NMR characterization of complex fluids by diffusion -- relaxation distribution functions

Many natural fluids are complex mixtures of different types of molecules. As an example, the molecular composition of crude oils typically consists of molecules with a number of carbon atoms that range between one to over 100. In addition to the diverse size, the constituent molecules can be classified into different chemical classes, such as saturates, aromatics, resins and asphaltenes. It is well known that measurements of diffusion and NMR relaxation times can give information on molecular size. We demonstrate that two-dimensional diffusion -- relaxation time distribution functions, f(D,T2), can provide a more unique fingerprint of complex fluids with information on both chain length distribution and chemical composition. The new approach is illustrated with results for different crude oils. The experiments were conducted at a Larmor frequency of 5 MHz and temperatures between 10 C and 58 C. The measurements show a strong correlation between the distributions of diffusion coefficients and relaxation times that are sample specific. The diffusion - relaxation correlation function provides information on the correlation between the rotational and the translational diffusion coefficients of each component of the fluid.