What is the Deepest Hole Ever Drilled?

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Russian stamp depicting deep hole in the Earth

Over 40 years ago, researchers in the Soviet Union began an ambitious drilling project whose goal was to penetrate the Earth’s upper crust and sample the warm, mysterious area where the crust and the mantle intermingle- the “Moho”. The deepest hole ever drilled by humans (Russians) is the “Kola Super deep Borehole”, on Kola Peninsula in the North West corner of Russia, located near Finland”. Russian scientists had to invent new ways of drilling, due to the depth of that area. A number of bore holes were drilled by branching from a central hole. The bore hole which reached 12,262 meters (40,230 ft) in 1989 is the deepest hole ever drilled, and the deepest artificial point on the earth. For two decades, it was also the world’s longest borehole, in terms of measured depth along the well bore, until surpassed in 2008 by 12,289 (40,318 ft) long Al Shaheen oil well in Qatar, and in 2011 by 12,342 meters (40,502ft) long Sakhalin-1 project.

 

Did Russia reach the Initial target depth?

The initial target depth was set at 49,000 ft. On 6th June 1979, the world depth record held by the Bertha Rogers hole in Oklahoma, at 31,440 ft was broken. In 1983, the drill passed 39,000 ft. in 27th September 1984, after drilling to 39,587 ft, drill string twisted off and was left in the hole. Drilling was restarted later and the hole reached 40,230 ft in 1989. By the end of 1990-1993, the depth of the hole was expected to reach 49,000 ft. However, because of the higher than expected temperatures, at this depth and location, drilling deeper was deemed unfeasible and the drilling was stopped in 1992. If the bore hole has extended down to 15 km, the projected temperature would have been 300 degree centigrade, well over the maximum operating temperature of the drill bit. Even though the borehole in question was 7.6 miles deep, it only penetrated a third of the Baltic continental crust, the layer of igneous, sedimentary rocks which form the continents and the areas of shallow sea beds close to their shores. The rock bottom of the hole was about 2.7 billion years old on a 4.6 billion year old planet, and samples brought from near the bottom contributed invaluably to the study of geology and geophysics at that time.

 

What were some interesting things about the Russian hole?

  • To scientists, one interesting findings from this well is that the change in seismic velocities was not found at a boundary marking Harold Jefferys, a mathematician’s hypothetical transition from granite to basalt (volcanic rock). One interesting thing found along the way was a fractured area of rock 3 to 6 miles beneath the surface that had been thoroughly saturated with water. This occurrence has never been observed any where. The most likely means that water, which was part of the chemical composition of the rock minerals, had been forced out of the crystals and could ascend due to an overlying cap of impermeable rock.
  • Another unexpected discovery was the large quantity of hydrogen gas, with the mud flowing out of the hole described as “boiling” with hydrogen.

 

What is the Longest Bore Hole Record?

  • The deep core-drilled Kola borehole cannot be directly compared with any other borehole in the world. As a deep core borehole a section of the rock drilled through is recovered from within the hole. It is this recovered drill core and the information it represents about the rock that was drilled through that is the most important feature of the borehole.
  • While the Kola Super deep Bore hole has not only been the deepest, but also the longest borehole in the world for years, in May 2008, a new record of length has been established by extended reach drilling (ERD) well drilled by Transocean for Maersk oil in the Al Shaheen oil field in Qatar. It was drilled to 40, 318 ft in only 36 days.
  • In January 28, 2011, Exxon Neftegas Ltd., operator of the Sakhalin-1 project, drilled the world’s largest extended reach well offshore on the Russian island of Sakhalin. It has surpassed the length of both Al Shaheen well and the Kola borehole.
  • By actual depth, the Kola Super deep Borehole still remains the world leader.
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