Aug 12, 2011
During the drilling of an oil well, crews set and cement in place a series of concentric pipes, or casings. These casings prevent the collapse of the geological formation into a well and also prevent potential leaks either in or out of a well. This structure results in a series of annuli (an annulus is the space between two concentric objects, such as between casing and tubing, where fluid can flow) generally filled with a heavy spacer fluid and/or packer or drilling fluid used to balance the pressure from the geological formation.
In the upper reaches of a deepwater oil well, casings are typically in a relatively cold portion of the geological formation (near freezing). The lower portions, where the oil is located, are as much as 25,000 feet below the ocean floor. These portions are extremely hot as a result of the natural geothermal gradient.
When crews first produce the oil well, the hot oil comes up from below, warming the whole casing string assembly (a long series of connected casings). Such heat causes the formerly cold drilling fluid in the annuli to undergo thermal expansion. In the industry, this phenomenon is known as trapped annular pressure or annular pressure buildup. The resultant pressure can lead to the collapse (or bursting) of the casing(s).
On land-based wells, crews can use a relief valve to manually alleviate pressure buildup. In deepwater locations, such valves are impractical, given that the actual wellhead at the sea floor can be as deep as 10,000 feet underwater. Moreover, there is no access to individual annuli, so it is not possible to relieve pressure simply through relief valves or through the use of a remotely operated vehicle.
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