ab initio and Molecular Simulation Studies of Organic and Inorganic Counterions' Effect on Anionic Viscoelastic Surfactant
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AbstractViscoelastic surfactants have a wide range of applications in the oilfield industry, including acid fracturing and matrix acidizing. A problem associated with acidizing is the tendency of the live acid or spend acid to form sludge which can reduce the effectiveness of the treatment or plug the formation or production equipments. Formation of sludge results from precipitation of insoluble materials upon interaction with acid. Presence of cations such as ferric will enhance the aggregation process. Emulsion and corrosion are other problems associated with well acidizing and it can impact flow to the wellbore. It has been reported that anionic surfactants can address some of the problems associated with well stimulation and align themselves due to intermolecular attraction or non-covalent bonds to form wormlike micelles. The micelles can be absorbed on the formation or further interact to form a network exhibiting elastic behavior dependent on type of rocks. This increase in viscoelasticity helps in many cases to control fluid placement while acidizing multiple zones with variation in permeability and it also has sufficient properties to suspend and carry the proppant into the fractured zone. Several previous experimental studies highlighted the significant role of counterions in determining the apparent properties of aqueous solutions that contained viscoelastic surfactants. Wormlike micelles are formed by aggregation of amphiphiles. Presence of co-surfactant, additives, salts or appropriate counterions helps promote growth of such micelles. The effect of counterions can be detrimental on the properties of the whole system. Understanding how macroscopic properties depend on intermolecular interactions for complex fluids is the main focus of this study.Density functional theory and molecular dynamics simulations are employed to better understand how organic and inorganic counterions can impact physical properties of viscoelastic surfactants and hence their functionality in carbonate reservoirs. Anionic surfactants with different head groups include acetic acid (AA), potassium butanoate (PB) and sodium butane-1-sulfonate (S1BS), together with their counterions are simulated over a wide range of parameters. Competition between mono-, di- and trivalent counterions was studied in the gas phase of three anionic surfactant head groups with organic counterions (sodium salicylate, NaSal, and sodium hydroxynaphthalene carboxylate, NaHNC) and inorganic counterions (Na+, Mg2+, Ca2+, Fe2+ and Fe3+). Binding energy and thermodynamic properties show that trivalent counterions are more strongly bonded than di- and monovalent ones. The results give important insight into the links between molecular structure of surfactant and ions present in solution, which will help in developing more systematic procedures for treatment optimization, and better design of more efficient/effective viscoelastic diverting systems.
