Microscopic flow simulation of viscoelastic fluid based on interface tracking method

Abstract

Microscopic residual oil in reservoirs after water flooding can be classified into four types: oil films, oil in the dead end, oil in pores throat, and oil clusters. The oil is trapped in pores throat when the viscous force is not sufficiently large to overcome the capillary forces with water flooding. The oil in the dead end is constrained by the rock configuration. The oil clusters mainly exist in the lower permeability portion of the porous media, which is constrained by the capillary force and rock configuration. In particular, the efficiency of water flooding is extremely low at the high water cut period. Recently, viscoelastic fluid flooding (polymer flooding) was widely applied to improve the recovery after water flooding in the petroleum industry, and the displacement efficiency was greatly improved. However, the poor understanding of viscoelastic fluid flooding restricts its further progress. The investigation of how the viscoelastic fluid mobilizes the residual oil is warranted. In this study, a direct numerical simulation method is employed to simulate immiscible two-phase flows in a micro channel. Viscoelastic effects are simulated using the Oldroyd-B rheological model. The position of the interface between two immiscible fluids is determined by using the phase field method. The model incorporates the wetting condition of the porous media. The dynamics of oil film and residual oil in the dead end are explored under the displacement of water and viscoelastic fluid. The forces exerting on the oil film in the micro channel are analyzed, which contribute to the difference of oil film movement and deformation between the water flooding process and viscoelastic fluid process. Moreover, the sweep efficiencies of water and viscoelastic fluid in the dead end are observed and compared. The concept of dimensionless velocity is introduced as the sweep boundary in the dead end to explain the discrepancy of recovery between water and viscoelastic fluid. The numerical solution used for the simulation is performed using a finite element method. Additionally, a benchmark of the flow of Oldroyd-B fluid past a cylinder between two parallel plates is given to validate the correction of the model and solution. Compared with water, viscoelastic fluid can more easily mobilize the residual oil which can greatly improve the displacement efficiency. The viscosity is not the only one mechanism of the viscoelastic fluid’s mobilization of the oil film; ‘elasticity’ of the fluid is another key driving mechanism which is reflected by the Weissenberg number. The horizontal stress difference of viscoelastic fluid acting on the residual oil film is considerably larger than that of water. The uneven distribution of normal stress caused by the viscoelastic fluid leads the residual oil film to deform significantly. Viscoelastic fluid can greatly improve the sweep efficiency in the dead end and this phenomenon is more obvious in the water-wet reservoir. The sweep boundary of viscoelastic fluid in the dead end is deeper than that of water which explain the reason why less residual oil trapped in the dead end with viscoelastic fluid flooding. The study provides effective guidance for tertiary oil recovery.