Offshore oilfields are characterized by loose sandstone reservoirs, strong heterogeneity and high injection and production intensity. Water channeling gradually develops after entering the high water cut stage, which weakens production performance. Current identification methods usually have high computational costs and low efficiency. A quantitative identification model of water channeling based on inter-well connection units has been established by simplifying the complex reservoir system into a connection network between injectors and producers, which can quickly and accurately obtain strength characteristic parameters for waterflow channels. In addition, a comprehensive evaluation factor M and classification standard for water channeling suitable for offshore heterogeneous reservoirs have been proposed. It indicates a thief zone when M is larger than 0.65, a predominant waterflow channel when M is between 0.55 and 0.65, and no water channeling when M is smaller than 0.55. The application of (an) offshore S oilfield demonstrates that the new method successfully identifies 18 segments of the thief zone and 19 segments of the predominant waterflow channel and improves computational speed by 100 times compared with the conventional numerical modeling method. This novel method allows for rapid and accurate identification and prediction of water channeling, including location, directions, and strengths, thereby providing timely and practical guidance for inefficient water channel treatment.

Alternating injection of different polymers can further trigger the potential of enhanced oil recovery effect by polymer flood, but such process cannot be perfectly numerically modeled by current simulators. Thus a sophisticated black-oil-based multi-component model for polymer flooding is proposed in this study, which can not only improve the computational efficiency over the compositional model, but can also better capture the complicated physiochemical features, such as multi-component polymer mixing, competitive adsorption and the resulting permeability reduction to aqueous phase. Next, the mathematical model is implemented with a finite volume method and an efficient three-dimensional, triple-phase, multi-component polymer flooding research code is developed when combined with the constrained pressure residual (CPR) preconditioned GMRES algorithm. Research on mechanisms and cycle number of polymer altering injection is conducted for heavy-oil reservoirs. The results show that alternating injection can improve compatibility between the strength of injected liquid and reservoir heterogeneity, which makes the change law of flow resistance force of each layer tend to be the same and then contain the conformance reversion, leading to the transformation of injection conformance of heavy-oil reservoir from ‘V’ type to ‘U’ type, and finally boost the mobilization of remaining oil of low-permeability zone. Proper altering cycle number can best suppress conformance reversion, and enhances liquid suction of low-permeability zone by more than 11.1% as well as recovery factor by 3.1%. These can provide technical guidance for sustainable and efficient development of heavy-oil reservoir.

Low primary recovery factor and rapid production decline necessitates the proposal of enhanced oil recovery methods to mobilize the remaining oil resource of tight reservoirs, especially for oil-wet ones, and wettability alteration by injecting a chemical agent such as a surfactant is a promising option. A discrete-fracture-network-based mathematical model is developed with consideration of the displacement mechanisms and complicated physical-chemical phenomena during EOR by wettability alteration, and this model numerically solved by the fully implicit method. Simulation cases are conducted to investigate the production performance and key factors of cyclic injection of a surfactant. Cyclic injection can significantly improve the production of oil-wet tight reservoirs, and the ultimate recovery factor can be increased by 10 percent. The reason is that a surfactant can alter the wettability of a reservoir from oil wet to medium or even water wet, which triggers spontaneous imbibition and favors oil movement from a matrix into a fracture. Better EOR results can be achieved with decreasing oil viscosity, increasing matrix permeability, or decreasing fracture spacing. Cyclic surfactant injection is applicable to reservoirs with an oil viscosity of less than 7 mPa·s, a matrix permeability bigger than 0.01 mD, or a fracture spacing smaller than 150 m. It is favorable for the wettability alteration method by maintaining capillary pressure and reducing residual oil saturation as much as possible.