Surfactant-Only Stabilized Dispersions of Multiwalled Carbon Nanotubes in High-Electrolyte-Concentration Brines

Multiwalled carbon nanotubes (MWNTs) exhibit promising properties for potential applications in oil production. Because of their substantial surface area, they could be used as carriers for catalysts or chemicals into subsurface oil and gas zones to change the properties of reservoir fluids or rock. A prerequisite for utilizing the MWNT in reservoir applications is to generate stable aqueous-phase dispersions that are well-dispersed and able to propagate successfully through the reservoir medium. In this study, different types of surfactants were investigated for their ability to disperse MWNTs in high-ionic-strength solutions typical of oil reservoirs up to 10% American Petroleum Institute (API) brine (8 wt % NaCl and 2 wt % CaCl2). Stable nanotube dispersions in deionized water were achieved with the anionic surfactants evaluated. Compression of the electrical double layer, however, at high ionic strength, e.g., >3 wt % electrolytes, led to rapid aggregation of the anionic surfactant-aided nanotube dispersion. This study showed that by dispersing nanotubes in nonionic surfactant such as alkylphenol polyethoxylates with a large number of ethylene oxide (EO) groups, stable MWNT dispersions were obtained in 10 wt % brine. In the sandpack column test, a binary surfactant formulation, which consisted of a nonionic surfactant and an anionic surfactant in the proper ratios, exhibited an excellent capability to propagate MWNT, with 96% of the injected nanotubes recovered in the effluent. The adsorption density of surfactants onto MWNT was determined to be 9 molecules/nm2 from the shift of the CMC value in the surface tension measurement. This study reveals that steric repulsion between the nanotubes could eliminate the aggregation of dispersed MWNT under the high-electrolyte-concentration condition, whereas nanotube–nanotube and nanotube–sand surface electrical repulsion could assist in the transport of the MWNT dispersion through porous media.