Comparison of NMR simulations of porous media derived from analytical and voxelized representations

Guodong Jin, Carlos Torres-Verdín, Emmanuel Toumelin
2009 Journal of magnetic resonance (San Diego, Calif. 1997 : Print)  
14 Keywords: 15 NMR response 16 Random-walk method 17 Grain pack 18 Voxel image 19 Surface-area effect 20 2 1 a b s t r a c t 22 We develop and compare two formulations of the random-walk method, grain-based and voxel-based, to 23 simulate the nuclear-magnetic-resonance (NMR) response of fluids contained in various models of por-24 ous media. The grain-based approach uses a spherical grain pack as input, where the solid surface is ana-25 lytically defined without an approximation. In the
more » ... ased approach, the input is a computer-26 tomography or computer-generated image of reconstructed porous media. Implementation of the two 27 approaches is largely the same, except for the representation of porous media. For comparison, both 28 approaches are applied to various analytical and digitized models of porous media: isolated spherical 29 pore, simple cubic packing of spheres, and random packings of monodisperse and polydisperse spheres. 30 We find that spin magnetization decays much faster in the digitized models than in their analytical coun-31 terparts. The difference in decay rate relates to the overestimation of surface area due to the discretiza-32 tion of the sample; it cannot be eliminated even if the voxel size decreases. However, once considering 33 the effect of surface-area increase in the simulation of surface relaxation, good quantitative agreement 34 is found between the two approaches. Different grain or pore shapes entail different rates of increase 35 of surface area, whereupon we emphasize that the value of the "surface-area-corrected" coefficient 36 may not be universal. Using an example of X-ray-CT image of Fontainebleau rock sample, we show that 37 voxel size has a significant effect on the calculated surface area and, therefore, on the numerically sim-38 ulated magnetization response.
doi:10.1016/j.jmr.2009.07.021 pmid:19665909 fatcat:2brjio3nbbdofgdvyik7xa2juy