Feasibility Evaluation of Natural Gas Immiscible Flooding for Tertiary Oil Recovery in Edge-Water Reservoir: A Case Study of X Oilfield

X Oilfield is a typical strong edge-water reservoir characterized by small formation dip angle, large aquifer volume, high porosity-permeability, and strong reservoir heterogeneity. Currently, it has entered the ultra-high water-cut stage, making it difficult to further enhance oil recovery using conventional technologies. To improve oil recovery, this study systematically investigates the feasibility of natural gas immiscible flooding for tertiary oil recovery in edge-water reservoirs through integrated laboratory experiments and numerical simulation, based on the geological and development characteristics of X Oilfield. The optimal flooding well pattern and injection-production parameters are also determined. The results show that: natural gas exhibits excellent solubility expansion and viscosity reduction effects. Switching to natural gas flooding after water flooding can still enhance oil recovery by 9.21% points, demonstrating significant potential for EOR even in the ultra-high water-cut stage. The optimal technical strategy includes: a staggered horizontal well pattern, conformance control before gas injection, cyclic gas injection, a gas injection rate of 0.08 HCPV/year, a liquid production rate of 8%, and converting injectors to producers when the producing gas-oil ratio exceeds 2000 m3/m3. For edge-water reservoirs, a synergistic development mode of “early-stage natural gas flooding + late-stage natural water flooding” is recommended, which proves to be both technically feasible and economically viable. This study provides a novel technical approach for further enhancing oil recovery of edge-water reservoir at ultra-high water-cut stage, and has important guiding significance for the efficient development of similar reservoirs.