Characterization of Pore to Pore Diffusive Exchange Using Nmr T2-store-t2 2d Experiments

Low field NMR T2 distribution is an efficient tool for characterizing pore size distribution in porous media. For multimodal systems, a new technique called T2-storeT2 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 T2-T2 maps. Qualitatively, a system is coupled when off-diagonal peaks are observed. Based on an analytical solution describing diffusional coupling between two pore sizes, we propose defining a coupling factor that quantify the degree of coupling between two pore populations, and allows an easy understanding of the complex analytical solution. This analysis allows understanding 1D T2 experiments as well, and provides some limitations of the T2 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 a double porosity carbonate. 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 T2-store-T2 experiments. However, in carbonates, more complex exchanges can occur between micro, meso and macro pores. INTRODUCTION Diffusional pore coupling is an issue of practical importance for the correct interpretation of NMR pore size distribution performed in the laboratory and in situ. At the extreme situation, a multimodal pore system may appear as unimodal due to pore to pore diffusional exchange and this has important interpretation consequences. This phenomenon has been studied initially to explain the temperature dependence of NMR relaxation data. Indeed, relaxation in porous media should not depend on temperature and this fact was put forward to justify the surface dominated regime [1]. However, in carbonates, a significant temperature dependence can occur, typically when comparing laboratory and log data performed respectively at 20 and 100°C. This is a direct effect of pore coupling and a solution to predict the shift of the T2 distributions was proposed [2]. Later, Anand et al. [3,4] proposed a 1D geometric model and a coupling parameter to describe the pore to pore coupling phenomenon and applied it to various situations. For bimodal pore systems, the shift of macro and micro pore NMR peaks can be predicted as a function of the coupling parameter comparing the characteristic relaxation rate to the diffusion rate. From the NMR point of view, the methods for observing the modification of the environment of spin carrying molecules has been evidenced very early in the