According to the fact that starting up pressure gradient exists when fluids flow in low permeability reservoir, a nondarcy flow mathematical model is established, in which the factor of starting up pressure gradient is included for low permeability reservoir. Software, by solving the partial differential equations by difference method, is developed, and is used to calculate development index and the distribution of saturation degree for low permeability reservoir. To obtain the sweep efficiency of water flooding accurately, this software is applied to simulate the distribution of saturation field in the conditions of different permeability and different well spacing. Results have shown that the sweep efficiency is reduced from 90.80% to 54.00%, recovery efficiency decreases from 26.14% to 10.65%, when permeability decreases from 50ÃÂÃÂ10-3 ÃÂÿm2 to 5ÃÂÃÂ10-3 ÃÂÿm2, given injector producer distance of 300 meters. It indicates the injector producer distance of 300 meters is not fit for low permeability reservoir in which the permeability is below 50ÃÂÃÂ10-3 ÃÂÿm2. In order to improve development efficiency, the distance between wells has to be reduced to enhance sweep efficiency and oil recovery efficiency.
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