stoicio's Journal: Drilling Holes in Earths Crust for Geothermal Energy

Current approaches to drilling into the Earth seem to center on duplication of technology used to drill for water or oil.
      Drilling into the Earth to produce
geothermal energy only requires that the material produced by drilling may be removed from the hole at a fast enough rate to allow drilling to be maintained.
      One of the main concerns of making bore holes into the Earth is the amount of torque required to produce the hole. It must be kept in mind that the average thickness of the earths crust is over 30Km. The thinnest parts of the crust are in the oceans near trenches (~20Km), continents are the thickest (~40Km).
      It's necessary to have power where population requires it. This means that most geothermal drills will be near the coasts or perhaps drilled the full 40+ Km depth near to inland cities.
      Drilling holes with conventional drilling equipment is possible but not likely. New drilling equipment is required to drill these massively deep holes.
      One idea is to use a smaller version of the machine used to create the chunnel from England to France. This machine was designed to cut through limestone (chalk) which is very soft compared to most rock. Certainly a similar design could be made to produce, smaller diameter, vertical bore holes. Guided by gravitational instruments this machine could move, nearly autonomously, downward, requiring only power to be supplied. This would also resolve the torque problem, since the torsion aspects of the machine are only the length of the machine itself.

      At this point the greatest problem would become the removal of drilled material from the bore hole.
      Pumping the ground up material, as mud, would provide the easiest method for removal.

      Consider the amount of material being removed from a 2 meter diameter bore, 40Km into the Earth. Using the formula for the volume of a cylinder and assuming the material is dried:

Volume = ((Pi*Radius)^2)*40000

Volume = 394784 Cubic Meters of material.

      This is a cube of material approximately 74 meters on a side that must be hauled away and dumped. This material could be used a construction material or to level and stabilize lands elsewhere.
    The calculated volume is optimistic. This doesn't take into account ground water or other fluids that may enter he drill hole and mix with the material to make mud. Some water may be added to aid in lubrication of the drill.