Nuclear magnetic resonance simulations of nano-scale cores and microscopic mechanisms of oil shale

The mineral composition and pore structure of organic shale are complex, and studies of the porosity, seepage characteristics, and pore structure by using nuclear magnetic resonance (NMR) can be of great significance for the identification and quantitative evaluation of shale oil reservoirs. In particular, pore-scale NMR numerical simulations and core NMR experimental analyses of organic shale can provide a theoretical basis for NMR log interpretations. An ideal digital core and actual shale digital core were constructed by simulating the deposition process and using computerized tomography (CT) scanning, respectively. Then, one- and two-dimensional NMR numerical simulations were carried out by random-walk method to study the NMR responses of cores under different compaction and water saturation conditions. The microscopic NMR numerical simulations showed that as the degree of compaction increased, the porosity of the cores decreased, amplitudes of the T(2)( )distributions decreased significantly, and T-2 distributions moved toward the direction of short relaxation. As the echo spacing decreased, the T-2 distributions shifted to the left, smaller pores were measured, and NMR porosities of shale increased; these findings are consistent with those of the rock NMR experiments. In two-dimensional T-2-D maps, the signal of the wetting fluid deviated from its free diffusion coefficient line, which was indicative of restricted diffusion in tight rock. The restricted diffusion effect of water was larger than that of oil. These microscopic numerical simulations provide a physical basis for interpreting NMR macroscopic responses, and the simulated NMR responses should be helpful for fluid typing in oil shale.