Abstract: According to the fundamental differences about the threshold pressure gradient existing in ultra-low permeability reservoirs given by domestic and foreign researchers, this paper proposes unsteady approach to test the threshold pressure gradient of saturated and unsaturated flow in the lab using the outcrop full-hole core with ultralow permeability of the Triassic in Ordos basin by means of fullyautomatic flooding system of AFS300TM provided by the Core Lab in U.S.A. The threshold pressure gradient in the saturated flow has been analyzed with the investigation of pressure variation from downstream and upstream of the core holder. The experimental results indicate that the threshold pressure gradient is not existent in terms of flowing of brine, kerosene and crude oil, and the interaction between the molecular of the fluid and the porous surface of the core plugs will not cause the threshold pressure gradient.

Key words: ultra-low permeability, saturated flow, unsteady state method, threshold pressure gradient