REMOBILIZATION MECHANISM OF MICROSCOPIC RESIDUAL OIL IN SANDSTONE RESERVOIRS AT THE HIGH WATER-CUT STAGE

As many of China's oil fields have entered a high-water and ultra-high water stage, the high-efficiency and precise remobilization of residual oil is currently the key difficulty in stabilizing production and increasing recovery in these mature oil fields. Due to the highly dispersed residual oil clusters, the trapping and mobilization mechanism is not fully understood and there is a lack of effective displacement of the discontinuous residual oil in the complex pore structure, resulting in high water consumption and low oil recovery. To reveal the formation and remobilization mechanisms of discontinuous microscopic residual oil, this study constructs a pore-scale oil-water two-phase flow simulation model combining the Volume of Fluid (VOF) method with capillary pressure filtering method, which is suitable for heterogeneous sandstone reservoirs in the late high water-cut stage. This model quantitatively reveals the quantitative relationship between pore topological parameters (coordination number, pore-throat ratio) and residual oil morphology. Breaking through the limitations of the homogeneous wetting assumption in sandstone, the coupling mechanisms between heterogeneous pores and heterogeneous wettability (based on in-situ contact angle) on residual oil distribution were investigated. The microscopic remobilization condition and displacement dynamics for different types of residual oil were systematically analyzed from a mechanical perspective. The results show that the continuity of the residual oil is influenced by pore structure heterogeneity. The pores with lower coordination number prefer to form network oil clusters, and the singlet oil bubbles is controlled by the average pore-throat ratio. Pores with good connectivity are the potential breakthrough positions for oil phase. The heterogeneous wettability will cause fluctuation and weakening of capillary force, leading to the advancement of high water-cut stage. At this point, pressure transport between the discontinuous residual oil clusters is invalid. The direction of the capillary forces then reverses and becomes the main resistance. The residual oil of network and multiple form affected by the boundary conditions is easy to be utilized. The enhanced driving force, coupled with the capillary force, can move the phase interface again, to overcome the viscous dissipation during the disconnection and transportation process, and to remobilize the network and multiple residual oil clusters. At the high water-cut stage, singlet oil can be divided into driving force control and tangential viscous force control, reducing the interfacial tension is the key to further recover singlet bubbles. This study provides a theoretical basis for adjusting the late-stage development measures and program design of water-flooding reservoirs.