The constant bottom hole pressure (CBHP) method of managed pressure drilling (MPD) has important advantages versus conventional drilling, especially for well control. One is the ability to stop formation flow by increasing casing pressure while continuing to circulate rather than shutting in the well. A requirement for applying this method is for the pressure rating of the rotating control device to exceed the maximum casing pressure experienced during kick circulation. This peak casing pressure is typically estimated by applying the gas law for a single bubble calculation or by using a computer simulation. This paper describes a new, step wise method that combines gas law calculations with empirical knowledge developed by Ohara (1996) of gas distribution and velocity based on full-scale kick circulation experiments. The new method was applied to predict the maximum casing pressure expected during kick circulation for a range of well geometries, kick sizes, fluid properties, and circulating rates. The results of applying the new method were compared to single bubble calculations, transient multiphase computer simulations, and the actual pressures recorded in test wells. The proposed method provides a simple, potentially more accurate way to predict the maximum casing pressure during circulation of a gas kick. Consequently, it can be used to determine the surface equipment pressure ratings and formation fracture resistance required to provide the desired kick tolerance to drill safely using MPD equipment for well control.
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