Low-Field NMR Spectrometry of Chalk and Argillaceous Sandstones: Rock-Fluid Affinity Assessed from T-1/T-2 Ratio

Nuclear magnetic resonance (NMR) procedure typically minimizes the effects of external magnetic field gradients on the transverse relaxation. Thus, longitudinal, and transverse, T-2, relaxation times should in principle be similar. However, internal magnetic field gradients related to minerals can shorten T-2, as compared to provided the saturating fluid has high affinity to the solid.

Consequently, the T-1/T-2 ratio should quantify the affinity between the mineral and wetting pore fluid, so we estimate wettability from logging data by comparing the T-1/T-2 ratio of oil and water peaks in the reservoir zone to the T-1/T-2 ratio in the water zone. We tested the hypothesis on core samples and used the predicted wettability to successfully determining the elastic bulk modulus of samples containing oil and water.In order to investigate the T-2-shortening, we performed 1D and 2D NMR experiments on samples of chalk, kaolinitic sandstone, and chloritic greensand, saturated either with water, oil or oil/water at irreducible water saturation.

The 1D NMR experiment involved determination of T-2 spectrum, whereas the 2D NMR experiments included determination of T-1-T-2 and D-T-2 maps, where D is the intrinsic diffusion coefficient.T-2 spectra show that in all water-saturated samples, surface relaxation dominates; in oil-saturated chalk and kaolinitic sandstone, bulk relaxation dominates; whereas T-2 of oil-saturated greensand shows surface relaxation in the part of the spectrum representing chlorite.

In all samples with irreducible water saturation, water shows surface relaxation, whereas oil shows bulk relaxation. In line with this observation D-T-2 maps of these samples show field gradient effects in the oil, but not in the water indicating that the water is trapped between solid and oil due to restricted diffusion.A T-2 shortening will increase the T-1/T-2 ratio, so we use the T-1/T-2 ratio obtained from T-1-T-2 maps as a measure of fluid-mineral affinity.

By this measure, the chalk shows high affinity for water, the kaolinitic sandstone has no clear preference for oil or water, whereas chloritic greensand shows different behavior for small and large pores. Small pores (fast-relaxing components) have T-1/T-2 = 2.0 when water saturated, but T-1/T-2 = 3.8 when oil saturated, indicating oil-affinity of chlorite.

By contrast, large pores (slow-relaxing components) have significant preference for water (T-1/T-2 = 2.2) as compared to oil (T-1/T-2 = 1.2 to 1.4).Overall, this paperprovides an insight into the preference of a mineral to a fluid and ultimately into determining the wettability and correct pore-fluid distribution of a reservoir rock.