A pore scale study of ferrofluid-driven mobilization of oil
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Ferrofluids are stable dispersions of magnetic nanoparticles in a liquid carrier. The easily controllable magnetization of ferrofluids has motivated their use in a broad range of applications, from the non-intrusive imaging of organic tissues to the sealing and stability improvement of bearings. More recently, experiments verified the feasibility of injecting nanoparticles with specifically-designed coatings into permeable rocks without significant retentions or permeability loss. The possibility of controlling fluid displacements in a porous medium by manipulating magnetic stresses with an applied magnetic field suggests ferrofluids have a potential for subsurface applications, particularly in the field of enhanced oil recovery. Although extensive research has been devoted to the mechanics of ferrofluids, this topic is never analyzed under the conditions that are likely to occur in the subsurface environments. In particular, research on the behavior of two-phase systems in which the ferrofluid is wetting is still lacking. This work has the goal of simulating immiscible displacements influenced by the magnetic stresses acting on a wetting ferrofluid at pore scale. The simulations are based on a model for the quasi-static displacement of the liquid interface that couples the effects of the fluid flow with the magnetic stresses. An approach for the numerical simulation is developed based on a level set method for tracking the interface displacement, which is suitable for the complex shapes appearing in the pore space. The explicit-jump immersed interface method, which handles irregular domains with non-conforming grids, is employed in specific versions for the fluid flow and the magnetic field. The results indicate that the magnetic stress distribution is strongly affected by the configuration of the magnetic field in the regions of proximity between liquid interface and the solid surface. By applying a field parallel to the flow path, the magnetic stresses push the non-wetting phase away from the solid surfaces resulting in an interface configuration that reduces the flow viscous stresses and thus favors applications related to the transport of non-wetting liquids. A field perpendicular to the flow path generates magnetic stresses that contribute to the mobilization of ganglia trapped in snap-off geometries, suggesting ferrofluids can be used to reduce residual oil saturations in reservoirs in synergy with other methods.