United Downs Deep Geothermal Power Project

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The United Downs Deep Geothermal Power (UDDGP) project is the first geothermal power plant in the UK. It is funded by a mixture of public and private funds to include the European Regional Development Fund, Cornwall Council and Thrive Renewables plc.

The aim of the project is to produce power and heat from the hot granite rocks beneath Cornwall at the United Downs Industrial Site near Redruth.

Two deep, directional wells have successfully been drilled; the production well to a depth of 5275m and the injection well to 2393m. Both wells have intersected the target Porthtowan Fault Zone located approximately 800m to the west of the site.

Cornish Geothermal Drill Rig

Project Timeline


The heat generated from the Earth is far greater than that required by the world’s population. It is also constant and can therefore produce clean renewable power and heat regardless of the weather conditions at surface.

UD Site Acquisition & Planning

The site was selected for the excellent geology, road access for the drilling rig, the connection to the grid and for being within an industrial landscape.

Secure Funding

It took 5 years to secure appropriate funding for the project from a combination of the European Regional Development Fund, Cornwall Council & Private Investors.

2012 - 2016
Funding Agreements Signed

Initial funding for the project was achieved from the European Regional Development Fund (ERDF), Cornwall Council and Thrive Renewables.

Procurement & Drilling

All contracts were tendered and awarded using European guidelines to ensure that they were awarded fairly and transparently.

Two Geothermal Wells Completed

The Innovarig used 38 drill bits to drill the wells. The production well is the deepest on UK soil at 5275m and the injection well is 2393m deep.

Final Reservoir Testing

A smaller rig was used to complete the wellhead and enable test equipment to be used to take core samples, temperature and flow test. Final production testing successfully produced the UK's first geothermal steam in July 2021.

Power Plant Design & Construction

A ‘Binary’ power plant will be used. This means that there are no ‘plumes’ of vapour and very little to see on the surface when the plant is finished.

The plant design is currently being finalised and construction is expected to start in 2023.



Drilling rig

The Innovarig is a new generation semi-automatic hydraulic rig constructed in 2007 by Herrenknecht vertical GmbH. It is the quietest rig of its size in Europe. Specifically designed for use in urban and noise sensitive areas.

Innovarig Technical Details: Height 51.8m, Maximum hook load 418 tonnes, Top drive 4,100 kN, Maximum trip speed 500 m/h, Drill pipe size 5 ½”.

Drill bits and casing spec

The drill bits were steel tri-cone with tungsten carbide inserts, Four sizes were used for the production well and three for the injection well. The wells are steel cased for most of their length with just the deepest section open to allow water to circulate.

Drill Bit Sizes
24”, 17 ½”, 12 ¼” & 8 ½”

Drill bits
Production Well Casing
Injection Well Casing

30” surface to 11m
18 5/8” surface to 245m
13 3/8” surface to 899m
Kick off point for directional drilling 3400m
9 5/8” surface to 3985m
8 ½” not cased to 5275m

30” surface to 11m
13 3/8” surface to 804m
Kick off point for directional drilling 1030m
9 5/8” surface to 1820m
8 ½” not cased to 2393m

Subsurface Analysis

0m – 210m

All metals and minerals ever mined were found in this zone, these include metals being explored today such as wolframite (tungsten ore) and cassiterite (tin ore).

Drilling fluid was lost in this section meaning we had intersected a fracture that was open. We know from mining records that there are many fractures known as cross courses above 1km, which is why mine shafts flood with warm water.

890m - 892m


An unexpected mineral lode was encountered, it was full of sulphide mineralisation like chalcopyrite, a yellow crystalline mineral consisting of a sulphide of copper and iron. It is the principal ore of copper with a hardness of 3.5 to 4 on the Mohs scale.
A curious pure white granite. This type of rock has been named ‘Granite C’ it only lasted for tens of metres before the rock type returned to a more typical heat producing granite.


4190m – 4300m

Throughout both wells many lodes with chlorite mineralisation were detected. Chlorite is the name of a group of common sheet silicate minerals that form during the early stages of metamorphism. Most are green in colour and found in igneous, metamorphic and sedimentary rocks.

The granite was very micaceous, which made the samples sparkle and shine. Any group of hydrous potassium, aluminium silicate minerals are called mica. This granite had a very high thorium (Th) content, which could help explain why the rocks are so hot at this depth.

4800m – 5275m

Binary power plant

The geothermal brine will not be fed directly into the turbine. Instead, the very hot brine passes through a heat exchanger to heat a secondary fluid that expands to drive the turbine (like steam) and is then condensed into a fluid again. The two circuits (geothermal and binary fluid) operate 24/7.

Binary Power Plant diagram

Natural and Induced Seismicity

GEL and seismicity

It’s not the unknown we fear, but what we think we know about the unknown

This short animation explains the difference between natural and induced seismicity, how GEL is working within guidelines and the measures being taken to reduce the possibility of induced seismicity.

Earthquakes in Cornwall

Seismicity can be complex and most people only know about the destruction of high magnitude earthquakes. But the UK experiences earthquakes frequently. Over 600 have been recorded in Cornwall since the 1980s. One potential cause of public concern with geothermal energy is the possibility of triggering small earthquakes.

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Natural or Induced

Induced seismic events are the same as natural ones, except that the trigger for the movement is human activity rather than a gradual build-up of geological pressure over time. In the case of geothermal projects, this can be caused by water pressure helping to unstick the rock along faults or small fractures. 

This is not the same process as Fracking where intact rock is hydraulically fractured.

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The strength of an earthquake is described by its magnitude. This is a logarithmic scale which means that a magnitude 2 is ten times bigger than a magnitude 1 and a magnitude 3 is ten times bigger than a magnitude 2 and so on. The largest earthquake ever recorded was in Chile in 1960, it had a magnitude of 9.5. This is about 30 million times bigger than a magnitude 2 event. 

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GEL has installed a network of seismometers that can detect earthquakes hundreds of times too small to be felt at the surface. GEL will automatically be alerted to seismic activity allowing mitigating action to be taken if necessary. Seismometers are also a valuable tool to help geologists and engineers understand the movement of water through the geothermal reservoir.

The British Geological Survey (BGS) report seismic activity on their websites for complete transparency and GEL will post seismicity information on social media.

If you have any concerns or question you should contact GEL using the information on the Contact Us page

There are two information sheets available for download below:

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Management and control of induced seismicity

UDDGP uses existing British Standards and Cornwall Council’s planning guidelines for blasting, quarrying and mining activity. Acceptable levels are based on how much ground vibration is measured, known as Peak Ground Velocity (PGV) rather than the magnitude of an induced seismic event. 

The GEL protocol uses PGV measurements to trigger changes in operational state. Typically, people cannot feel movements below 2mm/s but GEL will move from a ‘Normal’ to a ‘Caution’ operating state if any seismicity results in a PGV of more than 0.5mm/s as measured by the project’s seismometer network.

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