A Sea in Flames - Carl Safina [12]
This is certainly not the time for chest bumping or blurred authority. If you’re gonna release the parking brake, you’d better agree on who’s gonna be in the driver’s seat. And whoever grabs the wheel better know how to drive.
High-risk pregnancy enters labor. To determine if the cement job has worked and the well is sealed, rig operators can choose from several tests. On the Deepwater Horizon the engineers decide to do two kinds of pressure tests. In a “positive pressure test,” they introduce pressure in the well; if it holds, it means nothing’s leaking out from the well into the rock. They do this test on the morning of April 20, between about 11:00 a.m. and noon, roughly eleven hours after the cement job ends. It goes well; it seems nothing’s leaking out.
But the reason nothing’s leaking out may be that there’s pressure from oil and gas pushing to get in. So the engineers prepare to do a “negative pressure test.” A negative test is a way of seeing if pressure is building in the well, indicating that gas and oil are leaking in. That could mean the cement has failed.
To do a negative test, they close the wellhead, then reduce the downward pressure on the well by replacing some heavy drilling fluid with lighter water. Then they look at pressure gauges. If the pressure increases, hydrocarbons are entering, exerting upward pressure from below. What they want to see is zero pressure.
Until the negative pressure test is performed successfully, the rig crew won’t remove the balance of the heavy drilling mud that stoppers the well; that’s their foot on the brake.
Between 3:00 P.M. and 5:00 P.M., about fifteen hours after the cement job was finished, they start reducing the pressure by inserting seawater into the miles-long circulating-fluid lines. To make sure the drilling fluid and the seawater don’t mix, they precede the seawater with a spacer. The spacer they use contains that extra kill-pill material, the “snot.” And though a typical amount of spacer is under 200 barrels, this time it’s over 400 barrels because, remember, they’re trying to get rid of that leftover stuff.
There are various places all this fluid is getting to, because there are various lines and pipes going into and out of the blowout preventer. One such line is called the “kill line.” Another is the drill pipe.
A little before 5:00 P.M., they work for a while to relieve any residual pressure and are looking for the fluid to stabilize at zero pressure, indicated by a reading of zero pounds per square inch, or psi. They’ve got the pressure down to 645 psi in the kill line, but it’s at 1,350 psi in the drill pipe. So they try bleeding the system down, venting off some of that pressure. They achieve zero in the kill line. The drill pipe retains 273 psi. They need zero.
Over six minutes right around 5:00 P.M., the drill pipe pressure increases from 273 psi to 1,250.
The engineers tighten the blowout preventer’s rubber gasket and add 50 barrels of heavier-than-water fluid.
So the lines are filled with a variety of different fluids snaking through in segments: there’s a stretch of drilling fluid, or “mud,” a stretch of the unusual spacer material, followed by plain seawater. At this point in the circulation of the various fluids, the spacer—the “snot”—should be above the blowout preventer. But some of it has found its way into the blowout preventer and has entered one of the lines being tested.
The engineers see pressure building in the drill pipe, zero pressure in the kill line. They’re unsure what to make of that, so they repeat the test procedure several times. From shortly after 5:00 to almost 5:30, they get the pressure in the drill pipe down a little, from 1,250 to almost 1,200.
The Deepwater Joint Investigation panel asked Dr. John R. Smith, whose PhD is in petroleum engineering, to describe a negative test:
“If it’s a successful