Mechanistic Modeling of Wellbore Integrity During CO2 Injection in Deep Saline Aquifers

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.