The purpose of a vacuum degasser is to remove gas mixed with the drilling fluid before the drilling fluid goes over the shale shaker. A vacuum degasser works well in fluid with large bubbles of free gas. (Often the gas is starting to break free in the flowline.) A problem with the basic vacuum degasser is that the heavier gases will not rise and be dissipated in the air but settle around the rig.
An old-fashioned but effective vacuum degasser is made from a piece of 9- or 11-inch (228.6- or 279.4-mm) casing. An inlet, tangent to the side but tilted up about 5_, is welded into a 6-ft (2-m) length of casing about one third of the distance from the bottom. The mud entering the inlet spins, and the centrifugal force allows the gas to go to the center and out while the mud goes to the sides and down. A pipe on the top carries the gas away, and the bottom of the casing is open to the shale tank on the shaker.
The tangential intake is used on many land rigs where the vacuum degasser is installed in the possum belly (back tank) of the shaker. The tangential intake balances the force of the drilling fluid and expanding gas so that the gas-buster tank does not need heavy bracing.
Another version of the pipe vacuum degasser uses the same casing size as the standard vacuum degasser but directs the mud and gas mixture onto a blast plate or ‘‘baffle,’’ which breaks up the flow pattern and separates gas and rilling fluid. This system is unbalanced, and the pipe needs to be restrained.
In heavy mud and with higher than normal initial gel strength,the baffle system may cause some entrainment of the gas, which appears as gas cutting.
The offshore version of the vacuum degasser uses an 11-inch casing up to 20 feet tall ahead of the shale shaker. The offshore vacuum degasser is closed and usually has a U tube on the line to the shaker to build backpressure and force gas to the discharge line.