how do they drill for oil & gas?

Answer 1

Having worked on drilling rigs in my younger days....

A derrick is setup over the site where the hole is to be drilled. The derrick has a flat platform 10' off the ground, and a tower structure about 75-100' tall. It is anchored to the ground at the 4 corners of the base.

The drill bits are 12" in diameter near the surface and 8" in diameter when the depth is far below the water table. The bits look kind-of like metal fists with spiked protrutions. Some bits have 3 rotating parts, and the metal spikes interweave with each other.

The drill bits are threaded onto a large hollow tube that is about 4" in diameter and 50' or 60' long. A whole bunch of these drill pipes are stacked vertically against the derrick. The drillbit and 1st tube are lowered to the ground, and a clamp wraps around the pipe. A powerful motor spins the clamp assembly to spin the pipe and the bit. As the bit digs into the ground eventually it will get about 50' down. The next pipe is threaded into the top of the pip and bit below, and the spinning starts again. When another 50' depth is reached, another pipe is threaded onto the string, and on it goes.

One other thing that has been happening: While the drill bit is being rotated and cuts into the dirt and rock, special mud is being pumped into the top of the drill string. The mud flows down the pipes and through holes in the drill bit. Here it washes the cuttings away and there is enough pressure to force the cuttings back to the surface, where it is discharged into a plastic lined mud pit.

All the time the drilling is going on, an accurate depth is measured. The company geologist has already determined (based on other wells nearby) at what depth the gas or oil should be at, so when they reach that depth, they stop drilling.

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Answer 2

With a...uhm....drill. Seriously.

It's a really BIG drill, mind you.

Read more below:
http://science.howstuffworks.com/oil-drilling.htm