XXX oilfield is locate in northwest China. The Jurassic sandstone reservoir with huge natural edge water and bottom water energy began to put into production in 1992 and injection in 1995. The volume of natural aquifer body is 35.87 times the volume of reservoir. The intial reservoir pressure is 21.86MPa, and at present (June, 2008) the average reservoir pressure is 18.77MPa. The reservoir production rate began to decline in 1999, at present (June, 2008), the production rate decline is very rapid, the cumulative injection water is 480.98×104m3, the average water cut is 79.83%, the recovery is 44.04%, the number of the whole reservoir water wells is 17, the average bottomhole flow pressure for pumping well is 15.71MPa, the average bottomhole flow pressure for injection well is 27.23MPa, the number of the whole reservoir oil w ells is 95, and the well ratio of injection to production is 0.18 (1:5.56). There is a most pressing problems needed to address, which is that how to increase the water injection volume and at the same time consider the influence of natural water influx to make full use of the energy of natural aquifer body, so the problem of how to determine the optimal well ratio of injection to production cannot be avoided and ignored.
