Geomechanics of Post-Blowout Fracture Initiation and Broaching During Loss of Control in Offshore Wells

Abstract

During discharge, the drop in wellbore pressure may cause casing collapse at various locations along the cased part of the hole, exposing at those locations the formation to the fluid inside the wellbore. As a result, in the subsequent capping stage, the wellbore pressure buildup may exceed the pressure necessary to initiate a fracture either directly below the casing shoe, or at any point along the cased part of the hole where casing failure had occurred during discharge. Propagation of those fractures upwards towards the seafloor can provide a pathway for reservoir fluid broaching leading to an ecological disaster. The ability to model these fracture failures will help understand wellbore integrity problems from loss of control situations and predict the possibility of broaching during the post-blowout capping stage. Dimensionless plots are used to present fracture initiation for drilling and wellbore integrity engineers for different casing shoe depths. When targeting highly-pressured formations as in the deepwater Gulf of Mexico, wellbore architecture must be made with considerations of the wellbore pressure variations during the two loss of control stages (discharge and capping). Intermediate casing type and casing shoe depth decisions should be made reflecting such extreme scenarios.