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In the movie Deepwater Horizon they conducted a negative pressure test to see whether or not it would be safe to drill. But I cannot recall them explaining what such a test actually does or what it means. So my question is what is a negative pressure test and why did it seem like everything was okay when in reality it was not?

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A test to ensure that the casing and cement that separated the wellbore from the hydrocarbon-bearing formation could withstand that pressure differential without any leaks

The test is described here: The Failed Negative Pressure Test – a BP Investigation Report Analysis

The purpose of the negative pressure test on the Macondo well was to purposely lower the pressure inside the wellbore in a controlled manner to ensure that the casing and cement that separated the wellbore from the hydrocarbon-bearing formation could withstand that pressure differential without any leaks.

To do this however, it’s not a matter of simply turning a valve and watching the pressure go down. The 5000-foot vertical column of heavy, barite-saturated mud within the choke line, kill line, mud boost, drill pipe, and production casing must be replaced by a much lighter column of seawater, which in turn reduces the total hydrostatic head pushing on the sides of the wellbore.

During the negative pressure test, 1,260 psi of pump pressure was needed to overcome the difference in hydrostatic pressure between the column of seawater and the column of mud that was being forced back to the mud pits via the riser. Once the seawater/mud replacement was complete, the column of seawater was then isolated from the wellbore via the annular BOP and the 1,260 psi of head pressure differential was allowed to bleed off via the kill line causing a U-tubing affect within the wellbore.

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