TD-NMR
Pore Diameter Distributions:
When the pore space is saturated with aqueous phase, T2values in a pore diameter distribution (PDD) are directly associated with the number of protons within the pore space. In turn, the values of T2 relate to pore sizes [28] (Eq. (2)). Figure 3 (A-C) illustrates the T2 distribution and PDD of three core plugs with two surface relaxivities. The larger surface relaxivity is used to set the upper bound pore diameter to compare with the nominal resolution in MRI analyses in this section. This provides the upper resolution limit needed to visually resolve pores.
As Figure 3A shows how longer T2 values correspond to larger pore sizes and vice versa. The pore diameter of deionized water-saturated Berea Sandstone estimated by its T2distribution exhibits a trimodal distribution ranging from 3 μm to 100 μm (Figure 3A), suggesting that three populations of pore sizes can be observed. In order, a dominant peak (roughly centered around 400 ms) is associated with the largest pores (30 μm-100 μm). An intermediate peak (centered at roughly 150 ms) represents the mesopores, at a length scale from 10 μm to 30 μm. Finally a smaller peak (centered at roughly 40 ms) resides in the micropore region ranging from 3 μm to 10 μm. The connectivity among pores in a rock sample can be characterized by the connection among multiple peaks in PDD [29]. The existence of less defined or somewhat overlapping peaks in the Berea sandstone PDD likely reflects NMR diffusion coupling, which would imply well-developed connections among pores in this rock, most likely through pore-throat connections.
Figure 3B shows the PDD of Indiana Limestone with two bound values of surface relaxivity. In contrast to Berea Sandstone, the PDD of Indiana limestone contains more widespread multiple peaks. The rightmost peak, roughly centered at 1000 ms, represents the macropores, with pore diameters between 40 μm and 200 μm. The leftmost peak, located at approximately 50, indicates the pore diameter occurring in the range from 0.6 μm to 2 μm. The mesopores, ranging from 2 μm to 40 μm, occupy a relatively large portion of the pore volume in the Indiana Limestone. Furthermore, the multiple peaks above 2 μm confirm the high porosity of Indiana Limestone as the amplitude of the PDD peak is directly related to the porosity of the rock samples. The continuous PDD peak connection suggests the multiple well-connected pores of Indiana limestone.
Figure 3C shows a bimodal PDD for the Madison Limestone sample. More specifically, the predominant narrow (right-most) peak, at roughly 350 ms, represents the diameter of the majority of intergranular pores. This PDD is approximately 80.17 μm and a small portion of micropores with diameter less than 3 μm are also present and are shown by the most left peak. Additionally, the disconnected bimodal distribution in Figure 3C suggests a rather disconnected pore system in this sample compared to Berea Sandstone and Indiana Limestone (Figures 3A and 3B). The narrow peaks displayed by this sample suggest the presence of a more homogeneous pore space. It is worth mentioning that vugs (or dissolution pores) cannot be detected in the PDD plots, since the surface relaxation rate in the vugs is small, leading to signals too weak to be detected. Moreover, the porous medium of the Madison Limestone sample is dominated by intergranular porosity, and the vugs are randomly placed and surrounded by intergranular pores. Therefore, the signal from the vugs is further weakened by diffusion coupling between them [23,30].