The Circulation System

The mud circulation system is a complex rig system. The mud circulation system needs to pump drilling fluid (aka mud) from a stationary rig into and out of a rotating drill string, process the mud to remove rock cuttings, and control underground pressure to prevent blowouts and other problems.

Let’s follow the path of mud through the circulation system.

Three photos including the open water mud tank, the pipes connected to the tank, and the pumps that pulls mud from the tank.

The mud begins and ends its journey in the mud tank or mud pit. Recycled mud flows into the tank through the return line, and newly mixed mud can be added from the mud mixer.

Mud is pulled out of the suction pit by the mud pumps. After passing through the pumps, which impart pressure to the mud, the mud runs through the surface pipes and into the standpipe.

On a top-drive rig, the mud is then pumped to the top drive, where it passes through internal passageways within the top drive and into the top of the drill string.

On a kelly drive rig, the mud passes through the swivel  and gooseneck. These parts allow mud to flow from the non-rotating standpipe to the rotating kelly and drill string.

The mud flows through the inside of the drill string to the bottom of the well. After passing through the mud motor (if present), the mud passes through the drill bit through a set of orifices (called “nozzles” or “jets”) It rises through the annulus back to the surface and the flowline back through the shale shaker, other solids control system, and back to the suction pit.

The mud picks up any rock cuttings present on the bottom of the borehole and carries them along the annulus to the surface. The mud and cuttings flow out of the top of the well and are collected by the flow line in a large diameter section of pipe called the bell nipple.

The flow line carries the mud to the shale shaker as described earlier.

After the shale shaker, the mud might also pass through other solids controls systems such as a de-sander  and/or de-silter apparati. These remove fine sediments from the mud so that it can be recirculated through the system. After passing through these machines, the mud flows back to the mud tank or suction pit, where it begins the process all over again.

One of the most important parts of the circulation system is the wellhead and associated blowout preventer.

The wellhead and blowout preventer help drillers contain the fluids within the well, control underground pressure, and as the name suggests, to prevent blowouts – but they are only the last line of defense.

It turns out that the drilling fluids themselves play a major role in containing underground pressure. Since uncontrolled releases of formation fluids is the largest hazard encountered during drilling, the process of maintaining pressure control is the most important, and also a very challenging, part of drilling a well. That’s next.

Images: “Mud Tanks” by Michael Black ; “Trio Mud” by Michael Black; “Shale Shaker” by Michael Black; “Centrifuge” by Michael Black; “Blow Out Preventor Stack” by HHakim via iStock