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The legendary Kola superdeep borehole is about 12 km deep. To achieve such depth, a major difficulty is to make sure the borehole is vertical straight.

Intuitively, when the borehole is deep enough, it is very hard to make sure that the drilling bit is still moving downwards. Suppose it hits some hard rock, it can be easily deflected.

So, how straight is the Kola borehole? What technique is invoked in drilling straight downwards?

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    $\begingroup$ When you ask "what technique is invoked in drilling straight" do mean was invoked at the time of drilling the Kola deep hole or what is now available? Drilling techniques have advance much since the Kola deep hole was drilled. $\endgroup$ – Fred Feb 1 '17 at 6:23
  • $\begingroup$ @Fred I mean both. $\endgroup$ – S. Kohn Feb 1 '17 at 9:51
  • $\begingroup$ @Fred Can you suggest any book on drilling? $\endgroup$ – S. Kohn Feb 1 '17 at 9:56
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Borehole drifted from straight vertical by ~840 meters over 12,000 meters. All drill-bore wander side to side some and Kola borehole did not drift much until depth exceeded 5000 meters.

See page 102 in the linked document.

Typical rotatory drill heads designed for vertical drilling can be steer to a small degree by using a simple concept: point the bit in the direction that one wants to drill. A common way to achieve this is by the use of a bend near the bit in a down hole steerable mud motor. the bend points the bit in a direction different from the axis of the well bore.

By spinning the drill head at a different rate vs the drill stem, allows the bit to drill in the direction it points.

Reference: PDF document gives an excellent technical summary of the project.

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  • $\begingroup$ Can you give any more detail in this answer, rather than relying on the link? For example, for a layperson like me it's not clear what "vertical variance" is. Is this a margin of error? And, can you give any info on the other half of the question, of how drilling was directed? $\endgroup$ – Semidiurnal Simon Feb 4 '17 at 6:48

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