Ali Reza Mehrabi

Asphalt flocculation and deposition. III. The molecular weight distribution

Extensive new experimental data are presented for the molecular weight (MW) distributions of the asphalt and asphaltene aggregates formed when a solvent is injected into a crude oil. The effects of various factors such as the nature of the solvent, the solvent-to-oil volumetric ratio and ageing of the solution are all investigated. It is shown that if the asphalt- or asphaltene-containing solution has not been aged for long enough, a bimodal MW distribution is always obtained, which is due to the existence of two different types of aggregates with distinct structures and mechanisms of formation. However, after long enough the MW distribution is unimodal. Based on models proposed for the formation of the structure of the asphalt and asphaltene aggregates, which are based on the analysis of small-angle X-ray and neutron scattering and precipitation data, analytical equations are proposed for the MW distribution of the aggregates which provide accurate predictions of the data.

Percolation and flow in geological formations: upscaling from microscopic to megascopic scales

Modelling transport processes in a disordered system with broadly distributed heterogeneities, such as a field-scale porous medium, is often hampered by the fact that a very large set of equations (of the order of several millions) has to be solved repeatedly if detailed information about the systems morphology is available. Solving such a large set is often not practical. We discuss a novel and very efficient method for scale up of such systems such that no important information about their characteristics is lost at any length scale, and at the same time the number of transport equations to be solved is reduced drastically to a manageable level. The method, which is applicable to any type of heterogeneous medium, including one with percolation disorder, uses a wavelet transformation to coarsen the original fine-scale description of the system, such that finer resolution is maintained in regions of high transport properties, whereas coarser property description is applied to the rest of the system. The computational cost of the method is several orders of magnitude less than those of the most efficient methods currently available. The performance of the method is demonstrated by its application to calculation of the effective flow properties of several models of field-scale porous media. The method is equally applicable to other disordered media with broadly distributed heterogeneities, such as amorphous semiconductors.

COMMENT ON : DIFFUSION OF IONIC PARTICLES IN CHARGED DISORDERED MEDIA. AUTHORS' REPLY

This comment points out that simulation results of Mehrabi and Sahimi [Phys. Rev. Lett. 82, 735 (1999)] are iconsistent with exact bounds, renormalization group calculations, and previous numerical simulations.