NMR Studies of the Effect of CO2 on Oilfield Emulsion Stability

Formation of water-in-crude oil emulsions is a pervasive problem for crude oil production and transportation. Here we investigate the effectiveness of a comparatively low pressure CO, treatment in terms of breaking these water-in-crude oil emulsions. To this end, we used unique benchtop nuclear magnetic resonance (NMR) technology to measure the droplet size distribution (DSD) of the emulsions. Treatment with 50 bar CO2 for 2 h resulted in significant emulsion destabilization; this was replicated when CO2 was replaced by N2O, which has a solubility in both the aqueous and oil phases similar to that of CO2. Low solubility gases, N-2 and CH4, by contrast had no effect on emulsion stability. Treatment with CO2 was also found to have no effect on a model water-in-paraffin oil emulsion stabilized by a synthetic surfactant (Span 80). Collectively, this supported the hypothesis that emulsion destabilization results from CO2 precipitation of asphaltenes as opposed to emulsion droplet film disruption during depressurization, which are the two competing theories reported in the literature to explain the observed supercritical CO2 destabilization of emulsions. Treatment of a water-in-crude oil emulsion featuring partial removal of asphaltenes from the oil phase was consistent with this hypothesis, as the effect of the CO2 treatment on emulsion destabilization was significantly more pronounced.