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Comment by bhauth2 | Hacker News Reader

The article you linked is about conventional geothermal wells, which drill into reservoirs of hot water; they just have hotter water than has been typical. Extracting hot water is not limited by thermal conductivity of rock, but what Quaise plans to do is.

Enhanced geothermal involves fracking. Typical proposals involve creating crack paths between 2 nearby wells by fracking from both. It's been tested some but so far has not been economical.

Apart from the cost issues of enhanced geothermal so far, Quaise plans to drill deeper to higher temperatures to reduce power block costs. Sufficiently hot rock flows a little bit which makes fracking ineffective. Fracking also doesn't work as well with supercritical water. (If not drilling to rock hot enough to flow a little bit under high pressures, it would be much better to use conventional drilling techniques.)

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Thanks for the reply!

I'm afraid I'm not really following the argument though. I don't have the technical background to judge how economical EGS is or not, I just want to understand this energy-based argument. I checked to be sure, and Quaise's initial plans (per cofounder Matt Houde) are indeed EGS-based[1], just deeper. It seems correct to me that if they succeeded at building wells that way, they would produce at least comparable energy to standard wells.

It's entirely possible that EGS just doesn't work at really deep depths as you state here, but this seems like a qualitatively different argument to the one presented in the article.

[1] https://youtu.be/yz6rRw59Huw?t=675 "but what we're interested in in Quaise is this novel idea here all the way on the right which we call like to call superhot rock EGS systems"

Actually, they address the energy balance question at the end of that talk.

https://youtu.be/yz6rRw59Huw?t=3016 "[...] we could be using around five megawatts for the drilling process to drill our wells, and let's say we use that five megawatts over a year to drill three holes, so we get an injector and two producers. We predict that configuration of the two producers and an injector at superhot conditions can produce something around 50 to 100 megawatts of electrical energy, again owing to the benefits of producing this superhot, supercritical steam."

They also answer a question on borehole stability, admittedly claiming largely that they don't know.

https://youtu.be/yz6rRw59Huw?t=3254

> It's entirely possible that EGS just doesn't work at really deep depths as you state here, but this seems like a qualitatively different argument to the one presented in the article.

A different argument to the one presented in the article, you say. Huh.

I suppose I can't claim to know more about geothermal than the author of that blog post, but if you check again, you'll find it does mention EGS. Apparently something made the author decide the problems with Quaise using that are non-obvious enough to need explanation.

cf. "but I wish they finished writing the equations out"

The article mentions EGS but, as far as I could tell, only seemed to present it to contrast it with the claim that Quaise is using a worse single-bore strategy.

If I'm just misreading, and it sounds like you're saying I am?, it'd be really helpful to show your working so I can see where the models are differing. There is a factor 100 difference somewhere, it shouldn't be that hard to spot!

This has been amusing and all, but I'm the author of that blog post, and since arguments in a mere comment aren't as reputable I'd updated the post.

Thanks, that makes the position much clearer.

Some questions:

You say, “if there's enough pressure to make a little crack, then the fluid can instantly expand and immediately make a big crack”. My admittedly quite surface level view of the research is that it is viewed as feasible in this regime.

“Close to the brittle-ductile transition conditions of pressure and temperature, new findings suggest that fractures are sufficiently permeable to allow fluid circulation and, in case of insufficient fracture density, enhancement strategies are likely to be successful”

https://www.nature.com/articles/s41467-019-12146-0

I also believe that EGS fracture enhancement marginally prefers hydro-shearing (crack expansion) rather than hydro-fracking (crack formation), so if creating cracks is problematic, that leaves options open.

I also note that to my understanding gas fracking is already a thing, cf. nitrogen fracking. So gaseous behavior doesn't obviously seem like an instant write-off to me.

I'm sure that isn't convincing to you, but it's a bit challenging for me as a layman wrt. geothermal to see why I should trust your gut here, so to speak, and I'm wondering if you have a concrete argument I can evaluate on merits?

For example,

"The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10−16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT."

https://www.nature.com/articles/ngeo2879

I'm definitely not claiming to take these on faith, I'm just saying I haven't really been given a reason to believe those arguments are less trustworthy than your claims otherwise.

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