Abstract: Water injection is an important means to increase the production of tight sandstone reservoirs, which is widely used in tight oil and gas exploration. However, long-term water injection changes the physical and mechanical properties of formation rocks and further affects the production of oil wells. Comparison experiments were carried out on the same formation cores with water injection for 15 years and without water injection to explore the microscopic mechanism of the effect. The differences of elastic mechanics parameters, mineral composition and microscopic structure of the formation rocks before and after water injection were obtained through experiments. After the long-time water injection, the mineral composition and internal structure of rock were changed. Especially the loss of clay minerals in the form of cement and fillers caused a decrease in the cohesion among the rock particles and an increase in the porosity. After long-time water injection, clay minerals and calcite content decreased, the fillers and cements between rocks particles reduced, the small pores developed into large pores. Thus, the mechanical properties of sandstone were weakened and the deformation capacity increased.

Key words: tight sandstones, water injection, weaknening, mineral composition, microstructure