Calculation of diffusion coefficients for aqueous organic species at temperatures from 0 to 350 °C

Abstract

Evaluation of hydrocarbon transport through the pore spaces of saturated rock in the subsurface as a function of time and distance requires accurate values of diffusion coefficients for aqueous organic species. Analysis of aqueous tracer diffusion coefficients (D0) for normal alkanes, alcohols, amides, carboxylic acids, alkylbenzenes, and alkylnaphthalenes reported in the literature indicates that diffusional activation energies decrease with increasing temperature, reaching a constant limiting value of ~3300 cal mol−1 at temperatures a: 1̆00°C. This observation is consistent with the modified Arrhenius expression reported by Oelkers and Helgeson (1988). The Kirkwood-Riseman equation is used to predict D0 values as a function of the polymer chain length of hydrocarbons. Regression of experimental D0 data with a combined expression of the Kirkwood-Riseman and modified Arrhenius equations yields parameters which permit calculation of tracer diffusion coefficients for over 50 aqueous organic species at temperatures from 0° to 350°C and Psat. (psat refers to pressures corresponding to the liquid-vapor equilibrium curve for H2O at temperatures greater than 100°C and 1 bar at lower temperatures.) Resulting values of D0 permit evaluation of the extent of diffusional mass transfer in both contaminated near-surface environments and in the porewater adjoining oil field reservoirs. The computed tracer diffusion coefficients, which are qualitatively similar to D0 values previously calculated for aqueous ions, increase substantially with increasing temperature. For example, the tracer diffusion coefficient of aqueous toluene increases from 0.36 to 27.1 × 10−5 cm2sec−1 in response to increasing temperature from 0 to 350°C.