46th U.S. Rock Mechanics/Geomechanics Symposium,
2012. American Rock Mechanics Association
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Assessment of potential CO2 and brine leakage from wellbores is central to any consideration of the viability of CO2 sequestration. Most existing work on wellbore integrity has focused on field and laboratory studies of chemical reactivity. Very little work has been done on the impacts of mechanical stresses on wellbore performance. In this study, we use the coupled Thermal-Hydrologic-Mechanical (THM) computer code FEHM to simulate key features of a wellbore (casing, annulus and cement) embedded in a system that includes the upper aquifer, caprock, and storage aquifer. We consider two stress-permeability models: tensile-failure and shear-failure. Tensile failure represents the creation of annuli at the steel-cement or cement-rock interfaces. Shear-induced damage is similar to a Mohr-Coulomb slip mechanism used to represent displacements on faults. Two injection conditions were considered: constant pore-pressure above the failure threshold for the well and a constant flow condition with increasing pressure in a confined reservoir. The simulations show that both tension- and shear-failure modes lead to enhanced permeability of the wellbore system. Shear failure leads to a larger damage zone and generally higher leakage rates than tensile failure. In addition, the shear zone extends further into the caprock and is focused closer to the cement-steel interface.
Wellbore integrity is essential to demonstrating the viability of CO2 sequestration projects [e.g., 1]. Such stresses include increased pore pressure due to CO2 injection. Pore-pressure increases are usually designed to be less than about 0.6 times the principal stress (
σmax; often equivalent to the lithostatic load) to avoid fracturing the reservoir or caprock. Other sources of stress include thermal perturbations created by differences in temperature between the reservoir and injected CO2; internal pressure within the wellbore due to injection or mechanical integrity testing; and tectonic stresses in the geologic system.
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