We report here on production data from a North Sea oilfield that demonstrates that significant mineral-fluid interactions took place in the reservoir on a time scale of months. A time series of produced water compositions from the BP-operated Miller Field, in the UK sector of the North Sea, was generated over a period of more than 7 yr when seawater was being injected for secondary recovery and pressure maintenance. Deviation of the produced water chemistry from linear mixing of seawater and formation water indicates that mineral-water interactions took place in the reservoir. Of particular significance is the evidence for rapid reaction of injected water with silicates. The safety case for CO2 storage in such reservoirs is greatly facilitated if it can be shown to react with the host pore waters and rocks on a human time scale, and the results of this study indicate that this is indeed the case.