Abstract
Injection of chemically tuned, ‘smart’ water in oil reservoirs may increase both oil recovery rates and the total recovery (e.g. Morrow & Buckley 2011; Austad 2013; Zeinijahromi et al. 2015). This kind of water management has gained increased importance in the Danish North Sea reservoirs due to decreasing sweep efficiency in maturing oilfields. Knowledge about the compatibility of the injected water with local formation waters is, however, a prerequisite for successful implementation. Here, we present a regional overview of formation waters from oil reservoirs in the Danish North Sea, which comprise three main types of formation brine, and one type of modified seawater related to extensive water flooding. The water types show a distinct geographical distribution, which reflects original connate waters that are modified by saline brine being either depleted or enriched in SO42–. 
Highlights
Injection of chemically tuned, ‘smart’ water in oil reservoirs may increase both oil recovery rates and the total recovery (e.g. Morrow & Buckley 2011; Austad 2013; Zeinijahromi et al 2015)
The Principal Component Analysis (PCA)-2 axis displays high positive loadings of SO42– and high negative loadings of elements such as Ba, Sr, Mg and Ca.This could indicate that SO42– concentrations control the concentration of Ba2+, Sr2+, Mg2+ and Ca2+ due to the low solubility of e.g. barite (BaSO4) and anhydrite (CaSO4)
Water type 4 plots close to, or together with seawater with negative PCA-1 and positive PCA-2 scores in Fig. 2A, corresponding to low to medium salinity with high SO42– concentrations (Fig. 3). This water type occurs in the Dan, Halfdan and Skjold fields and is interpreted to be the result of decades of extensive water flooding performed by the operator (Energistyrelsen 2013)
Summary
In order to characterise the water types we have selected a total of 33 water analyses, 25 of produced water and six of formation water from North Sea wells (three core samples from the Francisca-1 well and production tests from the Boje-1, Elna-1 and M-9X, wells) and two analyses of seawater (North Sea mean water composition and a treated low-sulphate seawater), see Fig. 1. Samples analysed for Na, K, Ca2, Mg2, Sr2, Ba2, Cl–, and SO42– were used. Water density had been measured for most of the samples, it was estimated for four samples. The data were collected from Samuelsen et al (2009), Mackay et al (2012), and Undall-Behrend (2012) and from final well reports for the Boje-1, Elna-1, Francisca-1 and M-9X wells
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