Summary

“Worst-case discharge” is a relatively new term given to a hypothetical event that is deemed to be of low probability but of high impact. The consequences of such a discharge are substantial on drilling and completion operations for a specific well. It may also have significant implications on a fieldwide and, perhaps, companywide basis. This paper assesses, hypothetically, the circumstances that lead to such an event and the occurrence's subsequent impact on the subsurface. A hypothetical worst-case-discharge incident could be started by a kick (influx of high-pressure fluid into the wellbore during drilling). In such a hypothetical event, casing collapse could occur under the differential load (pressure within the casing is reduced significantly whereas pressure outside the casing remains high). After capping and controlling the well occur, pressure buildup occurs. Fluid could escape through the collapsed casing and migrate to surface, by means of conduits such as open annuli, unconsolidated sedimentary sequences, fractures, and salt sutures. For example, an open annulus can displace fluid to shallower horizons. Alternatively, the fluid pressure can propagate a fracture that could then broach to the surface or mudline. The paper reviews such a hypothetical event and discusses design changes targeted at mitigating and/or remediating potential negative impacts. The concept presented revolves around the positioning of casing shoes to force fracture propagation into particular horizons, thus minimizing and remediating the effects of the discharge event.

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