A Transient Flow Model for Investigating Parameters Affecting Kick Behavior in OBM for HPHT Wells and Backpressure MPD Systems

Abstract

Gas kicks can occur during conventional drilling operations, in which case the well has to be shut in and the kick circulated safely out of the system. In back pressure managed pressure drilling (MPD) systems, one can tolerate minor influx sizes before the well is shut in. However, when using oil based mud, solubility issues can complicate the picture making both kick detection and safe handling of kicks more complex. For sufficient large pressures the solubility can be infinite. A kick can also be taken undetected and reach the riser before the free gas suddenly emerge with a corresponding volume expansion that can unload the riser. Managed pressure drilling can be based on having a rotary control device on top of the riser, which makes it possible to add a backpressure on top to dampen the volume expansion, but possibly also change the depth where free gas emerges. It has been shown in another paper how one can use transient models to predict what kind of surface pressure is needed to maintain a constant bottomhole pressure when free gas emerge. In this paper, a fully transient model developed for a research cooperation between a research institute and Academia will be presented. The model includes behavior of kicks in oil based mud and solubility issues. The model will be used to study the behavior of kicks in riser when considering a backpressure MPD system with special focus on MPD from floaters. One objective will be to show how the results can be impacted by accuracy in the calculation method due to the amount of numerical diffusion present. Another objective will be to show how the PVT model adopted can affect the results. In addition, one will study the impact on results when varying physical input parameters like kick size, magnitude of pore pressure and the geometry of the riser.