Nanoparticle (NP) based enhanced oil recovery (Nano-EOR) has been considered as a promising future EOR strategy. However, although many mechanisms of Nano-EOR have been proposed, a lack of direct connections between the pore-scale mechanisms and the macro-scale oil recovery performance makes it hard to determine which mechanisms are dominant. In this work, we discovered a novel phenomenon of nanoparticle-crude oil interaction in pore-scale. Multi-scale experiments were conducted to connect this novel pore-scale phenomenon's role to oil recovery performance.

A microchannel with dead-end pore was used to observe crude oil-NP interactions, on which crude oil can be trapped in the dead-end pore with a stable crude oil-aqueous phase interface at the pore-throat. A glass porous micromodel was used to conduct oil displacement experiments. ASW was used as the secondary flooding fluid, and 2000 PPM negatively charged NP in ASW was applied as the tertiary flooding fluid. Saturation profiles were recorded and analyzed by advanced image analysis tools. A coreflood through the sandstone sample was also conducted with similar conditions to the micromodel-flood experiments.

A phenomenon that has never been reported was observed from the dead-end pore microchannel. It was observed that crude oil can considerably swell when contacting the nanoparticle aqueous suspension. In an ideal case (5 wt% NP in DI water), the oil volume more than doubled after a 50-hour swelling. The possible explanation for the crude oil swelling could be spontaneous formation of water droplets in the crude oil phase. NP can very likely affect the distribution of natural surfactants in crude oil (on the interface or inside oil phase), which breaks the water balance between aqueous phase and crude oil. This view has received support from quantitative experiments. It was shown from 2.5 D micromodel flood experiments that 11.8% incremental oil recovery comes slowly and continuously in more than 20 hours (40 pore volumes). From a saturation profile analysis, swelling of crude oil was found to improve sweep efficiency. Coreflood experiments also showed that the incremental oil was slowly and continuously recovered in about 20 hours during NP flooding. We propose that reduction of local water mobility by oil swelling in the swept region is the mechanism of sweep efficiency improvement.

Swelling of crude oil under a NP environment was observed for the first time, with a systematic theory proposed and examined by quantitative experiments. The micromodel flood and coreflood experiments showed slow incremental oil recovery with a similar time scale to the oil swelling. Image analysis on the micromodel flood demonstrated improvement in the sweep efficiency during NP flooding. The mechanism for this sweep improvement is proposed.

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