Design Factors Relating To Properly Design The Right Size of Mud Gas Separator for Drilling Rig

A mud gas separator or poor boy degasser or gas buster is one of the most critical well control equipment on drilling rigs. It is used to separate gas kick from drilling mud while circulating kick out of wells or circulating gas while drilling or workover operations. The mud gas separator used on drilling rigs is typically a vertical cylindrical vessel with many baffle plates inside because vertical vessels have small footprints. The drilling mud from the well goes into the mud gas separator and hits baffles. Then gas will be removed due to hitting action. The gas will go up and exist to atmosphere via a vent line at the top of the vessel. The drilling fluid drops after colliding baffles and exists the mud gas separator through the line and return to a mud pit.

Normally, hydrostatic pressure provided by mud leg or mud seal is the maximum allowable pressure in the MSG. Operating over the mud leg pressure will result in a blow-through situation which is the situation when gas from the drilling mud going through mud leg and returning back to the rig. This maximum operating pressure depends of fluid density in the MSG but normally the pressure is below 15 psis (1 bar). The friction pressure of gas flowing through the vent line must be less than pressure from the mud leg.

There are several factors that you need to consider when designing a proper size MGS for a drilling rig.

Kick volume – For the design purpose, gas volume is used for determining the size of MSG. Proper assumption of allowable gas kick volume is needed.

Mud leg height – The minimum mud leg height is determined by the vent line friction pressure and drilling mud density expected to see during well kill operation. Typically, oil density is used to determine the mud leg height because it is a worst-case scenario.

Vent line friction – size and shape of vent line affects the friction. There are several methods to determine friction of gas flowing through vent line.

Kill rate – The kill rate is required to determine gas flow rate through the MGS and it relates to the gas flow rate through MSG. More kill rate = more gas flow rate.

For full detailed calculation, you can find out from this SPE No. 20430 – Mud Gas Separator Sizing MacDougall.

What will be occurred if improper size of MGS is used?

• Unable to handle gas at a planned kill weight and gas will go back to the rigs.
• In order to control excessive gas because you cannot handle with MGS, the well must be choked back and it will result in high back pressure. With additional back pressure, the formation or casing shoe can be broken.
• The vessel and vent line can be damaged due to excessive flow.

Reference books: Well Control Books