Paramagnetic nanoparticles are potentially useful for formation evaluation and reservoir monitoring, as they can be induced to move by an imposed magnetic field. Nanoparticles can be designed to have long-term dispersion stability in brine with minimal retention in reservoir rock and with preferential adsorption at oil-water interface. When exposed to magnetic field, they generate sufficient interfacial movements for external detection.

When paramagnetic nanoparticles are either adsorbed at oil-water interface or dispersed in one of two fluid phases co-existing in reservoir rock pores, and exposed to external magnetic field, the resultant particle movements displace the interface. Interfacial tension acts as a restoring force, leading to interfacial fluctuation and a pressure (sound) wave. Our previous work provided theoretical explanations for the motion of the interface between a suspension of paramagnetic nanorods and a non-magnetized fluid in a cylindrical dish, as measured by phase-sensitive optical coherence tomography (PS-OCT). Here we report on additional experiments carried out with a range of in-house synthesized and surface-modified iron-oxide nanoparticles. The numerical method was improved to be volume conserving for more quantitative matching.

The measurements of interfacial motion by PS-OCT confirm theoretical predictions of the frequency doubling and the importance of material properties, such as magnetic susceptibility, for the interface displacement thus offering insights into behavior in real porous media. With the combined experimental and modeling work, strategies for improved nanoparticle design are developed so that the interfacial, thereby acoustic, response can be magnified.

This laboratory and modeling study is an important step to develop a magnetic field-based method for an accurate, non-invasive determination of multiphase fluids distribution in reservoir rock.