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.
Highlights
Surfactants are used in chemical enhanced oil recovery (EOR) to reduce the oil−water interfacial tension and mobilize residual oil
We showed that the low desorption rate is a consequence of the monomer−micelle equilibrium, the low critical micelle concentration (CMC) value of the surfactant, and the low volume-to-surface ratio in the porous rock.[32]
Reactive transport in calcite cores was modeled by considering calcite dissolution and the screening of a negatively charged surface
Summary
Surfactants are used in chemical enhanced oil recovery (EOR) to reduce the oil−water interfacial tension and mobilize residual oil. Adsorption is found to be high for anionic surfactants on the surface of carbonate rocks, which are almost entirely composed of soluble minerals such as calcite. The predominant cleavage plane of calcite is the (104) surface[1−7] (see Figure 1a), which exposes equal numbers of >Ca+ and >CO3− sites (approximately 5 sites/nm[2] or 8.22 × 10−6 mol/ m2). Because the number of positive and negative sites are equal, the (104) calcite surface has a net surface charge density of 0. Fractional surface charges of ±1/3 or ±0.25 may be considered when accounting for broken bonds or nonionic bonding to neighboring O sites.[5,8] For our discussion, it suffices to consider surface sites with integer charges and interactions between these sites and the species in solution
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