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Blowout prevention explained

Posted: December 12, 2011 - 12:21am

What is a blowout preventer, and what is a blowout?

A blowout preventer, or BOP, is the mechanical device intended to control and close off a well if there is an uncontrolled flow of oil and gas.

The BOP is positioned at the top of a well and just under the drilling floor of the rig while the well is being drilled. It contains mechanical “rams,” which are to be closed if there is an uncontrolled flow of fluids up the well.

The rams seal off the well, preventing the oil and gas from reaching the rig floor where a fire or explosion might occur or from where oil might spill off the rig into the environment.

The drilling fluid, or the “mud” is the first line of defense against a blowout. As the well is drilled, mud is pumped down the drill string to the drill bit at the bottom and returns up the well-bore in the space, or annulus, between the outside of the drill pipe and the casing, or the larger-diameter pipes, that line and protect the well.

The column of drilling fluid being pumped down, which would be a mile or more long when a well extends to 5,000 to 10,000 feet or more, has considerable weight and exerts a tremendous downward hydrostatic pressure in the well.

At the bottom, where there is an “open hole” with no casing, the circular tubing through which the well drilled, the hydrostatic pressure provides an “overbalance” of pressure that exceeds the natural pressure of the reservoir into which the well is drilled. The overbalance of pressure prevents oil and gas fluids from entering and coming up the well bore.

Sometimes the drillers encounter a gas “kick” or a surge of unexpected high pressure if they encounter a pocket of high-pressure gas. Shallow gas pockets are the cause of all of Alaska’s blowouts, except one.

Instruments in the well typically detect a rapid gas and pressure buildup and the drillers can counter this by closing the BOP and pumping heavier fluids under higher pressure through a choke, or restricted pipe, in the BOP. This is to overcome the pressure of the petroleum fluids and pushes them back down the well, bringing it under control.

Surveys of drilling locations are required to detect shallow gas hazards but sometimes drillers can be unpleasantly surprised. However, improvements in seismic technology, which can detect shallow gas hazards, and the increased regulatory scrutiny has greatly reduced the chances of a blowout in recent years.

If the unexpected upward flow of fluids is not brought under control, a blowout can result, potentially pushing the steel tubing, or pipe, back up out of the well, damaging the well, the rig and potentially causing loss of life or injuries to the drill crew.

The primary danger is the oil and gas itself, which are flammable and explosive. The mud system and the pressure it exerts is the primary method for controlling the well. The blowout preventer is a final defense, and a vital one.

Blowout preventers are used on both land and offshore rigs. In both cases the BOP is secured to the top of the wellbore. With a deep offshore well this would be at the sea bottom, with a “riser” or flexible pipe extending up to the drill rig on the surface. The riser provides a secure and enclosed path for the drill string, which rotates to drill the well, and drill fluids, or muds, that are pumped down the well to control pressure and then recirculated back to the rig at the surface.

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