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

Abstract

Abstract 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 (Swir) 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, Swir from NMR T2, 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 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. Introduction Oil base drilling muds have the advantage of higher drilling rate and better well bore stability1. They are often used in zones such as problem shale or deep, hot hole, which could not be drilled safely or efficiently with a water base mud2. Among their additives, primary and secondary emulsifiers are used to make water emulsified in the external oil phase, and oil wetting agents are used to make the drilled cuttings and density control particles oil-wet. Excess amount of emulsifiers and oil wetting agents is often added to maintain the stability and rheological properties of the mud system2,3. These emulsifiers and oil wetting agents are mainly the wettability alteration materials4. Potentially, these OBM surfactants may invade into the near well bore formation with the base oil and change the originally preferential water-wet mineral surface to mix-wet or preferential oil-wet5,6, especially during the spurt- loss period before the mud cake fully builds up. It was reported that OBM invasion altered the reservoir rock and affected formation evaluation from NMR well logging and special core analysis6–8. However, quantitative connection between the effects of OBM invasion on NMR derived parameters (Swir and permeability) and some independent wettability measurements has not been investigated. The objectives of this study are to investigate the quantitative wettability alteration by synthetic OBM surfactants, the effects of wettability alteration on NMR estimated Swir, and the modifications of NMR interpretation when wettability alteration occurs. Four typical OBM surfactant systems from the industry were used. A series of concentration of emulsifiers and oil-wetting agents in the base oil and different invasion volume were examined. It aims to provide some laboratory experimental results to help to minimize the NMR misinterpretation caused by the invasion of OBM drilling fluids.