When a drill hits an underground oil reservoir, the contents of the reservoir—oil and gas—will try to shoot up the drill's borehole. But why?
The pressure of the oil comes from the weight of the rock and water above it. Imagine a plastic bag full of water—that’s like an underground reservoir of oil. If you poked a tiny hole in the top of that bag, that would be like an oil well being drilled into the reservoir. If you pushed down on the bag, like the weight of rock pushing down on the oil, water would squirt out the hole. The harder you pushed, the harder the water would squirt out. The oil well in the Gulf of Mexico has 5000 feet of water and more than three miles of rock pushing down on it—that's a lot of pressure. Also, if an oil well has lots of methane gas trapped in it, the gas will expand when the well is tapped, sort of burping up the riser pipe.
If the pressure of the oil or gas gets to be too much for the drilling mud, a special device on the ocean floor called a blowout preventer immediately clamps the well shut, sealing it off. But the Deepwater Horizon’s blowout preventer wasn’t working right, and when a surge of pressure came up the riser, it didn’t shut off the well. Oil and gas shot up the riser and exploded on the Deepwater Horizon, setting it on fire, and causing it to sink.
When the drill rig sank, it took the riser down with it, breaking the tube in several places. The flowing oil began to leak through the multiple breaks in the riser, adding up to an extremely difficult situation.