Drilling the relief well (Image: Gerald Herbert-pool/Getty Images)

If you think threading a needle is difficult, imagine drilling into an 18-centimetre-wide cylinder 5500 metres below the sea floor.

That is the challenge facing BP engineers as they attempt to drill two relief wells in the Gulf of Mexico in a bid to stem the flow of oil from the stricken Deepwater Horizon. A relief well aims to bisect the original well casing, enabling engineers to pump in mud and concrete to seal up the well.

All previous attempts to plug the leaking well have failed, so relief wells are the last resort, says Greg McCormack at the University of Texas at Austin. “There isn’t anything else you can do.”


To reach the damaged well, the engineers are using a system called Measurement While Drilling (MWD) – accelerometers and magnetometers attached to the drill bit monitor the movement and direction of the wellbore and transmit this information back to the surface.

Muddy pulses

The system sends these messages by creating pulses in the flow of mud that is constantly pumped in and out of the borehole to clear away the rock cuttings, says Ken Arnold, who runs KACI, an oil industry consultancy in Houston, Texas.

The pulses are created by briefly closing and opening valves in the drill pipe to restrict the flow of mud – the resulting pressure fluctuations can be detected by sensors and interpreted by computers on the surface. “They provide the information needed to plot to within 10 feet of where the bit is the whole time,” he says.

Once close to their target, the engineers can home in on Deepwater Horizon by detecting fluctations in Earth’s magnetic field caused by the caisson – the steel and concrete foundation that surrounds the original well.

Once the drill reaches the leaking well, puncturing a hole in the casing and inserting the 18-centimetre cylinder will be tough, and may take a few attempts, says Arnold.

Mud pumped into the relief well through the cylinder will be pushed into the original well until its weight is sufficient to counteract the pressure in the oil reservoir, says McCormack. Once the oil stops flowing and the well is stabilised, it can be closed off permanently with cement, he says.

A spokesman for BP says the drilling, which began last month, is on track for completion in early August. The first relief well has now reached 4200 metres, while a second “insurance” well, is at 2700 metres, he says.