Meet the Circulating System

Like the circulation system in the human body, a rig’s circulation system ensures that the correct fluids reach the correct parts of the “body”.

The rig’s circulation system consists of several components. Together, they methodically and efficiently deliver drilling fluids into the wellbore throughout the drilling process. Drilling fluids serve a number of purposes, as we will see.

Drilling fluid, which drillers call “mud”, can be petroleum based, water based, composed of synthetic oil and even pneumatic, such as foams. Drilling fluids can also contain additional solids to obtain the desired density, thickness, viscosity and other properties. Regardless of their chemical composition, however, drilling fluids serve several functions.

First, as the fluid is pumped through the drill bit, it provides the hydraulic energy to operate the drill bit and other downhole tools. In so doing, the fluid also serves to cool and lubricate the drill bit.

As the bit drills the well, it grinds the solid rock into rubble called “cuttings”. The circulating drilling fluid carries these cuttings from the bottom of the well to the surface.

Without the fluid to bring them to the surface, these cuttings would collect in the wellbore and interfere with efficient drilling.

It is important to ensure that the new wellbore retains its shape and does not collapse. This is a third function of the drilling fluid.

Finally, thousands of feet of drilling fluid in a wellbore amounts to a considerable weight. This weight is important for the fourth major function of drilling fluids – to provide weight to counterbalance any tendency of the oil or gas to flow to the surface. Such an uncontrolled release in a live wellbore drilled conventionally can be quite hazardous. Consequently, this is a vital function of the drilling fluid.

The mud is stored in the mud tanks.

The mud pump is the heart of a rig’s circulating system. These devices are large reciprocating pumps which force the fluid from the mud tanks, up a standpipe, and through a high-pressure hose.

This flexible, high-pressure hose connects the standpipe to the swivel, allowing for vertical movement in the derrick. Through the hose, fluid is pumped into the drillstring.

The drill string consists of many lengths of connected drill pipe, drill collars, and downhole tools that can extend for thirty thousand feet, or approximately ten thousand meters. (You can learn more about drill pipe in the “Meet the BHA” and “Meet the Rotating System” modules.)

The drill string conveys the mud through the blowout preventer and down the wellbore to the drill bit. The fluid flows through the drillstring and out the drill bit nozzles at the bottom of the well. The return fluid path is through the annulus between the drill string and the borehole. Upon reaching surface, the fluid, now laden with rock cuttings from the bottom of the well, moves to mud-cleaning equipment and shale shakers.

As the name implies, shale shakers shake! These machines have for decades been the first line of defense in eliminating cuttings from the drilling fluid. Eliminating cuttings, also called “drilled solids” is critical to maintain an efficient drilling process.

Shakers are equipped with fine-mesh screens that allow fluid to pass through without the drilled cuttings. The shakers separate the rock cuttings from the fluid, allowing the often-costly drilling fluid to be recycled and re-used in the well, while isolating rock cuttings for proper disposal. Hydrocyclones, mud cleaners and centrifuges might also be used to remove additional solids returned from the sub-surface.

Following cuttings removal, the cleansed drilling fluid returns to the mud tank, ready for another cycle into the well.

Now that you’ve been introduced to the circulation system, please meet the rest of the rig through the links on the “Meet the Rig” page.