Abstract: Based on fractal theory and reservoir physics,and assuming that the amount of irreducible water adsorbed on the unit pore surface remains unchanged before and after rock deformation,the theoretical relationship on the change of irreducible water saturation with the drop of formation pressure was deduced and established,and the influences of stress sensitivity and microscopic pore structure heterogeneity on the irreducible water saturation were quantitatively analyzed. As the formation pressure decreases,the irreducible water saturation increases and the irreducible water volume decreases. The increase in irreducible water saturation will lead to the reduction in effective flow space for oil phase,which will narrow down the flow channels for oil and water in the pores,reduce the effective permeability of the oil phase,generate the threshold pressure gradient,and finally impact the fluid flow laws and capacity calculations.

Key words: fractal dimension, irreducible water saturation, stress sensitivity, pore throat structure