Deep-sea shallow CO₂ hydrate-based carbon sequestration technology possesses immense storage potential. However, it is constrained by engineering bottlenecks such as the poor initial flow capacity of weakly cemented sandstone reservoirs, resulting in generally low storage capacities for conventional wellbores. To overcome the contradictions of fluid barriers and spatial limitations, this study proposes a capacity-expansion and efficiency-enhancement strategy of "jet cavitation coupled with liquid CO₂ fracturing." Targeting the mechanical characteristics of shallow sandstone, such as low strength and high porosity, this study established and validated a fully coupled nonlinear Thermo-Hydro-Mechanical-Damage (THMD) model. This model accounts for both tensile-shear composite damage and the transient thermophysical evolution of liquid CO₂, systematically revealing the stress remodeling and micro-fracture nucleation mechanisms under different wellbore morphologies in heterogeneous formations. The results indicate that conventional circular wellbores exhibit a high fracture initiation threshold and a limited stimulated reservoir volume (SRV) due to their geometric symmetry. Conversely, the irregular wellbores formed by jet cavitation break the stress symmetry by virtue of geometric abruptions. They utilize local stress singularities to preferentially induce Mode I–II mixed (tensile-shear) fractures, thereby generating a highly dense, circumferentially dispersed fracture network. This mechanism significantly reduces the macroscopic breakdown pressure by 20%～35% and increases the fluid-solid contact area and effective storage volume by 1.87 times. Furthermore, the study found that matching a cavitation radius of 150 mm~200 mm can control the breakdown pressure within an economically and safely low range. Based on these laws, This study cross-coupled multidimensional geomechanical and thermodynamic parameters to construct a "dynamic safe injection window" control strategy centered on the comprehensive thermo-hydro-mechanical driving factor (). It also calibrated the critical threshold () for inducing fracture network densification. Under the premise of ensuring macroscopic geomechanical structural stability and avoiding caprock breakthrough risks, this strategy achieves multi-objective synergistic control to maximize carbon sequestration capacity. It provides crucial theoretical support for the optimization of engineering parameters and the development of specialized downhole equipment for highly efficient deep-sea carbon sequestration.
 

Deep-sea shallow carbon sequestration; Jet cavitation; Liquid CO₂ fracturing; THMD coupled model; Dynamic safe injection window