If you’ve ever tried to dig a hole by hand, you probably got tired doing it. That is because it took energy to make the hole. In this Drilling Minute, we’ll look at how energy is used to drill an oil & gas well.
Weight on Bit (WOB)
In some ways, drilling a well is like using a handheld drill. In order to make a hole, you have to push the bit into whatever you are drilling. In oil & gas drilling, we call this “weight on bit,” commonly abbreviated as WOB. Just like pushing on the drill, WOB applies force to drive the bit ahead into the formation. But where does the weight come from?
If you added up the weight of all of the components of the drill string, you would have what we call the “string weight,” which is usually hundreds of thousands of pounds. But not all of this is used to drill; most of it is suspended from the derrick, which is known as the “hook load.” In fact, the structure of the derrick is actually designed to support this heavy load. The total string weight minus the hook load is the WOB.
Just pushing on a drill doesn’t make a hole; you have to pull the trigger to turn the bit. The force that causes the bit and drill string to turn is called torque. Torque is provided by the rig on surface by the top drive or Kelley, depending on the type of rig. Surface torque is used to turn the entire drill string. In many wells, a down-hole motor is also used, which converts the energy in the fluid flow into torque to turn just the bit. Whether at the surface or bit, torque is what generates RPM.
Last but not least, hydraulic energy is what forces fluid to circulate through the drilling system. Drilling hydraulics are used to evacuate the drilled formation, and clean, cool, and lubricate the drilling components and the well. The rig pumps supply hydraulic energy by putting the drilling fluid under pressure to circulate.
These energy sources are often referred to as parameters because they can be controlled to improve drilling performance, which is the topic of the next Drilling Minute.