Surfactant adsorption is one of the significant problems of enhanced oil recovery (EOR) operations.
One method of enhanced oil recovery (the extraction of previously unobtainable petroleum from an oil deposit using non-traditional methods) is the injection of surfactants into the field to lower interfacial tension, change the wettability, and control mobility in order to increase the amount of oil recovered from a previously water-flooded reservoir. One of the main challenges in this process is the loss of surfactant due to adsorption on formation rocks. In fact, high adsorption of surfactant can make the chemical EOR processes economically-unfeasible.
The net charge of most typical rock/mineral surfaces is strongly dependent on pH; that is, above a certain pH known as point of zero charge (pHpzc), the net charge of the surface is negative and below this pH the surface charge is positive. The first adsorbed layer of surfactant is mostly governed by electrostatic attraction. Once this first adsorbed patch forms on the surface, a second layer of surfactant may be adsorbed by surfactant tail-tail hydrophobic interactions at high anionic surfactant concentrations. Adsorption of this second layer helps to change the net surface charge to negative, thereby, repelling further anionic surfactant molecules and saturating the adsorption.
Traditionally, alkali has been used as a sacrificial agent to decrease the adsorption of anionic surfactants on rocks. The addition of alkali increases the pH and thus it can make the surface charge negative (pH>pHpzc), which causes electrostatic repulsion between the rock/mineral surface and anionic surfactant, leading to a significant decrease in adsorption of anionic surfactant.