Surfactant screening for enhanced oil recovery involves studies of fluid-fluid and fluid-rock interactions to optimize the surfactant formulation for the reservoir of interest. In this paper, we utilize low field nuclear magnetic resonance (NMR) technology to monitor the phase behavior microstructure of fluid mixtures developed for high salinity and medium-range temperature conditions. A surfactant mixture of unknown composition was selected since it was developed for high salinity and temperature reservoirs.
Surfactant performance was monitored over 20 days using phase behavior bottle tests. Crude oil was mixed with surfactant solutions of different salinities to study the different emerging phase behaviors. Salinity scans were carried out using synthetic seawater. Also, samples were aged at 25 and 68 °C to monitor the progression of emulsion phase behavior. In addition to visual inspection, NMR measurements, in the form of 1D brine profiles along the samples, were conducted to detect the observed dispersed phases throughout the aging process and evaluate the associated oil-water interfacial tension (IFT).
At both temperatures, all samples formed water-in-oil (W/O) and oil-in-water (O/W) emulsions, except for the 4.5% salinity sample at 25 °C. Also, at 25 °C, both types of emulsions were shrinking with aging while forming a secondary W/O emulsion in-between the two phases. On the other hand, at 68 °C, emulsion break-down was accompanied by the formation of a middle O/W emulsion for 3 and 4.5% salinities while a middle W/O emulsion was emerging in 7.5 and 9% salinity samples. For the 6% salinity sample, two types of secondary emulsions, W/O and O/W, were developing in the middle of the sample.
Low field NMR techniques provide a nondestructive and robust method of monitoring surfactant performance, including dispersed volumes, emulsion stabilization time, emulsion type and surfactant potential in lowering oil-water IFT. In addition, they are capable of microscopic phase behavior monitoring, which is not possible by conventional visual monitoring technique.