When drilling like 12 km deep into the ground, how is it possible that a 12 km long pipe (drill string) is able to turn the drill bit AND be pushed down enough to drill??

Question

A 12 km long pipe seems like a ridiculous length for any of that to be possible. Isn’t it like trying to drill a hole with a 258 ft long piece of spaghetti?

In: 1335

Answer 1

The drill rig doesn’t actually push the pipe down at all. In fact, a drill rig is a glorified crane. The weight of the drill string alone can be a million pounds whereas only on the order of 25,000-50,000 pounds of weight pushing the bit is needed to drill. This is accomplished by lowering the drill string and letting some of its own weight push the bit.

Rotating on the other hand is accomplished two ways. First off, the drill rig itself can spin the drill pipe. It takes on the order of 20,000 ft-lbs of torque to turn the pipe due to friction but it has the power to do it. The second method of rotation is called a mud motor. A motor can be placed at the bottom of the drill string. Mud that is pumped through the drill string spins the motor independently of the drill string.

EDIT: A few more details for the interested:

>Isn’t it like trying to drill a hole with a 258 ft long piece of spaghetti?

Yes. As mentioned in the first paragraph, at no point does the drill rig push the pipe. It uses a little bit of the drill string’s own weight to push the bit. The term for lowering the pipe so that it rests on some of its own weight is called “slack-off.” As far as the rig is concerned, it is *always* suspending the drill string like a crane suspends a weight in air. If you hold a wet noodle in the air and lower it to touch your dinner plate, you are always holding the noodle up. The more noodle that you lower onto the plate, the less weight you are holding and the more weight that the noodle is pushing against the plate.

As for rotation, steel is incredible at being able to transfer torque, but at those depths it will twist around 10-15 times before the bottom of the drill string starts turning.

Others mention stabilizers, all stabilizers do is keep the pipe in the center of the hole. Keeping it concentric reduces vibration and some wobble at the bottom of the drill string.

A follow-up question could be, “How do you control where the bit goes?” This process is called directional drilling. The simple explanation is that you **push off the wellbore in the opposite direction that you want to steer the bit**. There are many sophisticate tools to figure out where the bit is, what direction it is pointing, and to get it to drill in a desired direction.