Establishment of Stability Evaluation Criteria for the Oil–Gas Interface during Gas-Cap Gravity Flooding in Oil Reservoirs with Strong Edge-Water Drive

[Image: see text] A three-dimensional (3D) experimental model considering the influence of water energy is established to innovatively analyze oil–gas interface stability, and it is found that a stable gas cap with sufficient driving energy can be formed only under a stable oil and gas interface. So, the bidirectional displacement through the gas cap and edge water can be realized during the late development stage in the strong edge-water drive reservoirs. Results show that the recovery of bidirectional displacement can reach 60%, while the recovery of an unstable interface is basically below 50%. The recovery degree under a stable oil–gas interface is more than 25.63% than that of the unstable interface. Based on the Navier–Stokes equation, a “steady flow velocity u(s)” formula that considers the influence of multiple factors is defined, and the ratio of the flow velocity at the oil–gas interface to the gas injection rate is controlled between 0.6 and 0.85 to determine the criteria for the stable oil–gas interface. At the same time, the differentiation-hindered quasi-number (Gr) is defined to clarify the influence mechanism of various mechanical factors on gravity differentiation and the difficulty of oil and gas replacement. When Gr is equal to 1, the oil–gas interface reaches stable migration conditions. Larger formation dip angle, higher permeability, smaller oil viscosity, and lower gas injection rate are the favorable conditions for forming stable flow. The findings are recommended to be used in reservoirs with relatively homogeneous reservoirs, certain dip angles, and moderate or low oil viscosity.