Injecting low salinity water into a petroleum reservoir to improve oil recovery has been studied extensively over recent years as a low cost enhanced oil recovery (EOR) process. Extensive chemical analyses have been performed on the effluent water from low salinity waterflood experiments which reveal the extent of interaction between the injected brine, the oil and the rock matrix. However, there has been little work reported on the impact of the injected fluid composition on the nature and composition of the oil recovered. This paper details an investigation on how the waterflood medium affects the chemistry of the produced oil, which is important for understanding the mechanism by which the additional oil is released. Produced oil samples were analyzed using High Resolution Mass Spectrometry (HRMS) which essentially measures the mass of individual molecular species very precisely, which makes it possible to assign a unique elemental composition (e.g. carbon, hydrogen, oxygen, nitrogen and sulfur content) to each mass. Additionally, by careful control of the ionization procedure, it was possible to identify acidic and basic polar species, as well as neutral aromatic hydrocarbons. The data indicates that the composition of the produced oil changes during the reduced salinity waterflood, with an increase in the CxHyO2 species occurring. These molecular species, compared to the secondary high salinity flood, are released as the tertiary low salinity injection water passes through the core; they then decline towards the end of the waterflood. In contrast, there appears to be little change in aromaticity, sulfur and nitrogen containing species during the flood. The fact that the produced oil is enriched predominantly with CxHyO2 species is consistent with the multiple ion exchange and local pH rise mechanisms proposed previously.
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