The Effect of Micrite and Micro-porous Skeletal Grains on the NMR Response of Carbonate Rocks

Abstract:

Nuclear magnetic resonance (NMR) measurements are commonly used to obtain an estimate of the pore size distribution in a saturated material. The NMR relaxation time T₂ is inversely proportional to the surface-area-to-volume (S/V) ratio of the pore space. An estimate of pore size is obtained by defining a geometrical parameter (α) linking pore radius to (S/V) ^-1. For example, for a spherical pore α =3; i.e. the radius is equal to 3(S/V)^-1.

Carbonate rocks have a complex pore structure, being composed of many different materials. In this study we assessed the use of NMR to differentiate between the water held in the pore space in micrite (carbonate mud) and in the pore space of micro-porous skeletal grains, two major constituents in carbonates. We expected to find significant differences in NMR T₂ measurements on these two materials, given that the pore and particle size of the micrite is ~100 times smaller than that of the skeletal grains. However, we found that the T₂ distributions for the two materials overlap. Measurements of porosity and measurements of surface area with a nitrogen BET method showed that S/V of the two materials is very similar. The skeletal grains have very high surface area due to surface roughness and micro-porosity. The geometric factor (α), relating (S/V) ^-1 to pore radius, was found to be two orders of magnitude larger for the skeletal grains than for micrite. We concluded that knowledge of the geometric value is needed in order to accurately estimate pore size from NMR T₂ data.