Abstract：According to the results of mine test and lab experiment，non-Darcy seepage flow will occur during the exploitation of reservoirs with low permeability and extra-low permeability，especially the crude oil seepage flow，which will have a obvious startup pressure gradient. Considering that the seepage flows of the oil and water have startup pressure gradients and that rock has stress sensibility，the three-dimensional oil-water two-phase mathematical model is built，and combined with the characteristics of the low permeability reservoir¢s water injection exploitation. The finite difference schemes of the seepage mathematical model are obtained by the finite difference method；and the numerical simulation software is developed. The numerical simulation results of the production history of Yuan 121–3 block of Zhaoyuan Oil Field in Daqing are compared and successively verified with the field production data. Based on the five-point injection-production well network，a square injection-production unit is designed. Under the condition of keeping bottom pressures of producer and injector wells constant，the space between the injection and the production well is changed；and the formation fluid pressure，fluid pressure gradient，formation absolute permeability，and the spatial distribution of the startup pressure gradient are obtained by using the numerical simulation software when oil well begins to produce water. At last，the effective space between the injection well and the production well is achieved to provide references to the network arrangement of the injection and production wells for the extra-low permeability reservoir.
