(596g) Ab Initio and Molecular Simulation of Viscoelastic Surfactants Solutions: Effect of Counter Ions

Viscoelastic surfactants have a wide range of applications in the oilfield industry including: hydraulic fracturing and matrix acidizing. It has been proposed that surfactant molecules align themselves and aggregate due to intermolecular attraction or non-covalent bonds to form wormlike micelles as the ionic strength or pH increases (pH > 2). The micelles can further interact to form a network exhibiting elastic behavior. This increase in viscoelasticity helps to divert treating fluid while acidizing multiple zones with variation in permeability and it also has sufficient properties to suspend and carry the proppant into the factured zone.

Cationic and amphoteric surfactants have been extensively used to enhance well performance in carbonate geology. Several previous studies highlighted the significant role of counter ions in determining the apparent properties of aqueous solutions that contained viscoelastic surfactants. The effect of counter ions can be detrimental where the surfactant would not form a gel or precipitate, as in the case of ferric iron, in the formation and causes formation damage. Understanding how macroscopic properties depend on intermolecular interactions for complex fluids is a difficult task.

Molecular dynamics and molecular mechanics simulations are used to better understand how counter ions can impact physical properties of viscoelastic surfactants. Different molecular structures of cationic and amphoteric surfactants together with their counterions are simulated over a wide range of parameters. Simulation results such as density profiles, and radial distribution functions show a clear correlation between the surfactant head group and counterions. The results give important insight into the links between molecular structure and diverter (surfactant) phase behavior, which will help in developing more systematic procedures for treatment optimization, and the molecular design of more efficient/effective diverting systems.