Using Worst Case Discharge (WCD) Estimations to Prevent Underground Blowouts After Well Capping

Abstract

Abstract As part of post-blowout contingency planning, it is mandated by U.S. laws to perform "worst case discharge" (WCD) flowrate and volume calculations a-priori to spudding an offshore well. During well capping attempts following loss-of-well-control, an underground blowout (tensile-fracture initiation) may occur, leading to reservoir hydrocarbons broaching the seafloor. To prevent such scenarios, WCD estimations coupled with critical discharge flowrates, used in order to assess potential capping stack (CS) shut-in strategies vis-à-vis the number of shut-in steps and their duration. Development of workflows for calculating the critical discharge flowrate, below which fracture initiation takes place during post-blowout capping, also used to determine the optimal CS shut-in strategy. Assuming a robust casing architecture, the casing shoe depth is the primary line of defense against loss of control-induced fracturing. Fracture initiation and subsequent broaching, although narrowly avoided during the Macondo disaster, is a real possibility in overpressurized, stacked sequences encountered in deepwater drilling operations Reservoir depletion models are coupled with wellbore geomechanics to derive closed-form expressions for critical discharge flowrates used to indicate whether the wellbore pressure buildup induced by a specific CS shut-in schedule will exceed the local fracture initiation pressures. Sensitivity analyses are conducted to evaluate the impacts of several in-situ geomechanical parameters, acting as independent variables, along with the casing-shoe depth. Hence, basin-agnostic physics-based models can be derived that describe the dependency of the critical-discharge-flowrate values on parameters (such as the number of steps and the time duration-per-step of the CS shut-in), controlled by these practitioners on the surface are presented. These models can assist contingency planning via comparisons between these critical discharge flowrate values (variable with respect to the extent of the post-blowout reservoir depletion and casing-shoe depth) and the law-mandated, pre-spudding WCD estimates with the help of modern data analysis tools for building useful tables and figures.