To clarify the formation mechanisms of limited injectivity and pressure buildup risks during large-scale CO? geological storage in ultra-low permeability saline aquifers of the Ordos Basin, and to define engineering safety boundaries, this study relied on a million-tonne-level CCUS demonstration project in the Ordos Basin. By integrating multi-source data including 200 km² high-resolution 3D seismic surveys, well logs, and production/injection test data, a high-precision 3D facies-controlled geological model was constructed. The model includes 14 sublayers and accurately characterizes the spatial architecture of multi-stage channel sand bodies and mudstone barriers. Long-term numerical simulations at the million-grid scale were then conducted to investigate reservoir pressure evolution patterns and engineering parameter thresholds for CO? storage under ultra-low permeability conditions of 3.0–6.0 mD. The results show that: Although the conventional "five-well collaborative" scheme meets the initial injection rate requirement, it leads to a persistent high-energy superposition state of 26.0–27.0 MPa in the core reservoir area during long-term storage, directly threatening caprock integrity. The active pressure management strategy incorporating production wells (i.e., the five-injection and two-production (5I2P) well pattern) pre-releases reservoir energy via peripheral production wells while maintaining the connectivity of facies-controlled heterogeneous flow fields. This strategy significantly reduces the overall reservoir pressure by 15%–18% (with the core area pressure dropping to 22.0–24.0 MPa) without inducing fluid breakthrough. Furthermore, the water production rate is positively correlated with the pressure reduction efficiency but exhibits a distinct marginal effect, and the optimal single-well water production rate in the study area ranges from 1000 to 1500 m3/d. It is thus evident that the active pressure management strategy is an effective measure to control pressure buildup and ensure caprock integrity during CO? storage in ultra-low permeability saline aquifers. This study quantifies the engineering boundaries of low-permeability CO? storage systems using field data, providing direct geological basis and parameter support for well pattern deployment and safety regulation of industrial-scale CCUS projects in the Ordos Basin and other regions with similar geological conditions.