A dynamic model for underbalanced drilling has been developed in a joint industry project. The model is of use for planning an operation, and especially evaluations of dynamic pressure and flow effects. Use of the model can help prevent excessive pressure transients during operations, which will have negative effects on well productivity.

Experiments have been performed in a vertical, extensively instrumented, test well. The experiments focused on dynamic operations; such as kick-off or unloading, changes in liquid and/or gas injection rates and pipe connections as well as "steady state" periods. The tests were performed with either"parasite string" gas injection, or drillstring gas injection.

Simulations with the dynamic model has been performed, using experimental quantities as input (geometries, liquids, rates etc.)

Results show that the model performs very well for "parasite string"gas injection. For drilling gas injection system, the model

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