Analysis Of Nmr Diffusion Coupling Effects In Two-Phase Carbonate Rocks: Comparison Of Measurements With Monte Carlo Simulations

Nuclear Magnetic Resonance (NMR) is the only logging technique available to estimate pore-size distributions. However, quantitative interpretation of NMR data can become uncertain in carbonate rocks because of unaccounted diffusive coupling between existing pore scales. The objective of this study is to assess the relative importance of diffusive coupling and temperature using practical examples encountered in the interpretation of NMR data. The core of our work is based on the analysis of experimental NMR measurements and their comparison with numerical simulation results. Our numerical simulation algorithm consists of Monte-Carlo random walks in three dimensions and was specifically designed to account for two-phase fluid saturations in the presence of a bimodal poresize distribution. We present numerical simulation results that reproduce MR/L experimental data acquired in carbonate rocks exhibiting bimodal pore-size distributions and two-phase fluid saturations. Simple interpretation models are derived to include NMR coupling effects by way of cross-interactions between the fluids borne within the different types of pores. Such models have been subsequently used to assess the importance of diffusion coupling. Experimental data acquired from rock core samples was also used to assess the influence of temperature on the estimation of movable fluids volumes otherwise determined assuming a constant formation T2cutoff. It is shown that temperature has moderate effect on T2 distribution, T2~, off, and BVI determination. The diffusive coupling effect is more significant on mapping I"2 distribution to pore size distribution than on the determination of BVI.