Classification of and changes within formation water types in Danish North Sea Chalk: A study of the Halfdan, Dan, Kraka and Valdemar oil reservoirs

The ionic composition of produced water from four oil producing chalk fields in the Danish North Sea has been interpreted to shed light on the temporal and regional variations in the water composition. The data includes 8749 samples, covering 157 wells, analyzed for eight ions. The data dates back to the start of production (1972-2007) and the most recent data is from 2015. The variations are viewed in a regional context with respect to subsurface geology, fluid migration and production strategy since parts of the fields have been extensively water flooded by injection of seawater. We present results from both time series analysis and from multivariate statistical methods and show that produced water across the four fields, up to 55 km apart, show gradual changes in the composition reflecting regional scale hydrodynamics as well as subsurface geology. We use multivariate statistical analysis to extract end-member compositions to facilitate comparison between basins and for an evaluation of scale and corrosion risk. The data analysis shows five main water types. These are mainly differentiated by salinity and the concentrations of the divalent cations. The study is discussed in the context of examination of interconnection between close-lying fields including aquifer support and distribution of principal ions associated with corrosion and scale formation.