Introduction to Geothermal Drilling

When deciding whether or not a geothermal system is right for your home, a major factor that must be a consideration is the issue of geothermal drilling. In order for geothermal to work, a hole (typically in the 6” diameter range, more on this later) must be drilled into the earth for quite some depth. This might not sound like a large hole because, quite frankly, it’s not. However, it takes major equipment to bore the hole. The machines that do this are called geothermal drill rigs.

Geothermal drill rigs are vehicles that are approximately the size of concrete trucks. The difference is that the back of the rig holds a gear box equipped with many ~30 ft. cutting shafts. This gear box can be tilted perpendicular to the ground so it can make a straight boring. A cutting head will be attached to the bottom of the first cutting shaft. It will then be drilled into the ground. Drilling will cease for about five minutes. During this time, the first shaft will be disconnected, and the gear box will rotate a new shaft in to place. The (second) shaft will be attached to the original shaft, and drilling will continue. The process of disconnecting and reconnecting new shafts will continue until the appropriate depth is drilled. The operation will be reversed so the rig retrieves all the shafts in the ground and the gear box finishes in the original position.

If this isn’t making sense, please oblige as I use an analogy. Think of using a screwdriver to drive a screw into a piece of wood. You would twist the screwdriver and the screw would begin penetrating the piece of wood. However, if this screw had to go several feet into the piece of wood, imagine taking an extension bit and putting it on the screwdriver. The screwdriver is still doing the “work” but the extra extension bits allow it to go deeper than normal. Geothermal drilling is basically an extremely large version of a screwdriver attached to a large truck.

How Deep Are Geothermal Wells?

Geothermal Wells are typically anywhere from 150 feet deep to 400 feet deep. Some drilling companies have equipment that can drill wells deeper than 600 feet, but they are not typical. In fact, wells deeper than 400 feet are typically only used on larger commercial project sites and are carefully evaluated before the building is constructed. The deeper the well, the more other things come in to play (such as subsurface conditions, earth type, etc). Geothermal drilling equipment typically does not drill any less than 150 feet, as this can be done by traditional potable water well drill rigs.

As a rule of thumb, assume that 200 feet of well depth can provide you with 500 square feet of HVAC in your home. If your home is 1500 square feet, a good rule of thumb is to assume you’ll need 2 separate wells, each 300 feet deep. As I mentioned, this is simply a rule of thumb. You would need to conduct a proper HVAC heating and cooling load calculation on the building in question and also conduct a thermal conductivity test either on your site or see reports from nearby bores.

What is the Cost of a Geothermal Well?

One of the biggest “con” of geothermal is the price (although it’s cheaper long term!). This is the same of most newer technologies regardless of the field you’re in. A rule of thumb for drilling a geothermal well in the United states is $5,000 per well. Shorter wells will be a little less, deeper a little more. The large cost is what you pay to get the drill rig on site in the first place. The “mobilization” fee is why the per well cost is so high. Geothermal drilling is becoming more popular, but it’s unlikely that you will have several companies in your city. Thus, the cost will be higher if the driller will have to travel a long distance to get to your project site.

If you are talking about drilling a larger project (greater than 10 wells) your “per well” cost will go down. In cases like this, the driller is likely to make one single trip to the site and park the rig on site overnight. It is not uncommon to see costs closer to $3,500 per well on larger projects. 

For a typical home, the $5,000 per well figure would include everything OUTSIDE your home. This would be drilling the well, installing the geothermal pipe, backfilling the well, and filling it with water. The major cost this quote does not include is the geothermal heat pump and the connected ductwork. 

Wrap Up

Geothermal Drilling is quite an interesting process all things considered. To think that a piece of equipment capable of drilling 600 feet into the earth can drive beside you on a standard interstate road is pretty cool. It is a costly operation, but with proper sizing of your well field (depth and number of wells) it is an essential part of choosing the right geothermal system appropriate for you to achieve long term energy savings.