Characterisation and selection of demulsifiers for water-in-crude oil emulsions using low-field ~1H NMR and ESI-FT-ICR MS

摘要

Water-in-oil emulsions constitute a widespread issue for the oilfield industry. Indeed, the phases of water-in-oil emulsions must be separated as soon as possible in the production chain. These emulsions can be fairly stable due to certain intrinsic components in crude oil; therefore, adding chemicals -demulsifiers - to aid this separation is very common. Selecting the ideal demulsifier can save time and money, and in this context two-dimensional (2D) low-field nuclear magnetic resonance (NMR) is a promising technique because it allows for the separate study of the oil and water phases without the need for physical separation. In this study, four laboratory-made emulsions were used to investigate the demulsification process via low-field ~1H NMR, allowing for the investigation of the correlation between the diffusion coefficient (D) and transverse relaxation time (T_2) in 2D D-T_2 plots. The three commercial demulsifiers used in this process (A, B and C) were characterised by positive-ion electrospray ionisation (ESI(+)) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The ESI(+)-FT-ICR mass spectra recorded for the commercial demulsifiers showed distinct chemical profiles, with two samples (A and B) being composed of surfactants containing isoprene and glycol propylene oligomer species, where the ions detected in the m/z 200-700 region were separated by m/z 42 and 58 units, respectively. A more abundant and complex chemical profile was observed for sample C, in which pyridine and sulphoxide analogue species were identified. The 2D D-T_2 plots clearly showed that the separation of water and oil phases can be monitored and allowed for the classification of the demulsifiers according to their performance in the physical separation of the phases: demulsifier A ＞ demulsifier B ＞ demulsifier C.