Abstract: In response to the problem that the change law of reservoir characteristics before and after water flooding in the C₆ low-permeability reservoir in W Oilfield is unclear, the X-ray diffraction, scanning electron microscope, casting sheet, mercury injection experiment, phase permeability experiment and real sandstone microscopic seepage experiment were applied to study the change law of reservoir lithology, physical properties and seepage characteristics before and after water injection, and to analyze the mechanism and influence reasons of reservoir characteristics change. The research result shows: After water flooding, the total amount of clay minerals shows a decreasing trend as a whole, and the content of calcite increases. Among the clay minerals, the content of illite decreases and the content of chlorite increases slightly; Before and after long-term water flooding of rock samples, the median radius of pore throats of rock samples with good physical properties becomes larger, the permeability increases, and the heterogeneity becomes weak, while the median radius of pore throats of rock samples with poor physical properties becomes smaller, the permeability decreases, and the heterogeneity becomes stronger. The phase-permeability curve after flooding shifts to the left as a whole, the hydrophilicity of the rock weakens, the residual oil saturation increases, the water breakthrough time is earlier, and the water cut rises faster. The long-term scouring of injected water will cause certain damage to the reservoir, and the displacement effect of water-flooding oil will become worse. The research results provide a basis for the description, development effect evaluation and development plan adjustment of low permeability reservoirs.

Key words: low permeability reservoir, water flooding, mineral composition, physical properties, seepage characteristics