Drilling a geothermal well is much like drilling any other kind of well. Each well comes with its 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 and well casing process.
Before drilling can begin, a rig must be selected that is appropriate to the geological conditions being targeted, quiet so that it does not disturb our neighbours and very safe to operate. In Cornwall, to reach temperatures of around 180oc the rig must be capable of drilling to approximately 6,000m (over 3 miles!). This means that the rig needs to be tall and strong enough to support the weight of a 3-miles steel drill pipe, as well as drill bits, steering mechanism and stabilisers.
As drilling technology improves, rigs will become smaller and more efficient. The rig used at United Downs was the Innovarig, it was designed to be quiet and compact for use in urban environments.
The Drilling Process
Drill Bits
There are many different types of drilling methods, some of which use rotating bits that grind through the rock and others use percussion or hammering to break the rock.
To penetrate the hard granite, quartz, and other solid mineral deposits found under Cornwall, rotating tricone drill bits made of steel are used. These tricone’s are covered in “teeth” or “buttons” which are inserts made of tungsten carbide, a hard-wearing alloy made of tungsten and carbon. As the drill bit rotates, each of the three cones turn, grinding the rock into small pieces.
The production well will use four sizes of drill bits. The wells start off being drilled with a 24″ bit, when the well has been drilled to the desired depth at this size, the drill bit is removed from the well to allow steel casing to be inserted and cemented into place. The 17.1/2″ size drill bit is then used, the well is again steel cased before the 12 1/4″ bit is used then cased, the 8.1/2″ bits are used to drill the open hole section of the well. The completed production well will have 3 layers of steel casing. The injection well is drilled using just the 3 smaller sizes of drill bit and will, therefore, have 2 layers of steel casing.
Both wells are drilled vertically to begin with into the solid granite, they then deviate into a J shape to cut across the fractured zone. The end of the wells at United Downs are 800m away from the well heads on the GEL site.
Geothermal Wells
A standard geothermal system is made up 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 fracture zone in the open hole section and travel upwards to the surface within the cased sections of the well. Although the geothermal fluid is under pressure, it is not enough to push the fluid to the surface, so an Electrical Submersible Pump (ESP) will be inserted into the well around 1km below the surface to pump the geothermal fluid to the top of the well. Once the fluid has been used at the surface, it will then be reinjected through the injection well. The pressure in the system is enough to force the cooled fluid down the injection well. Once each well has been completed a wellhead is installed ready for well testing and connecting to the power plant.
Geothermal wells are regulated in the same way as oil and gas wells by the North Sea Transition Authority and the Health & Safety Executive (HSE). GEL 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.
