The term is loosely applied to the assembled collection of the smuggler pool, drill collars, tools and drill bit.
The drill string is hollow so that drilling fluid can be pumped down through it and circulated back up the annulus (the void between the drill string and the casing/open hole).
The drill string is typically made up of three sections:
- Bottom hole assembly (BHA);
- Transition pipe, which is often heavyweight drill pipe (HWDP);
- Drill pipe;
Running a drill stringMost components in a drill string are manufactured in 31 foot lengths (range 2) although they can also be manufactured in 46 foot lengths (range 3).
Each 31 foot component is referred to as a joint.
Typically 2, 3 or 4 joints are joined together to make a stand.
Modern onshore rigs are capable of handling ~90 ft stands (often referred to as a triple).
Drill pipe, HWDP and collars are typically racked back in stands in to the monkeyboard which is a component of the derrick if they are to be run back into the hole again after, say, changing the bit.
The disconnect point (break) is varied each subsequent round trip so that after three trips every connection has been broken apart and later made up again with fresh pipe dope applied.
Stuck drill string
A stuck drill string can be caused by many situations.
- Packing-off due to cuttings settling back into the wellbore when circulation is stopped.
- Differentially when there is a large difference between formation pressure and wellbore pressure. The drill string is pushed against one side of the well bore. The force required to pull the string along the wellbore in this occurrence is a function of the total contact surface area, the pressure difference and the friction factor.
- Keyhole sticking occurs mechanically as a result of pulling up into doglegs when tripping.
- Adhesion due to not moving it for a significant amount of time.
Once the tubular member is stuck, there are many techniques used to extract the pipe.
The tools and expertise are normally supplied by an oilfield service company.
Two popular tools and techniques are the oilfield jar and the surface resonant vibrator.
Below is a history of these tools along with how they operate.