BP engineers were this week preparing to inject a combination of slurry and concrete deep into the well that is continuing to release hundreds of thousands of litres of oil into the Gulf of Mexico every day.
The oil company is preparing to inject 50,000 barrels (8M litres) of heavy drilling mud into the well via a blowout preventer, a large valve that was designed to seal off the wellhead but failed for unknown reasons.
The fluid will be pumped at such a rate and pressure that it should stem the flow of oil from the well. Concrete will then be pumped down into the well bore to permanently seal it. Drilling mud, or slurry, is usually a clay and water mix known as bentonite. It can be made heavier by adding chemicals like barite.
The heavier the slurry, the greater the pressure it will exert on the oil in the well. The slurry will be pumped into the blowout preventer via a manifold on the seabed from a surface vessel named Q4000.
The manifold will distribute the fluid into choke-and-kill pipelines that run into the bottom of the blowout preventer. Valves on the choke-and-kill lines will enable engineers to pump at a maximum rate of 40 barrels per minute − far more rapidly than the flow of oil from the well.
A BP spokesman said that the company was “quite comfortable” with the technology but that the method has never been used at the 1.5km depth needed here. As a result it is unknown how long the process will take, and how much slurry will be needed.
The process, dubbed “top kill”, may also need to be accompanied by a “junk shot” technique if too much of the drilling mud proves to be rising upwards with the oil flow instead of downwards into the well bore.
The junk shot involves the injection under pressure of a variety of materials − including pieces of tyres, golf balls and rope − into the blowout preventer above the choke-and-kill lines, to seal off upward flow.
Onshore tests over recent weeks have proven that the junk shot materials can fill various sized spaces in the blowout preventer until the flow is stopped. The junk shot will be continued until it is successful or deemed to be ineffective. If the procedure is successful the blowout preventer will be removed for examination.
The oil spill began last month when the sinking of the Deepwater Horizon drill rig caused the 1.5km long riser pipeline − which connects the wellhead on the sea bed to the rig on the surface − to rupture. There are multiple fractures in the riser pipe.
BP has been attempting to stem the flow using a 100mm diameter riser tube inserted into the open end of the riser pipe. It has also placed a steel box, dubbed the “top hat” on top of this connection to collect more of the leaking oil and allow it to be pumped to the surface.