Adsorption of a Switchable Cationic Surfactant on Natural Carbonate Minerals

Summary A switchable cationic surfactant (e.g., tertiary amine surfactant Ethomeen C12) was previously described as a surfactant that one can inject in high-pressure carbon dioxide (CO2) for foam-mobility control. C12 can dissolve in high-pressure CO2 as a nonionic surfactant and equilibrate with brine as a cationic surfactant. Here, we describe the adsorption characteristics of this surfactant in carbonate-formation materials. The adsorption of this surfactant is sensitive to the equilibrium pH, the electrolyte composition of the brine, and the minerals in carbonate-formation materials. Pure C12 is a nonionic surfactant. When it is mixed with brine, the solution has a high pH and limited solubility. However, when the surfactant solution in brine is equilibrated with high-pressure CO2, the pH is approximately 4; the surfactant switches to a cationic surfactant and becomes soluble. Thus, the adsorption is also a function of pH. The adsorption of C12 on calcite at low pH is low (e.g., 0.5 mg/m2). However, if the carbonate formation contains silica or clays, the adsorption is high, as is typical for cationic surfactants. The adsorption of C12 on silica decreases with an increase in divalent (Ca2+ and Mg2+) and trivalent (Al3+) cations. This is because of the competition for the negatively charged silica sites between the multivalent cations and the monovalent cationic surfactant. An additional effect of the presence of divalent cations in the brine is that it reduces the dissolution of calcite or dolomite in the presence of high-pressure CO2. The dissolution of calcite and dolomite is harmful because of formation damage and increased alkalinity. The latter raises the pH and thus increases the adsorption of C12 or even causes surfactant precipitation.