Casing Design Load Case for Shut-In After Worst Case Discharge Using a Progressive Depth Analysis Approach

Abstract

The US Bureau of Ocean Energy Management (BOEM) defines Worst Case Discharge (WCD) as the maximum foreseeable daily flow of oil from a well, commonly referred to as a "well blowout." When casing design engineers refer to "WCD", they focus on the impact of shutting in a maximum flow case on casing stresses as opposed to "volume" of discharge. This is sometimes referred to as Worst Case Discharge Pressure (WCDP) or Maximum Discharge Casing Pressure (MDCP) or Worst Case Discharge Load (WCDL). The US Bureau of Safety and Environmental Enforcement (BSEE) may refer to this pressure as Pressure Gradient (PG). Some international operators refer to "WCD" only as a load case and not a discharge volume so the term "WCD" may mean different things to different people. A new approach is presented to augment casing design load methodology associated with shut-in of an openhole flow event and where multiple hydrocarbon zones are exposed in an openhole section. The proposed methodology determines the casing design loads for the highest potential flowing temperature and highest shut-in at capping pressure case establishes stresses on the existing casing and verification of casing hanger lift-off. Uniform understanding and resolution of volume-based assessment and well survivability considerations in current practice are discussed. The Progressive Depth Approach focuses on determination of the highest stress by considering the progressive contribution of multiple reservoirs flowing during an uncontrol flow event and subsequently shut-in. The load case is conceptually structured in two events: 1) uncontrolled hydrocarbon flow through exposed casing strings with related discharge to ambient conditions (seabed in subsea wells) and 2) shut-in of the well with capping installed at wellhead. From a casing design and well integrity viewpoint, the load case enables quantification of stress changes along existing casing strings and at the casing hanger to assess well survivability. Thermal effects typically generate compressive forces (hot cases) and induce annular fluid expansion in casing annuli (if sealed) potentially leading to hot casing collapse scenarios in combination with severe draw-down pressures in the inner wellbore. Axial forces generated at the casing hanger need careful evaluation relative to lockdown mechanism ratings in a subsea wellhead. Operators may incorrectly assume that shut-in of the deepest reservoir in an open-hole section generates the highest stress casing load. Rather it could occur from a shallower reservoir depending on formation pressures, fluid composition and contribution of the remaining reservoir exposed (commingled fluid and flow conditions). An advanced load methodology to apply flow conditions and shut-in of an open-hole flow event given multiple reservoir zones for the determination of maximum thermal effect and shut-in pressure at capping wellhead condition is presented herein.