In this paper, we examine wellbore integrity during carbon dioxide (CO2) injection in deep saline aquifers, by modeling stress-distribution evolutions within the casing-cement sheath-rock formation (C/CS/RF) system. For our analysis, a mechanistic model is used, which considers a total of eleven ("10 + 1") modes of mechanical degradation assessing each of the three layers of the C/CS/RF system discretely.
The integrity of the wellbore is assessed by modeling the casing layer as a thick-walled cylinder and the adjacent-RF layer as a poroelastic solid, accounting for fluid infiltration into and out of the pores in close proximity to the CS layer. The magnitude of the normal-effective stresses at the C/CS and CS/RF interfaces provide calibration parameters for the stress distributions within the intermediate-CS layer, honoring linear elasticity. This novel method is used to determine the initial state of stress within the C/CS/RF system with balanced conditions inside the wellbore, following cement setting. Using input data from the literature, 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 subsurface aquifer.
In closed-aquifer configurations, disking failures along with radial and shear cracking tendencies are indicated within the CS layer, providing potential pathways for CO2 leakages back into the atmosphere. In open-aquifer configurations, the three aforementioned tendencies for mechanical degradation remain, albeit at a smaller degree. The generated stress distributions demonstrate no indication of inner debonding along the C/CS interface, while the outer-debonding limit is approached on the CS/RF interface, but never exceeded. Moreover, no tensile failures (via longitudinal or transverse-fracture initiation) is expected along the CS/RF interface, nor casing failures (related to compressive/tensile loads, collapse and burst stress loads). Finally, none of the scenarios considered are expected to generate seismic activity along preexisting faults (PEFs) near the injection well.