You take a pole and poke a hole. This is the essence of oilfieldology. What I will try to lay out here is a simple explanation of how this is done with the understanding I have never worked offshore.
First of all, to dig a hole, one must be able to get the dirt out. The drill bit is always quite a bit bigger than the diameter of the pipe it is affixed to, and it has small holes, called jets, between the cutting heads. While drilling, fluid is pumped through the center of the drill pipe and comes out of the jets in the drill bit. All of the cuttings from the drill bits work are then carried toward the surface, flowing in the extra space between the outside walls of the drill pipe and the walls of the hole itself.
The first part of drilling requires no drilling mud. Water is sufficient for "red bed’ formations. A great big drill bit spinning fast makes short work of this. Eventually harder ground is found and changes are necessary.
The surface hole needs to be "cased." To keep it from falling in, casing pipe is put in place. It is just slightly smaller than the diameter of the bit, but much larger than the drill pipe. At the bottom of the casing is a plug with a big hole in it. As you run casing, the hole is full of water, the hole in the bottom of the casing allows the water in and allows the casing to sink
Once the casing is almost at the bottom of the hole (you don’t really want it setting on the hole floor) the cementing begins. Somebody with a slide rule and counting skills, figures out how much area exists between the outside casing wall and the actual hole wall. This is how much cement is pumped into the casing. A plug (more like a plunger) is placed on top of the cement and water is pumped behind the whole mess.
This forces the cement through the hole at the bottom of the casing and back up around the outsides, between the hole wall and the casing wall. The idea is to forever case the hole in steel pipe from the hole collapsing.
Now the process is the same, with some minor differences for intermediate and completion casing. Of course, the number one thing is high pressure gas that you done drilled into from coming to see you in a very rude fashion.
The magic is in the drilling mud. Brought by trucks (or boats) in 50 and 100 lb sacks, drilling mud is very expensive. Barite and Gel and Cypan and caustic soda and mica are just some of the dry ingredients that make up drilling mud. Gell makes it thick and slick, good for slicking up hole walls and carrying out cuttings, Barite is for weight, to hold done mother natures indigestion.
Weighted mud getting kicked around makes me nervous. Yes, mixed mud is weighed on a set of triple beam scales that would make a drug dealer green with envy. And you can watch weighted mud get kicked around. Mud levels are watch very closely because, well, if all of a sudden your mud levels are going up above ground, that means something below ground is coming to see you very soon.
Now with the Deepwater Horizon blowout, they had just cased and capped the well with cement and were replacing the drilling mud, you know, that which holds down high pressure gas, with saltwater, which was nearly 40 percent lighter.
They do this with pumps bigger than large trucks. Pumps that move volumes quickly. They depressurized about a mile of pipe by nearly 40 percent based on a cement plug installed by Halliburton. The Blow Out Preventers should have been able to catch this and shut in the well.
The floormotors ran away with the high concentration of natural gas. All of these are equipped with flapper valves on the intake for just this reason. I have so many questions and am not confident that any answers forthcoming will be credible in the face of liability exposures.
I hope this info is helpful and feel free to ask questions in the comments–I’ll answer them if I can.