Prediction of molecular diffusion coefficients of gases in hydrocarbon liquids at high pressure and temperature

• Main Author: Emadi, Mohammad Ali
• Published: University of Surrey 2004
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.731074

Mass transfer by molecular diffusion is the basic physical mechanism underlying many important areas of chemical engineering and petroleum engineering. In recent years, the problem of mass transfer by the mechanism of molecular diffusion in oil reservoirs, when a non-equilibrium gas is injected into the reservoir, has become increasingly important. In oil recovery projects, gas is injected into oil reservoirs for different reasons such as maintenance of reservoir pressure and enhanced recovery of oil. In these two cases the rate of dissolution of a gas such as methane in a quiescent body of hydrocarbon oil is controlled primarily by the rate of diffusion of the dissolved gas from the gas-oil interface into the body of the liquid phase. In all the above situations, molecular diffusion of gas into liquid or transfer of dissolved gas between enriched and heavier liquid phase due to differences in compositional gradients between gas and liquid phases is important. The most important property required to determine the rate of mass transfer between the two phases in all these cases is the molecular diffusion coefficient at high pressure and temperature. The present investigation is aimed at a systematic study of the mechanism of molecular diffusion of gases in liquids by measuring the diffusion coefficients of methane in dodecane and in a typical Iranian crude oil up to a pressure of 35 MPa and at several temperatures. All tests are conducted in an accurate high-pressure diffusion cell with "finite-domain" moving boundary behavior. The data acquired is used to assess the predictions of various available correlations for diffusion coefficients. Several liquid hydrocarbon swelling tests comprising dodecane and a typical Iranian crude oil as liquids and methane as gas are performed and swelling heights of liquid as a result of gas molecular diffusion are measured at various temperatures (T=25°C to82°C ) and pressures (P=3.2 to 35 MPa), characterized by a sharp increase in volume followed by gradual increase toward the saturation concentration of methane in the liquid phase. A mathematical model is developed to calculate diffusivities using semi-infinite boundary conditions, hi this model, a variable power profile with time is introduced to allow for the moving interface boundary. The current solutions offer significant improvement over those in previous literature that assume a steady-state interface boundary condition with a parabolic concentration profile. The proposed model offers excellent predictions of experimental data for diffusion coefficients of methane-dodecane and crude oil systems. A computer program using a neural network model is set-up to predict the dimensionless diffusivity for special use with more complex systems such as crude oil reservoir flows in fractures and matrix conditions. The results obtained by this software show about +/- 2% deviation in comparison with the experimental data from the diffusion cell experiments.