In the past four decades, several computer programs have been developed to predict steady-state fluid flow in coiled tubing (CT) applications. They are useful in designing the ultimate treatment phase of an intervention but questions remain concerning how their results are affected by the transient nature of the overall job. Any change in the operating conditions on surface or down hole will introduce a transient response. Common treatments such as liquid unloading, under balanced drilling with nitrified fluid, well control, cementing and hole cleaning may never reach a steady state. Only a transient simulation can determine whether a steady state is achieved. If it is not, the simulation can be used by job programmers to develop procedures that will deal with transient phenomena.

This paper describes a transient multi-phase flow simulation that has been developed based on the correlations used to describe steady-state multi-phase flow. The software was extensively validated against data collected on a full scale flow loop, field operations, and other transient software. Several field cases are used to illustrate the transient nature of CT applications and the benefits of using the transient simulation for the operational design, training and job execution.

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