Abstract: After a long-term scouring by injected water, the remaining oil distribution is scattered and vertical water flooding is extremely complex in reservoirs developed by water flooding, especially in the reservoirs with thick oil layers. For quantitative study of vertical sweep degree between injection and production wells in thick oil layers, motion equations of water droplets in planar and vertical directions were built based on seepage theory and giving due consideration to the oil-water two-phase flow, effective permeability, and density variation with saturation. A complete set of mathematical models describing the vertical sweep between injection and production wells in thick oil layers was developed. Taking L Oilfield as an example, according to the analysis results, the following findings could be found: a greater injection-production rate, a greater viscosity of crude oil, tighter injection-production well spacing, a lower permeability, a lower water cut, and a higher permeability change degree meant a higher vertical sweep degree of the injected water. Most affected by the thickness of the oil layers was the migration path of the water droplets instead of the sweep degree. Moreover, the injection-production rate limit in thick oil layers was 180 m³/d, and the viscosity limit of crude oil was 200 mPa·s.The limit of injection-production intensity was 3.00 m³/(d·m) when the oil layer thickness was fixed, and the limit of oil layer thickness was 30 m when the injection-production intensity was fixed. The results show that for the thick oil layers developed by water flooding, the injected water infiltrates downward due to gravity, which reduces the vertical sweep degree and recovery. In a nutshell, quantitatively evaluating the vertical sweep degree between injection and production wells in thick oil layers can provide theoretical guidance for understanding the remaining oil distribution and improving the vertical sweep degree.