Geothermal Drilling

“Utilising Britain’s leading oil & gas drilling expertise for a low-carbon, geothermal future”

Drilling a geothermal well is much like drilling any other kind of well. Each well comes with it’s own unique set of challenges depending on the geology being drilled through and the infrastructure constraints at the surface. It is the art of the drilling manager and their expert team to guide a drilling rig safely through the drilling process.

Before drilling can begin, a rig must be selected that is appropriate to the geological conditions being targetted. In Cornwall, to reach temperatures of around 180oC the rig must be capable of drilling to approximately 5,000m (or 3 miles!). This means that the rig needs to be tall enough to support the weight of a 3 mile long steel drill string as well as drill bits and well casing. 

As drilling technology improves, rigs become ever smaller and quieter. The rig at United Downs reached 55m high and was designed to be quiet and compact for use close to populations. Several years on and rig technology continues to bring significant improvements in noise and scale.

The drilling rig at the United Downs geothermal project near Redruth, visible amongst the relics of past heavy industry.

The Drilling Process

Drill Bits

There are many different types of drilling method, some of which use rotating bits which grind through the rock and others use percussion or hammering to break the rock.

To penetrate the hard granite of Cornwall, rotating tricone bits made of steel were selected for United Downs. These bits are covered in “teeth” or “buttons” which are inserts made of tungsten carbide, a hard-wearing alloy made of tungsten and carbon.

The four types of drill bit used at United Downs. From left to right these are 24", 17.5", 12.25" and 8.5" bits.

Many different bits are used to drill a single well, particularly when drilling through very hard granite which wears the teeth down extremely quickly. In addition, in the near-surface section of the well, a larger bit is used compared to at the bottom of the well. At the surface, a larger well is required to stabilise the softer material with greater volumes of cement and steel casing, and it also provides more space to accommodate specialist equipment. However, a smaller bit is more manoeuvrable, allowing directional drilling to target a fault zone, and a smaller hole means less rock to remove and greater casing strength to combat the increased pressure at depth.

Geothermal Wells

A standard geothermal system is made of two wells, a production well and an injection well. This is called a Geothermal Doublet. The fluid will enter the production well from the rock formation in the open hole section and travel upwards to the surface within the cased sections of the well. Once the fluid has been used at the surface, it will then be reinjected through the injection well.

A geothermal well is very similar to an oil and gas well in construction, containing layers of casing and cement around open hole through which geothermal fluid will flow. As the wells are targeting hot water and not flammable gas or oil, there is far less danger of blowout or gas pockets.

However, geothermal wells are regulated in exactly the same way as oil and gas wells by the Oil & Gas Authority (OGA) and the Health & Safety Executive (HSE). We must also work closely with the Environment Agency (EA) to ensure there is no risk of pollution to the surrounding environment. This regulation ensures the safety and effective operation of all drilling and well activity at geothermal developments in the UK.

The wellheads of the geothermal doublet at United Downs, with the injection well in the foreground (UD-2) and the production well in the background (UD-1)
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