An analysis approach to assess borehole stability following a hypothetical blowout from representative deepwater scenarios is presented. It addresses whether imposed underbalanced conditions cause sufficient instability that the borehole bridges-over and the well kills itself. The approach uses a series of interrelated analyses: (i) analyses of the kick and blowout development are performed predicting how bottom pressure and in-flow velocity changes over time; (ii) underbalanced wellbore failure in exposed shales and sands is then determined; (iii) cavings and produced sand volumes are calculated from the estimated failure zone, and the transport of these materials in the borehole is determined from the predicted hydrocarbon flow rates; and (iv) bridging tendency is assessed by considering the concentration of cavings in either the enlarged borehole or in flow-paths within the well casing or annuli.

To the best knowledge of the author, the proposed analysis presents the first in-depth study of transient wellbore instability, sand / cavings transport and bridging tendency during a blowout. Analyses applied to a typical deepwater blowout scenario suggest that bridging leading to self-killing can occur only in a small number of situations. This differs from the more widely published data from shallow water Gulf of Mexico Shelf wells which show that self-killing is likely in shallow-hazard scenarios.

Important conclusions from the study are: (1) bridging and self-killing can occur in kicks resulting from a catastrophic loss of riser integrity, due to the loss of the riser margin causing underbalanced conditions in openhole sections of the borehole; and (2) bridging and self-killing is more likely to occur in a well control event that develops while drilling-ahead, due to plugging of the borehole/drill-pipe annulus. Bridging is less likely to occur if a kick develops with the drill-pipe not in the open-hole interval. For self-killing to happen this study concludes that it has to occur during the time that the kick is developing – i.e. before hydrocarbons reach the wellhead. Once the kick has fully developed into a blowout it is predicted that typical high productivity deepwater reservoirs will have attained sufficient borehole flow velocity (in the absence of major constrictions to flow) that spalled or produced formations will be transported from the wellbore without bridging. Once a blowout has occurred, therefore, it is largely too late to consider future bridging as a means of terminating the flow, at least in the short-term.

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