Abstract: At the present, to the research flowing through porous media in most of low permeable reservoirs, the relation between permeability and startup pressure gradient is dained by core flooding experiment, which is timeconsuming and highly costed, and also, the factors taking into consideration are simple. Through the core flooding experiment, the core samples are from a certain low permeable reservoir in west China and with different permeability, processing and regression analysis of the data are carried on based on the mechanism of fluid flowing through low permeable porous media, a new physical model is set up, a new method studing startup pressure gradient is presented, and the mathematical model soluting startup pressure gradient of low permeable reservoir is gained. From the model, it can be derived that the main factors impacting startup pressure gradient are driving pressure gradient and the fluid mobility, there is a positive relation between startup pressure gradient and driving pressure gradient, and a negtive relation between startup pressure gradient and the mobility. By means of the mathematical model soluting startup pressure gradient and practice of low permeable reservoir, the influence of startup pressure gradient on single well productivity, ultimate distance between producer and injector and water content increasing rate are studied.

Key words: low permeability, nonlinear fluid flow, startup pressure gradient, ultimate distance between production wells and inj, water content ratio