OR/17/006 Conclusions: implications for planning of a geothermal research facility

Monaghan, A A, Dochartaigh, B O, Fordyce, F, Loveless, S, Entwisle, D, Quinn, M, Smith, K, Ellen, R, Arkley, S, Kearsey, T, Campbell, S D G, Fellgett, M, and Mosca, I. 2017. UKGEOS - Glasgow geothermal Energy Research Field Site (GGERFS): initial summary of the geological platform. British Geological Survey Open Report, OR/17/006.

Gap analysis

A great deal of legacy shallow borehole data (less than 50 m) and mine abandonment plan data are available for the Clyde Gateway area, as well as existing baseline datasets for monitoring (e.g. BGS systematic geochemistry, hydrogeological monitoring in superficial deposits). However, some clear data gaps are apparent from the summary of diverse data sources currently available to BGS as described in this report (Table 22). The most critical gaps at this planning stage for the Glasgow Geothermal Energy Research Field Site (GGERFS) are:

• Borehole, geophysical well log and seismic data to define the bedrock lithology and fault structure at depths greater than a few hundred metres, leading to uncertainty for deep borehole prognosis and understanding of deep fault structure
• Bedrock hydrogeology data, temperature and hydrogeochemistry data within or close to the area of interest leading to uncertainty in any geothermal modelling and planning for monitoring

Table 22    Examples of main data gaps identified.

Type	Data gap	Implication	Mitigation	New data collection would involve
Bedrock geology	Lack of borehole, mining data at greater depths than 435 m	Uncertainty for planning of boreholes >435 m in depth	Use surrounding geology, boreholes, wells	New ‘deep’ borehole
Faults	Lack of seismic data to define fault geometry at depth	Uncertainty for planning of boreholes >435 m in depth	Mine plan data characterises faults at <300 m. Seismic from nearby gives indication of fault geometries	New ‘deep’ borehole, new seismic reflection data. ?Other geophysical techniques
Rock properties	Rock property data (e.g. porosity, permeability, temperature very sparse at depths greater than a few hundred metres)	Borehole and geothermal productivity planning difficult	Use data from comparable depths/stratigraphy from across Midland Valley of Scotland	New ‘deep’ borehole testing and sample analysis
Hydrogeology	Lack of bedrock monitoring boreholes	Lack of data on groundwater levels, chemistry, temperature through time	Talk to Councils and SEPA	New characterisation boreholes and adoption of existing boreholes
Site specific geophysical datasets	No BGS data currently located	Techniques could provide a range of interpretations, primarily on shallow subsurface	Further BGS, Council etc requests if appropriate

BGS knowledge gaps: BGS has extensive data holdings, information and knowledge to define a geological platform for the Clyde Gateway area. Much of that knowledge is systematic and regional as opposed to local and site specific. BGS is strongly aware that input of data and knowledge from stakeholders (academic, Government and local Government, industry, local people) are required, where they bring both specialist and local expertise. Examples of this include aspects of research into geothermal energy from mine waters, the site specific knowledge of Local Authorities, site investigation contractors, consultants, etc. Stakeholder input will therefore form important next steps in development of GGGERFS in the Clyde Gateway area.

Science questions revisited

As a result of gathering BGS data and knowledge concerning the geological platform for the Clyde Gateway area, some detail has been added to the initial science questions described in section Initial examples of science questions at ‘ESIOS Clyde’ in the Clyde Gateway area. A vision statement for GGERFS has also been prepared (Appendix 3).

Minewater geothermal

The research will be built around the measuring and monitoring of minewater flow in a complex system of multiple mined seams, which in turn lie within a complex, faulted succession:

• Do faults act as barriers or conduits for flow?
• How far does pumping/heat extraction at one site influence heat and fluid flow at another site?
• What are the hydrogeological characteristics and impacts of mine workings of different styles/ages (e.g. longwall vs stoop and room); and what is the interaction between these and the surrounding un-mined aquifer
• What are the controls of temperature distribution through mine workings?

This will require the identification of patterns of recharge, fluid flow pathways, conduction and convection processes, and thermal breakthrough through detailed assessment of all hydrogeological and mine dewatering information, and additional borehole and outflow measurements coupled with modelling of the flow system.

• What volume/percentage of the groundwater and/or mine waters can be abstracted and returned annually without affecting long term heat transfer?
• Can we predict if, and over what timeframe, there will be thermal breakthrough from returning cold water to mines?

Hot sedimentary aquifer geothermal

• Are sandstone aquifers: hot enough; permeable enough; and have enough volume for continued geothermal supply?
• What are the roles of: mudstones etc and other seals in compartmentalising the resource and influence on resource potential; and of faults and other discontinuities as barriers of, or conduits to, flow

General

• What monitoring is required to address complexities of the geothermal gradient in a substantially anthropogenically-influenced groundwater environment
• What potential is there, and on what scales, for the subsurface storage of heat and transfer of heat
• What is the potential for linkage(s) of any strategic heat resource (heating, cooling, heat storage) to a developing District Heating Network

Identified Data gaps

• Aquifer hydraulic properties — transmissivity, permeability, storativity; flow types
• Key groundwater flow pathways and connectivity — through the unmined and mined Carboniferous aquifer; between Quaternary and Carboniferous aquifers; through and related to artificial ground; different recharge sources
• Groundwater flow rates
• Hydrogeological properties of faults and fault zones
• Constraints on the geothermal gradient in non-mined areas, and identify how the geothermal gradient is altered through mining; understand possible impacts of pumping (therefore recharge rates and system sustainability, changing temperatures)
• Heat flow measurements to depth via multi-depth equilibrium temperature measurements and conductivity measurements from core or chippings. This would improve the heat flow data set for Scotland and improve estimations, including an understanding of the conducting/insulating behaviour of certain formations such as the bulk behaviour of the Scottish Coal Measures Group
• Data to investigate the hypothesis that the high heat flow beneath Glasgow is due to groundwater upwelling
• Data to understand the impacts of stratigraphical and faulted surfaces on temperature distributions