How deep does geothermal borehole drilling go?

In recent years, geothermal borehole drilling has become an increasingly common requirement in both domestic and commercial-scale projects.

With ground source heat pumps in particular, these infrastructural investments can feel quite substantial, and a lot of people will have a few questions about how things are expected to go. 

Here, we take a look at geothermal borehole depth in particular, and answer your most common questions as we go. 

How deep do they go?

While the precise depth of a borehole will vary depending on a range of factors (that we’ll explore later on), most domestic options will typically be between 50 and 200 metres in depth. For commercial boreholes, some can go a little deeper, but for most purposes, drilling beyond 200 metres tends to become inefficient.

Where additional energy is required, rather than drilling deeper, it’s normally the case that additional holes will be drilled. This allows for additional heat extraction without having to bring in additional equipment to facilitate deeper drilling. 

How wide are they?

These boreholes don’t typically need to be that wide – just enough to pump liquid around the loop system once it’s been installed. In most cases, around 150 mm, or 6 inches, is enough. 

Using drills of this diameter, it typically doesn’t take more than a day or two to drill the boreholes. 

What actually is a borehole?

Knowing what a geothermal borehole is and roughly how they work helps to understand why they’re dug to the depths that they are. These boreholes are drilled so that loops of piping can be installed into them, with fluid then being circulated through that loop. 

Because the temperature of the ground below the surface – typically below about 15 metres – remains at around 10-12 centigrade throughout the year, the liquid that’s pumped through the loop can absorb that heat and bring it up to the heat pump at ground level. 

What affects the depth of the borehole?

There are a number of factors impacting borehole depth, including:

• Heat requirements: Larger buildings will typically require more energy, in turn necessitating deeper boreholes.
• Geology: Different rocks have varying levels of thermal conductivity. This means that geological factors can also affect the depth of the borehole. 
• Land availability: If a property has high heat requirements, but not enough land available to drill multiple, shallow boreholes, then it can be necessary to drill deeper.

You’ll likely find out all of these variables when you have your initial geothermal surveys carried out.

Site assessment

All of these projects will start with a thorough set of surveys and assessments. Technicians will come to your property and determine your property size, energy efficiency, available drilling sites and surrounding geology, before making a more specific, informed plan on what steps to take going forward. 

Geothermal boreholes can be excellent additions to your property, providing you with sustainable, affordable sources of energy. It’s surprisingly simple to have them installed, and if you have any questions, it’s absolutely worth reaching out for more information.