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.
Rationale
Fully formulated oils (FFOs) are integral to automotive lubrication; however, detailed compositional analysis is challenging due to high levels of chemical complexity. In particular, existing mass spectrometric approaches often target particular FFO components, leading to poor analytical coverage of the overall formulation, with increased overheads and analytical timescales.
Methods
Herein we report the application of a commercially available SICRIT SC‐20 dielectric barrier discharge ionisation (DBDI) source and Thermo Fisher Scientific LTQ Orbitrap XL to the analysis of an FFO. Nitrogen was used as a discharge gas for the DBDI source, and was modified using a range of commonplace solvents to tailor the experimental conditions for the analysis of various components.
Results
The reported method allowed analysis of a range of FFO components of interest, encompassing a wide range of chemistries, in under 1 min. By modifying the discharge gas used for ionisation, experiments could be optimised for the analysis of particular FFO components across positive and negative ion modes. In particular, use of water vapour as a discharge gas modifier with positive ion mode mass spectrometry permitted concomitant analysis of antioxidants and base oil hydrocarbons. Furthermore, case studies of selected linear alkanes and alkenes profile the differences in the range of ions formed across these saturated and unsaturated aliphatic compounds, giving insight into the fate of base oil hydrocarbons in FFO analyses.
Conclusions
A rapid method for analysis of FFO compositions has been developed and provides coverage of a range of components of interest. The results indicate that the method presented may be of utility in analysis of other FFOs or similarly challenging complex mixtures.
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