Sweet-Spot Zonation-Guided Cyclic Gas Injection for Offshore Buried-Hill Condensate Reservoirs: Integrating Enhanced Recovery with CO2 Storage

Abstract

Fractured buried-hill condensate reservoirs are often developed under near-dew-point conditions, where retrograde condensation and preferential fracture channeling jointly reduce sweep efficiency and impair deliverability. This study proposes a sweet-spot-guided differentiated cyclic injection workflow that couples multiscale fault-fracture characterization, integrated geological-engineering sweet- spot zonation, and compositional simulation. The reservoir is partitioned into three functional zones by clustering the integrated sweet- spot attributes, enabling spatially differentiated injection timing and intensity. A progressive set of development strategies is evaluated in a commercial compositional simulator, evolving from natural depletion to continuous injection and finally to an integrated differentiated cyclic injection regime with a staged transition from hydrocarbon gas to CO2 injection. Mechanism analysis introduces dew-point exposure, defined as the time integral of pressure deficit below the dew point in the near-well domain, to quantify thermodynamic impairment. Simulation results show that low-connectivity zones exhibit the highest sensitivity to dew-point exposure, and delayed pressure maintenance in these zones leads to stronger condensate accumulation and larger recovery penalties. The integrated differentiated cyclic injection strategy reduces dew-point exposure, improves volumetric sweep, and increases the gas and condensate recovery factors to 62.3% and 32.1%, compared with 52.1% and 24.6% for continuous injection. For seasonal peak-shaving operation, an optimal cushion-gas ratio of 55%-60% balances working-gas efficiency and minimum reservoir pressure relative to the dew point. Over the 20-year evaluation period, the strategy enables approximately 26 Mt of cumulative CO2 storage while maintaining recovery performance.