Lost circulation due to induced fracturing is a costly problem in the petroleum drilling industry. After a discussion of the processes of fracture initiation and propagation, this paper reports a case from the Norne field offshore Norway. A production well suffered lost circulation during drilling at a mud density significantly below what had been perceived as the local fracture gradient.

Striking similarities were found between the lost circulation incident and the results of an extended leak-off test that had been performed at the same depth in an offset appraisal well just months earlier. Both datasets indicated that the formation at the relevant depth had a lower-than-prognosed minimum in situ stress, but a high fracture initiation threshold. This additional barrier to lost circulation was irretrievably lost once a fracture had initiated and penetrated a certain distance away from the borehole.

Based on this understanding of the nature of the problem, and an updated set of operational constraints, the drilling operation progressed to the prognosed total depth of a well section that could otherwise have been lost. Future wells will be based on an updated set of pore pressure, collapse pressure, minimum stress and fracture pressure gradients that were established after the incident.

The case illustrates that the conventional methods of determining the pore pressure and fracture gradient may carry considerable uncertainty.

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