Applicability Criteria and Development Characteristics of CO2 Gravity Flooding in Low-Permeability, Steeply Dipping Reservoirs

CO2 gravity flooding can establish a stable artificial gas cap through gravity segregation and selectively mobilize attic oil, making it an effective EOR option for low-permeability, steeply dipping, strongly water-sensitive reservoirs. Establishing a stable artificial gas cap is therefore critical to field performance. To define the applicability criteria and development characteristics of CO2 gravity flooding, this study uses the Yi 59 block of the Moliqing Oilfield in Jilin Province as a representative case. Based on a pore-scale force analysis of free-phase CO2 in porous media and considering key parameters including reservoir dip, permeability, pore-throat radius, interfacial tension, and fluid-density difference, a method was developed to calculate the minimum petrophysical threshold for immiscible CO2 gravity flooding. The calculated minimum pore-throat radius for effective gravity flooding in this block is 0.52 μm. Oil–gas interfacial tension is the dominant control on artificial gas cap formation, and reducing it promotes the development of a stable gas cap. Three-dimensional physical model experiments and compositional reservoir simulations show that reservoir permeability directly controls gravity-flooding performance: as permeability decreases, gravity segregation weakens and gas cap formation becomes more difficult; as permeability increases, upward gas migration becomes more stable, and displacement performance improves markedly. Permeability exceeds 3 mD in most reservoir intervals of the Yi 59 block, satisfying the petrophysical requirements for effective CO2 gravity flooding and indicating good potential for field application. By integrating theoretical analysis, laboratory experiments, and numerical simulation, this study clarifies the dominant mechanisms and applicability limits of CO2 gravity flooding in the block, providing a theoretical basis and technical guidance for optimizing CO2 enhanced oil recovery (CO2-EOR) in the Yi 59 block and in steeply dipping, low-permeability continental reservoirs with closely comparable geological conditions.