Produced water analyses from the Birch Field, UK North Sea have been interpreted and combined with simulation results to explain the causes of changes in produced water compositions over time. Preliminary conclusions are that two formation waters are present in the oil leg, both trapped at the time of oil emplacement. Lower salinity formation water has been expelled from the Kimmeridge Clay Formation (KCF) and dominates shallower sections of the reservoir. Higher salinity formation water is thought to be ancient Brae aquifer water and dominates the deeper sections. Some lateral variability in formation water compositions is evident. Produced water from individual wells is a mixture of the lower salinity formation water, higher salinity formation water and injected water. Trends in produced water compositions over time reflect a relative decrease in formation water production and increase in injection water production. Depending on the constituent, reactions occurring as a result of injection of water into the reservoir also affect the composition of produced water. The results have challenged previous concepts relating to water production at Birch and will be considered in scale management plans in future. They can also be used to constrain reservoir simulations, to aid enhanced oil recovery decisions and to provide more reliable tracking of injection water and formation waters entering the production wells at Birch.
More generally, this study has demonstrated the importance of evaluating produced water analyses as early as possible after water breakthrough. Integration of reservoir simulation studies with the interpretation of produced water analyses can provide information that benefits scale management, STOIIP calculations, reservoir models, and tracking of injection water as well as providing analogue information that can help reduce uncertainties associated with the development of deep water and marginal subsea fields.