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Abstract: Lamadian Oilfield has entered a high water-cut stage after long-term water flooding, and injected water circulates in high-permeability channels. Constant-rate and constant-pressure mercury injections were used to study the change in the physical properties of the reservoir during development by water flooding and that in the features of the pore structure of the reservoir before and after long-term water flush. The constant-rate mercury injection analysis shows that the throat radii of main infiltration paths are enlarged and the pore radius distribution remains constant after long-term water flooding, suggesting that the infiltrating features are controlled mainly by throats instead of pores. The constant-pressure mercury injection analysis shows that water flooding enlarges the pore-throat radius median and maximum radius, increases large throats, and the corresponding pore-throat distribution frequency and permeability. Both methods reveal that pore throats are enlarged after long-term water flooding and large pore throats are the main path of fluid infiltration. Key words: high water cut, long-term water flooding, pore structure feature, pore throat, constant-rate mercury injection analysis, constant-pressure mercury injection analysis