Effects of OBM Invasion on Irreducible Water Saturation: Mechanisms and Modifications of NMR Interpretation

摘要

In this study, the effects of synthetic oil base mud (OBM) surfactants (emulsifiers and oil-wetting agents) on wettability alteration, NMR response and irreducible water saturation (S_(wir)) were systematically examined with Berea and limestone cores. Results show that the originally strongly water-wet Berea and limestone cores are altered to be intermediate-wet or oil-wet by OBM surfactants. As a result, S_(wir) from NMR T_(2,cutoff) model with the default assumption of water-wetness generally underestimates the measured value. The magnitude of underestimation depends on three parameters: 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. The mechanisms of Swir underestimation and modifications of NMR interpretation when wettability alteration occurs were investigated. In the case of an oil-bearing zone at irreducible water saturation, OBM invasion does not significantly decrease the actual Swir, but changes the water and oil relaxation time distributions due to wettability alteration. This is visualized by the diffusion editing technique. When wettability alteration occurs (water-wet to intermediate-wet or oil-wet), a T_(2,cutoff) value larger than the one based on waterwetness is needed because the irreducible water relaxes at a longer relaxation time. Correlation between this modified T_(2,cutoff) value and the Amott-Harvey wettability index was found.