[anakaine]

From the thesis: https://www.proquest.com/openview/624989df3cdd8055a6cee9affc...

"For the application in EGS drilling, this device uses a metallic waveguide to carry the millimeter wave (MMW) beam to a standoff distance from the crystalline rock. Argon gas is used as the waveguide fill medium due to its ability to stay transparent to MMW’s at such deep depths and thus higher pressures [12]. Purge gas is also used to pump out the excess material that has been transformed into smaller particles (Figure 2.4). "

As a former geologist involved in drilling, thats going to get real expensive, real fast, in terms relative to regular mechanical drilling thanks to the requirement for argon. Perhaps theres an economically efficient changeover point at depth as mechanical drilling becomes less capable due to increasingly plastic deformation.

[MadnessASAP]

You don't need a significant flow of argon, just enough to keep unwanted gasses out of the waveguide.

It's possible there exists a material that is transparent to mm waves, airtight, and can survive the conditions at the bottom of the hole. In such a case they could cap the waveguide and prevent any gas leakage.

I'm quite sure Quaise is well aware that Argon isn't cheap and are already exploring multiple avenues for reducing its usage.

It is interesting that they have to use Argon instead of the more typical Nitrogen or SF6. A waveguide with such a significant pressure differential is decidedly unusual and a unique challenger for what they are doing.

[pfdietz]

SF6 is a horrifically powerful greenhouse gas, so I doubt it could be used. Its GWP is somewhere around 23,000 on a 100 year timescale.

[MadnessASAP]

Oh yeah, there's no shortage of reasons not to use SF6. Even in conventional waveguides, as far as I know most designs these days prefer nitrogen or dried atmospheric air.

[audunw]

There is definitely an economic changeover point, I’m sure I read they will use conventional drilling down to a certain depth, before switching to MMW

I doubt argon is the purge gas.

[tekacs]

Yeah, they say in their launch video for Project Obsidian (https://www.youtube.com/watch?v=xmrna_r_b3A) that they'll drill the first 3km using conventional rotary drilling and mmWave beyond that.

I'd be curious if anyone (perhaps the parent) knows why – my assumption is that it's more expensive and/or not as reliable to drill higher up with mmWave, not least because the ground might be uneven materials, etc., and then it becomes something predictable and harder to rotary drill lower down, incl. as you would spend more time doing things like replacing bits low down and sending things up and down?

[tomtom1337]

You mean the argon gas used as medium specifically? I assume the purge gas is something else, cheaper?

[anakaine]

If the goal is to simply purge the content of the hole, compressed air is typically sufficient. That said, the wider the hole, and the deeper it is, the harder it is to lift material on air.

To be clear though, I'd love to have one of these rigs on my old project and compare rate of progression and hole quality. Particularly when establishing the hole in sedimentary gravels and clays. I imagine casing will still be required.

One thing that I'd be concerned about is the ability to collect samples if most of the material is being vaporised or melted. Similarly, the cooking of the side of the hole on the way down could make geophysical responses much more difficult to interpret. Sonic velocity would probably increase, televised would probably be harder to interpret, harder to spot hydrothermal infill in sedimentary cover, would it affect gamma tools (probably not)

Edit: also wondering how the hole holds up around aquifers. Does the super heating cause wall instability immediately above the non geothermal aquifers as superheated steam is created? How does this affect the hole stability if we are not casing?

Edit 2: if we are not casing, how does the hole hold up around aquifer sands, loose fill, fractured or brecciated mass?

Edit 3: Also! Do we ream open the top of the hole to down past the last aquifers before the geothermal horizon? If not, how are we stopping stopping aquifers interplay and interaquifer contamination?

[fleetwood]

i think they plan to drill with a traditional rig until they get deep/hot enough to necessitate a switch to mm wave

[tomtom1337]

Great response! I'm just a layman here (former material scientist) but it's fun to think about this stuff!

[mzhaase]

Maybe you could hook up a mass spectrometer to the purge gas to get real time composition.

[nerdsniper]

perhaps, but usually things like "which fossil species are present" are also utilized to figure out what's going on near the drill bit, like if you're trying to reach oil deposits right along the edge of an old riverbed.

Some shale formations in Michigan, for example, sometimes requires drilling to a 4" thick target. You don't know the exact depth because the depth of that 4" thick layer can vary by many feet from an another spot 100m north/south.

I'm aware that if you search "thickness of Antrim shale" or "thickness of Collingswood shale", Google will happily tell you that it's 20-40 feet thick, but for modern drilling techniques, the economics of the well depend on hitting a much more narrow target than that, which can be delicately guided in by analyzing fossils that come up.

[westurner]

Why mmwave instead of ultrasonic? FWIU 28 kHz shreds the quartzite in granite?

[hangonhn]

Isn't the idea here to gasify the rock by essentially microwaving it? With ultrasound, wouldn't you still need to remove the leftover rocks?

[saltcured]

Naively, I wonder how much the density of argon gas helps here, in terms of being able to recover and reuse the argon gas in a relatively closed-loop system.