Coupling reservoir depletion and geomechanics to assess risks during post-blowout well capping: Case study on Macondo

Abstract

Reservoir depletion can be consequential to wellbore integrity after a blowout, especially offshore. A prolonged post-blowout discharge extends reservoir depletion. “Underground blowouts” (tensile-fracture initiations) occurring after well capping, or shear-driven slow slippage of naturally-occurring pre-existing faults (PEFs) in the near-well vicinity, can compromise post-blowout wellbore integrity. Upward propagation of the initiated tensile fractures may trigger seafloor broaching by reservoir hydrocarbons.

This study examines reservoir depletion analytically, evaluating associated geomechanical implications on the in-situ reservoir conditions and assessing the likelihood of tensile-fracture initiation (oriented longitudinally or transversely-to-the-wellbore) during post-blowout-well-capping operations, in addition to shear-driven slow slippage along PEFs in the near-well vicinity. A set of calculational procedures and thinking sequences are presented, necessary for encompassing the primary effects of post-blowout reservoir depletion on the in-situ stress state and the limits of tensile and shear failures that could aid in the appropriate blowout-contingency decision-making.

Our novel, physics-based scheme (analytical-coupling approach) is applied to parameters from the MC 252–1 “Macondo Well” blowout from April 20, 2010 and the targeted M56 oil reservoir in deepwater Gulf of Mexico (GoM). The reservoir rock is modeled as a porous-permeable medium, considering fluid infiltration to-and-from the pressurized wellbore. The likelihood of an underground blowout correlates with the shut-in wellbore pressure buildup, after successful well capping.

Elevated reservoir depletion via higher post-blowout-discharge flowrates and longer post-blowout-discharge periods (in terms of time duration) are shown to reduce the shut-in wellbore pressure buildup against time following well capping. The “critical discharge flowrate,” an established predictive indicator for underground blowouts following shut-in of an installed subsea-capping stack (SCS) is employed, using data from the post-blowout-discharge period, suggesting underground blowouts to be highly-unlikely for the set of parameters assessed. Finally, the Mohr-Coulomb criterion indicates that shear-driven slow slippage along PEFs in the near-well vicinity is also unlikely, considering the Macondo Well's bottomhole-wellbore-pressure history in the aftermath of the blowout.