As a reservoir is depleted, both pressure and production rates decline and lost circulation or differential sticking problems can hamper or prevent drilling operations undertaken to enhance production, such as in-fill drilling, drilling horizontal sections through the reservoir or re-entry operations. Underbalanced drilling is a workable method to drill in such depleted or low-pressure fields. Another benefit of underbalanced drilling is that it can potentially prevent or reduce near wellbore permeability damage (skin), thus enhancing production, reducing stimulation requirements and delaying water or gas break-through time due to the lower pressure drop required for the same flow rates.

In order to design and realize a successful underbalanced drilling project many important elements must be considered conjointly. This is especially important for UBD operations offshore from floating vessels, where the number of critical issues increases significantly. An integrated analysis method has been developed which incorporates these elements, including multiphase flow calculations, gas and liquid injection rate optimization, hydrostatic gradient and frictional pressure loss calculations, cuttings transport, reservoir fluid influx (gas, oil, or water), and operational procedures such as tripping and liquid unloading.

Software has been developed based on this method so that an engineer can quickly and easily consider and compare the use of different light-weight fluids or to see the effect when varying diverse parameters. The software can model gasified liquid, foam, mist air or non-Newtonian single phase mud for upward or downward vertical and inclined flow as well as horizontal flow. A cuttings transport model resulting from full-scale experimental work is incorporated to delineate the limits of adequate hole cleaning and the effect of operational procedures can be studied.

Different field cases have been modeled with the software and the results have compared well with actual data. A gasified fluid example is included to illustrate the potential of the software.

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