Revisiting Post-Blowout Wellbore Integrity: Mechanistic Modeling of the Stress Evolutions Within the Casing-Cement Sheath-Rock Formation System

Abstract

Abstract Eleven ("10 + 1") mechanisms of mechanical degradation that can compromise the wellbore integrity are evaluated by modeling the stress evolutions within the casing-cement sheath-rock formation (C/CS/RF) system. Past attempts from the published literature overlooked the need for establishing an initial stress state that yields stability following cement setting (wait-on-cement). A novel approach is used to calibrate for the initial stress distributions assuming effective-radial-stress-and-displacement continuity along the CS's interfaces with the steel casing and the adjacent-RF. Established analytical expressions form what is generally recognized as the basic stress analysis inside the casing walls and the RF region adjacent to a well. A calibration procedure is necessary to extend the stress model into the CS layer between the casing and the adjacent-RF considering effective-radial-stress-and-displacement continuity. Our mechanistic model is capable of evaluating against four failure mechanisms in the casing layer considering collapse, burst, tensile, and/or compressive stress loads, four failure mechanisms in the CS layer (inner or outer debonding, radial cracking, disking, and shear cracking), along with longitudinal and transverse tensile/"Mode I" fracture initiation in the RF layer (an "underground blowout"), and finally potential reactivation via shear-slippage of natural, pre-existing faults at close proximity, that could trigger seismic events. Post-blowout-reservoir depletion, followed by pressurization after successful capping stack shut-in presents one of the biggest challenges vis-à-vis wellbore integrity. A case study is presented, assessing the wellbore integrity during the aftermath of the MC 252-1 "Macondo Well" blowout from April 20, 2010, in the deepwater Gulf of Mexico (GoM). The results differ from those of previous studies and we think are more realistic due to the improved approach used for calibrating for the initial stress distributions. Wellbore integrity is maintained during the heavy-pressure depletion that the reservoir experienced throughout the 86.2-day post-blowout-discharge period and the subsequent wellbore-pressure buildup that followed the successful well capping.