A Sea in Flames - Carl Safina [3]
Equally amazing as how deep, is how narrow. At the seafloor—atop a well 2.5 miles long—the top casing is only 36 inches across. At the bottom it’s just 7 inches. If you figure that the average diameter of the casing is about 18 inches, it’s like a pencil-width hole 184 feet deep. Nine drill bits, each progressively smaller, dig the well. The well’s vertical height gets lined with protective metal casings that, collectively, telescope down its full length. At intervals, telescoping tube of casing gets slid into the well hole. The upper casing interval is about 300 feet long. Some of the lower ones, less than a foot across, are 2,000 feet long. The uppermost end of each casing will have a fatter mouth, which will “hang” on the bottom of the previous casing. You will make that configuration permanent with your cementing jobs. The casings and drill pipes are stored on racks, awaiting use. Casings are made in lengths ranging from 25 to 45 feet; the drill pipe usually comes in 30-foot joints. They are “stacked” in the pipe racking system. You assemble three at a time and drop approximately 90 feet in, and then repeat. When you get ready to put the casing in, you pull all the drill pipe out. Rig workers also remove the drill pipe from the hole every time the drill bit gets worn and needs changing or when some activity requires an open hole. Pulling the entire drill string from the hole is called “making a trip.” Making a trip of 10,000 feet may take as long as ten or twelve hours. When you want to start drilling some more, you have to reassemble the drill pipe and send it down.
On drillers’ minds at all times is the need to control the gas pressure and prevent gas from leaking up between the outside of the casings and the rock sides of the well. At each point where the casing diameter changes, the well drillers must push cement between the casings and the bedrock wall of the well. This cements the casings to the well wall. It controls pressure and eliminates space.
Drillers continually circulate a variety of artificial high-density liquid displacements or drilling fluids, called “mud,” between the drill rig and the well. The circulating fluid is sent down the drill pipe. It causes the drill bit to rotate, then leaves the drill bit and comes up to the surface, carrying the rock and sand that the drill bit has ground loose. Because the well is miles deep, the fluid creates a miles-high column of heavy liquid. (The drilling fluid is heavier than water. Imagine filling a bucket with water and lifting it; then imagine that the bucket is three miles tall. It’s heavy.) That puts enormous downward pressure on the entire well bore. As the drill digs deeper, the drilling “mud” formulation is made heavier to neutralize the higher pressures in the deepening depths. But that heavier fluid can exert so much pressure on the shallower reaches of the well (where the ambient pressure is less) that it can fracture the rock, damage the well, seep away, and be lost into the rock and sand. Steel casings can protect weaker sections of rock and sand from these fluid pressures.
Because things fail and accidents happen, a 50-foot-high stack of valves sits on top of the well on the seafloor. Called a “blowout preventer,” it is there to stop the uncontrolled release of oil and gas when things go wrong in a well. If something goes seriously wrong below, the valves pinch closed, containing the pressure. The blowout preventer is relied on as the final fail-safe.
Designs vary. This rig had a 300-ton blowout preventer manufactured by Cameron International. A blowout preventer’s several shutoff systems may include “annulars,” rubber apertures that can close around any pipe or on themselves; “variable bore rams,” which can seal rubber-tipped steel blocks around a drill pipe if gas or oil is coming up outside it; “casing shear rams” or “super shear rams,” designed to