Gc. Borgia et al., Changes of the local pore space structure quantified in heterogeneous porous media by H-1 magnetic resonance relaxation tomography, J APPL PHYS, 90(3), 2001, pp. 1155-1163
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.