Changes of the local pore space structure quantified in heterogeneous porous media by H-1 magnetic resonance relaxation tomography

Magnetic resonance imaging and relaxation analysis are combined in a spatia lly resolved technique (relaxation tomography), which is able to quantify t he parameters connected to the local structure in the internal regions of a porous material saturated by water, giving information on the pore space s tructure beyond the nominal instrumental resolution. Voxel-by-voxel longitu dinal (T-1) and transverse (T-2) relaxation curves are acquired in order to obtain T-1, T-2 and S(0) maps, where S(0) is the extrapolation to zero tim e of the total equilibrium magnetization corrected for T-2 decay. The propo sed method permits evaluation of the porosity (ratio of pore space to total volume), at different length scales, from the sample to the voxel, not all achievable by traditional methods. More striking is its ability to describ e how porosity is shared among different classes of surface-to-volume ratio s of diffusion cells (the regions that the individual water molecules, star ting at their particular positions, can experience by diffusion before rela xing). This is a consequence of the fact that relaxation times of water con fined in a porous material can, under favorable circumstances, distinguish regions with the same local porosity but with different pore sizes and conn ections. So, parameters can be introduced, such as the microporosity fracti on, defined as the fraction of the "micropore" volume with respect to the t otal pore volume, and several voxel average porosities, defined as the aver age porosities of the voxels characterized by particular classes of diffusi on cells. Moreover, the imaging methods enable us to get all this informati on in a user-defined region of interest. The method has been applied to qua ntify changes in the structure of carbonate cores with wide distributions o f pore sizes induced by repeated cycles of freezing and heating of the samp le. With freezing, the microporosity fraction decreases significantly; the voxel average porosity of voxels with T-1 shorter than for free water tend to decrease; and the distributions of porosity as functions of T-1 show a t rend, with much more signal with the T-1 of free water, in accordance with the picture suggesting large vugs breaking, with fractures contributing to homogenizing the structure of the pore space and favoring coupling between neighboring pores. (C) 2001 American Institute of Physics.