Days 77-83: getting physical

I mentioned earlier that one of my favorite classes is the course on surface geothermal exploration.  I thought I’d share a little bit of why.

Prospecting for geothermal energy shares some of the same uncertainty and risk as prospecting for oil.  You’re looking for a reservoir of valuable fluid buried underneath miles of rock and dirt, and it will cost you a lot of money to get at it.

0 - early drill
Geothermal drilling operations. Note the truck on the left of the photo for scale. Image credit: Orkuveita Reykjavíkur.

If you’re trying to use geothermal power to produce electricity, you want your wells to produce as much high-pressure, high-temperature steam as you can get.  Volume, temperature, and pressure are interrelated in a system and are collectively referred to as enthalpy, a word that sounds very much like “entropy” but means not at all the same thing.  If you want to produce electricity, high enthalpy is good.  High entropy, not so much.

Using geothermal power to produce electricity means taking the steam and using it to spin a turbine.  There are different strategies for placing the turbines: you might, for example, put a relatively small turbine – one capable of generating 3-5 million watts (megawatts, usually abbreviated “MW”) on or next to the well.  These wellhead generators are produced by several different companies: one of them is here in Iceland.  Green Energy Geothermal (GEG) makes modular wellhead generators that are easily transported in standard 20-foot shipping containers and assembled on site.  There are dozens of GEG-produced wellhead generators currently in use at the Olkaria geothermal field in Kenya.  They’re relatively inexpensive (about $2 million USD per MW of capacity) and easy to operate.

Cover Art
Wellhead generator schematic. Image credit: Green Energy Geothermal.
Wellhead generators in Olkaria
Wellhead generators in operation at the Olkaria geothermal field in Kenya. Image credit: Green Energy Geothermal.


If you want a larger capacity power plant, you’ll probably need to gather the steam from several wells and use them to spin bigger turbines, like the 30 MW turbines at the Svartsengi Power Station near the international airport in Iceland, or the 45 MW turbines used in the country’s largest geothermal plant,Hellisheiði, located about 30 minutes’ drive from the capital.  For larger projects like these, a well is considered successful when it produces steam that can generate 7 MW of electricity or more.

Pipes bearing steam and geothermal fluid to Hellisheiði. Image credit: author.
Svartsengi power station turbine. Image credit: author.

Drilling a geothermal well can easily cost upwards of $3-5 million US dollars.  The exact price depends on many factors, including how far you have to drill and the sort of material you’re drilling through.  At those prices, a few unproductive wells can make a profitable enterprise into a money-losing disaster … and a large plant requires several dozen wells.  Ideally, you want all of them to be successful – but you can’t measure their steam output until after they’re drilled.

So that’s the challenge:  how do you determine whether a well will be successful before you start drilling?  To get what you want, it helps to know what you need.  We’ll look at that in my next post.

3 thoughts on “Days 77-83: getting physical

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