Poor Boy Degasser or Mud Gas Seperator, located downstream of the choke manifold, is a vertical vessel used to separate any gas from drilling fluid during well control situation. Once the gas has been separated, it can pass through the vent line in the derrick. Alternatively, as long as it’s a safe distance from the rig, it could even be vented.
With mud’s separators, there are two main types. Also known as a ‘poor-boy’ and a ‘gas buster’, the more common of the two is called an atmospheric mud/gas separator. However, some mud/gas separators are designed to operate at moderate back pressure. Although these will mostly operate under 100 psig, it’s possible to come across those that work at the atmospheric gas vent line pressure plus the vent line friction drop. The simple diagram of poor boy degasser is show in figure 2.
Figure 2 – Poor Boy Degasser (Mud Gas Separator) Diagram
As long as they have a liquid level control, all separators can be referred to as pressurized mud/gas separators. Ultimately, there are benefits and drawbacks to both pressurized and atmospheric mud/gas separators. Despite the differences, both types also have some common requirements. For example, the capacity of the separator may sometimes be exceeded, or a malfunction may be experienced. With this in mind, both must have a by-pass line to the flare stack as a precaution.
When the drilling fluid impinges on the vessel’s wall, certain precautions should also be taken to prevent erosion. In case of plugging, easy cleanup should be possible with lines and vessels and these are more provisions to consider. For well testing operations, the rig mud/gas separator isn’t recommended for use unless it has been specifically designed for use in these conditions.
When a kick is being displaced, the mud/gas separator should be lined up constantly. As well as removing large gas bubbles from mud, the separator is also used to cope with gas flows as the influx reaches the surface.
Often, there are questions regarding the volume of gas with which each separator can safely cope. There will be a limit, and there’s a danger of gas getting into the shaker header box if this limit is breached. When it comes to calculating a maximum gas flow rate for each separator, an estimate can be made.
What factors will limit this flow rate?
Mainly, this will come from the relationship between the hydrostatic head of fluid located at the mud outlet and the back pressure at the outlet to the vent line. At times, there’s a risk the back pressure at the gas flow will be greater than (or even equal to) the mud outlet’s available hydrostatic head. When this occurs, the shaker head tank may be in danger.
At all times, minimizing risk should be a key objective and this can be aided with a large ID and straight vent line. Elsewhere, the mud outlet should be set up with a hydrostatic head of at least ten feet.
Furthermore, it’s important to keep an eye on the pressure gauge used for back pressure. If registered readings are showing pressure close to the discharge line’s hydrostatic head, this is a warning of a so-called ‘blow-through’. When large amounts of condensate or oil are displaced to the surface, the maximum hydrostatic head may not be equal to that of mud.
When the separator is in danger of breaching the safe operating limit, check that the well isn’t over-pressured and close in the choke. Alternatively, switch the flow to the burn pit or overboard line.
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Separation, H. (2019). Mud Gas Separator, Poor Boy Degasser | H-Screening. [online] H-Screening. Available at: https://www.h-screening.com/poorboy-degasser/ [Accessed 31 Mar. 2019].