Abstract: According to the Kozeny equation of the simplified model for capillary bundle in rock pore structure, on the assumption that the thickness of irreducible water homogeneously adhering on capillary wall keeps unchangeable, this article presents the theoretical formula for irreducible water saturation variation with formation pressure fall, and the calculated irreduclible water saturation variation is determined by core samples from five gas wells. The case analysis indicates that the irreducible water saturation is gradually increased with the formation pressure fall-off. This is significant to guide the rational and efficient development of the producing water gas wells in sandstone gas reservoir.

Key words: sandstone gas reservoir, formation pressure, porosity, permeability, rock bulk strain, irreducible water saturation