[nucleative]

Why is the cost of drilling so high?

I'm sure the rigs are an expensive capital outlay, but do the operators require advanced skills?

Once a hole is drilled, does it have a limited lifespan, or is that investment something that can be hypothetically used for eternity?

[Bedon292]

Drilling to the depth needed to get high enough temperatures for energy generation can be hard. Depending on how active the area is. Iceland, which has tons of heat close to the surface can do it with relative ease. But elsewhere you may have to be drilling far deeper through much harder rock to get there. In some locations you may have to drill down many km at 10s of meters per day through the hardest rocks. It could potentially take months to hit the right depth.

You also need materials to ensure that the well doesn't collapse on itself, and stuff like that as well. I would expect the hole itself should have an extremely long lifespan, but I don't really have much to back it up.

Its not for electric generation, but for Geo HVAC the ground loop itself has a 50 year warranty and quoted 150+ year lifespan. The house will probably be long gone before the loop would go bad.

[jofer]

The operators very very very very much require advanced skills. Don't confuse roughneck work with drilling engineers. Drilling engineers also aren't by any means the only advanced skillset required. It requires a large team of highly educated (postgraduate degrees usually required) and highly paid professionals.

The materials you're using when drilling are expensive. The steel casings that you're putting down the hole are specialized alloys. You're using very large amounts of expensive specialized fluids to fill the hole during drilling (drilling mud) that are relatively hazardous and need to be treated after use. You're going through large numbers of very expensive consumable parts (e.g. drill bits are _not_ cheap, and there are a lot of even more expensive consumables). A large amount of the equipment and consumables are custom built for the specific conditions you're working in. There are certainly cases where you can use things that are a bit more of a commodity, but deep and hot wells usually don't fall into that category. And yes, there are a ton of companies that specialize in making all of these things either en masse or custom built to order, but they're still expensive. You're also going to need to run logs (i.e. downhole measurements) using a wide variety of very expensive and very specialized tools.

It's not just a hole. You're also building a subsurface structure and collecting information about the subsurface.

Any well has a limited lifespan. Geothermal wells aren't that different from hydrocarbon wells in this sense. You're not actually able to extract heat from the bulk of the rock mass. Rocks are _very_ poor conductors of heat. So you're extracting heat from relatively tiny depth within the rock mass along the surface of a pore network. You're flowing water from one well (an injector) to another well (a producer). As that flows and temperatures and water chemistry change, various minerals precipitate out and block the pore throats. Even in cases where you can avoid those reactions, you eventually block different pores and fracture networks with debris. You can and do use techniques to restore that (aka "workovers"). However, they are expensive and have diminishing returns. And even if none of the before-mentioned things happen, you'll eventually hit diminishing returns on the temperature you can produce, as the heat can't migrate from the bulk of the rock to the water-rock interface very quickly.

Either way, eventually you drill new injectors and producers. The cycle you need to do that on varies quite a bit depending on the specifics of the project. In some cases, it's a couple years, in others it's a decade or two. Either way, you need continued redevelopment plans to continue production.

All of that is not a problem and is work as normal. I'm just trying to lay out what subsurface projects entail (be they geothermal, oil, methane, or even large scale water production). It doesn't mean geothermal is impractical or overly expensive (far from it). But it does mean that there are very large initial captial expenses and then periodic capital expenses needed at later dates to continue optimal operation.