Underbalanced drilling (UBD) and managed pressure drilling (MPD) techniques provide the capability to precisely control bottomhole pressure (BHP). During upfront planning and engineering, as well as during the actual operation, the calculation of the BHP using a wellbore hydraulics simulator is crucial because it helps to determine the operational drilling parameters necessary to achieve the desired objectives, equipment requirements, and operational limits.

Several wellbore hydraulics simulators and multiphase flow pressure gradient methods are available in the industry to facilitate the calculation of the BHP, especially when using gasified fluids in both MPD and UBD, and when accounting for formation influx when drilling underbalanced. Furthermore, before beginning operations, the wellbore hydraulics model is calibrated using the actual readings from the pressure while drilling (PWD) sensor. Simulations were performed for several well designs to compare results and to evaluate the possible underlying causes of the differences among simulators and correlations.

Although the primary components in the BHP calculation include density, frictional pressure losses, and applied surface back pressure (SBP), another component that is usually overlooked during planning and engineering is the surface pressure losses because a considerably larger surface pipe line diameter is typically used for UBD. A case study illustrates the effect on the target BHP for an underbalanced operation when using a smaller pipe diameter. Field data are also used to illustrate the calculation of surface pressure losses; a comparison of the calculated and measured values is also evaluated for different scenarios.

This paper focuses on a comparative study of the various methods and correlations available in some wellbore hydraulics simulators and evaluates the results for different wellbore scenarios using MPD and UBD. The study evaluates actual field measurements, including PWD, surface pressures, flow rates, and standpipe, for some of the scenarios. Conclusions and recommendations address the gaps and areas of improvement that should be considered for future developments.

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