Thermo-mechanical assessment of wellbore integrity in CO2 geological storage: cement-sheath cracking and interface debonding

Abstract

Long-term sealing performance of a CO2 injection well depends strongly on the mechanical integrity of the cement sheath and its bonds with the casing and formation. This study develops a static, three-layer thermo-poroelastic analytical screening model to evaluate prescribed pressure-increase and uniform-cooling end states under isotropic and anisotropic horizontal stresses. A tension-positive, compression-negative effective-stress convention is used throughout. Radial- and axial-tension initiation are screened from the hoop and axial effective stresses, whereas in-plane shear potential is evaluated with a radial-hoop Mohr-Coulomb safety margin. Interface initiation is evaluated with a shifted quadratic nominal-traction (QUADS) index, defined as the conventional quadratic sum minus one. The interface calculation applies this initiation surface to analytical Lamé tractions and does not use cohesive elements or solve a traction-separation law, damage evolution, or microannulus aperture. For the isotropic case, cooling from 80 °C to 32 °C at zero overpressure produces in-plane shear-criterion exceedance near the casing-cement interface and a C/CS shifted QUADS index of 1.49. Increasing the pressure increment reduces shear-criterion exceedance and interface-initiation potential but shifts hoop stress toward tension; with the deliberately conservative tensile threshold of 0 MPa, the radial-tension criterion is exceeded at the higher-pressure states. Under horizontal-stress anisotropy, SHmax/Shmin = 1.2, the fields become strongly azimuth dependent, and the C/CS shifted index remains slightly positive at Δp = 800 psi. The results reveal a trade-off between suppressing cooling-induced shear/interface initiation and increasing tensile-initiation potential. The model is intended for rapid screening and requires site-specific calibration before design use.