Wellbore Integrity During Blowouts: Broaching Prevention and Control

Abstract

Abstract During loss of well control events, fracture initiation occurring during the post-blowout capping stage following uncontrolled discharge, can lead to reservoir fluids broaching to the seafloor. A classic example is Union Oil's 1969 oil spill in Santa Barbara channel, where fracture initiation at various locations caused thousands of gallons per hour to broach in the ocean floor over a period of a month before it could be controlled (Mullineaux, 1970; Easton, 1972). The impacts on California's oil industry are still felt strongly today. Disasters as such could be prevented if the effects of the post-blowout loss of well control stages (uncontrolled discharge and capping) are incorporated into the shut-n procedures and the wellbore architecture. Analytical models are used to simulate the loads on the wellbore in the different stages during loss of control and predict capping pressure build-up during the shut-in to indicate fracture initiation during the capping stage. Using these models, critical capping pressure and subsequently critical discharge flowrates is calculated for a well below which fracture initiation would occur. A hypothetical case study with typical deepwater Gulf of Mexico parameters is performed demonstrating the likelihood of fracture initiation during different discharge flowrates, discharge periods and shut-in methods (abrupt/"hard" or multi-stage/"soft"). Further discussion addresses reservoir depletion during the discharge stage preceding the capping, as well as the conditions necessary for upward propagation of these fractures towards the seafloor. Through these fractures fluids from the reservoir ultimately broach into the seawater. The ability to model these fracture failures will enhance the understanding of wellbore integrity problems induced during loss of control situations and create workflows for predicting possible broaching happening during the post-blowout capping stage early on. Dimensionless plots are used to present fracture initiation for different scenarios useful for drilling and wellbore integrity engineers when making contingency plans for dealing with loss of well control situations.