The mechanism of the fluid flow in low permeability reservoirs is different from that in middle-high permeability reservoirs because of the existence of the Threshold Pressure Gradient （TPG）. When the pressure gradient at some location is greater than the TPG, the fluid in porous media begins to flow. By applying the mirror image method and the principle of potential superposition, the steady-state pressure distribution and the stream function for infinite five-spot well patterns can be obtained for a low permeability reservoir with the TPG effect. Based on the streamlines distribution, the flowing and stagnant zones in five-spot well patterns can be clearly seen. By the definition of the effective startup coefficient （SUC）, the ratio of the flowing and stagnant zones can be calculated accurately. It is shown that the SUC for five-spot well patterns is not constant, but decreases with the increase of the di- mensionless TPG. By increasing the effective permeability of the formation （such as by the acid treatment and the hydraulic fracture）, in increasing the injection-production differential pressure or shortening the well space （such as by infilling well）, the SUC can be improved. The results of the sensitivity analysis show that a better choice for the SUC enhancement is to shorten the well spacing for small permeability reservoirs and to increase the pressure difference for large permeability reservoirs. This streamline approach can be used to determine the distribution of remaining oil and provide guidance for infilling well.
