Fundamentals of Fluid Flow in Porous Media
Diffusion Coefficient: Measurement Techniques
It is noteworthy that there is no well-established and universally applicable technique for measuring the molecular diffusion coefficient. Unlike the measurements of viscosity or thermal conductivity, for which standardized techniques and equipment are readily available, the measurements of mass transfer characteristics are often more difficult due to difficulties in measuring point values of concentration and other issues which complicate this transport process. Considerable efforts were made to determine diffusion coefficient for diffusion of solvent in the oil experimentally. Different experimental methods can be classified into direct methods and indirect methods. Also there are some empirical correlations based on the experimental results that could be used under some condition to find the dispersion coefficient.
Direct methods evaluate the diffusion coefficient by measuring concentration of the diffusing species (solvent) as a function of depth of penetration. Such methods are often more reliable and include the wide variety of physicochemical methods like mass spectrometry, radio-active tracer technique, spectrophotometry etc. The diffusivity is estimated by using compositional analysis techniques (Schmidt 1989). The drawbacks of direct methods are expense, time consumption and many of them are system-intrusive.
Indirect methods measure the changes of one of the system parameters that depend on the diffusion rate. These parameters could be the rate of change of solution volume or movement of the gas-liquid interface, rate of pressure drop in a confined cell which is known as pressure decay method, rate of gas injection from the top to a cell in which the pressure and solution volume are kept constant, magnetic field characteristics, computed tomography (CT) analysis and dynamic pendant drop analysis. The advantage of these methods is that they do not need to determine the change in composition. In this section, the more common and modern methods will be introduced.