Cainan oilfield is located in northwest China. The Jurassic sandstone reservoir with huge natural edge water and bottom edge water energy began to put into production in 1992 and injection in 1995. The volume of natural water is about 35 times the volume of reservoir. The reservoir production rate began to decline in 1999, at present, the production rate decline is very rapid, the average water cut is 79.83% and the recovery is 44.04%. There is a most pressing problems needed to address, which is that the water injection and the natural water influx how to influence the reservoir pressure system respectively, including the pressure maintenance level and the pressure exhaustion level, etc.

No reservoir engineering method is found to discriminate effectively the influence of water injection and natural water influx upon reservoir pressure system. A new method based on material balance principle is presented to resolve the above practical problem. Study is taken on from two aspects that fluids flow in and out of the reservoir. In terms of "in", the injection water and the influx water are divided into two parts according to their function in the reservoir: one part plays the role of maintaining the reservoir energy, the other plays the role of driving fluids. In terms of "out", the production fluids are produced due to two factors: one is the driving of the injection water and the influx water, the other is the reservoir energy depletion. After a series of deriving and deduction of complex formulas, the contributions of the water injection and the natural water influx to the reservoir pressure system respectively are calculated by use of the actual history production data. The results available can be used in guidance and reference to the next development adjustment for stabilizing production.

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