UK Geothermal Well Heats up World’s Largest Indoor Rainforest

Sept. 29, 2023
Eden Project in Cornwall, UK was built over a reclaimed china clay pit 2 km from St. Blazey and incorporates two enclosures of adjoining domes which emulate natural biomes

Eden Geothermal Ltd. (EGL) is the United Kingdom’s first operational deep geothermal project to come online in 37 years.

A 5-kilometer geothermal system heats Eden Project Biomes, the world’s largest indoor rainforest, and Growing Point nursery, both built on a former china clay pit in Cornwall, UK. The Growing Point nursery is between the geothermal site and Biomes and eliminates a pre-existing off-site nursery, reducing food-miles and dependence on fossil fuels.

Eden Project Phase 1 delivers heat using a coaxial circulation system. Phase 2 will include a second well to supply heating and electrical generation. Deep granite drilling for Phase 1 concluded in 2021, and EGL completed the heat mainline and plant in 2023. The project started generating heat June 19.

EGL is a three-way partnership between Eden Project Ltd., EGS Energy Ltd., a geothermal development and consultancy group, and BESTEC (UK) Ltd., affiliated with BESTEC GMBH, a specialist geothermal developer and drilling advisor.

Eden Heat Reservoir

The drilling target was the Great Crosscourse natural fault bisecting St Austell granite in the Cornubian Batholith, a large granite mass formed some 280 million years ago. It contains tourmaline veins and has a surface outcrop.

The geothermal gradient, which get hotter as it goes deeper (in this case, 38.6 °C/km, 101 degrees Fahrenheit per kilometer), superheats water. The Grand Crosscourse contains natural fractures, does not require fracture stimulation, and therefore is not covered under the same regulatory framework as shale gas exploration projects. It is regulated by the local authority with similar monitoring and managing protocols to mining and quarrying. Water circulates in a closed loop about 4 km deep before directly contacting the granite and will have no impact on local aquifers.

The well, the longest geothermal well in the UK, contains a deep single-well coaxial heat exchanger. Water is injected down the annulus, heated at the reservoir, and produces inside vacuum-insulated tubing (VIT) to retain the heat. The water passes through a surface heat exchanger where it is cooled and subsequently reinjected into the well. A 3.8-km insulated main line delivers heat at about 85° C to Project Biomes and the nursery.

Drilling Deep

Drilling of Well EG-1 began in May 2021 and was completed in October of that year. The deviated well was drilled to 4,871 meters deep and 5,277 meters total measured depth using conventional drilling but with the drilling fluids cooled at surface during the later stages to minimize fluid degradation at high temperatures. Steel casing lines the upper three sections to isolate the circulating water from the surrounding rock and earth, but the bottom section is an open hole directly exposed to the granite.

The well was drilled by the Bentec 450. The drilling machine produces lower emissions, is certified under European Union directive for safety in explosive atmospheres, and has a 450-metric ton hook load capacity to drill to depth. Penetration rates in the granite were typically 3-4.5 meters per hour.

The well was drilled vertically to 1,706 meters, at which point the drilling angle was kicked off, deviating east away from vertical and towards the target zone in the Great Crosscourse fault. The open hole section was drilled out in the last 1,417 m of the well. Drilling took 162 days.

Testing Parameters of Geothermal

Heat and permeability comprise the two key elements for a successful deep geothermal energy system. Well tests quantified these parameters and provided information on petrography, hydrogeology, and geo-mechanical conditions of the rock at 4,500 m. Tests were designed to:

• Characterize the permeability-impedance of the target zone. 

• Evaluate the spatial extent of fracture distribution and orientation in the target zone. 

• Characterize flowing zones and evaluate the far-field flow contribution. 

• Generate models from the data to understand the regional geothermal resource.

Well tests were conducted at production and reinjection rates. A 3,200-cubic-meter lagoon served as a water storage site during the tests. The lagoon also contained and treated water released from the well during maintenance or normal running.

Well tests started in late January 2022 and lasted several weeks. Further tests were carried out in August, September, and November of that year. Permeability was derived from both water injectivity and production tests.

Injection tests were carried out first using cold water injected from the surface at low pressure and low flow rates. Over time, gradual and incremental flow rate increases were conducted to assess the well’s ability to accept fluid. Pressures and flow rates were continuously monitored. Microseismic monitors in the vicinity of the well site provided real-time data during drilling and testing. These data outlined the spatial extent of the geothermal heat source and aided control of injection and production during well testing to minimize seismic impact.

Production tests assessed the well’s ability to produce fluid. After the injection tests, a compressor injected air 600-700 m down open-ended tubing placed in the top of the well to reduce the column weight, induce a drawdown within the borehole, and initiate flow. Hot water subsequently flowed from the formation surrounding the open-hole section up the well. Produced water was directed to a unit to separate air and discharge water into the lagoon. Test duration was usually short (12-24 hr) and could produce steam at surface. During the test, downhole pressures and flow rates were continuously monitored to provide well productivity data.

Heating System

The well uses a coaxial circulation system to extract heat and supply it to the Eden Project. VIT installed to 3,850 meters produces water via an electric submersible pump. A plate heat exchanger at the surface extracts heat from the water which returns through the annulus to be reheated at depth during a continuous circulation operation.

The heat main line consists of two 6-inch steel pipes (flow and return) with a thick layer of insulation increasing total diameter to 11 in. The lines link the geothermal well with heat loads at Growing Point and the Eden Energy Centre using heat exchangers (Fig. 5). Cooled water returns to the geothermal site to be reheated. The heat main is below ground except for a short stretch behind Eden Project’s main office building.

EGL completed the heat main and plant in 2023. The project started generating heat on June 19.

Phase 1 will last a year and is expected to reduce greenhouse gas emissions by about 900 metric tons over a conventional heating system. Phase 2 will include a second well and water will be circulated through natural fractures in the Great Crosscourse from the original well to the new well. If a suitable connection to the grid is made,

Phase 2 will incorporate a generator to supply about 20 MWh of electricity.

(Story repurposed with permission of Oil and Gas Journal).