Coupled Thermo-Hydro-Mechanical Modeling of CO2 Leakage Through a Micro-Annulus in an Abandoned Wellbore

Abstract

Abandoned wells pose a significant leakage risk for geological carbon (CO2) storage because defects at the casing–cement interface can create preferential pathways for upward CO2 migration. Although thermo-hydro-mechanical (THM) studies have extensively investigated wellbore integrity, leakage pathways are commonly represented using equivalent cement permeability or prescribed leakage fluxes, limiting the ability to distinguish hydraulic leakage behavior from mechanical response. This study develops a coupled THM model that explicitly represents a pre-existing casing–cement micro-annulus as an independent hydraulic domain within a two-dimensional axisymmetric wellbore system. Sensitivity analyses were performed to evaluate the effects of injection pressure, micro-annulus aperture, and cement stiffness on leakage behavior and stress redistribution. Increasing the aperture from 0.5 to 1.0 mm increased effective permeability from 2.08 × 10−8 to 8.33 × 10−8 m2 and increased the average micro-annulus velocity from 2.85 to 11.42 m/s. In contrast, increasing injection pressure from 15 to 25 MPa increased the maximum von Mises stress from 33.52 to 47.58 MPa, with negligible influence on leakage velocity. Increasing the cement’s Young’s modulus from 10 to 30 GPa reduced the maximum von Mises stress from 39.73 to 28.14 MPa without affecting hydraulic conductance. The combined pressure–aperture analysis further showed that the highest leakage capacity did not correspond to the highest stress concentration. These results demonstrate that micro-annulus geometry primarily controls leakage conductance, whereas injection pressure and cement stiffness primarily control the mechanical response. Within the fixed-aperture thermo-hydro-mechanical (THM) framework adopted in this study, this mechanistic separation provides a physically transparent basis for evaluating CO2 leakage through pre-existing casing–cement micro-annuli in abandoned wells. The proposed framework also provides a foundation for future model developments incorporating stress-dependent aperture evolution, multiphase flow, and coupled geochemical processes.