Abstract: Based on the previous studies available, a new method for calculation of rational well spacing density and ultimate well spacing density of a field or reservoir after development by water drive process for a number of years. This method not only takes account of the investment, overhead expenses per year and the interests of crude oil vendition income, but also considers the injection-to-production-well ratio, remained oil distribution and production decline rate, etc. This method can be used to demonstrate if the old field patterns are infilled or not for purposes of avoiding the risk of uncertain investment to the maximum.

Key words: water drive, water flood, well spacing density, recovery factor