Abstract
The effect of the electrolyte composition of produced water in oil & gas production facilities was investigated by developing a laboratory scale oil/water separation setup and a related procedure, coupled with an analytical method based on the OSPAR reference method for oil-in-water measurement. The experimental plan was aimed at investigating the effect of the ionic strength, for a given electrolyte, and of different electrolytes, for a given ionic strength, on the separation of oil droplets from the aqueous phase. The oil-in- water concentration was reported against settling time, for a number of different ionic compositions. It was observed that increasing the ionic strength increases the separation efficiency, with more pronounced differences for settling times closer to those typical for the three phase separators in use in the field. For example, for a settling time of 5 min the increase in oil-in-water concentration caused by reducing the ionic strength from seawater level down to zero resulted in a reduction of the separation efficiency up to 37%. This result clearly shows the importance of the electrolytes in the kinetics of oil/water separation, for typical residence times in use in offshore production facilities. Changing the type of electrolyte for a given ionic strength gave results that are not conclusive and worth investigating further. In addition to the separation kinetics, the interfacial tension (IFT) between the crude oil and the different aqueous electrolyte solutions was measured. Only slight differences in the IFT were observed, thus suggesting that other parameters (e.g. ?- potential) are expected to be more relevant in explaining the observed differences in the separation kinetics. The developed experimental method for measuring the separation kinetics proved reliable and can serve as a basis for further investigations aimed at comparing the separation rate of actual brines vs. modified brines, i.e. brines with the composition that can be expected as a consequence of the application of enhanced oil recovery methods, such as SMART water injection.
