Impact of Nanoparticles Shape on the VES Performance for High Temperature Applications
Authors: Hanafy Ahmed , Najem Faisal , Nasr-El-Din Hisham A. 
Published: April 2018

In recent years, viscoelastic surfactants (VES) seemed like an optimum solution for fracturing fluids. The technology was introduced to replace heavily damaging polymers. VES low thermal stability, high cost, and incompatibility with acid additives limited its application in the field. This work aims to investigate the crosslinking of the VES micelles using different shapes of silica and iron oxide nanoparticles to reduce the VES loading and extend its thermal stability.

This work utilized surfactant templating and ultrasonicated co-precipitation methods to produce a specifically tailored mesoporous silica and magnetite nanorods respectively, which were mixed with an anionic VES using ultrasonic bathing. Both spherical and rod-shaped particles of silica and iron oxides were examined to investigate the particle size, shape, and surface charge impact on the degree and the strength of the VES micellization. The studied particles physical properties were assessed using zeta potential, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The rheological performance of the VES mixtures were evaluated at 280 and 350°F through various shearing and heating ramps. The mixture microstructure was investigated using a polarizing microscope before and after the heating process. The produced network between the VES micelles and the nanoparticles were examined using TEM to describe its nature.

The interaction between the nanoparticles and the anionic VES is controlled by the VES concentration, the particle shape, and the temperature range. Although the spherical particles failed to cross link the VES at a concentration of 2 wt%, it succeeded to extend the thermal stability of the VES at a concentration of 4 wt% up to 350°F. The nanorods succeeded to enhance and extend the thermal range of the VES system at only 2 wt% VES. Both shapes of particles performed similarly at 4 wt% VES and up to 280°F. The addition of 7 pptg of silica nanorods extended the thermal stability of the 4 wt% VES, which exhibited and held an apparent viscosity of 200 cp for 2 hours. The addition of rod-shaped particles contributed to stronger micelle to micelle entanglement, especially at VES concentration of 2 wt%. The nanoparticles resulted in secondary networking that contributed positively to the viscosity of the mixture. The rod-shaped particles showed lower thermal stability at 350°F. They maintained 50 cp compared to the total failure of the VES by itself with 10 cp at 350°F. The polarizing microscopy, the TEM, and the DLS analysis showed that the enhancement in the apparent viscosity comes from closely packed structures of nanoparticles in surfactant strings.

This research shows the importance of the selected nanoparticle size, shape, and surface charge on the rheological performance of VES. It lays out a route to synthesize custom built nanoparticles to accommodate the chemistry of surfactants for higher performance and lower cost. This work has implementations in both self-diverting acid systems and fracturing fluids.