Taking an offshore deep extra heavy oil reservoir from Bohai Bay as an example, the approach to determine gas-liquid phase equilibrium constants of multi-agent-heavy oil system is established, the nonlinear interpolation method of fluid viscosity considering the diffusion of gas molecules is proposed, and the numerical simulation model of typical FG-SAGD wellpair characterizing the FG-phase behavior is established. based on the actual operation parameters, the performance of pure-steam SAGD and FG-SAGD is compared, which indicates high heat utilization efficiency and oil/steam ratio by FG-SAGD, and 3 cycles of CSS followed by FG-SAGD is optimized. Further more, the key geological parameters of structure dip angle, distance from edge aquifer, distance from bottom aquifer, buried depth, distance from faults and effective thickness are integrally and quantitatively simulated and analyzed, upon which the screening criteria for FG-SAGD deployment is formed, which has significant guidance for the effective development of hardly recoverable deposits in deep extra-super heavy oil reservoirs.

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