Abstract: The calculation of dimensionless water influx rate has to be made for both reserves estimation and performance prediction in process of development of natural water drive reservoirs. However, it is not practical and not precise to calculate the dimensionless water influx rate by available table-lookup method. Based on the theory of mathematical statistics, the widely applied empirical equation method of van Everdingen-Hurst dimensionless water influx rate calculation for infinite radial water drive and finite natural water domain is evaluated. And a new method for calculation of finite confining natural waters' dimensionless water influx rate is then proposed in this paper.

Key words: reservoir, water drive, performance analysis, water influx rate