The mud/gas separator is designed to provide effective separation of the mud and gas circulated from the well by venting the gas and returning the mud to the mud pits. Small amounts of entrained gas can then be handled by a vacuum-type degasser located in the mud pits. The mud/gas separator controls gas cutting during kick situations, during drilling with significant drilled gas in the mud returns, or when trip gas is circulated up.
This topic discusses design considerations for mud/gas separators. The purpose of this topic is to allow drilling rig supervisors to evaluate mud/gas separators properly and to upgrade (if required) the separator economically to meet the design criteria outlined in this topic, and to provide office drilling personnel with guidelines for designing mud/gas separators before delivery at the drill-site.
The operating principle of a mud/gas separator is relatively simple. The device is essentially a vertical steel cylindrical body with openings on the top, bottom, and side, as shown in Fig-1. The mud and gas mixture is fed into the separator inlet and directed at a flat steel plate perpendicular to the flow. This impingement plate minimizes the erosional wear on the separator’s internal walls and assists with mud/gas separation. Separation is further assisted as the mud/gas mixture falls over a series of baffles designed to increase the turbulence within the upper section of the vessel. The free gas is then vented through the gas vent line, and mud is returned to the mud tanks.
Operating pressure within the separator is equal to the friction pressure of the free gas venting through the vent line. Fluid is maintained at a specific level (mud leg) within the separator at all times. If the friction pressure of the gas venting through the vent line exceeds the mud-leg hydrostatic pressure within the separator, a blow-through condition will result sending a mud/gas mixture to the mud tanks.
As one can readily see, the critical point for separator blow-through exists when peak gas flow rates are experienced in the separator. Peak gas flow rates should theoretically be experienced when gas initially reaches the separator.