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Solubility of petroleum in oil-field waters as a function of the oxidation state of the system

¹ Department of Geology and Geophysics, University of California, Berkeley, California 94720-4767

Aqueous solubilities of liquid nonane (C₉H₂₀ [liq]) with activities ranging from 1.0 in pure nonane to 0.1 and 0.01 in crude oil have been calculated at 120 °C and 400 bar as a function of oxygen fugacity (f_O2(g), where g stands for gas) in a generic oil-field water containing representative concentrations of NaCl, CaCl₂, MgCl₂, and FeCl₂ in equilibrium with minerals in the system CaO-MgO-FeO-CO₂-O₂-H₂O. The calculations indicate that in the presence of calcite, the oxidative solubilities resulting from the formation of aqueous CO₂, n-carboxylic acids, and other oxidized organic aqueous species from n-alkanes in crude oil are several orders of magnitude greater than their stoichiometric counterparts. For example, the oxidative solubility in the aqueous phase of nonane with an activity of 0.1 in crude oil coexisting with calcite at the oil-water interface increases dramatically with increasing log f_O2,( g) at 120 °C and 400 bar from a stoichiometric solubility of 0.26 ppm to an oxidative solubility of 320 ppm at log f_o2 (g)= -56, which is typical of the fugacity of oxygen in hydrocarbon reservoirs at 120 °C and 400 bar. Because oil-field waters are generally acidic, production of carbonic acid by the oxidative dissolution of crude oil in hydrocarbon reservoirs favors generation of secondary porosity at the oil-water interface, which may be mediated by hyperthermobarophilic microbes.

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