M. Liebum, Q.P. Nguyen
The University of Texas at Austin,
Keywords: viscoelasticity, surfactant, rheology, molecular self-assembly, shear thinning, protonation
Summary:Viscoelastic solutions (VES) are used in many applications including oil and gas operations regarding reservoir stimulation and enhanced oil recovery to modify rheology properties in the subsurface. In general, the viscoelastic property of the surfactant solution is derived from surfactant molecules self-assembling intro cylindrical and worm-like micellar structures with polymer-like characteristics . Factors such as changes in surfactant concentration, salinity, temperature, pH, and shear rate impact the growth of micellar structure, therefore understanding the effects of each parameter can provide insight on how to effectively control or “fine-tune” viscoelasticity for a given surfactant under a set of conditions. These tuning parameters are called triggers when viscosity show levels of high dependency on a certain factor, for instance, changes in salinity is the most common trigger to induce viscoelasticity in a solution among ionic surfactants since the addition of salt reduces the electrostatic repulsion of the molecules resulting in aggregation of the micelles to form elongated and entangled structures. Most surfactant structures will have some degree of trigger-induced parameters, but a proprietary cationic surfactant investigated in this work, poses triggers for all the factors listed above . This study reflects the work completed to develop and control viscoelastic properties with solutions ranging from 0.2% to 2% wt. surfactant and 5 % to 25% wt. NaCl. Rheology analysis to decouple effects was conducted based on steady-state shear rate. The results suggest that salinity has the highest impact inducing the viscoelastic property, while an increase in surfactant concentration magnifies this viscoelastic behavior (Figure 1). Additionally, the effect of pH coincide with protonation property common among amine surfactants at low shear rates (Figure 2). Moreover, unusual viscosity behavior was exhibited for changes in temperature noted by a few researchers . In all, the ability to fine-tune viscoelasticity and rheological behavior based upon altering the solution’s surfactant concentration, salinity, pH, and temperature is an engineering novelty to optimize performance and efficiency.