A new method for predicting injection multiples of extreme displacement in waterflood reservoirs

Abstract

The theoretical relationship between water injection multiple (i.e. injected pore volume) and water saturation is inferred from theoretical concepts of reservoir engineering. A mathematical model based on core displacement tests is established for the entire injection process that satisfies both initial displacement and extreme displacement, simultaneously. The results show that prior to the flooding, the water injection multiple has a linear relationship with the water saturation, and the utilization rate of the injected water is the highest. As water breakthrough at the production end, the water-cut increases, and the injection multiple increases exponentially while the utilization efficiency of the injected water gradually decreases. When the injection multiple approaches infinity, the utilization efficiency of the injected water gradually decreases to 0, by which time the water-cut at the production end is always 1. At this time, the water saturation no longer changes, and the water flooding recovery rate reaches its limit. Based on the experimental test data, a mathematical model of the entire process of injection multiple and water saturation is established, which has high fitting accuracy that can predict the injection multiple in the different stages of development of a mature oil reservoir. The dynamically changing index of the injection water utilization efficiency in reservoir development by reactive water flooding can be obtained through reasonable transformation of the mathematical model. This is of great significance in guiding evaluations of the effects of reservoir development and formulating countermeasures.