The sizing calculation of a traditional Mud Gas Separator (MGS) becomes crucial when flowrates as high as drilling flowrates could be utilized to circulate out formation fluids using a fully automated Managed Pressure Drilling (MPD) control system. With the technological advancements to detect influxes, some of today’s MPD systems are equipped with algorithms to automatically apply surface backpressure (SBP) in order to restore the overbalance while retaining the ability to circulate the small volumes of formation fluids (influx) that entered the wellbore, prior to achieving the overbalance (Patil 2018). Since the algorithms of MPD systems are designed to assist in minimizing influx size while the system achieves the necessary overbalance, circulation rates as high as drilling rates are typically used to maintain relatively lower Surface Backpressures (SBP) while circulating influxes with MPD systems.
From the MPD perspective, the liquid and gas handling capacity of the MGS influences the MPD Influx Management Envelope (IME) which is designed to determine the operational limits of the influx volumes and pressures (Patil 2018). The MGS liquid and gas handling capacities influence these circulation rates to manage the peak liquid and gas flowrates below the calculated threshold of the MGS capacities. Reducing the circulation rates increases SBP requirements which directly affects the MPD IME.
This paper focuses on evaluating the methodology described in SPE 20430 (MacDougall 1990) for calculating the MGS liquid and gas handling capacity when relatively higher flowrates are used to circulate formation fluids through the MGS. It compares the method to estimate friction pressure loss for gas exiting the vent line that is described in SPE 20430 to an iterative and complex simulation model that includes pressure, temperature, gas density, compressibility etc. Furthermore the paper illustrates the setup and the results obtained when using a CFD (Computational Fluid Dynamics) simulator to qualitatively assess the velocities and separation inside the MGS in comparison to the average velocities limited to 8.4ft/min (MacDougal 1990) inside the MGS which would prevent separator blow-through condition.