Nuclear magnetic resonance (NMR) measurements are considered among the most reliable methods to evaluate porosity and pore-size distribution in fluid-bearing rocks. However, in reservoirs with complex pore geometry, there is still a challenge to accurately interpret NMR data to evaluate petrophysical properties of these reservoirs. In this paper, we propose to inject superparamagnetic iron oxide nanoparticles (SPION) into rock samples with multiple-porosity system (including natural fractures) and then quantify their impact on NMR measurements. The comparison of NMR data before and after nanoparticle injection improves characterization of pore-size distribution and pore connectivity.

The objectives of this paper are (a) to improve assessment of pore-size distribution and pore connectivity in the heterogeneous multiple-porosity system containing natural fractures, and (b) to enhance the reliability of reservoir characterization in challenging reservoirs such as complex carbonates and organic-shale formations. We conducted coreflood experiments to inject SPION in rock samples from sandstone, carbonate, and organic-shale formations, and obtained NMR T2 distribution before and after SPION injection. We also numerically simulated the NMR responses in different rock samples using a random walk algorithm. The comparison of simulated NMR T2 distribution before and after nanoparticle injection confirmed the experimental results well.

The results of laboratory experiments show non-uniform distribution of SPION in the porous media. We observed that the long-relaxation-time peaks in NMR T2 distribution significantly shifts to short-relaxation-time in the presence of nanoparticles, indicating that inter-connected large pores/fractures are most easily pervaded by SPION. However, the original short-relaxation-time peaks remained at the same positions with almost the same amplitudes and shapes after SPION injection, indicating that small pores are not pervaded by SPION. The same accumulative volume of water in the rock before and after SPION injection indicates that SPION pervasion in the rock only results in the shift of T2 relaxation time, but does not affect estimates of total porosity. We conclude from the experiments and numerical simulations that natural fractures, interconnected and isolated large pores, and small intra-granular pores can be accurately differentiated in NMR T2 distribution with the aid of SPION.

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