Degassers are necessary to remove entrained gas bubbles from the mud. Gas-cut mud will impair the performance of centrifugal pumps. Since all solids removal equipment beyond the shakers requires a pump, the gas must be removed before it reaches these devices. If left unchecked and pumped downhole, the entrained gas will reduce mud density, which will, in turn, reduce the hydrostatic head in the wellbore.
The fundamental principle for all degassers is that gas bubbles must reach
the liquid-gas interface before they will burst. Any action which brings these gas bubbles to the surface will result in degassing. Four basic mechanisms exist for bringing gas to the surface:
- increase the bubble size by drawing a vacuum.
- create a thin film.
- create turbulent action.
- impart centrifugal force on the mud to drive the gas bubbles to surface.
There are two basic types of degassers: atmospheric degassers and vacuum degassers. Tests conducted by Amoco Production Research have shown that vacuum degassers provide superior performance in the presence of
higher mud weights and yield points greater than 10 lb/100 ft2. Atmospheric degassers are acceptable for unweighted muds with low yield points. The overall ranking of degasser models resulting from experimental data is given in Table 1.
|Table 1 Ranking of Degasser Models|
|Tillet Gas Hog||Atmospheric|
|Aipu Solids Control||Vacuum, Atmospheric|
|* Not tested but similar in design to Drexel-Brandt|
Degasser Placement and Operation
- Provide enough degasser capacity to treat at least 100% of the circulation rate. Be aware that actual processing rates for gas-cut mud are much lower than claimed rates for water.
- Degassers should be located downstream from the shale shakers and
upstream of any equipment requiring a centrifugal pump. The degasser
suction should be installed downstream of the sand trap. The suction
entry should be approximately 1 ft from the floor in a well-agitated
- The equalizer flow between the degasser suction and discharge must be high. There should be a visible backflow across the high weir, indicating full processing of the circulation rate. If equalization is low, the light gas-cut mud entering the suction compartment may not be able to displace the heavier mud returning from the discharge compartment. As a result, the light mud may overflow the suction compartment. Fig. 1 illustrates correct fluid routing for degassers.
- Atmospheric degassers should discharge horizontally across the surface of the tank to allow large gas bubbles to break out. Vacuum type degassers should discharge below the mud surface with the flow turned up towards surface.
- Vacuum degassers must take power mud suction from their discharge compartment. Power mud is the mud pumped at high velocity through an eductor to create the vacuum in the degasser tank. Taking suction upstream will likely result in the pump becoming gas-locked. Suction from further downstream will likely cause mud to bypass the hydrocyclones.
- The power mud centrifugal pump must supply the necessary feed head. Install a pressure or head gauge to monitor the feed head at the eductor.
The high weir helps ensure complete processing of gas cut mud.
- Degassers are used to remove entrained gas bubbles from the mud to prevent impairment of centrifugal pump performance, a reduction in mud density and a subsequent reduction in hydrostatic head in the wellbore.
- There are two basic types of degassers: atmospheric and vacuum. Vacuum degassers are recommended for weighted muds and yield points over 10 lb/100 ft². Atmospheric degassers are acceptable for unweighted, low viscosity muds.
- An overall ranking of degasser models resulting from experimental data is provided in this chapter. Vacuum degassers are generally superior.
- Provide enough degasser capacity to process over 100% of the circulating rate.
- Locate the degasser downstream of the shakers and upstream of any centrifugal pumps.