Wellbore-Stability Assessment for Self-Killing Blowouts
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 156330, ’A Wellbore-Stability Approach for Self-Killing-Blowout Assessment,’ by Stephen M. Willson, SPE, Apache Corporation, prepared for the 2012 SPE Deepwater Drilling and Completions Conference, Galveston, Texas, 20-21 June. The paper has not been peer reviewed.

An analysis approach was developed to assess borehole stability following a hypothetical blowout from representative deepwater scenarios. It addresses whether imposed underbalanced conditions cause sufficient instability that the borehole bridges over and the well kills itself. Analyses applied to a typical deepwater blowout scenario suggest that bridging leading to self-killing can occur only in a few situations. This finding differs from widely published data from shallow-water Gulf of Mexico shelf wells that show that self-killing is likely in shallow-hazard scenarios.

Introduction

Uncontrolled influxes of formation fluids into a borehole (a kick) develop into a blowout when the drilling fluid is fully displaced from the well and the formation fluid exits the well. In the case of subsea deepwater wells, the wellhead is at the seafloor and is connected to the drill rig by a marine riser. Depending upon the integrity of the well and riser following a blowout, hydrocarbon discharge may be at the seafloor, at the rig floor, or at some point in between. An influx of hydrocarbons, especially gas, is more severe than an influx of overpressured water. Occurrences of overpressured water (i.e., shallow water flows) or shallow-gas influx are well understood from a shallow- hazards perspective. Another possible route for hydrocarbons to reach the mudline is by way of an underground blowout. An underground blowout occurs when the hydrostatic pressure in the borehole exceeds the fracture gradient of exposed formations. In these circumstances, the increased borehole pressure from actuating the blowout preventer (BOP) to stop flow up the wellbore can cause fracturing of the formation and, depending on the overburden geology, can result in fractures propagating around the wellbore to seabed.

As a kick develops, drilling fluid is displaced from the wellbore by the influx of less-dense fluids that have a formation pressure greater than the hydro-static pressure exerted by the drilling fluid. Thereby, the bottomhole pressure (BHP) in the wellbore decreases during the developing kick, which accelerates the influx. When all the drilling fluid is displaced from the wellbore, unrestricted open flow occurs (i.e., a column of hydrocarbon fluid to the wellhead) and the blowout continues until terminated by natural causes or external intervention. Natural causes that terminate a blowout include depletion (depressurization) of the formation, water breakthrough (particularly in shallow-hazard gas blow-outs), wellbore collapse, or bridging at choke points resulting from an accumulation of produced formation solids in the hydrocarbon flow. The size, productivity, and overpressured nature of many deepwater reservoirs preclude depletion and water breakthrough as viable self-killing methods within acceptable time scales.