Multiphase flow simulation in an annulus configuration
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On the 20th of April 2010, explosion and fire killed 11 people on the Deepwater Horizon oil rig in the Gulf of Mexico. After two days in flames, the rig sank and left the riser (the pipe going up from the seabed) on the sea floor leaking oil and gas from the well through the gap between the drilling riser and the drill-string (rod holding the drillbit) inside it. The accident is known as the Macondo accident. Immediate concerns were raised regarding the amount of oil being released into the Gulf of Mexico killing fish and seabirds. The size of the oil spill has also played a significant role in the court proceedings where the operator BP at the end accepted to pay a fine of more than 18 bn US dollars. One of many techniques that were applied was based on using simulation software to match the observed frequency of plugs of oil flowing out of the sunken riser. There are, however, serious flaws to this approach. The most important is the lack of reliable models for multiphase flow in annuli, that is, flow of oil and gas in the ring shaped space between the drillstring and the outer pipe. Commercial simulation software packages for flow of oil and gas in pipes lack the ability to correctly predict pressure drop in annulus flow, even when the flow contains only gas or only liquid. When both gas and oil are flowing in the pipe at the same time, which is called multiphase flow, the models are even worse. Well design is thus based on crude formulas with large uncertainty and high error margins. This may lead to inefficient well design and high risk of production problems. ANNULUS will bring a step change in the scientific knowledge in this field and provide data and physical understanding that can be used as a basis for developing more usable and accurate commercial simulation software in the future. Improved models can be used by operators and contribute to significantly safer and more efficient and profitable well operations and production.