As a kid, you probably blew bubbles in a drink through a straw. Pretty easy, right? You were using pressure to force air through the system. In this Drilling Minute we’ll learn how drilling hydraulics are used to do the same thing to carry the drilled cuttings from the bit back to the surface of the hole.
When you first start digging any hole, it is easy to just toss the dirt to the side. But the deeper you dig, the more energy it takes to get the cuttings all the way back out of the hole. In the early days of drilling, a type of bucket called a bailer was used to collect cuttings to drag back to the surface every few feet. Today, most oil & gas wells pump fluid through the drill string that then carries the cuttings back up the hole.
Using fluid is much more efficient, but moving all that rock thousands of feet straight up still requires a lot of energy. Just like your lungs pushed air through the straw, rig pumps are used to provide the necessary pressure and flow to evacuate the cuttings. Rig pumps can often provide thousands of pounds of pressure per square inch, but how much pressure do we need?
Every time a fluid passes through something, like drill pipe, there is a pressure loss. Think of it like friction of the fluid against the sides of the pipe. The longer or more narrow the straw, like a coffee stirrer, the harder it is to blow through it. In drilling, pressure is lost through each of the thousands of feet of drill pipe and BHA, and then again all the way back up the annulus.
All that pressure is required just to push the fluid through the system. But don’t forget that the fluid being pumped to the bottom of the hole has the entire weight of the fluid column and cuttings above it – hundreds of thousands of pounds – pushing down on it. This is known as head pressure.
Blowing bubbles was pretty easy, right? But if you had a drink that was about six and a half feet tall and put a straw all the way to the bottom, you wouldn’t be able to do it-your lungs can’t provide enough pressure to overcome the system losses and head pressure! In the same way, if the rig pumps can’t provide enough pressure, there is no flow to carry the cuttings.
Usually the rig pumps have some pressure to spare, so what do we do with it? Well, one option is to adjust the nozzles in the bit. This is done to optimize the use of drilling hydraulics to clean the bottom of the hole, like putting your thumb over the end of a water hose.
Cuttings evacuation is critical in order to drill a hole, but drilling fluid has several other responsibilities as well, which is the topic of the next Drilling Minute.