Eni Near Balance Drilling (e-nbd™) is an innovative drilling process to maintain constant bottom hole annular pressure at all times while circulating, and to manage annular dynamic hydraulic pressure profile accordingly. The operational benefits of this process are measured on field operations through the enhancement of borehole quality, the reduction of NPT and the ability to reach targets otherwise inaccessible.

e-nbd™ can improve drilling operations safety, especially by reducing the probability of the most significant threats, which is a blowout event. These benefits are not measurable on operations, unless having a huge number of statistical records.

Nonetheless the probability of a complex event like a blowout can be effectively calculated through dedicated fault tree models.

Eni drilling department has developed a replicable blowout probability model that is suitable for most of the current drilling and completion situations. This model is offering a high number of possible operating scenarios and detailed settings, in order to guarantee the accuracy of each analysis.

This model has been used to address the benefits of e-nbd™, in order to compare analogous wells to be drill with or without e-nbd™. In both situations results were analysed to measure the overall impact on blowout probability and to further address the sensitivity to technical / non technical factors.

By comparing two situations using a probabilistic fault tree, this paper shows that the key factors affecting a blowout probability are notably impacted by the use of e-nbd™. These key factors are not only technical, but also organizational and human.

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