What is a degasser on a drilling rig?

A degasser is equipment used to remove entrained gas in drilling fluid so it prevent or minimize reduction of hydrostatic pressure due to gas cut mud. When drilling mud passing over the shale shakers while drilling, gas will normally be released. However, the wellbore could receive additional volumes of gas and these need to be removed from the mud. If not removed from the circulating system properly, recirculation of mud containing gas will reduce the well’s hydrostatic head. With a degasser, this can eliminate or minimize loss of hydrostatic pressure.

Figure 1 - Degasser (Courtesy of NOV)

Figure 1 – Degasser (Courtesy of NOV)

Mounted over the active pit, degassers are essentially a one-stage liquid/gas separator. With a maximum lift to the inlet of around ten feet, mud vacates the submerged pipework in the mud pit and enters the degasser. From here, a three hp electric motor will power a vacuum pump and this should be mounted atop the degasser itself. By the pump, the vacuum is then applied to the vapor space.

Ultimately, the range applied by the vacuum will depend on the density of the mud passing through. In most cases, it will offer between 2-5 pounds per square inch (between 8 and 15 inches of mercury). In terms of extracting gas from mud flows, 900 gallons per minute is likely to be the maximum rate.

Figure 2- Degasser Diagram

Figure 2- Degasser Diagram

Sometimes, the mud’s separator won’t be able to remove extra small gas bubbles in the drilling fluid due to their size. Therefore, a degasser can remove these bubbles; depending on the project, a certain degree of vacuum can assist with removing entrained gas. In order to reduce the risk of gas entering the pit after breaking out of the drilling fluid, the drilling fluid discharge line from the mud/gas separator and the drilling fluid inlet line to the degasser should be within close proximity to one another.

Additionally, degassing for all drilling fluid can be ensured by increasing the drilling fluid throughput capacity beyond the maximum flow rate from the well. After entering the top of the degasser, there’s a pipe closed in at the far end through which the mud needs to flow; to form an open trough, the pipe’s top is cut away horizontally. Over inclined plates, the mud should spill over the trough and this allows the gas to be released as the mud spreads over a large surface area. In the vapor space, the vacuum plays an important role too as the mud carries over the plates as evenly as possible. Once this process has occurred, the gas can either be burned away from the rig or vented through a vent line from the tank.

What happens to the mud now free from gas?  Well, it now has a normal weight and is allowed to fall to the bottom of the degasser’s cylinder. Before entering the mud stream, it travels through a 5/8th-inch extractor jet. After passing through, there’s no danger of the mud being at a higher pressure than the vacuum pressure.

Mud can then be sent to the active pit, but not before the safety valves jump into action. If the working pressure reduces, these valves stop the mud from coming into contact with the vacuum pump and therefore being sent back up the degasser.

References

Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.

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