Abstract: Aimed at the forming mechanism of the single phase and oilwater two phases threshold pressure gradient (TPG) and the influence on the law of flow through porous media, displacing experiment is done to the typical cores with ultralow permeability from block 119 at a certain oilfield with simulated oil, formation water and injected water respectively through the method of stable "pressure differenceflow capacity" measuring TPG, and the TPG value of single phase and oilwater two phases flow through porous media, the changing law of TPG is analyzed. Both physical simulation and theoriodical research indicate that the relationship between TPG measured with different media and permeability is power function, and the exponent sign is close to -1. Meanwhile, oil water two phases TPG is much greater than single phase one because of capillary force and Jiamin effect. Finally, the application of TPG in ultralow permeability reservoir exploitation is described.

Key words: lowpermeability, threshold pressure gradient, nonDarcy flow, production decline