A unique simulation model developed for an underbalanced primary cementing of a 7 inch liner at the Gullfaks field in the North Sea is described and used for design, during operations, and post-processing. The value of a close cooperation between modelling experts and operational people is demonstrated and emphasised, and operational recommendations related to the use of advanced computer models before and during operations are given. Due to very small margins it was desirable to avoid pressure changes in the open hole section while circulating the cement. This was obtained by circulating fluids out through a choke that was accurately regulated to maintain constant bottom hole pressure throughout the circulation phase. The fluids in front and after the cement had relatively low density to compensate for the high density of the cement, and choke regulation was crucial before the cement was in place. The new transient cement displacement model has dynamic choke regulation to make it possible to design choke operations accurately. It is fast and flexible to allow adoption to unforeseen changes in plans and setup. The results of this paper have applications in any cementing operation where managing bottomhole pressure with a choke is required. The model can be applied in any such operation. The paper illustrates how to use managed pressure drilling ideas when cementing a well with very narrow pressure margins. The work is considered to be the first step towards managing cementing in combination with managed pressure drilling

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