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 redevelopment led by Reading Borough Council, where an ageing gas-based heating system was being replaced. The open loop geothermal scheme was scoped as a pilot, designed not just to serve the theatre, but to prove whether a wider system could support future residential and commercial development around the site.
From the outset, this scheme needed to do more than function; it had to demonstrate that open loop geothermal could be delivered reliably, within tight physical constraints, and in a way that could scale.
The system was built around a paired abstraction and reinjection borehole, supported by a shallow monitoring well to understand how higher-level groundwater behaved during operation. In an urban setting, that detail matters because perched water can introduce both variability and contamination risk if not properly managed.
Both production boreholes were drilled to 100 metres into the chalk aquifer using a large-diameter commercial approach. The holes were started at 18-inch diameter to allow for the installation of 14-inch permanent steel casing to a depth of three metres, in line with the design specification. That casing was fully grouted to seal out superficial and higher-level deposits, maintaining the integrity of the deeper aquifer and preventing any vertical migration of poorer-quality water.
During drilling and geophysical surveying, a risk became clear. Flint within the chalk presented a credible threat to borehole stability, with the potential to fall into the open hole and damage infrastructure. We identified that early and worked with the design team to adapt the solution in real time, installing a plastic screen and casing system to line the borehole. That protected the submersible pump, maintained borehole integrity, and ensured the system would perform as intended over the long term.
Following construction, the boreholes were acidised to open up blocked fractures within the aquifer and maximise achievable yield. Pump testing was then carried out using both step and constant rate testing, in line with the Environment Agency conditions set out in the permit to drill. This stage is essential for any abstraction borehole, whether for water supply or open loop geothermal, as it provides the data required for the Environment Agency to assess aquifer performance and determine abstraction licensing.
However, testing brought a further constraint. Under normal conditions, pump test water would be discharged to the local sewer network, but in this case, the available capacity wasn’t sufficient to accommodate the required flow rates. Rather than delay or compromise the programme, Drift designed and installed a temporary containment solution using pillow tanks, allowing discharge to be controlled and managed within consent limits. It was a practical, site-led solution that ensured testing could proceed without disruption.
The boreholes delivered a sustained yield of 16 litres per second, with reinjection confirming the aquifer could support a balanced system. The scheme was completed on programme and within budget, successfully proving the concept.
The project also gained wider recognition, including coverage by the BBC, reflecting its role in demonstrating how open loop geothermal can be delivered in complex urban environments.
This was a scheme where we identified risks early, adapted the design to suit the ground, and solved site constraints - from flint instability to discharge limitations - to deliver a system that works long-term.
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