Abstract
Mass-action equilibrium equations give a good description of cation-exchange effects in laboratory floods with solutions containing sodium, calcium, and magnesium cations. These equations can help when designing prefloods for surfactant and polymer processes. processes. Cation exchange in the presence of a surfactant system is found to be complicated significantly by interaction between surfactant and divalent cations. The evidence suggests that a divalent cation-surfactant "complex" may be a new exchangeable species. Both divalent cation and surfactant transport are described qualitatively by a model incorporating Langmuir chemisorption and a divalent cation-surfactant complex. Surfactant adsorption in Berea rock was reduced by a factor of one-fifth by reducing divalent-cation concentration in the surfactant from 300 ppm to zero, dissolving carbonate minerals from the core, and converting clays to the sodium form.
Introduction
Chemical flood performance is a relatively sensitive function of the ionic composition of a chemical system. Initial composition can be controlled, but control during traverse of the reservoir is extremely difficult. Recognized underground process that may alter ionic composition include mixing with in-situ waters, partition of dissolved gases and polar materials between the crude oil and the slug, dissolution of minerals, chromatographic lag of the surfactant, and cation exchange between the surfactant slug and reservoir clays. Wilson recently reviewed these and other in-situ processes affecting the performance of polymer, caustic, and surfactant floods. performance of polymer, caustic, and surfactant floods. Melrose et al. described a complex model involving six equilibria that explained substantial increases in the concentration of NaHCO3 observed during a pilot polymer flood. Partition, ionization, solubility, and cation-exchange processes were included in the model and, after due consideration of the additional effects of reservoir heterogeneity and fluid mixing, the authors estimated that in-situ viscosity of the polymer solution was only about 25% that of the injected solution. More recently, Smith discussed a model describing cation exchange during preflooding. For one planned project, he showed that 2 PV preflush were required before effluent divalent-cation concentration reached the injected level. He concluded that calcium and magnesium ions could be treated as a single specie and that idealized estimations of preflush efficiency could be made from the model, if adequate experimental information for the reservoir and pertinent waters were available. Predictive techniques pertinent waters were available. Predictive techniques resulting in a continuous description of the ionic environment of the chemical slug as it traverses the reservoir would allow more accurate performance prediction and provide improved slug-design criteria. This paper is a progress report on experimental efforts to understand and predict cation exchange and other interrelated equilibria. Companion papers describe the application of chromatographic principles to the problem and a simulator that, given the principles to the problem and a simulator that, given the correct equilibria description, can predict ionic environment.
CATION-EXCHANGE ISOTHERMS
Two-Cation Exchange
Cation-exchange equilibria have been described by empirically and theoretically derived equations. Using double layer theory, Bolt derived a relation for sodium-calcium exchange that simplifies into
(1)
where C1 and C3 are the concentration of calcium and sodium in the equilibrium solution, respectively. C1 and C3 are corresponding concentrations on the clays, and the subscript g indicates Gapon equilibria. In this simplified form, the expression was derived much earlier by Gapon. Magistad et al. reported additional data supporting the applicability of the Gapon equation.
SPEJ
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