2013: Advent Oil & Gas: Well drilling – bits and string

by on December 5, 2013

I’ve already mentioned drilling as part of exploration and appraisal, but how does it actually work?  The well-publicised image of an oil derrick is only the visible part of a complicated engineering system, intended to drill through immense thicknesses of rock and to extract valuable oil and gas in a (hopefully) safe and economical manner.

The key part of a rig is the drill string, an assembly of thick-walled pipe that runs from the surface all the way to the drill bit at the bottom of the well.  Its operation involves a number of interdependent mechanisms:

  • The drill bit at the bottom uses conical toothed rollers to pulverise the rock.
  • This is turned by the drill pipe, which is itself turned by a top drive or rotary table at the surface.  Effectively the pipe serves as the axle transmitting torque to the bit.
  • Drilling fluid or ‘mud’ is pumped down through the pipe; it flows through the hollow bit and returns to the surface in the annulus between the pipe and the sides of the borehole.  This useful stuff controls the pressure in the well, stabilises the bore, cools and lubricates the bit and drill pipe, carries away rock chippings, and often provides additional power to the bit through a hydraulic ‘mud motor’.  The chemical additives used to control the viscosity, pressure characteristics, and other properties of the fluid tend to make it toxic, so it must be carefully contained.

Above the drill string is the wellhead.  This is a substantial component that serves as pressure containment for the well, including a rotating seal for the drill string, as well as a specialised valve called a blowout preventer (BOP) designed to stop pressurised oil or gas from escaping.  (The absence of BOPs in the early days of oil led to wasteful and dangerous ‘gushers’, as you might see in old cartoons.  These days blowouts are rare, but a poorly designed BOP failed to prevent the infamous Deepwater Horizon disaster in 2010.)  The wellhead is also used to support the steel casing that lines the borehole.  On offshore rigs, the wellhead is often located on the seafloor.

As the drill cuts deeper into the rock, the drill string is lowered by a the drill line, a cable running from the top of the derrick.  As this happens the pipe is extended in stages by screwing sections onto the top above the wellhead.  Once a well is complete, the drill pipe is hoisted back up and dismantled for re-use due to the huge length of pipe (and therefore expense) involved.

The record depth stands at 10,680 metres from wellhead to the bottom – and this was on one of the Tiber wells in the Gulf of Mexico, where the wellhead was 1,259 m below sea level!  How do you drill a well eight miles below the surface?  Find out about platform technology in the next article…

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