Precipitation and Dissolution of Minerals During Waterflooding of a North Sea Oil Field

摘要

A long-term study of produced water chemistry from a NorthSea field was used to investigate the mechanisms of watermixing and water-rock interaction in the reservoir. Seawaterflooding has continued throughout much of the productionlife. Detailed repeated sampling of the produced water wasundertaken and has produced an extensive dataset, yieldinginformation on water chemistry variations in space and time.The dataset documents both fluid mixing in the field and thephysical, chemical and thermodynamic response of the systemto the injection of seawater. Analysis of the data establishesthe nature of the controls on the composition of the scaleproneformation water, and enables an in-depth look at thefluid-rock interactions occurring in the reservoir during awaterflood.Changes in produced-water chloride concentrationthrough time reflect changing proportions of injected seawaterand formation-water, revealing differing patterns of injectedwaterbreakthrough over the field. However, parallel changesin the concentrations of less conservative fluid componentsprovide evidence of fluid-mineral interactions that occurred inthe reservoir on the timescale of the waterflood. For example,calcium is enriched in the produced fluid relative to a linearmixture of original formation-water and seawater, whilemagnesium is depleted, probably reflecting dolomitisation ofcalcite and growth of clay. Barium and sulphate are stronglydepleted due to precipitation of barite. However, massbalance highlights an additional sink for sulphate, possiblyreduction to sulphide. Excess silica present in the producedfluid is ascribed to dissolution of silicate phases in thereservoir. Concentrations demonstrate that the produced wateris always close to quartz saturation at reservoir temperature,irrespective of the proportion of seawater produced.Analysis of produced water chemistry providesinsights into the inner workings of the reservoir system duringa waterflood. Study of individual dissolved species relative tolinear mixing lines between injected and formation water allows measurements of the nature and amounts of dissolutionand precipitation reactions affecting scaling ions within thereservoir. This allows for greater understanding of thecontrols on water composition and of the nature of watermixing in the reservoir, leading to improved prediction andplanning of scale occurrence, prevention and remediation.