Interfacially active carbon nanotube hybrids (nanohybrids) exhibit promising properties for potential applications in reservoir systems. They could be used as modifiers of transport properties as well as nanoscale vehicles for catalyst and contrast agents. In situ catalysis might be used to modify interfacial tension and wettability of the rock wall. The main requirements for any of these applications are the ability to form stable dispersions and to effectively propagate through the reservoir porous medium under the temperature and salinity conditions that are typical in commercial operations. In this work, suspensions of purified multi-walled carbon nanotubes (P-MWNTs) in deionized water and high-salinity brine have been prepared using two commercially available polymers, polyvinyl pyrrolidone (PVP) and hydroxyethyl cellulose (HEC-10). Stable dispersions were put in contact with crushed Berea sandstone, quantifying the amount of nanotubes lost from suspension to estimate the adsorption of these nanotubes from suspension onto the walls of the reservoir rocks. Adsorption isotherms were measured from room temperature up to 80 °C from aqueous suspensions with salinities up to 10%. These studies demonstrate that combining these two polymers stabilizes suspensions in high-salinity water and minimizes adsorption on the sand walls. It is proposed that this optimized behavior is due to additive electrostatic and steric repulsions. While the polar PVP helps disaggregation by effectively wrapping individual nanotubes (primary dispersant), the bulky HEC-10 inhibits the reaggregation in saline solutions (secondary dispersant). Column experiments were conducted to study the propagation of these suspensions through porous media. It was found that a small amount of nanohybrids adsorbed to the sand will be able to saturate available adsorption sites, resulting in subsequent injections of nanohybrids to be propagated completely through the column without adsorption. In that sense, we were able to reach 100% of the injected concentration with a low particle concentration of 100 ppm and total particle adsorption to the sand of less than 10% at room temperature.

Carbon nanotubes (CNTs) exhibit promising properties for potential applications in oil and gas reservoirs. CNTs can be used as delivery vehicles for contrast agents or catalyst nanoparticles deep inside the reservoir. Dispersing 100 ppm of CNTs in deionized water is easily achieved by sonication of CNTs using properly selected surfactant or polymer solutions. These surfactants and polymers are non-covalently adsorbed to the nanotube surface, inducing dispersion stability. In oil reservoirs, high salinity is the norm; therefore, because the electrostatic double layer is compressed as a result of the high ionic strength found in a typical reservoir brine, colloid CNT dispersions lose stability and CNTs flocculate and precipitate. To maintain a stable colloidal dispersion of CNTs, a dispersant with functionality providing steric repulsion between the dispersed tubes is needed to prevent aggregation. In this work, suspensions of multi-walled carbon nanotubes (MWNTs) were generated using two polymers, gum arabic (GA) and hydroxyethyl cellulose (HEC-10), in 10% API brine (8 wt % NaCl and 2 wt % CaCl 2 ). GA was used as a primary dispersant, which is able to debundle the tube aggregates. After the first sonication with GA, the secondary dispersant, HEC-10, is added to provide the steric repulsion needed to keep the tubes dispersed in high-salinity brines. Polymer adsorption to the nanotube surface was observed using scanning electron microscopy. Focusing the electron beam for an extended period of time induced damage to the polymer layer around the individual nanotubes, leaving the tubes intact, as clear evidence of polymer adsorption. Adsorption experiments showed low to negligible adsorption of MWNTs to crushed Berea sand at 80 °C in both 10 and 20% brines. Dispersion injection in column and coreflooding tests showed successful propagation of CNT dispersions through porous media, with total nanoparticle recovery exceeding 80% in reservoir rock. This work demonstrates the potential of using polymer-stabilized carbon nanoparticle dispersions in a range of applications to advance current oilfield technology.

Carbon nanotube hybrids (CNTs) have attracted research interest due to their interfacial activity. CNTs can stabilize emulsions and foams and can be used as contrast agents or tracers in rock matrix. In addition, catalytic functionalities can be attached to the nanotubes making them delivery vehicles for catalyst into zones deep inside the reservoir. Generating stable dispersion of CNTs in harsh reservoir conditions has been the main challenge for utilizing the tubes in in-situ reservoir applications. This is because the dispersed tubes tend to form aggregates that settle down in the presence of high ionic strength (high salinity) brines. In this work, stable dispersion of carbon nanotubes prepared in reservoir fluids is realized by successfully separating individual tubes using such additives as polymers and surfactants. For example, the CNTs would be well dispersed via sonication with highly polarizable polymer such as polyvinyl pyrrolidone (PVP) or Gum Arabic (GA). To mitigating their agglomeration, a secondary additive such as hydroxyethyl cellulose (HEC) polymer is also added to provide adequate steric repulsion for their propagation in porous media at high salinity brines. The nanotube dispersion generated using these dual polymer system is able to deliver successfully through both consolidated cores (200mD permeability of Berea sandstone) and sand pack experiments (4D permeability) with minimal retention at mimic reservoir conditions (65°C and brine compositions of 8% NaCl and 2% CaCl2). Results of the eluted nanoparticles indicate that greater than 80% recovery of injected concentration observed in both consolidated and non-consolidated porous media. Small adsorbed amount of nanotubes is capable of saturating the adsorption sites inside porous media resulting in complete propagation of subsequent injections; this is corroborated by nanotube concentrations approaching 100% of the injected concentration after few pore volumes of injection. Experiments also demonstrated that in the presence of residual oil inside crushed Berea sandstone sand columns, the extent of nanotubes adsorption to the oil/water interface is a function of the level of oil saturation. This work is providing insight about the full potential of using carbon nanotubes in oilfield development applications.

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