geothermal Exploration Well as a Deep Borehole Heat Exchanger: Understanding Long-Term Effects of Lithological Layering, Flow Direction, and Circulation Flow Rate

Abstract

In the drive to achieve net-zero carbon emissions, decarbonisation of heating is essential. This can be facilitated by geothermal energy, but drilling geothermal wells is associated with high risks and costs. The use of preexisting wells (e.g., exhausted hydrocarbon wells or failed geothermal exploration boreholes) offsets this cost while potentially turning liabilities into assets. The Newcastle Science Central Deep Geothermal Borehole (NSCDGB) is a geothermal exploration well that was drilled to target the Carboniferous Fell Sandstone Formation at 1418.5 to 1795 m depth. However, low hydraulic conductivities prevented the development as a conventional “wet” geothermal abstraction well; therefore, new alternative methods of development are being explored. This work investigates the repurposing of the NSCDGB as a deep borehole heat exchanger (DBHE), focusing on the sustainable operation of the system in the long term by employing a constant heat load designed to contribute to local buildings or a heat network. Numerical modelling was undertaken by using OpenGeoSys software to analyse the thermal and hydraulic performance of the system. Both homogeneous and heterogeneous models were developed to compare the influence of lithological layering in contrast to a homogeneous (nonstratified) subsurface geological model. Results from homogeneous simulations modelling the DBHE to a depth of 922 m show that a 50-kW heat load can be supported for a lifetime of 25 years. This corresponds to a 65-kW building load when coupled to a heat pump with a coefficient of performance of 4.33. Thus, the DBHE could meet up to 72% of the heat demand of the adjacent urban sciences building. Rather than being a purely hypothetical case study, this work considers a real existing borehole, adjacent to a building cluster which could make use of the geothermal heat. Heterogeneity, which has been considered for the first time at the NSCDGB site, exhibits a minor impact in comparison to homogeneous simulation results. Flow direction and mass flow rate also exhibited small effects on the system performance, whereas if the exploration well could be repurposed to increased depths, the heat load could be increased. This is the first study of a coaxial DBHE at the NSCDGB site considering long-term effects of mass flow rate, heterogeneity, and flow direction. The study evaluates the feasibility of repurposing an exploratory geothermal well in the UK as a DBHE that can be used as a low-carbon heat source for space heating, thus converting liabilities into potential “green energy” assets.

Decarbonising heating and cooling using temperature setback and geothermal energy

“…The model was set up with homogenous geology (i.e., constant parameters of the rock) to a depth of 922 m and a circulation flow rate of 5 l/s was applied to the wellbore (see [15], [16], [17], [18] IOP Publishing doi:10.1088/1742-6596/2600/2/022015 3 heating and cooling using the heating and cooling coefficient of performance relations listed for heat pumps in [19]. As preliminary studies have highlighted that it is unfeasible to meet the building demand using the DBHE at Newcastle [18], the building load has been scaled down by a factor of five to meet a 'pre-heat', heat pump system. Therefore, only a 5 th of the building demand will be met and additional heat sources will be required.…”

“…A constant heat load (CHL) simulation was also conducted where a 50-kW thermal power was imposed at the top of the DBHE. This corresponds to a constant building load of c. 65 kW [15], [16], [18]. [20]).…”

“…A CHL simulation for the duration of the year was also performed (see [16] or [18] for further details). This showed a rapid cooling of the circulation fluid within the DBHE.…”

Generic modelling to develop thermal yield nomograms for coaxial deep borehole heat exchangers (DBHEs)

“…More recently, researchers have considered whether the use of BHEs to greater depths is feasible (Chen et al, 2019;Cai et al, 2021;Brown et al, 2023a;Kolo et al, 2023a). Beyond depths of around 500 m, the hydraulic resistance associated with U-tubes becomes unacceptably high, leading to excessive use of parasitic electrical power in circulation pumping (Olsson, 2018a,b,c;Wirtén, 2018;Brown et al, 2024).…”

“…The borehole in which the coaxial DBHE is to be installed may be purpose-drilled for heat extraction, although analyses suggest that the ratio of drilling cost to heat yield is still too high in many countries and geological environments for this to be cost-effective (Banks, 2023a). The concept is, however, of interest to hydrocarbon and geothermal companies as a DBHE could, in principle, be installed in unsuccessful hydrocarbon exploration boreholes, mature or abandoned hydrocarbon boreholes (e.g., Westaway, 2018;Hu et al, 2020;Watson et al, 2020;Brown and Howell, 2023), or sub-commercial conventional geothermal exploration wells (Brown et al, 2023a;Kolo et al, 2023a), provided that diameter and environmental considerations allow.…”