Using Pre-Spud Worst-Case-Discharge Predictions to Prevent Underground Blowouts After Offshore-Well Capping

Summary In the aftermath of a blowout, a well undergoes a period of unrestricted fluid discharge (de facto primary recovery), followed by pressure buildup after its shut-in, which can adversely impact wellbore integrity. As part of blowout-contingency planning, U.S. laws mandat performing “worst-case-discharge” (WCD) flowrate and volume calculations, before spudding an offshore well. During well-capping attempts following loss-of-well control, an underground blowout may occur (starting by tensile failures on the wellbore walls), leading to reservoir hydrocarbons broaching the seafloor. To prevent such unfortunate scenarios, critical discharged-mass quantities (expressed as variations in thepost-blowout discharge flowrate at standard conditions against time) are compared with these pre-spud WCD predictions to assess potential subsea-capping-stack (SCS) shut-in strategies to safely cap a blown-and-discharging well. The thrust of this study is the development of hybrid data-driven/physics-based workflows for calculating critical discharged-mass values (below which underground blowouts must be expected) to help facilitate the selection of appropriate well-capping strategies in complex offshore-blowout scenarios. Assuming a robust cemented-casing architecture, the casing-shoe depth is the primary line-of-defense against underground blowouts and subsequent seafloor-broaching events, with Union Oil’s 1969 “A-21” well blowout in California’s Santa Barbara Channel being the quintessential example and similar events fortunately avoided during the aftermath of the 2010 MC 252-1 “Macondo Well” blowout. Analytical and numerical reservoir-depletion modeling is coupled with near-wellbore geomechanics to derive closed-form expressions for critical discharged-mass calculations that can be used to indicate whether the pressure buildup inside the wellbore induced by a specific well capping strategy will exceed the local fracture initiation pressures. Derived basin-agnostic models describe the dependency of the critical discharged-mass values on parameters controlled by practitioners on the surface, such as the SCS shut-in timing and schedule. These models can assist blowout contingency planning via comparisons between these critical discharged-mass quantities (which vary with the extent of the post-blowout reservoir depletion) and the law-mandated, pre-spud WCD predictions with the help of empirical models composed by a physics-based core, complemented by numerically obtained data.