Quantitative analysis of diffusional pore coupling from T2-store- T2 NMR experiments

Abstract

Low field NMR T 2 distribution is an efficient tool for characterizing pore size distribution in porous media. In NMR relaxation experiments, we measure the magnetization decay characterized by the so-called T 2 relaxation time, resulting essentially from liquid–solid interactions of the spins carried by molecules and exploring the pore space by diffusion. For multimodal systems, a new technique called T 2-store- T 2 allows the analysis of diffusional pore to pore exchange that is extremely useful for the characterization of connectivity. This technique uses 2D inverse Laplace techniques and produces T 2– T 2 maps. Qualitatively, a system is coupled when off-diagonal peaks are observed. Based on an analytical solution describing diffusional coupling between two pore populations, we propose defining a coupling factor that quantify the degree of coupling between the two populations, providing an easy understanding of the complex analytical solution. This analysis allows understanding 1D T 2 experiments as well, and indicates some limitations of the T 2 characterization when interpreted as a pore size distribution. We provide examples of bimodal pore structures in which we applied our methodology: a clay gel system, a shaly sandstone and two double porosity carbonates. These systems are also described by conventional techniques (mercury injection, SEM visualization) and illustrate weak, intermediate and strong coupling. Despite the presence of distribution of pore sizes, the two pore system exchange model gives satisfactory results for the quantitative analysis of the coupling and T 2-store- T 2 experiments. In carbonates, more complex exchanges can occur between micro-, meso- and macropores.