Mechanistic assessments of wellbore integrity for geologic carbon sequestration in deep saline aquifers via bulk-CO2 injection

This paper examines wellbore integrity during bulk injection of carbon dioxide (CO₂) in deep saline aquifers, by modeling the associated stress-distribution evolutions within the three-layer casing-cement sheath-rock formation (C/CS/RF) system. A novel scheme, incorporating a total of eleven (“$10+1$″) failure mechanisms is used to assess each layer of the C/CS/RF system, discretely.

The magnitude of the normal-effective stresses along the C/CS and CS/RF interfaces provide a set of calibration parameters to model the stress distributions within the intermediate-CS layer via estimating “free strains” created during the cement-curing process, while the integrity of the C/CS/RF system is assessed over a 30-year period of bulk-CO₂ injection in a closed (bounded system) and an open (unbounded system) aquifer. Disking failures, along with radial and shear-cracking tendencies, predicted within the intermediate-CS layer in closed-aquifer configurations, provide potential pathways for CO₂ leakages back into the atmosphere. The three aforementioned failure tendencies remain in open-aquifer configurations also, albeit to a smaller degree.

The generated stress distributions indicate no-inner debonding tendency along the C/CS interface, with the outer-debonding limit approached at the CS/RF interface, yet not exceeded. Furthermore, no tensile failures (via longitudinal or transverse-fracture initiation) are predicted from the CS/RF interface, nor any casing failures (related to compressive/tensile and collapse/burst stress loads). Neither of these scenarios (closed or open-aquifer configurations) is expected to trigger seismic activity along pre-existing faults (PEFs) in the CO₂-injection well’s vicinity. Finally, CO₂/brine displacement is overviewed using analytical and numerical means, as an alternative approach for potentially increasing storage efficiencies without wellbore-integrity compromises.