A mathematical model which describes the dynamics of gas being circulated out of a riser is presented. The model is valid for conventional drilling where no backpressure is applied, controlled mud level (equivalent to dual gradient drilling), managed pressure drilling with backpressure, and with a riser gas handler. The presented model is validated with data from a field trial where gas was circulated down the drillstring and into an open riser with a reduced riser liquid level (controlled mud level). Underlying assumptions makes the models conservative. The model is suited to evaluate the procedures for use of a riser gas handler, and an example with automatic control of the rig choke shows how a gas kick can be safely circulated from the riser.

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