Abstract: To address the unclear understanding of formation mechanism and occurrence of remaining oil in the late stage of water flooding of low-permeability sandstone reservoirs, a three-dimensional digital core model of the reservoir was established based on Micro-CT scanning technology to extract the inter-connected pore structure and quantitatively characterize the microscopic pore structure of rock samples, and an unstructured tetrahedral grid model was established, taking the low-permeability sandstone of Weizhou 11-1 and Weizhou 12-1 oil fields as the study object. On the basis of N-S equation and level set algorithm, a mathematical micro seepage model of oil-water flow was established, and the finite element method was used to solve the model, the micro water-oil displacement flow was simulated. The results of the study show that the pore throat size and connectivity are the key factors to determine the permeability of the reservoir. Viscous fingering can be found in the micro water-oil displacement simulation. The remaining oil after water flooding are mainly in the form of reticulated remaining oil, remaining oil in pores with dead ends, remaining oil in small pore throats, remaining oil in parallel pore channel, and oil film. With the improved water injection intensity, the reticulated remaining oil and retaining oil in small pore throats can be recovered effectively. After the oil-water viscosity ratio is improved, the sweep volume can be increased. With enhanced wettability, the remaining oil in pores with dead ends and in oil-philic pores can be recovered effectively. The results of the study are of guiding significance for further developing the remaining oil after water flooding in low permeability reservoirs.

Key words: digital core, pore structure, flow simulation, micro water-oil displacement, remaining oil distribution