Gas–water two-phase displacement mechanism in fractal structure of tight sandstone gas reservoir based on nuclear magnetic resonance experiment

The pore structure of tight sandstone gas reservoirs exhibits significant complexity, giving rise to intricate gas–water two-phase flow behavior during reservoir development. This paper studied the gas–water two-phase seepage process in tight sandstone through nuclear magnetic resonance experiments combined with fractal theory. The results showed that the fractal dimension of small pores was significantly smaller than that of medium and large pores. As the permeability of tight sandstone increased, the fractal dimensions across different pore scales also increased. The gas–water relative permeability experiments showed that the increase in core permeability led to a decrease in bound water saturation and an increase in residual gas saturation. The gas relative permeability decreased significantly, while the liquid relative permeability showed a certain upward trend. With the increase in displacement pressure, the fractal dimensions of different pore scales increased, and the liquid distribution became more complex, among which the change of the fractal dimension of small pores was significant. The heterogeneity and complexity of large pores had the most significant impact on residual gas saturation and isoelectric point, while the fractal dimension of small pores had the highest linear correlation with irreducible water saturation. This study revealed the microscopic mechanism of gas–water two-phase migration in the fractal structure of tight sandstone, providing a theoretical basis for the efficient development of tight sandstone gas reservoirs.