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Section 500 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Degassers

January 1998 Page 1 of 4

Degassers 1 Introduction .........................................................................................................................1

2 Placement and Operation ...................................................................................................2

3 Summary..............................................................................................................................3 FIGURES Fig. 1. Correct degasser operation. .........................................................................................3 TABLES Table 1 Ranking of Degasser Models .....................................................................................2

1 Introduction 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: 1) increase the bubble size by drawing a vacuum, 2) create a thin film, 3) create turbulent action, and 4) 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 2 higher mud weights and yield points greater than 10 lb/100 ft . 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.

CONFIDENTIAL

Section 500 SOLIDS CONTROL HANDBOOK

January 1998

Schlumberger Dowell

Degassers

Page 2 of 4

Table 1 Ranking of Degasser Models Manufacturer

Type

Drexel-Brandt

Vacuum

Derrick*

Vacuum

Wellco

Vacuum

Sweco

Vacuum

Burgess

Vacuum

Swaco

Vacuum

Totco

Vacuum

Tillet Gas Hog

Atmospheric

Drilco

Atmospheric

Sweco

Atmospheric

Judco

Atmospheric

* Not tested but similar in design to Drexel-Brandt A complete list of available degassers and their processing capacities are listed in Appendix F, Equipment Specifications.

2 Placement and Operation 1.

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.

2.

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 compartment.

3.

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.

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Section 500 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Degassers

January 1998 Page 3 of 4

Fig. 1. Correct degasser operation. Note: The high weir helps ensure complete processing of gas cut mud. 4.

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.

5.

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.

6.

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.

3 Summary ·

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 2 points over 10 lb/100 ft . Atmospheric degassers are acceptable for unweighted, low viscosity muds.

CONFIDENTIAL

Section 500 January 1998

SOLIDS CONTROL HANDBOOK

Degassers

Page 4 of 4

Schlumberger Dowell

·

An overall ranking of degasser models resulting from experimental data is provided in this chapter. Vacuum degassers are generally superior. A comprehensive list of available degassers is listed in Appendix F, Equipment Specifications.

·

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.

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