Restricted diffusion effects on nuclear magnetic resonance DT2 maps

The 2D nuclear magnetic resonance diffusion-relaxation experiment (NMR DT2) has proven to be a powerful method to characterize complex fluids. Molecular components with distinct diffusion coefficients are shown on DT2 maps as separate peaks. In porous media such as reservoir rocks, molecular diffusion is restricted such that the apparent diffusion coefficient is time dependent and the diffusion behavior is non-Gaussian. Such restricted diffusion effects can manifest on the DT2 maps and complicate the interpretation of the results, but so far, they have not been systematically investigated. We used controlled laboratory experiments to demonstrate the influence of non-Gaussian restricted diffusion on NMR DT2 maps under various conditions and to show how restricted diffusion effects on DT2 maps can be distinguished from multiphase fluids. NMR DT2 experiments were carried out on a series of water-saturated packs of glass beads and two rock cores. The results revealed the important role of two critical length scales controlling the restricted diffusion effects on NMR DT2 maps: the molecular diffusion length l(D) during the NMR diffusion encoding time and the characteristic pore size d(pore). For l(D) << d(pore), the effect of non-Gaussian diffusion was negligible and the NMR DT2 map showed only one peak. As l(D) approaches d(pore), an additional peak with a smaller diffusion coefficient emerged (resembling the DT2 map of an unrestricted two molecular components fluid), and its relative intensity was maximized (to similar to 17%), when l(D) approximate to d(pore). As l(D) further increased, the relative intensity of the additional peak started decreasing, in contrast to the scenario of DT2 maps of multiphase fluids. We determined the extent and influence of restricted diffusion on NMR DT2 maps, and we informed the interpretation of NMR DT2 measurements, which are commonly used to quantify gas, water, and oil signals in reservoir rocks.