Proving Open Loop Geothermal in a Constrained Urban Environment

This project was delivered in Reading, where an open loop geothermal system was installed for the Hexagon Theatre within a constrained urban environment as part of a wider decarbonisation programme.

The Hexagon Theatre formed part of a major redevelopment led by Reading Borough Council, where an ageing gas-fired heating system was being replaced. The open loop geothermal scheme was conceived as a pilot project, designed not only to serve the theatre, but to prove whether a wider groundwater system could support future residential and commercial development across the surrounding site.

From the outset, this project needed to do more than simply function. It had to demonstrate that open loop geothermal could be delivered reliably within a constrained urban environment and provide a scalable solution for future decarbonisation.

Designing an Open Loop Geothermal System for a Complex Site

The system was built around a paired abstraction and reinjection borehole, supported by a shallow monitoring well used to understand the behaviour of higher-level groundwater during operation.

In urban environments, this level of investigation is essential. Perched groundwater can introduce variability and contamination risks if it is not properly identified and managed during design and construction.

Both production boreholes were drilled to 100 metres into the chalk aquifer using a large-diameter commercial drilling approach.

Protecting the Aquifer and Borehole Integrity

The boreholes were started at 18-inch diameter to accommodate 14-inch permanent steel casing installed to a depth of three metres, in accordance with the design specification.

The casing was fully grouted to seal off superficial deposits and higher groundwater horizons, protecting the integrity of the deeper aquifer and preventing vertical migration of poorer-quality water.

Adapting the Design to Ground Conditions

During drilling and geophysical surveying, a significant risk became apparent.

Flint bands within the chalk presented a credible threat to borehole stability, with the potential for material to collapse into the open hole and damage installed infrastructure.

Rather than simply record the issue, we identified the risk early and worked with the design team to adapt the construction approach in real time. A plastic screen and casing system was installed to line the borehole, protecting the submersible pump, maintaining borehole stability and ensuring the system would perform reliably over its operational life.

Borehole Development and Pump Testing

Following construction, the boreholes were acidised to open blocked fractures within the aquifer and maximise achievable yield.

Step testing and constant rate pump testing were then completed in accordance with the Environment Agency permit to drill.

This stage is fundamental to any commercial abstraction borehole or open loop geothermal system. The results provide the hydrogeological data required by the Environment Agency to assess aquifer performance and determine future abstraction licensing.

Solving a Site Constraint Without Delaying the Programme

Testing introduced another practical challenge.

Under normal circumstances, pump test water would be discharged to the local sewer network. However, the available sewer capacity could not accommodate the required discharge rates.

Rather than delay the programme or compromise the testing regime, Drift designed and installed a temporary containment system using pillow tanks. This allowed discharge to be carefully controlled and managed within consent limits, enabling the full testing programme to proceed without disruption.

Proving the Resource

The completed boreholes delivered a sustained yield of 16 litres per second, while reinjection testing confirmed the chalk aquifer could support a balanced open loop geothermal system.

The project was completed on programme and within budget, successfully demonstrating that the site could support both the theatre's heating requirements and the wider redevelopment ambitions.

National Recognition for The Project and Drift Services Group

The project also received wider recognition, including coverage by the BBC featuring Drift, highlighting its role in demonstrating how open loop geothermal can be successfully delivered in challenging urban environments.

A Practical Engineering Approach

This project demonstrates the way Drift approaches every scheme.

Rather than simply drilling to a specification, we work with the realities of the ground. By identifying risks early, adapting the design where required and solving site constraints - from flint instability to discharge limitations - we delivered an open loop geothermal system designed to perform reliably for the long term.

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