Deep Geothermal Single Well heat production: critical appraisal under UK conditions

Abstract

The idea of Deep Geothermal Single Well (DGSW) heat production has existed for many years, but with no consensus regarding its potential applicability: proponents have made claims regarding thermal outputs that appear exaggerated, whereas detractors have stated that the concept can never be economic unless the capital cost of drilling has already been discounted. However, because this technology offers the potential of delivering geothermal heat projects 'off the shelf' with a minimum of site-dependent research, the possibility exists of achieving cost-effective solutions. The present study sets out to investigate this topic subject to environmental and subsidy regimes applicable in the UK; the results might also be useful for other jurisdictions. Under these conditions, the variant of the technology with greatest potential for cost effectiveness is the hcDGSW, or conductive DGSW with heat production via heat pump. Analytic modelling enables the physics of the heat-exchange processes within a hcDGSW to be approximated. It is thus established that this option can indeed be cost-effective under the current UK subsidy regime for deep geothermal heat, provided boreholes are deep enough and in localities where the geothermal gradient is high enough. The environmentally optimum operational mode (optimizing savings in CO2e emissions) involves heat production at a lower rate than the economically optimum mode (maximizing profit). If such projects are subsidized from public funds, then a particular operational mode might be specified, maybe as a compromise between these optima. After the 20 year duration of the subsidy, the technology might well no longer be economic, but the infrastructure might be easily repurposed for seasonal heat storage, thus offering the potential of making a significant long-term contribution to sustainable future heat supply. These preliminary results indicate that more detailed appraisal of this technology variant is warranted. Introduction Most deep geothermal heat production projects are based around the concept of well doublets, in which production of thermal water from one well is balanced by injection of the water (after heat is supplied from it to the heat load) in a second well. The injection of water serves two purposes: it avoids the drawdown in hydrostatic pressure that would result in the geothermal reservoir rocks if only a production well were in operation; it also avoids the need for treatment of the produced water before surface disposal, which in many circumstances would otherwise be necessary. Typically one or both of the injection and production wells are deviated so they reach the geothermal reservoir some distance (up to ~2 km; e.g., Smit, 2012) apart. Over time a project of this type gradually extracts heat from the reservoir rocks surrounding the production well; heat production usually takes place far faster than the associated 'thermal recharge' by upward heat flow from the Earth's interior. The cold thermal front emanating from the injection well will therefore eventuall...

Repurposing Hydrocarbon Wells for Geothermal Use in the UK: The Onshore Fields with the Greatest Potential

“…An alternative to re-drilling a well might be to re-complete the well to implement a closed-loop wellbore heat exchanger system [32,33]. This would depend on favourable geological conditions and local heat requirements [34]. Application of this technology would remove the cost of drilling a new deep geothermal well.…”

“…In the latter case, source temperatures are known but the thermal power output has to be estimated. To provide a common basis for comparison for heat sources with a wide range of temperatures, in accordance with previous works (e.g., [34,77]) thermal power output for potential projects will be estimated using Equation (1) by equating ∆T to the difference between source temperature T O and annual mean ambient temperature T S , and are reported in Table 8. The resulting estimates of thermal power output are thus idealized upper bounds, which exceed those that might be calculated assuming a specific end use of the energy as in Tables 6 and 7.…”

Low-carbon GeoEnergy resource options in the Midland Valley of Scotland, UK

“…MacDonald et al, 2004). Some work has been carried out on single borehole concepts in the Midland Valley which do not require permeability at depth, however the long term sustainability of these concepts is yet to be proven (Westaway 2018). The temperature and therefore depth targeted depend on the requirements of the district heat network that the resource would provide heat to.…”

“…Law et al (2015) predicted a 2 km borehole could yield 400 kW of heat as an upper bound estimate. These single-well concepts remain experimental and the long-term sustainability has been questioned (Westaway, 2018).…”

Conjugated Numerical Approach for Modelling DBHE in High Geothermal Gradient Environments

“…Enhanced geothermal systems (EGS) is aiming to potentially double the worldwide power production from geothermal resources [2], but is still technically discussed due to induced seismicity, poor rock connectivity, and low recovery factors [3]. In this respect, closed-loop unconventional geothermal well designs are able to overcome these issues [4][5][6], with economic benefits and CO 2 emission savings [7], notably from very deep drilling into favorable higher geothermal gradient (>45 • C/km).…”