Mud gas separator located at downstream of the choke manifold is one of the important well control equipment that you need to focus. It separates gas out of the mud after the gas comes out of hole. Gas will be vent to atmosphere via the vent line in derrick (offshore operation) or the line away from the rigs (land operation) and the mud will be returned back to the pit. In the oilfield, people have several names for the mud gas separator as “poor boy degasser” or “gas buster”. While drilling, the mud gas separator should be lined up at all times and filled with the present mud weight currently used.
The concept of this equipment is density difference between liquid and gas. When the mud coming out from the choke manifold goes into the mud gas separator, mud will hit the baffle plates which are used to increase travelling time and allow gas to move out of the mud. Gas which has lower density than air will move up and mud will goes down due to gravity (see – Figure 1). Mud leg will provide hydrostatic pressure in order to prevent mud going through the separator into the rig.
Figure 1 – Mud Gas Separator Diagram
The mud gas separator is strictly used for well control only. There are some events that this equipment is utilized in a well testing operation. Additionally, inspection for the gas buster must be performed frequently the same as other well control equipment. People tends to forgot about this since it is just only a vertical separator tank without high pressure specification like BOP, choke manifold, valves, etc. Erosion is one of the worst enemies to the vessel like this and it can be seen at the points where the drilling mud impinges on the internal wall vessel.
Each mud gas separator has limitation on much it can safely handle volume of gas. If the volume of gas exceeds the maximum limit, gas can blow through into the rig. It is very important that you must estimate gas flow rate on surface based on pit gain and kill rate in order to see how much expected gas on surface. This will be the limitation on how much you can circulate for well killing operation. You can estimate the volume gas on surface by using well control kill sheet provided by several companies. You need to ensure that the vent line should be as straight as possible with a larger ID in order to minimize back pressure while venting the gas out.
The pressure gauge should be installed on the mud gas separator and frequently calibrated. This is very important because you will use this gauge to monitor the gas blow through situation. If the pressure in the vessel is more than hydrostatic pressure provided by mud leg, gas will blow through the vessel. By carefully monitoring, you will be able to react in a timely manner. If the pressure gauge shows you that pressure will be reach the limit, you should reduce current circulation rate to control volume of gas coming into it.
There are several considerations for mud gas separator design. You need to know the reservoir gas because if it is a sour gas (H2S), the mud gas separator should be able to handle this. The normal vessel will not work safely with H2S. Even though it has a name as “poor boy gasser”, it does not mean that it can be built in a cheaper way by some machine shops. The vessel must be fabricated to meet ASME specification and there must be a third party to certify it. What’s more, the scheduled preventive maintenance must be in place and strictly followed.
How Much Pressure Will Cause Blow Through?
This is a very important concept of blowing through while performing well control. Mud leg length is 20 ft and mud weight is 9.5 ppg (see – Figure 2)
Figure 2 – Mud Gas Separator and Blow Through
Pressure to blow through = Hydrostatic pressure provided by mud leg
Pressure to blow through = 0.052 x 9.5 x 20 = 9.88 psi
Reference books: Well Control Books