Abstract: To address the issues of unclear understanding of the variation law of clay minerals and micro-porosity structure within low permeability tight sandstone reservoirs before and after water flooding, the Chang 2 and Chang 6 Reservoirs in Yanchang Oilfield of Ordos Basin were taken as research objects. The types and contents of rock and clay minerals, the characteristics of microscopic pore throat structure are studied by using experimental methods such as casting thin sections, X diffraction and high pressure mercury intrusion, and the variation law of reservoirs before and after water flooding was analyzed. The results show that the microscopic heterogeneity of Chang 6 tight reservoir is stronger than that of Chang 2 low permeability reservoir. After water flooding, the content of illite/Montmorillonite mixed layer and illite in Chang 2 Reservoir increased, chlorite content and illite/smectite mixed layer ratio decreased, and the content of illite/smectite mixed layer, chlorite content and illite/smectite mixed layer ratio in Chang 6 Reservoir decreased. When the pore throat radius is large, the injected water improves the pore throat. On the contrary, the pore throats are damaged by the injected water. The larger the core permeability is, the faster the pressure propagation velocity is, the faster the injected water advances, and the more water is visible at the outlet end. This study can provide technical reference for water flooding development of low permeability tight sandstone reservoirs.

Key words: low permeability sandstone, tight sandstone, water flooding, reservoir characteristics, microscopic pore throat variation, Ordos Basin