Restricted diffusion effects on nuclear magnetic resonance DT2 maps

Abstract

The 2D nuclear magnetic resonance diffusion-relaxation experiment (NMR [Formula: see text]) has proven to be a powerful method to characterize complex fluids. Molecular components with distinct diffusion coefficients are shown on [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] maps under various conditions and to show how restricted diffusion effects on [Formula: see text] maps can be distinguished from multiphase fluids. NMR [Formula: see text] 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 [Formula: see text] maps: the molecular diffusion length [Formula: see text] during the NMR diffusion encoding time and the characteristic pore size [Formula: see text]. For [Formula: see text], the effect of non-Gaussian diffusion was negligible and the NMR [Formula: see text] map showed only one peak. As [Formula: see text] approaches [Formula: see text], an additional peak with a smaller diffusion coefficient emerged (resembling the [Formula: see text] map of an unrestricted two molecular components fluid), and its relative intensity was maximized (to [Formula: see text]), when [Formula: see text]. As [Formula: see text] further increased, the relative intensity of the additional peak started decreasing, in contrast to the scenario of [Formula: see text] maps of multiphase fluids. We determined the extent and influence of restricted diffusion on NMR [Formula: see text] maps, and we informed the interpretation of NMR [Formula: see text] measurements, which are commonly used to quantify gas, water, and oil signals in reservoir rocks.