Study of the water displacing oil process in low permeability porous media based on digital rock technology

Abstract

Unconventional oil has played increasingly important roles for the sustainable development of human society. Understanding the two-phase displacement mechanism in low permeability porous media is therefore essential for the efficient development of unconventional reservoirs. In this study, the water displacing oil process and characteristics in low permeability cores are numerically investigated based on digital rock technology (DRT). The complex internal structure of the low permeability core is directly constructed based on micro-CT images and the pore-scale two-phase displacement behavior is mechanistically studied by solving the Navier-Stokes equations which incorporate the interfacial tension (IFT) and wettability effect. The oil recovery rate obtained from the simulation results agrees well with that obtained from the experimental results under the same pressure drop condition. Parameter studies on the effect of the displacement pressure, IFT, and fluid-rock wettability on the oil recovery ratio are further performed. The numerical results show that the micro-throats in the rock have a significant impact on the recovery ratio. Larger displacement pressures can overcome the capillary force in the micro-throats, thus enhancing the oil recovery due to the expanded sweep region of the displacing fluid in the core spaces. IFT and wettability affect the magnitude and direction of the capillary force in the pore-throats, and a smaller IFT and a higher wetting angle can lead to a higher displacement efficiency. More specifically, it is revealed changing the pore wettability from oil-wet to water-wet could improve the oil recovery ratio significantly. In addition, a numerical simulation of the nanoparticle solution displacement process has been performed to study the synergistic effect of IFT reduction and fluid-rock wettability improvement on oil recovery enhancement. It is demonstrated that DRT can substantially apply in mechanistical studies of the displacement process. in low permeability porous media. The findings of this study can help better understanding of the water displacing oil mechanisms in the complex pore scale geometries and thereby contribute to the low permeable oil recovery practices.