Abstract: The oilOgas system and the porous media are interaction system. The three phases of vapor , liquid and solid (rock) will coOexist in the porous media when the pressure is under the sat uration pressure. Since the pore radius is very tiny in the porous media , the capillary pressure should be considered. The phase equilibrium calculation model concerning capillary pressure is es
tablished in this paper. The capillary pressure , the saturation pressure , and the liquid content of the constant compositional expansion are calculated by using this model for one black oil reservoir , one volatile oil reservoir and two gas condensate reservoirs. The conclusion of the capillary pressure effect on the ilOgas system is summerized at the end of this paper.

Key words: porous media, oil and gas reservoir, phase equilibrium, capillary pressure