NMR surface relaxation, wettability and OBM drilling fluids
Hirasaki, George J.
Doctor of Philosophy
NMR surface relaxation mechanisms and their dependence on temperature were investigated. Paramagnetic ions are a very important factor of surface relaxation. The mechanism of the surface enhanced dipole-dipole interaction is negligible when the paramagnetic ion mechanism is strong. In such case, surface relaxivity of both water and oil depends only weakly on temperature. However, in cases when paramagnetic ion mechanism is weak, the contribution from surface enhanced dipole-dipole interaction could dominate the total surface relaxation rate. In such case, surface relaxivity of both water and oil decreases with temperature. Effects of synthetic oil base mud (OBM) surfactants on wettability alteration, NMR response and irreducible water saturation (Swir ) were systematically examined. The originally strongly water-wet Berea and limestone cores are altered to be intermediate-wet or oil-wet by OBM surfactants. As a result, Swir from NMR T 2, cutoff model with the default assumption of water-wetness generally underestimates the measured value. The magnitude of underestimation depends on the type of OBM surfactants, their concentration in the flushing fluid, and the flushing volume. The magnitude of underestimation correlates with the Amott-Harvey wettability index. These results suggest that the effects of OBM invasion on estimation of Swir can be minimized by controlling the volume of OBM invasion and the concentration of OBM surfactants. Mechanisms of Swir underestimation and modifications of NMR interpretation when wettability alteration occurs were investigated. In the case of an oil-bearing rock at Swir, OBM invasion does not significantly decrease the actual S wir, but changes the water and oil relaxation time distributions due to wettability alteration. This is visualized by the D-T 2 map. When wettability alteration occurs (water-wet to intermediate-wet or oil-wet), a T2, cutoff value larger than the one based on water-wetness is needed because the irreducible water relaxes at a longer relaxation time. Correlation between this modified T2, cutoff value and the Amott-Harvey wettability index was found. Three unusual NMR properties were observed for the filtered OBM filtrates due to the unexpected presence of trace amount of paramagnetic particulates. Interactions between the filtered OBMF and rock samples can cause wettability alteration, enhanced surface relaxation and increased internal gradient strength.