The production chemistry data contains a wealth of knowledge on the physicochemical interactions of the formation and injected water with the reservoir rock and the hydrocarbons. This is even more pronounced in the highly reactive chalk formations of the Danish North Sea. The interpretation of the data is, however, not trivial due to the short-circuiting of the injected fluid into the production well in the fractured reservoirs, the injection of an often unknown mixture of the formation and sea water, and the reactive flow of brine in the carbonate system that continuously alters the water composition. A reactive transport model that is coupled to the multiphase flow of fluids in a well-characterized geology is a tool that can facilitate the interpretation of the production chemistry data. Our objective is to analyze the production chemistry and water cut data by constructing a reactive transport model that takes into account all the chalk-oil-brine physicochemical interactions. To that end, we use a transport model that is coupled to a surface complexation model, with parameters that are optimized by fitting the model to the chromatographic and zeta-potential data. We also include the dissolution and precipitation rate of different minerals (calcite, magnesite, and anhydrite) in the model. Moreover, we link the chalk and oil altered surface composition in the presence of sea and formation water on the transport properties of the aqueous and oleic phases in the chalk reservoir. To validate the model, we apply it to the Halfdan field, where no short-circuiting occurs due to the near piston-like displacement of the injected seawater in several sectors of the reservoir; moreover, in the Halfdan field data, clear trends in the produced water composition and the water-cut are identified in several production wells.

Our results show that the observed trends in the field data, i.e., a jump in the water-cut followed by an increase in the concentration of certain ions in the produced water, can be explained by our reactive transport model. Considering the so-called smart water effect of the seawater observed previously in the chalk outcrops, we also suggest possible mechanisms for the –possible- improved recovery of oil due to the interactions of the seawater with the chalk-oil-formation water system.


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