Chemicals such as anionic surfactants and polymers often contain groups that complex divalent ions such as Ca2+. The formation of divalent ion complexes can decrease emulsifying or viscosifying power and lead to adsorption or precipitation. This is particularly relevant in chemical enhanced oil recovery, where high viscosities and low interfacial tensions are required for mobility control and the formation of oil–water microemulsions, respectively. In this work, we use a Ca2+-sensitive dye to determine the Ca2+ concentration and Ca-complex formation constants in solutions containing complexing agents. This method can be used to rapidly screen the affinity of different chemicals to form Ca-complexes in low-salinity solutions. The complex formation constants can be implemented into chemical flooding simulators to investigate the interplay with mineral dissolution and cation exchange and model adsorption processes.

The monomer−micelle equilibrium is shown to be responsible for an asymmetry between surfactant adsorption and desorption rates. When a solution containing micelles is brought into contact with a solid surface, the micelles dissociate to supply monomers that adsorb to the surface. When the same surface is subsequently exposed to a surfactant-free solution, desorption occurs slowly because of the higher affinity of the monomers to remain to the surface than to form micelles. As a result, the number of monomers that desorb is limited by the critical micelle concentration (CMC) of the surfactant. This effect is particularly pronounced for surfactants with low CMC values and in systems with high surface-to-volume ratios, such as porous media. A generic model is developed and applied to simulate the Ca 2+mediated adsorption and desorption of surfactants in limestone cores.

No abstract

Surfactant adsorption and ion exchange on calcite surfaces are investigated by means of flooding experiments in Estaillades limestone cores. We show that, in the 5000 mg/L salinity range, effluent concentrations can be captured in a reactive transport model that includes calcite dissolution, ion exchange, complexation of divalent ions, and the monomer−micelle equilibrium. In this model, ion exchange and adsorption are described by the screening of the negative surface charge by cations and surfactant−divalent ion complexes. We further show that adsorption can be reduced by a factor of 10 by coinjection of surfactant and polyacrylate or a polyacrylate preflush, which is attributed to a combination of divalent ion complexation and adsorption to the rock surface.