How straight is the Kola superdeep borehole?

Question

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?

Answer 1

So, how straight is the Kola borehole?

The Kola well drifted from the vertical by an average of five degrees down its passage to a maximum of 17 degrees at 10,000 to 10,500 meters; at that point, the drill bit was 840 meters to one side of the wellhead. The lighter weight of the drill string decreased the wear on the string itself, and on the casing and wall of the well brought about by friction; considerable wear caused the well to drift as drilling proceeded.

What technique is invoked in drilling straight downwards?

A reducing gear, under a bottom-hole turbine in the drill string, lowered the rotation of the bit three to four times below that of drills in conventional wells, thereby increasing its rotation momentum. A hydraulic feedback line in the pumping system, transmitting pressure surges in the drilling mud, controlled the rotation of the turbine and the string, and therefore the speed and moment of the drill bit.