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I recently read on another SE site that there's a limitation on how deep we can dig into the earth.

A Google search reveals that temperature is the problem.

I quote from a Popular Mechanics article which speaks on the Kola Superdeep Borehole

The limiting factor is almost always the temperature. The Kola was abandoned in 1992 when workers hit 356 degrees Fahrenheit, far hotter than they had anticipated. The tunnel was eventually closed off in the early 2000's.

356$^\circ$F is 180$^\circ$C, for those of you who don't get Fahrenheit, like me.

Iron's melting point is well above 1000$^\circ$C, and I think it is okay to assume most of our machines have iron as a major constituent. Alloys with iron will only have higher melting points.

Why is 180$^\circ$C a problem? Why can't we dig deeper?

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    $\begingroup$ I read the article, I believe the temperature issue has been mis-entered because they seem low to me, you could check out the Geothermal Gradient. On the other hand when metals get closer to their melting points their properties change. Finally the temperature issue might not be due to the metal but instead due to trying to deal with superheated water associated with the "mud" they run through the drill to lubricate and remove the waste from the hole. $\endgroup$ – Friddy May 22 at 16:23
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    $\begingroup$ I think it's not so much a matter of "can't", as it is "it would cost more than we want to spend". Iron may melt above 1000∘C, but it softens at much lower temperatures. The drilling bits (at least for conventional oil well drilling) aren't simply solid blocks. They have moving parts that might be adversely affected by thermal expansion, the cutting parts are often things like tungsten carbide or diamond, &c en.wikipedia.org/wiki/Drill_bit_(well) $\endgroup$ – jamesqf May 22 at 18:23
  • $\begingroup$ A quibble about terminology. The deepest humans have dug is 3900 m at the Tau Tona Mine in South Africa. The longest hole drilled is supposedly the Kola Super Deep Hole. I can vaguely recall reading about a hole drilled later that was longer, but I have forgotten the details. Given that such long holes are never vertical for their entire length - they all bend eventually. I question whether the stated 12 262 m depth of the hole is actually its ** length**. If so, being a bent/curved hole would reduce it depth. $\endgroup$ – Fred May 22 at 19:14
  • $\begingroup$ @Fred I meant by all means "drill". As for the hole bending, it certainly might be possible. $\endgroup$ – William R. Ebenezer May 22 at 19:22
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    $\begingroup$ nature.com/articles/d41586-019-01551-6 - a very relevant recent article $\endgroup$ – Gimelist May 30 at 11:06
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Here is a thermal profile of the borehole:

(Source: Fig 5 in Yuri A. Popov, Sergei L. Pevzner, Vyacheslav P. Pimenov, Raisa A. Romushkevich, New geothermal data from the Kola superdeep well SG-3, Tectonophysics, Volume 306, Issues 3–4, 1999, Pages 345-366, ISSN 0040-1951, https://doi.org/10.1016/S0040-1951(99)00065-7.)

Figure 5: Thermal Profile of Kola Super Deep

This borehole is (almost) vertical and not an angle, so its depth is true (11662 m) and it is quite warm down there, around 180 degrees Celsius as shown in the above figure. I assume temperatures were similar in other shafts drilled at maximum depth at 12262 m.

The equipment they used initially for the 1982 borehole down to 11662 m was made to be able to work in thermal stability up to 200 degrees Celsius (Source: Kozlovsky, Yevgeny A., and N. I. Adrianov, eds. The superdeep well of the Kola Peninsula. Springer Berlin Heidelberg, 1987.) Consequently I assume the equipment used for the later shaft that reached 12262 m was of similar or better make than the original.

The problem is not the temperature against the barrel only: there is heat due to the shear (rotation) and let's not forget the weight of the shaft down the base. This is why the thermal stability of the system is to be considered in addition to the resistance of single parts of the drill system. So while hotter than they expected, it was within the operating range conform to the specifications of the make - but yet very near the thresholds and thus problematic.

The problem lied also with the rock being more and more plastic deep down in the borehole, due to pressure, porosity and water, making it a very difficult challenge to keep the borehole open each time the shaft need to be replaced or removed, adding difficulties to this already never-been-done-before project.

So this sum of challenges are probable reason why the project was interrupted before reaching the initial goal of 15 km.

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